AGR 3102- Prinsip Sains Rumpai
PJJ/UPMET
Unit 1
INTRODUCTION TO WEED SCIENCE
Introduction to Unit
Students will be introduced to weed science study and the importance to study weeds.
Objectives to Unit
1. Define the terminology of pest and differentiate between weed, crop and plant.
2. Explain the distribution of weed species all over the world.
3. Discuss the effect of weeds in agricultural and non-agricultural aspects.
4. Learn about the concept and importance of weed in our life.
Topic 1: Introduction to Weed Science
a.
What is Pest?

Any organism which can become an enemy to human.

Potentially threaten crops, beauty of the surrounding, health etc…

They interfere with human activities, need and desire.

Consist of animals, insects, microorganisms, WEEDS.
b.
Plants, Crops & Weeds

Plants: all living creatures in Plant Kingdom/Plantae (either wild or
cultivated).

Crops: all plants which are cultured/cultivated.

Weeds: plants which interfere with human needs (direct or indirect).

All crops/weeds are plants.
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What is Weed???

Often defined as “a plant out of place”, or “a plant growing where it is not
wanted”.

From above definition, we can classify weeds into 2 categories: plants
regarded as weed as an occasional nuisance, and true weed plants.

True weeds/weedy plants possess certain definable characteristics that set
them apart from other plant species.

Best defined as “plants that are unintentionally planted, competitive,
persistent, and pernicious, and are undesirable because they interfere with
human activities” (WSSA).
d.
What is Weed Science???

Weed science is the discipline that investigates the biology and ecology of
weeds and how best to manage these plant species for the betterment of
mankind.
e.
Weed Distribution

Almost 6%, or 18,000 species, considered/behave as weeds (A global
compendium of weeds. 2002 - R.P. Randall.)

From 18,000, 202 spp. considered as problematic and herbicide resistance
weeds (account for 90% of world crop losses):
http://www.weedscience.org/In.asp

12 crops from 5 families provide 75 % of the world food and the same 5
families provide many of the worst weeds.

Our major crops and weeds share certain characteristics, and perhaps a
common origin.
f.
Invasive Plants/Weeds

Defined as an exotic (or alien) species that is non-native to the ecosystem and
whose introduction causes, or is likely to cause, economic or environmental
harm or harm to human health.
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Invasive species can be thought of as biological pollutants, with potentially
severe impacts on the ecosystems they inhabit.

Can be problems because:
a.
Destroy wildlife habitat.
b.
Reduce opportunities for hunting, fishing, and other recreational
activities.
g.
c.
Displace threatened or endangered species.
d.
Reduce plant and animal diversity because of weed monocultures.
e.
Cost millions in management and loss of productivity
Impact of Weeds in Agriculture

Regarded by many farmers as the crops' greatest pest.

Weeds compete with the cultivated crops for nutrients, moisture, sunlight, and
space.

They shelter/host pests and diseases that attack the crop.

Reduce crop yields and farmers’ incomes.

Reduce product quality (grain contamination).

Impede water flow in drainage ditches, irrigation canals, and culverts.

Controlling weeds can be a lot of works and costly.

Economic losses due to weeds: 60-70 % up to 100% (differs according to
crop).

Some weed spp. can make animals ill after digestion. Example: L. camara.
The toxic its foliage contains cause hepatotoxicity (liver damage) and
photosensitivity (skin and sight) in grazing animals such as sheep, goats,
bovines, and horses (Barceloux, Donald G. 2008. Medical Toxicology of \
Natural Substances: Foods, Fungi, Medicinal Herbs, Plants, and Venomous
Animals. John Wiley and Sons. pp. 867–868).
h.
Impacts of Weeds in Non-agriculture Areas

Landscapes, golf courses
1. Reduce aesthetic values.
2. Less tolerant of traffic than turf species: cause a less uniform playing
surface.
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Roadsides, rights-of-way
1. Interfere with vision, create hazards (cars and trees don’t mix).
2. Trees grow into powerlines.
3. Dried weeds can be a fire hazard along railroads, buildings.
4. Provide habitat for wildlife that then may enter roads, fields, etc.

Aquatic weeds:
1. Interfere with recreation (swimming, boating, fishing).
2. Interfere with commercial navigation, clog water intake pipes, etc.
3. Disrupt habitat for other fish, waterfowl, other organisms.
4. Detract from the aesthetic appeal of a body of water.
5. Stunt or interfere with a balanced fish population.
6. Fish killed due to removal of too much oxygen from the water. Oxygen
depletion occurs when plants die and decompose.
7. Photosynthetic production of oxygen ceases, and the bacteria, which break
down the plant material, use oxygen in their own respiration. Light is
blocked thus preventing photosynthesis by any living plants or algae.
8. Produce quiet water areas that are ideal for mosquito breeding.
9. Certain algae can give water bad tastes and odors.
i.
Concept of Weeds

In general, weeds are common plants. Concept of weeds was established as the
human civilization began.

Weeds can be found or grow in common places and almost everywhere.

In the end, it all depends on ones’ interpretation on its existence and function.
One can regard a plant (ie. Asystasia spp.) as a weed, and one can use the
same plant to feed his/her farm animals or as a cover crop in the plantation.
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Conclusion to Unit
By knowing, studying weeds:
-
understand and balance the advantage & disadvantage of an entity and
deal with it appropriately for our own good.
-
understand the concept of diversity of God’s creation.
-
accept that weeds are parts of ecology/environment.
Exercise / Activity
1. Define ‘weedy rice’
2. List 5 weed species that can be found in:
a. Rice field
b. Plantation
c. Landscape area
d. Vegetable farm
e. Orchard
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Unit 2
BIOLOGY, ECOLOGY AND WEED DISTRIBUTION
Introduction to Unit
This topic will cover the biology, ecology and establishment of weeds.
Objectives to Unit
1. Explain the biology and ecology of weed species.
2. Explain the methods of reproduction and dissemination in weed.
3. Discuss the weed establishment process.
Topic 2: Biology, Ecology and Weed Distribution
a.
Basic Weed Biology and Ecology

Terrestrial: plants that live on land: plantation, landscape, farm, field etc…

Aquatic: plants that live in or around water: rice field, swamp, drainage
system, canal.

Herbaceous plants: plants with non-woody aerial stems that usually die down
each year.
a. Mainly reproduce by seed (although can also produce vegetative structures).
b. Can be annual/seasonal, biennial or perennial.

Woody plants: plants with woody aerial stems that persist from year to year.
All of these are perennials. Includes trees and woody shrubs. Often reproduce
by seed, but may also regenerate from buds and spread by means of root
sprouts.
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Seed-producing plants (phylum = Spermatophytes) comprised of two classes,
the Angiosperms and the Gymnosperms. Angiosperms are plants whose seed
are borne within a mature ovary (fruit). Most weeds are Angiosperms.

The Angiosperms are further divided into two subclasses: Monocotyledoneae
(monocots) and Dicotyledoneae (dicots).

Monocots: plants whose seedlings bear only one cotyledon (seed leaf).
Typified by parallel leaf venation. Includes grasses (Gramineae) and
sedges (Cyperaceae). Other monocot families which contain problem weeds:
Juncaceae, Liliaceae, Commelinaceae, Typhaceae, Lemnaceae, and
Alismaceae.

Dicots: plants whose seedlings bear two cotyledons. Typified by net-like leaf
venation. Commonly called broadleaves.
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Classification of Weeds

Several methods of classification:
1. Leaf shape
* parallel - monocots
* netlike - dicots
2. Life cycle: Life cycles of many species may vary with environment or among
individual plants:
* annuals/seasonal – plant that completes life cycle in a single growing season
(within a year or less)
* biennials – plants that need 2 years to complete life cycle
* perennials – plant that can live indefinitely. Most have vegetative
reproductive structures
– can be simple (upright, no underground lateral branching-side
shoot, bud) OR creeping perennials (have creeping vegetative
reproduction structures).
3. Root system
* tap root (deep rooting, usually woody weed plants/dicots and perennial
weeds).
* fibrous root (shallow rooting, common for seasonal weeds or monocots).
4. Habitat/ecology
* Dry land weeds
* Aquatic weeds (floated or submerged weeds)
5. Taxonomic
* Fami
Biotype
- Biotype: a group of genetically identical plants (possess a similar genotype)
within a species. The most commonly studied weed biotypes are those that
possess a herbicide resistance gene. Also called “microspecies”.
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How Weeds Interfere/Become Part of Human Activity

Plants which follow human civilization.

Prior to disturbance and intrusion of landscape by man, most native weedy
species were plants that colonized areas where natural disturbances (fire,
floods, etc.) had killed native vegetation (i.e. forest).

Later, human replaced those weedy plants with cropping/livestock.

Invasion of weeds into a disturbed area is the first step in SUCCESSION.

In the end, weeds become ESTABLISHED and COMPETE with human for
both survival and need time after time.

Keep changing their appearance and existence according to human practices
and environmental changes.
d.
Weed Establishment

How weeds reproduce, disperse, increase and colonize a habitat.

Factors influencing establishment:
- Transportation/spread of reproductive materials
- Viability of reproductive materials
- Suitability of an area for reproduction
- Ability to colonize and compete (ecological competence)

Methods of Weed Establishment - via propagation:
1. Sexual reproduction:
- requires pollination of a flower, leading to seed production.
- main reproduction for most weeds for survival.
- weeds are very prolific seed producers.
- this contributes to “weediness” of a species and increases the
difficulty of control.
- allows recombination of genes to create individuals with new
combinations of traits (diversity).
- easy target for chemical, biological & mechanical controls.
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2. Asexual reproduction:
- does not require flowers and pollination, seed are not produced.
- also called vegetative reproduction. Creeping perennial have various
vegetative organs for reproduction.
- higher tolerance to control tactics.
- enhanced competitiveness.
- fewer reproductive units compare to seed production, i.e.
- spread less rapidly than seeds.
- much less genetic variation within the population than from plants
emerged from seed.
- types of vegetative part:
a) Stolons - aboveground, horizontal stems that root at the
nodes (Asystasia spp., bermudagrass.
b) Rhizome - underground, thickened stems that grow
horizontally in the upper soil layers. Imperata cylindrica,
ferns.
c) Tubers (stem tubers) - enlarged stolons or rhizomes with
compressed internodes, e.g. Cyperus esculentus, wild
potato.
d) Bulbs - short stem with fleshy leaves or leaf bases for
carbohydrate storage that are located at the base of the stem
or below the soil line, e.g., wild garlic, onion.
e) Budding/creeping roots - modified roots that can store
carbohydrates and grow both vertically and horizontally.
Can give rise to shoots, lack of nodes an internodes. Canada
thistle, Solanum carolinense.
e.
Methods of Weed Dissemination/Spread

Types of dispersal:
A. Seed
1) Natural dispersal: seeds have specific modification:
a) Wind
b) Water
c) Animals
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2) Artificial dispersal (a result of man’s activities):
a) Machinery
b) Crop seed
c) Livestock feed, hay and straw, manure
d) Other human activities (nonfarm & recreation etc)
B. Vegetative Reproduction Structures
1) Natural dispersal:
Usually short distance as compared to seed dispersal. Results from
encroachment or “creeping” of weed as it grows and produces additional
vegetative reproductive structures.
2) Artificial dispersal (related to man’s activities):
Can be short or long distance. Examples include the following:
a) Dragging bermudagrass’s stolons or congograss/lalang’s rhizomes on tillage
equipment
b) Movement in soil for construction, landscaping, roadside maintenance
c) Movement in nursery stock or sod
f.
Successful Characteristics Ensuring Weed Establishment

Weeds possess one or more of the following characteristics that allow them to
survive and increase in nature (weediness characteristics):
a) Prolific/abundant seed production.
b) Ability to occupy areas disturbed by humans.
c) Persistent and long-term survival seeds (seed dormancy and high
viability).
d) Presence of vegetative reproductive alongside the sexual reproductive
structures (especially creeping perennials).
e) High capability to compete with crops.
f) Fast/rapid growth.
g) Short time to reproduction (short time to flowering or between
flowering and seed maturity).
h) Adaptations for short and long-distance seed dispersal.
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i) Mimicry with crops
If both the weed and crop have envolved with the same agronomic or
environmental pressures, weed and crop may share many
morphological and physiological similarities (eg. Identical life cycle
and life style –rice and Sambau), seed maturity coincides with crop
harvest
j) Plasticity of weed growth
The ability of a weed species for rapid phenotypic adjustment to
environmental change
Eg. Weed can respond to canopy shade-by undergoing rapid stem
elongation or undergo sun-shade leaf transition for maximum light
interception
Conclusion to Unit
Weeds can be classified based on their biological and ecological characteristics. Weed
plants can reproduce and disperse by 2 main methods: by seeds and vegetative
reproduction parts.
Importance: By knowing how weeds are reproduced and dispersed, one may be able
to take steps to reduce or avoid further spread.
Exercise / Activity
Define C3 and C4 plants in relations to weed – crop competition
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Unit 3
WEEDS AND HUMAN
Introduction to Unit
This topic will cover the role and impact of weeds to human.
Objectives to Unit
1. Describe the impact and benefit of weeds to human.
Topic 3: Weeds and Human
a.
Type of Losses Caused by Weeds to Crop Production
1. Direct losses.

Reduce crops yield through:
Competition-weeds compete with crops for light, water, nutrients, gasses,
space.
Allelopathy-process by which a plant releases into the environment (air or
soil) an organic chemical (an allelochemical) that adversely affects the growth
and development of surrounding plants: Mikania micrantha significantly
affected soil nutrients and N2 transformation (Bao-Ming Chen et al. 2008;
Biological Invasions 11 (6):1291-1299).

Reduce harvesting efficiency

Reduce quality of harvested crop
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2. Indirect losses.

Increase crop production costs (herbicides, herbicide application, cultivation,
additional land preparation, equipment costs, labour cost).

Crop damage by the application of control methods.

Weeds may be alternate hosts for insects, microbes carrying diseases, or
nematodes that can potentially attack crops.
b.
Beneficial of Weeds???

Stabilizing & adding organic matter to soils – dead weed plants can become
compost, fertilizer, mulching agent.

Providing habitat and feed for wildlife (i.e. nectar for bees).

Agronomical value: prevent soil erosion, nutrient (N2) fixation and retention,
i.e. creeping legume weeds as groundcover crops: Mimosa spp., Asystasia spp.
Centrosema pubescens, Calopogonium mucunoides.

Ornamental purposes/aesthetic qualities – some weeds produce beautiful
flowers: water hyacinth.
Landscape plants: Acacia spp. and Lantana camara.

Products for human consumption: Limnocharis flava, maman (Cleome
gynandar, C. pentaphylla), fern-pucuk paku.

Medicinal use: senduduk (Melastoma malabathricum), dukung anak
(Phyllantus spp.), rumput jarum mas (Striga asiatica), L. camara
(Barreto F. et al. 2010. J Young Pharm 1;2(1):42-44 ).

Some can potentially become pasture crops (forage) or fodder (hay, silage) for
animals, i.e. Paspalum spp., Digitaria spp. Cynodon dactylon.

Biological resource: germaplasm for medicinal use or a genetic reservoir for
improved crops.
c.
•
Weeds Interference on Crops
Negative Interference
–
Competition, Allelopathy and Parasitism
–
Competition
Is the process that occurs when the combined resource demands of
plants within a given area exceed the available growth supply.
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Amensalism (Allelopathy)
A depressive effect of a plant upon its neighbors
Allelopathy- allelochemicals/toxic substances release by a plant that
harm/kill others. (Mikania micrantha)
–
Parasitism
A plant living in, on, or with another plant at whose expense it obtains
food, shelter, or support (Striga, Orobanche, Fern)
•
Positive interference
–
Commensalisms, Protocooperation and Mutualism
–
Commensalisms
Only one organism is stimulated by the presence of the other an
inhibited by its absence whereas the other or host is unaffected
(epiphyte)
-
Protocooperation
Both organisms are stimulated by the association but unaffected by its
absence (intercropping, mycorrhizae)
-
Mutualism/Symbiosis
Obligatory types of relationship
The benefits gained by each partner
In the event that one partner absent, they both suffer (Lichen alge and
fungus)
Conclusion to Unit
Weeds primarily cause problems to human by invading and infesting human space
and need. However, some weeds can also be useful to us. The keyword is “taking
advantage of their presence”.
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Unit 4
WEED CONTROL
Introduction to Unit
This topic will cover the history and development in weed control, as well as principle
and concept of weed control.
Objectives to Unit
1. Explain the history of weed control.
2. Describe the concept, principle and strategies in weed control.
Topic 4: Weed Control
a.
History of Weed Control

When did it start?
- Together with the history of agriculture.

When?
- Approx. 10,000 years ago (Hamilton 2009).

Changed following the development of science and technology.

Chronology:
 10,000 B.C. - hand labor, substituted fingers with a sharp stick to
remove weeds; centuries later metal hoe developed.
 1000 B.C. - Animals used for cultivation & weeding.
 100 A.D. - Romans used sea salt as a herbicide (first herbicide).
 1908 - Bolley in the U.S. selectively controlled winter annual weeds
with table salt (NaCl), iron sulfate, copper sulfate, and sodium arsenite.
A major breakthrough in weed control.
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 1941 - 2,4-D synthesized providing selective control of certain
broadleaf weeds in grass crops (first commercial herbicide).
 1970's - glyphosate (Roundup) available-world’s most important and
best selling herbicide (control broad-spectrum of weed spp.) – Powles
2008 (Pest Manag. Sci. 64:360-365).
 Mid 1990’s - GM crops available.
b.
Approaches to Weed Control/Management

Three approaches:
1. Eradication:
a) Complete elimination/annihilation of all live plants, plant parts, and
seeds from a defined area.
b) Infers that the weed will not reappear unless introduced.
c) The ideal of weed control but is rarely achieved, because:
i. Very difficult and expensive (costly-labor cost).
ii. Practical only in a small and isolated area.
2. Prevention:
a) Often most practical approach to stop the spread of weed.
b) Preventing the introduction, establishment, or spread of weed species
in areas not currently infested.
c) Usually easier than controlling after weed establishment.
3. Control:
a) Management of weeds to minimize weed density, competition and
growth, and provide greatest economic return from a crop; limiting
seed infestations and production; what we are mainly doing now.
b) Consists of several methods: mechanical, cultural, biological,
chemical, IWM.
c.
Weed Control Practices/Strategies

“No control” practice: there is no weed control method applied to weeds
interfering with crops in the field. As a result, weeds vigorously emerge and
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compete with crops, causing crop to suffer and in the end, only minimum or
no production obtained from the crops. Loss in crop yield is between 85-100%
under this condition.

“One-time” control practice: control method is only applied once to control
weed. This control method is either applied in the early period of crop growth
(crop is still small), or during crop establishment (crop is actively growing), or
in the late period of crop growth (crop at the flowering stage). As a result,
higher yield will be produced by crops compared to the “no control” approach.
However, crop is still severely affected by weed competition during the noncontrol period. Loss in crop yield is between 50-70%.

Describe “continuous” control practice: weed is being continuously controlled.
In this method, crop will be always free from weed infestation during their
growth. The highest yield will be obtained from “continuous” control, and
only small yield reduction will be observed (less than 25%). However, this
control approach will greatly cost time and money spent on weed control,
especially on labour, machineries, and herbicide costs.
d.
Critical Period of Weed Control???

The period of time in which weed control is necessary to avoid significant
yield loss (Nazarko et al., 2005).

OR alternatively: an interval in the growth cycle of the crop when it must be
kept weed-free to prevent significant yield losses.

Different crops are susceptible to interference from weeds at different times,
but critically during crops establishment.

Importance:
- Timing of control applications is crucial for controlling weeds effectively
and preventing excessive interference with the crop.
- Knowledge of the critical period of weed control can also allow for reduced
herbicide use.
- Can also save time and money spent on weed control.
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Hypothetical yield of crop as affected by length of early period of weed control:
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Hypothetical yield of crop as affected by length of early period of weed interference:
Hypothetical critical period of weed control:

However, continuous weed control is still needed:
- Reduce weed seed production & seed bank.
- Avoid movement/transfer of other pests from weeds to crops.
- Avoid contamination of weed parts/seeds in the crop yield.
- Increase crop harvest efficiency.
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Conclusion to Unit
There are three (3) major approaches used in weed control and management. Critical
period of weed control is a control practice practised in accordance to at which crop
growth stage is highly affected by weed interference (when a weed is likely to cause
problems to crops, and when it can be left alone).
Exercise / Activity
1. Provide data on the percentage yield loss due to weed competition in :
a. Oil palm
b. Rice
c. Maize
d. Sugarcane
e. Vegetable
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Unit 5
METHODS OF WEED CONTROL
Introduction to Unit
Weeds can be controlled using four (4) methods: mechanical, cultural, biological and
chemical. In this topic, all weed control methods, together with their advantage and
disadvantage will be discussed. In addition, the Integrated Weed Management (IWM)
will also be discussed.
Objectives to Unit
1. Explain the concept and approach of different weed control methods.
2. Describe the IWM practices and its advantages.
Topic 5: Methods of Weed Control
a.
Mechanical Weed Control

Objective: to injure/kill weed plants physically and use physical barrier to
impede weed growth.

Consists of:
1. Hand pulling and hoeing: can be expensive in certain countries (labour
cost). Good for garden & small areas.
2. Tillage (including seedbed preparation): destroys weeds by burial,
exposing weed seeds, pull-out de-rooting.
3. Mowing and slashing: more effective for tall-growing weeds; repeated
mowing may prevent seed production. More effective on broadleaf weeds
than on grasses because grasses have a lower growing point.
4. Weed seed destroyer (HSD) – in Australia.
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5. Burning and heat:
-
Steam sterilization of potting media for nursery operations; kills weed
propagules (veg. parts), insects, pathogens.
-
Burning of wheat straw before planting next-season crops will kill
most emerged weeds and seeds; lose benefit of allelopathy from straw;
also eliminates potential for organic matter increase.
-
Windrow burning in Australia.
-
Rice straw burning in Malaysia.
6. Mulching
-
Reduce/block light and air penetration into soil: impede/prevent weed
seed germination and growth.
-
Materials used: hay, woodchips, rocks, sawdust, plastic mulch used in
horticultural crops (tomatoes, chillies and bell peppers, etc.)
b.
Cultural Weed Control

Objective: provides conditions more favorable for crop growth, less favorable
for weeds. Involves agronomic practices and crop management. Grow a
healthy crop to best compete with weeds.

Consists of:
1. Grow Healthy & Competitive Crops:
Ø Select competitive & resistant varieties.
Ø Provide optimal growing conditions for crops – suitable soil type,
pH.
Ø Irrigate and fertilize crop, not weeds.
Ø Early/delay sowing date – e.g. in rice farming.
Ø Intercropping – using empty space between crop rows.
Ø Crop rotation: life cycles of problematic weeds also often match life
cycles of crops. Crop rotation can disrupt weed life cycles. Crop
rotation: diversity in herbicides use.
Ø High seeding rate - higher seeding rates Increase planting density
and planting uniformity & enhance competitiveness of the crop – no
space for weeds.
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2. Cover Crops:
a. Usually used to compete with the weeds infesting an area.
b. Helps prevent weed emergence and/or reduces growth of weeds.
c. Cover crops residue can exclude light, provide a physical barrier to
weed emergence, and affect soil temperature and moisture, become
fertilizer to crops.
d. Allelopathic chemicals from the cover crop residue may also inhibit
germination/growth of weeds.
3. Water Management:
a. Flooding used in rice production for weed control.
b. Many weeds are not tolerant to anaerobic conditions, ultimately leads
to their death. Rice can survive flooding- maybe less suitable for
aquatic weeds.
c. Water deprivation can be a weed control method in arid, irrigated
areas, such as withholding water (timely irrigation) at a critical stage
for weeds.
c.
Biological Weed Control

The action of parasites, predators, or pathogens to maintain another
organism’s population at a lower average density than would occur in their
presence (Zimdahl 2007).

Objective: use of a living organism (a natural enemy) to reduce weed
populations to economically acceptable levels.

Key concern of selecting control agent:
1. Must be very host specific (does not harm crops or other beneficial
vegetation).
2. Synchronization of life-cycle with targeted weeds.
3. No changing of host and target

Control agents - insects, pathogens, fish, mammals, and birds.
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
PJJ/UPMET
Characteristics of chosen control agent:
a. Ability to kill weed & stop weed reproduction
b. Ability to spread and find targeted weeds
c. Adaptability to environment and changing climate
d. Ability to reproduce in sufficient numbers to control the weed – esp.
insects

Examples of biocontrol agent and weeds that are usually controlled by the
particular agent will be given.

Advantages of bio-control:
a. excellent alternative in low cost areas such as rangelands, forests, aquatics.
b. environmentally safe - no contamination of ground water, soil, etc.
c. cost is relatively inexpensive & non-renewing.
d. can be reasonably permanent.
e. risks are known and evaluated before release.

Disadvantages of weed bio-control:
a. complete control is difficult, slow and no guarantee.
b. agent introduced potentially attack desirable species from the same
family/genus.
c.
agent sometimes inadaptable to the changing environment.
d.
narrow spectrum of control: one type of control agent = one/two weed
species.
e. agents may not be compatible with other components of pest management,
such as use of insecticides or fungicides or crop rotation.
f. initial investment can be very high and time consuming.
g. there can be natural enemy for the agent (esp. insects).
h. establishment may fail for many reasons (environment, agent
‘disappeared/moved’ to other places, unknown reasons…).

Reality of biological control:
a. Although the concept of biological control have been some outstanding
successes, generally not feasible for intensive agricultural areas.
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b. The concept is acceptance of a low level of weed population or the
existence of monoculture weed spp., so the biological control agent can
survive and take action, but…
c. This may not be acceptable in intensively managed systems. Biological
control is better suited to non-intensive production systems such as
rangeland, aquatic sites, or to natural areas.
d.
Chemical Weed Control

Definition: use of herbicides to control weeds.

Objective: to kill or prevent or interrupt weed growth chemically/using
chemical substances.

Advantages of chemical control:
a. Various types available. ‘Magic’ tools that kill unwanted plants and
leave desired plants/crops relatively unharmed.
b. Provide a convenient, fast, economical, and effective way to help
manage weeds.
c. Allow fields to be planted with less tillage = less soil erosion. Without
herbicide use, no-till agriculture becomes impossible.
d. Save time: farmers can perform other tasks.
e. Relatively cheap (not so cheap nowadays, but still helps reducing
labour cost).
f. Can be used in both extensive agricultural and less extensive non
agricultural areas.

Advantages of chemical control:
1. Costly: added cost on sprayer, herbicide manufacture.
2. Environmental persistence: source of pollution (air, soil, underground
and aboveground water). Herbicide carryover affect other plants/crops.
3. Weeds evolved resistance to herbicides (especially the ones with a
single MOA).
4. Poison to human and animals: residues in food crop and herbicide drift
into water supply (majority cases are from insecticides – Zimdahl
2007).
5. Direct source of danger to people’s health:
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-
Made from hazardous, deadly and poisonous chemicals.
-
Can be direct source of danger to human.
-
Some posses ‘burn’ effect if have direct contact with
skin/eyes, some have unpleasant smell & toxic if inhale, taken
or have contact.
6. Can become source of misuse:
-
Careless during preparation and application:
i. Using wrong dosage (not follow the recommended rates).
ii. Wrong chemical for wrong crop.
iii. Failure to do calibration.
iv. Preparation and spraying equipment not in proper order, not
clean from previous herbicides.
v. Condition during spray is not suitable (weather, temperature
etc.).
vi. Ignorant of the chemical toxicity level and activity – less
care during handle.
e.
Integrated Weed Management

Definition: combination of multiple management methods to reduce weed
population to an acceptable level while preserving the quality of existing
habitat, water, and other natural resources.

Involves 2 critical elements:
-
using multiple control tactics (cultural, mechanical, biological, and
chemical control methods).
-
integrating a knowledge of weed biology and ecology into the
management system.
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
PJJ/UPMET
Examples of IWM approach for:
1. Rice
Critical period of weed
20-60 DAS
control
Cultural method
1) Healthy and highly competitive variety
2) Flooding
3) High density
Mechanical method
1) Land preparation (ploughing)
2) Hand weeding
3) Rotary weeder
Chemical method
Apply pendimethalin 1.0kg/ha on 5 DAS or
Pretilachlor + safener (Sofit) 0.45kg/ha on the
day of receipt of soaking rain, and apply
Nominee (bispyribac-sodium) 0.35kg/ha at 7-15
DAS)
Biological method
Release Tilapia spp. to control aquatic and .
floating weeds such as Azolla and Salvinia.
Remarks
Can also include preventive method:
1) Certified seeds
2) Clean planting and tilling machines
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2. Sugarcane
Critical period
4 to 5 months after planting
of weed control
Cultural method
High density planting
Mechanical method
Remove the weeds along the furrows with
hand hoe. along the ridges on 25, 55 and 85
days after planting for removal of weeds and
proper stirring
Chemical method
1. Pre-emergence herbicides like atrazine
(2 to 3 kg/ha), Simazine (2 to 3 kg/ha),
Alachlor (1.3 to 2.5 kg/ha) etc. will
generally last for 8 to 12 weeks
2. To obtain best results, sequential
application of preemergence and post
emergence herbicides like Glyphosate
(0.8 to 1.6 kg/ha) Paraquat (0.4 to 0.8
kg/ha).
Biological method

None
Advantages of IWM:
1. Can develop a long-term weed management plan, that considers all
available management control techniques to control weeds.
2. Likely to hinder the adaptation of weeds to any single control tactic, i.e.
chemical control-herbicide resistance.
3. IWM takes a common-sense approach to weed control (helps reduce
hazards to people, animals and the environment).
4. IWM works well for agricultural, home, garden and workplace
environments.
5. Avoid unknown problems in the future i.e. some biological agent may
become invasive spp., insect may mutate and change their diet attack other
crops and plants; mechanical will reduce soil nourishment and cause
erosion; water scarcity is a major issue nowadays.
6. Most important, IWM helps decrease herbicide use.
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Conclusion to Unit
•
Controlling weeds is important especially in agriculture production.
•
No proper control: they may take over your field = left with little or no yield!!!
•
Important to control weeds at the right time, before they become a problem.
•
Do not allow them to compete too long with the crops, and.....
•
Do not let them grow long enough to produce seeds!!!
•
Every control methods has its advantages and limitations; IWM necessary for
successful management.
•
The IWM in agricultural production holds great promise for reducing crop
yield losses.
Exercise / Activity
Give examples of Integrated Weed Management Program in Malaysia
\
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Unit 6
HERBICIDE
Introduction to Unit
Chemical weed control using herbicides is a major mean for weed control nowadays.
This topic will cover all aspect about herbicides.
Objectives to Unit
1. Explain the history, development and classification of herbicides.
2. Describe the herbicides characteristics, formulations, calibration and active
ingredients calculation.
3. Discuss the role of adjuvants on herbicides efficacy.
Topic 6: Herbicide
a.
Introduction to Herbicide

History of Herbicides:
 1900 - sodium arsenite to control aquatic plants in waterways in the US.
 1925 - sodium chlorate was first used for killing weeds.
 1934 - sodium nitrocresylate, as the first selective weed killer in France.
 1940s – first organic herbicide 2,4-D was commercialized providing
selective control of certain broadleaf weeds in grass crops.
 1960's – pre-emergence herbicides trifluralin and atrazine introduced.
 1974 - glyphosate (Roundup) available - among the world’s most
important herbicides -S.B.Powles. Control broad-spectrum of weed spp.
 As in 2010, more than 200 herbicides from 22 mode-of-actions available
in the market.
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
b.
PJJ/UPMET
Examples of herbicide in Malaysia will be given in the laboratory practical.
Herbicide Nomenclature

Active ingredient - the chemical in the herbicide formulation primarily
responsible for its phytotoxicity.

Trade name - a trademark or other designation by which a commercial product
is identified.

Common name - each herbicidal chemical has one common name assigned to
it. In many cases, the common name is a simplified version of the chemical
name.

Chemical name - the systematic name of a chemical compound according to
the rules of nomenclature of the International Union of Pure and Applied
Chemistry (IUPAC), Chemical Abstracts Service (CAS) or other authorized
organizations.
c.
Herbicide Toxicity

Toxicity: danger/poison level.

Based on oral LD50 values on the test population (animal).

The lower the LD50 value, the less chemical that is required to reach lethality
(LD5010 is more toxic than LD50100).

LD50 value measurement: mg herbicide / kg test animal.

Info: Material Safety Data Sheet (MSDS) or Safety Data Sheet (SDS) for each
herbicide.
Classification of herbicide toxicity based on the oral LD50 value
Class
LD50
Solid form
Colour
Code
Toxicity
Warning
<5
LD50
Solution
form
<20
Ia
Black
Very high
toxicity
Ib
5-50
20-200
Red
II
III
50-500
500-5000
200-2000
2000-10000
Yellow
Blue
IV
>5000
>10000
Green
High
toxicity
Toxic
Moderately
toxic
NA
Extremely
dangerous,
poison
Danger,
poison
Harmful
Caution
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AGR 3102- Prinsip Sains Rumpai
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Comparison of oral LD50 values for commonly used herbicides and consumer goods
Herbicide
LD50
Common consumer chemicals LD50
Paraquat (Gramoxone)
~100
Nicotine
9-10
Pinoxaden (Axial)
>3100
Bleach
192
2,4-D
666
Caffeine
192
Atrazine
3090
Household ammonia (10%)
350
Glyphosate (Roundup)
4900
Table salt
3000
Glufosinate
>430
Codeine (pain and cough
427
relief)
d.
Herbicide Classification

Selectivity: Selective vs. Non-selective
a. Selective: toxic to some species and much less toxic to others at a given
dosage. A selective herbicide kills weeds but not the crop.
b. Non-selective/broad-spectrum: kills or severely damages all or most
species. A non-selective herbicide kills weeds and potentially crops.
c. A selective herbicide can be made non-selective by applying improperly or
by applying high rates. Selectivity is usually rate dependent.

Activity: Contact vs. Systemic
a. Contact herbicides: destroy only that plant tissue touched by the chemical.
Effective only on tissue contacted by application of herbicide. Very little
movement (translocation) within the plant. Full spray coverage essential
(because no translocation, must get spray deposited on most of plant).
Usually exhibit acute effects - kill rapidly. Effective for annuals, usually
ineffective for perennials (because no translocation to underground
reproductive organs). Can be selective or non-selective.
b. Systemic herbicides: absorbed by roots or above-ground parts (depends
upon particular herbicide and application method), translocated within
plant. Good spray coverage is necessary with all herbicides, but less
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critical than contact herbicides because of systemic herbicides can be
translocated. Usually exhibit chronic effects - slow acting. Effective on
annuals, some are effective on perennials; effective on perennials because
of translocation to underground organs. Can be selective or non-selective.

Residual activity: Residual vs. Non-residual
a. Residual: Herbicide retains activity on susceptible weeds for some time
following application. Length of residual depends upon the particular
herbicide and the application rate.
b. Non-residual: Herbicide has activity only on weeds present at time of
application. Does not provide control of weeds that emerge after
application.

Type/time of application
a. Pre-plant incorporated: applied to soil and mechanically incorporated into
the top 2 to 3 inches of soil before the crop is planted.
b. Pre-plant: applied prior to planting the crop.
c. Pre-emergence: applied to the soil prior to emergence of the crop or weed.
d. Post-emergence: applied after emergence of the crop or weed.

Methods of application (refer to the ways herbicides can be applied)
a. Broadcast: applied over the entire field.
b. Band: applied to a narrow strip over the crop row.
c. Direct: applied between the rows of crop plants with little or no herbicide
applied to the crop foliage.
d. Spot treatment: applied to small, weed-infested areas within a field.

Herbicides Chemistry and Mode-of-action (MOA)
a. Common method of grouping herbicides: chemical families and MOAs.
b. Classification based on similarity/dissimilarity of the herbicides chemistry.
c. Herbicides with a common chemistry are grouped into “a family.”
d. Different families have the same mode of action, grouped into “similar
MOA.”
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e. Based on the MOA: classification by a numerical system (WSSA) and
alphabetical system (HRAC).
f. Importance: helpful in knowing what groups of weeds are killed,
specifying application techniques, diagnosing herbicide injury problems,
and minimising herbicide resistant weeds.
g. 2 types of herbicides:
Inorganic compounds* - “old school”
i.
-
Copper sulphate, ammonium sulphate, ammonium sulphamate,
ammonium nitrate & ammonium thiocyanate.
-
Act as desiccant/desiccating agent.
-
Highly persistent in soil & highly toxic to other organisms.
-
Widely used in 1900s for aquatic and woody terrestrial plants,
many no longer applicable as herbicides nowadays.
ii.
Organic compounds
-
Extensively synthesized following 2,4-D & other PGRs
commercialization.
-
Widely used in agric and non-agric areas.
-
Differ from one to another by their chemical characteristic and
MOAs.
I.
Based on the MOAs, divided into several important groups:
Plant Growth Regulators (PGR) / Synthetic Auxin
•
First commercialized in the 1940s to primarily control dicot
weeds in wheat, corn, rice, and many cereal crops.
•
MOA: response similar to those of natural, growth-regulating
substances called auxins.
•
Selective systemic, post-emergence: effective on annual &
perennial broadleaf plants and usually have no activity on
grasses or sedges, except at high application rates.
•
Short soil residual; average half-life of 5 - 14 days.
•
Comprise of phenoxy-carboxylic acid, benzoic acid, pyridin
carboxylic acid, quinoline carboxylic acid & phtalamates.
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II.
PJJ/UPMET
Amino Acids Biosynthesis Inhibitors
•
Divided into 2 major MOAs: Acetolactate synthase (ALS)
inhibitors and 5-Enolpyruvylshikimate 3-phosphate synthase
(EPSPS) inhibitors. Post-emergence, systemic.
•
ALS inhibitors effective mostly on annual broadleaves
(selective), while EPSPS inhibitors have activity on grasses,
sedges, and/or perennial dicot plants (non-selective).
•
Consist of sulfonylureas, imidazolinones, triazolopyrimidines,
pyrimidinyl thiobenzoate (ALS) & glycines (EPSPS).
•
These herbicides cause the shutdown of metabolic activity with
eventual death of the plant.
•
Glyphosate – single EPSPS herbicide, for example, is a broadspectrum herbicide and has activity on all types of plants.
•
ALS herbicides and glyphosate considered among the safest
herbicide because ALS & shikimate-pathway exists only in
photosynthetic plants and not in animals.
•
Soil residual: half-life varies among herbicides, generally about
2 months – 3 years (long persistence).
III.
•
Fatty Acids (ACCase) Biosynthesis Inhibitors
Selective-commonly referred to as graminicides (grass killers),
but have no activity on other monocots (i.e. sedges). Also have
no activity on dicots. Post-emergence, systemic.
•
They inhibit meristematic activity, stopping growth almost
immediately, especially young grass.
•
Comprise of aryloxyphenoxypropionates (APPs),
cyclohexanediones (CHDs), and phenylpyrazoline (PPZ).
•
FOPs are sold as esters of their acids. The esters are rapidly
absorbed by plant foliage. Once inside the plant, the ester is
converted to the acid by carboxyolesterase. The acid is
considered to be the phytotoxic form of these herbicides.
•
Soil residual: half-life 3-5 days (DIMs), 17 days (PPZ), 9-60
days (FOPs).
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IV.
PJJ/UPMET
Seedling Growth Inhibitors (Root & Shoot)
•
2 types of MOA: microtubule inhibitors and cell wall
(cellulose) biosynthesis inhibitors.
•
They inhibit root and shoot development (depending upon
where they are absorbed) by interfering with cell division in
meristematic areas.
•
Effective only on germinating, small-seeded annual grasses and
some broadleaves (seedling selective, systemic, preemergence).
•
Have little to no post-emergence activity.
•
Consist of dinitroanilines, nitriles, pyridines, benzamides,
carbamates and benzoic acids (DCPA).
•
Depending upon the herbicide and crop, often incorporated into
the soil (esp. dinitroanilines).
•
Have relatively long residual activity (half-life more than a
year).
•
V.
Water insoluble and are subject to photo-degradation.
Seedling Growth Inhibitors (Shoot)
•
Divided into 2 MOAs: lipid synthesis inhibitors (not ACCase)
& cell division inhibitors (VLCFA synthesis). Applied as preemergence/planting.
•
Considered to be seedling shoot growth inhibitors. These
herbicides cause abnormal cell development or prevent cell
division in germinating seedlings.
•
Seedling-selective & systemic. Most effective on annual
grasses and sedges, some can control small-seeded annual
broadleaves.
•
Lipid synthesis inhibitors need soil incorporation, absorbed
readily by emerging shoots.
•
Cell division inhibitors absorbed by emerging shoots and roots.
•
No control of established weeds.
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•
PJJ/UPMET
Have a relatively short persistence in soil. Half-life 4-8 weeks
for VLCFA inhibitors and 3-6 weeks for cell division
inhibitors.
•
Consist of chloroacetamides, oxyacetamides, acetamides
(VLCFA inhibitors) & thiocarbamates, phosphorodithioates,
benzofurans (lipid synthesis inhibitors).
VI.
Photosynthesis (at PS II) Inhibitors
•
Divided into 2 types: mobile/slow acting – soil applied
herbicides (pre-& post-emergence); and non-mobile/rapid
acting – foliar applied herbicides (post-emergence). Both
systemic.
•
Triazines, triazinones, uracils, phenyl-carbamates, ureas,
amides (soil applied) & nitriles, phenyl-pyridazines,
benzothiadiazinones (foliar applied) are structurally diverse
chemical groups but have similar MOA.
•
These herbicides are effective primarily on annual broadleaves
(selective), however at certain rates, some provide control of
grasses as well.
•
Foliar applied; absorbed by leaf, block the photosynthetic
process so captured light cannot be used to produce sugars.
•
Soil-applied, these herbicides permit normal seed germination
and seedling emergence, but when the seeds’ food supply gone,
the seedlings die of starvation.
•
These herbicides are more effective on seedling weeds than on
established perennial weeds.
•
Some herbicides under ureas family such as prometon and
tebuthiuron are considered soil sterilants.
•
Relatively long persistence. Soil half-life: 60 – 120 days.
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VII.
PJJ/UPMET
Cell Membrane Disrupters
•
Consist of two MOA: Protoporphyrinogen IX oxidase (PPO)
inhibitors (selective) and PS I inhibitors (non-selective). Both
post-emergence.
•
PPO inhibitors: diphenylethers, phenylpyrazoles, Nphenylphthalimides, thiadiazoles, oxadiazoles, pyrimidindiones
& triazolinone; PS I inhibitor: bipyridiliums (paraquat &
diquat).
•
PPO inhibitors control almost all broadleaves, although some
have some activity on grasses.
•
PS I inhibitors provide broad-spectrum control of many
different species.
•
Paraquat: extremely toxic, no antidote so far. 5-10 ml fatal to
human.
•
Referred to as contact herbicides & kill weeds by destroying
cell membranes.
•
They appear to burn plant tissues or desiccation within hours or
days of application.
•
A very good coverage of the plant tissue and bright sunlight are
necessary for maximum activity.
•
The activity of these herbicides is delayed in the absence of
light.
•
PS I inhibitors are extremely tightly bound to soil colloids,
especially clay (no root uptake). Because of binding to soil,
bipyridyliums have no phytotoxicity in soil (no/very limited
soil activity).
•
Soil half-life for PPO inhibitors: varies among chemical
groups. Can be as short as 5 days or long (100-280 days).
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VIII.
PJJ/UPMET
Pigment Inhibitors (Bleaching Agents)
•
Divided into 2 MOA: Carotenoids synthesis inhibitors & HPPD
(4-hydroxyphenyl-pyruvate-dioxygenase) inhibitors.
•
These herbicides provide control of many annual broadleaves
and some grasses (non-selective, systemic, some pre-, some
post-emergence, some both).
•
Consist of pyridazinone, pyridinecarboxamide, triazole,
isoxazolidinone, diphenylether (Carotenoids synthesis
inhibitors) & triketone, isoxazole, pyrazole (HPPD inhibitors).
•
Referred to as “bleachers” since they inhibit carotenoids
(chlorophyll protector) biosynthesis or the HPPD enzyme by
interfering with normal chlorophyll formation.
•
Without carotenoid pigments, the sun damages chlorophyll
pigments and the plant becomes “bleached” and dies.
•
Absorbed by roots and shoots (pre-) or leaves (postemergence).
•
IX.
Soil half-life ranging from 25-145 days.
Phosphorylated Amino Acid (Nitrogen Metabolism) Disrupter
•
Phosphinic acids derived herbicide glufosinate-ammonium.
Provides broad-spectrum control. Group H (HRAC) & 10
(WSSA).
•
It affects growth by disrupting nitrogen metabolism (glutamine
synthesis), important for amino acids synthesis.
•
Generally considered a contact herbicide (thus, much more
effective on annuals than perennial weeds) although has slight
translocation (systemic) throughout the plant.
•
Good spray coverage and sunlight are important for maximum
efficacy.
•
Little to no soil activity because the herbicide is very rapidly
degraded by microorganisms.
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X.
PJJ/UPMET
Unknown Herbicides
•
Group of herbicide chemicals with unknown MOA, but likely
differ from aforementioned herbicides. Basically contact but
some have limited apoplastic translocation.
•
Contain arylaminopropionic acid, pyrazolium, organoarsenical
& other unknown family.
•
Relatively non-selective, post-emergence, & many are foliarapplied products.
•
DSMA and MSMA have no soil activity and tightly bound to
soil, others unknown.
•
Dazomet and metam are considered soil fumigants. These
products are applied to the soil and covered with a gas-tight
tarp. Kill everything in the soil.
e.
Herbicide Formulations

Active ingredient (a.i.) is the chemical that has phytotoxicity & controls the
target weed.

Herbicide product rarely made up only of a.i.

Often diluted in water or a petroleum solvent, and other chemicals are added
before sale.

Other chemicals (additives) such as adjuvants, extenders or diluents (no
herbicidal activity) are known as inert ingredients.

Active ingredients are identified on the label; inert ingredients are not
identified.

Herbicides are formulated to make the product easier to mix & apply, increase
its efficacy, improve formulation stability & improve shelf-life.

Herbicides are sold in various formulations, depending upon:
(1) the solubility of the active ingredient in water
(2) the manner in which the product is applied (i.e. dispersed in water or
applied in the dry form).
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
PJJ/UPMET
Several available herbicide formulations and abbreviations:
1. Emulsifiable concentrate (E or EC): a concentrated liquid (a.i., petroleum
solvents, emulsifier), water based/soluble.
2. Water Soluble Concentrate/Solution (WSC/S): a liquid formulation (a.i.,
water, sometimes a surfactant), water soluble.
3. Oil Soluble (OS): a liquid formulation (a.i. dissolved in oil or some other
organic solvent. Must be applied in an oil-based carrier such as diesel fuel
or kerosene.
4. Liquid Flowable (F or LF): thick slurry-like liquid (a.i., water, and
stabilizers), water solute. Spray tank agitation is necessary to avoid
settling.
5. Wettable Powder (W or WP): a dry powder (a.i., a diluent, and
surfactants), water solute. Spray tank agitation is necessary to avoid
settling.
6. Soluble Powder (SP): a dry formulation that contains a high percent
(>50%) a.i, water solute. Need initial agitation to dissolve.
7. Water-Dispersible Granules (WDG) & Dry Flowable (DF): a dry
formulation, water solute. Easier to handle and measure than wettable
powders. Spray tank agitation is necessary to avoid settling.
8. Granules (G): a dry formulation (a.i., coated or adhered to some type of
inert granule). Applied just as they are purchased with no mixing. Special
granular spreader is required.
9. Pellets (P): a dry formulation - similar to granules only much larger.
Pellets are applied directly to the target area by hand or with special
spreaders.
f.
Adjuvants

Definition: “any substance in other formulation (in our case, it’s herbicide) or
added to the spray tank to enhance or modify that formulation performance or
application characteristics”.

Some people call spray additives.

Main functions:
1) improve ease of application/handling
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2) enhance product efficacy (increase foliar adhesion and cover, uptake or
translocation of herbicides).

Formulation vs. Spray Adjuvants:
a. Formulation adjuvants
Those included in the formulated herbicide product by the herbicide
manufacturer.
b. Spray adjuvants
Added to the mixing tank primarily to improve herbicide performance,
aid in mixing, reduce drift, reduce foaming, etc. Commonly used with
post-emergence herbicides.

Our focus will be on spray adjuvants.
Numerous types (or functions) of adjuvants

Acidifiers
Humectants
Buffering agents
Nitrogen fertilizers
Colorants, dyes
Penetrants
Compatibility agents
Spreaders
Crop oils
Water conditioners
Stabilizers
Dispersing agents
Wetting agents
Stickers
Drift inhibitors
Emulsifiers
Surfactants
Antifoaming agents
UV absorbents
For simplicity, categorized into 2 basic groups:
1. Activator adjuvants:
 Spray adjuvants used to enhance the biological efficacy of the
herbicides.
 What they do: result in more herbicide being deposited, adhered,
retained, penetrated & absorbed into the weeds.
 What they overdo: can, and often do, result in more herbicide also
being absorbed by the crop.
 Consequence: while increasing weed control, can also increase crop
injury by the herbicide.
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 Examples of activator adjuvant:
Surfactants
Derived from surface active agent .
Reduce water surface tension and improve spray dispersion
2 groups: non-ionic type; organo-silicones.
They are good dispersing agents, stable in cold water, and have
low toxicity to both plants and mammals.
Surface tension is so reduced that the spray solution can
penetrate the stomata on the leaf surface.
Stickers
Cause the herbicide to adhere to the plant foliage.
Reduces the possibility that rain will wash it off before the
herbicide can penetrate.
Many blended with wetting agents so that they both increase
the spray coverage and provide better adhesion action.
Crop Oils
Crop oil concentrates, and methylated seed oils, like
surfactants, improve the spreading of the herbicide solution.
Being oil instead of water, they keep the leaf surface moist
longer than water.
Allow more time for the herbicide to been absorbed, thus
increasing the amount that will enter the plant.
Penetrants
In general, penetrants make herbicides “hotter” than it normal
condition.
Improve cuticular penetration by softening, plasticizing, or
dissolving cuticular waxes; allowing herbicide to move
underneath.
Penetrants are often a complex mixture of surfactant and oils
(paraffinic petroleum or modified vegetable).
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2. Utility/Spray Modifier Adjuvants:
 Spray adjuvants used to modify the physical characteristics of the
spray mixture.
 They improve ease of application/handling of the herbicides.
 Examples of spray modifier adjuvant:
Drift Inhibitors
Drift inhibitors or thickeners are used to control drift.
These may be powders, granules, or liquids that cause the spray
solution to be more cohesive/thickened; less subject to wind
shear as it leaves the nozzles.
Also reduce the amount of very small spray droplets.
Antifoaming Agents
Air gap filling or mechanical agitation in partially full tanks
will cause excessive foaming.
Foaming: interfere with herbicide flow and spray.
Antifoaming agents: silicone containing products.
They cause rupture of the air bubbles and breakdown of surface
foam.
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Herbicide Calculations

Two (2) methods of herbicide calculation:
a)
active ingredients (a.i.)
b)
acid equivalent (a.e.)
a) Active ingredient (a.i.) calculation
 a.i. is always identified on the herbicide label.
 Often expressed as either a % or in g/L or g/kg.
 Calculations based on dry or liquid formulation.
 Several calculations to determine the amount of a.i. applied. One of the
easiest calculations:
g a.i. applied per ha = kg or L of product applied X g active ingredient
ha
L or kg of product
 Examples for calculation of dry formulation:
- You are given a herbicide, tradename Plantgard with 30% 2,4-D
(note: 30% [w/w] in trade formulation = 30g a.i. in 100g product or
300g a.i. in 1kg product).
- The spray recommendation is 2.5kg product / ha.
- How much is the 2,4-D in the product when we apply 2.5kg of
Plantgard per ha???
Calculation:
g a.i. applied per ha =
kg or L of product applied X g active ingredient
ha
2.5kg Plantgard
ha
L or kg of product
X
300g 2,4-D
1kg Plantgard
Answer = 750g a.i. 2,4-D per ha applied when spray at recommended
rate of 2.5kg Plantgard per ha.
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 Examples for calculation of liquid formulation:
- You are given a herbicide, tradename Gramoxone with 35% paraquat
(note: 35% [w/v] in trade formulation = 35g a.i. in 100mL product or
350g a.i. in 1L product).
- The spray recommendation is 3kg a.i. paraquat / ha.
- How much is the Gramoxone required when we apply 3kg a.i.
paraquat per ha???
Calculation:
g a.i. applied per ha =
kg or L of product applied X g active ingredient
ha
L or kg of product
3kg paraquat per ha = ҳL Gramoxone X 350g paraquat
ha
1L Gramoxone
Answer = 8.57L Gramoxone is required for 1ha when spray with 3kg
a.i. paraquat per ha
b) Acid Equivalent (a.e.) calculation
 Herbicides a.i. usually in parent acid form (its herbicidally active
form), but many are formulated as a derivatives (i.e. esters, salts,
amines).
 Alterations with herbicide molecules that are acids (at carboxyl
[COOH] structure).
 Why would a herbicide be formulated as a derivative (ester, salt,
amine, etc.) of the parent acid?
- Increase the ability of the herbicide to penetrate/absorb through the
leaf/root much more effectively.
- Increase the water solubility of the herbicide.
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 a.e. calculation indicates the amount of an acid herbicide in a
formulation.
 Some labels indicate both a.i. and a.e. contained in the formulation,
while others list only one or the other.
 Sometimes, the numbers in formulation do not indicate gram active
ingredient per L or kg, but rather the acid equivalent per L or kg.
 If the g a.e. is specified on the product label, to determine the g a.e
applied per ha is substitute g a.e for g a.i. in the equation presented
previously.
 If not specified, calculate a.e. first using this equation:
acid equivalent (%) = molecular weight of the acid – 1 X 100
molecular weight of the herbicide
*molecular weight of the acid has to minus (-) 1 because one H atom is
missing when formed derivative (H+ was replaced by the
salt/ester/amine i.e. NH4+ / Na+).
 Examples of a.e. calculation:
2,4-D herbicides can be found in ester or amine formulation
2,4-D dietanolamine salt = 326 mw
2,4-D acid (pure form) = 221 mw
% of a.e. 2,4,-D = 221-1 X 100 : 67.5%
326
If in a 2,4-D formulation contains 700g dietanolamine salt per L, the g per
L a.e. 2,4-D in the formulation is:
67.5 (a.e.) X 700g/L (2,4-D salt formulation) = *472.5g/L
100
*472.5g is the actual a.i. 2,4-D (in a.e.) that cause phytotoxicity to
plants/weeds
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Now, how much g a.e. 2,4-D per ha is applied if the spray dosage
recommendation is 5L product per ha?
g a.e. applied per ha = kg or L of product applied X g acid equivalent
ha
L or kg of product
g a.e. applied per ha = 5L X 472.5g
ha
L
= 2362.5g a.e. 2,4-D per ha is applied when sprayed at the recommended
rate of 5L/ha product.
h.
Calibration

A technique to help you calculate how much water / chemical mix your
sprayer puts out to ensure that the correct rate of chemical is applied to the
target plant.

Calibration will be emphasized on knapsack sprayer.

Calibration procedures:
1) Measure the spray width of the nozzle(s) on a dry surface (in m).
2) Spray a test area at the intended pressure and walking pace. Record distance
(in m) covered in one minute (min).
3) Measure the nozzle output in L over one min in a measuring jug (L/min).
Repeat all steps at least twice.
4) The spray volume can be calculated by the following formula:
Application rate (L/ha) = nozzle output (L/min) x 10,000 m2
spray width (m) x walking speed (m/min)

Examples of calibration calculation will be given in the laboratory class.
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Conclusion to Unit
1. Basic structural components of most organic herbicides are carbon chains
(aliphatic groups) and rings (aromatic groups).
2. Chemistry of a compound determines how the herbicide will act in biological
and physical systems such as plants, animals, soils, and water.
3. Come tend to be degraded readily by microbes e.g. glyphosate, glufosinate,
and several thiocarbamate herbicides.
4. Compounds with aromatic structures and halogen substitutions tend to be
longer lived than straight chains, e.g. chloroacetamide, triazine, sulfonylurea,
dinitroaniline herbicides.
5. The more chlorinated a compound the longer it will persist in soil.
6. The substitutions and alterations that can be made to an organic acid illustrate
the effects that chemical structure can have on important herbicidal and
mixing properties.
7. Most herbicide labels specify the type and amount of adjuvant to use.
8. Be sure to include the proper adjuvant(s) for the herbicide being used.
9. Failure to follow the recommendations can result in poor weed control or
excessive crop injury.
10. Calibration is simply the accurate determination of the spray volume delivered
by the sprayer and verification that the spray pattern is correct.
11. A sprayer that is not properly calibrated may result in wasteful and harmful
over-application of the herbicide, or reduced effectiveness due to underapplication.
12. Herbicide sprayers should be calibrated at least once per season, more often if
the sprayer is used frequently.
Exercise / Activity
Lasso 500EC (a.i. alachlor) recommends to spray 30mL Lasso for every 5L water.
1ha area required 450L water. Calculate:
a.
How much Lasso is required per ha?
b.
How much a.i. alachlor is applied when spray at recommended rate per ha?
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Unit 7
HERBICIDE ACTIVITIES IN PLANT AND INTERACTION
WITH THE ENVIRONMENT
Introduction to Unit
This topic will discuss the absorption and translocation of herbicides in plant, the
injury symptom related to each mode-of-action, basis of selectivity in herbicides,
evolved herbicide resistance in weed populations and environmental fate of herbicides
in the soil.
Objectives to Unit
1. Explain the herbicides absorption and translocation process in plants.
2. Explain the injury symptoms related to herbicide modes-of-action.
3. Describe the basis of herbicide selectivity and evolution of herbicide
resistance in weed populations.
4. Describe the herbicide interactions and activities in soil.
Topic 7: Herbicide Activities in Plants and Interaction with the Environment
a.
Herbicide Absorption

Absorption: entry of herbicide into the plant. Often called “uptake”.

How herbicides enter plant?
1. Aboveground organs: stem, flower, buds, FOLIAGE (main herbicide entry).
2. Belowground organs: seeds, stem, SHOOTS, ROOTS (main herbicide entry).
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Foliar-Applied Herbicides Absorption

Foliar-applied herbicides are mainly absorbed through:
1. Leaves: most important entry site; main target for post-emergence
chemical weed control.
2. The entry sites are channels through cell wall, trichomes, stomata, and
cracks in cuticle.

Fate of herbicides applied to plants via foliage:
1. The herbicide may volatilize and be lost to the atmosphere.
2. The herbicide may be washed off the leaf surface.
3. The herbicide may remain on the outer surface and dry up / thicken.
4. The herbicide may be photo-degraded (broken down by sunlight).
5. The herbicide may penetrate the cuticle, but remain absorbed in the
lipid components of the cuticle.
6. It may absorb and penetrate the cuticle and then enter the apoplast or
symplast, where it may be subject to translocation and metabolism.

Biotic factors affecting foliar herbicide absorption:
 Cuticle thickness (waxiness of leaf surface): the thicker the slower
absorption.
 Presence of trichomes: the more trichomes the less herbicide retention on
the leaf surface. But can be site of absorption too.
 Damage to leaf surface (cracks in cuticle): the more cracks the more
absorption.
 Physiological state of plant: good condition, high absorption.
 Age of leaf or plant: the younger the higher absorption, but not too
young…
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Non-biotic factors affecting foliar herbicide absorption:
 Light intensity: higher intensity = higher p/synthesis = higher absorption.
 Temperature: higher tempt = faster drying time = lower absorption. Too
low = plant less active = low absorption.
 Humidity: high humidity = stomates open, cuticle not so thick (hydrated
by the humidity) = high absorption.
 Precipitation/rain: depending on herbicides’ activity. Systemic = more
affected (6h to absorb); contact = less affected (30 min to absorb).
 Wind: too windy = high herbicide drift = less deposition = less
absorption.
 Herbicide formulation: ester/amine/salt formulation better than pure acid
formulation.
 pH: low pH = high solubility in lipid = high absorption.
 Adjuvants: help a lot!!!

The 3 barriers to herbicide absorption:
-Cuticle
-Cell wall
-Plasmalemma (cell membrane)
-Cuticle:
•Main barrier to absorption
•Function: prevent dehydration of the leaf
•Content: wax, cutin and pectin (hydrophobic/lipophilic) and cellulose
(hydrophilic/lipophobic)
•Herbicide movement via diffusion
•Thus: only oil-soluble herbicides can easily penetrate the cuticle
layers
•How about water-soluble herbicides: Surface Active Agent – reduce
water tension
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-Cell wall:
•Function: provide rigidity to the cell and the plant
•Content: cellulose with water filled interspaces - hydrophilic in nature
•Herbicide movement via diffusion
-Plasmalemma:
•Function: containment. It holds things in the cell. It is comprised of
lipids/fats – lipophilic
•All herbicides must enter the cell to be active, therefore, it must cross
the cell membrane/plasma membrane
•Major movement via diffusion
c.
Soil-Applied Herbicide Absorption

Absorption for herbicides with soil activity.

Underground parts need to have contact with herbicides via:
1. interception - the underground plant parts intercept herbicide molecules in
the soil.
2. mass flow - herbicide moves to the underground parts of the plant in the
soil due to transpiration.
3. diffusion - the herbicide moves along a concentration gradient to plant.

Sites of herbicide absorption in soil:
1. seed absorption
2. root absorption - major mode for broadleaves
3. shoot absorption - major mode for grasses

Absorption via foliage or soil complete when herbicide released into
cytoplasm in the cell. Next is herbicidal action or move symplastically or
apoplastically or both of the herbicides throughout the plant.
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Hypothetical digram representing the foliar absorption aspects of cuticlecell wall-protoplasm structure; route entry of (a) polar and (b) non polar
Herbicides - in leaves, stem, flowers, fruits
d.
Herbicide Translocation

Translocation: movement of herbicide from site of entry to other locations
within plant.

Translocation pathways:
apoplastic
symplastic
both

Apoplast is the non-living, continuous network of cell walls, intercellular
spaces, and xylem tissue that transport water and mineral nutrients from roots
to shoots.
o Apoplastic translocation: translocation via the apoplast.
o Xylem mobile herbicides move through the apoplast.
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o Apoplastically mobile herbicides follow the same pathway and move
in the same direction as the net movement of water within the plant.
o Transpiration is the driving force.
o Soil-applied herbicides are primarily apoplastically mobile.
o Apoplast movement would be acropetal in the plant.
o Foliar-applied herbicides pathway: cell walls ≡ plasmalemma
intercellular spaces ≡ enter xylem (moved upward).
o Soil-applied herbicides pathway: root hair ≡ intercellular spaces ≡ cell
walls of cortical cells ≡ endodermis ≡ diffuse through casparian strip
≡ enter xylem (moved upward).
o Casparian strip - impermeable barrier in the root. Made of waxes. The
function of the casparian strip and leaf cuticle is the same - to prevent
dehydration. Thus, casparian strip also a main barrier to herbicide
movement across root. Herbicides must diffuse through the casparian
strip or go around it which slows down movement.
o An apoplastically mobile herbicide applied to the soil will enter the
roots, move up the stem, and accumulate in greater quantities in leaves
with the highest transpiration rate (see picture below).
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o An apoplastically mobile herbicide, applied as a single droplet to a
leaf, will move outward toward the tip and margin of the leaf and will
accumulate around the leaf margin (see picture below).

Symplast is the continuous network of living cells, intercellular protoplasmic
connections (plasmodesmata), and phloem tissue that transports assimilates
(food) from the site of production (leaves) to the site of use (root and shoot
meristematic areas, developing fruit, and storage organs such as rhizomes).
o Symplastic translocation: translocation via the symplast.
o Phloem mobile herbicides move through the symplast.
o Symplastically mobile herbicides follow the same pathway in the plant
as movement of assimilates.
o This is often referred to as a "source-to-sink" movement, with the
leaves being the "source" of food production and apical growing
points, buds, roots, and underground storage organs, such as rhizomes,
being the "sinks" or areas utilizing the food produced by
photosynthesis.
o Translocation of herbicides in the symplast is bidirectional. It can be
acropetal or basipetal.
o Symplastically mobile herbicides are almost always applied postemergence.
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o Symplastic herbicide movement would result in injury symptom in
new growth of terminals.
o A symplastically mobile herbicide applied to a single fully expanded
leaf will move out of that leaf and move both up and down the stem via
the phloem into active areas such as apical growing points, expanding
young leaves, developing seed or fruit, and root tips (see picture
below).
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Routes of translocation of herbicides in plants through either symplast (floem)
eg.glyphosate, apoplast (xylem) eg. Diuron or both eg. Dalapon, Dicamba,
DSMA, MSMA, Picloram
e.
Herbicide Mode of Action and Injury Symptoms
Plant Growth Regulators (PGR) / Synthetic Auxin
Injury Symptoms:
Affected broadleaf plant leaves become crinkled, strap shaped, stunted, and
malformed; leaf veins appear parallel rather than netted, and stems become crooked,
twisted, and brittle, with shortened internodes.
Amino Acids Biosynthesis Inhibitors
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Injury Symptoms:
Affected plants stop growth almost immediately after foliar treatment (2-4) days.
Established perennials in two to four weeks.
Plants become straw coloured several days or weeks after treatment, gradually turn
brown, and die.
Fatty Acids (ACCase) Biosynthesis Inhibitors
Injury Symptoms:
Growing points are killed first, resulting in the death of the inner leaves.
Older, outer leaves of seedlings appear healthy for a few days, but eventually they
also wither and die.
After several weeks, the growing points begin to rot, allowing the inner leaves to be
pulled out of the whorl.
Sensitive grasses commonly turn a purplish colour before dying.
Seedling Growth Inhibitors (Root & Shoot)
Injury Symptoms:
Seeds will germinate, but they either fail to emerge or emerge as stunted seedlings
that have thickened, shortened lower stems, small leaves, and short roots.
Seedlings eventually die from lack of moisture and nutrients because of the restricted
root system.
Seedling Growth Inhibitors (Shoot)
Injury Symptom:
Grass seeds normally do not germinate.
If they do, young leaves fail to unfold and rolled.
The leaves of broadleaf plants turn dark green, become wrinkled, and fail to unfold
from the bud.
The roots become shortened, thickened and brittle.
Photosynthesis (at PS II) Inhibitors
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Injury Symptoms:
Early annual/seasonal seedling growth appears normal, but shortly after emergence
leaves become mottled, turn yellow to brown, and die.
For established plants, the oldest leaves turn yellow on the leaf margins first, the veins
remain green, and eventually the plant turns brown and dies.
Perennials starve very slowly because they have large energy reserves in roots or
rhizomes to live on while photosynthesis is inhibited, but death is certain.
Cell Membrane Disrupters
Injury Symptoms:
Cellular breakdown from destroyed cell membranes, allowing cell sap to leak out.
Effected plants initially have a “water-soaked” appearance, followed by rapid wilting
and “burning,” or leaf speckling and browning.
Plant death occurs within a few days.
Pigment Inhibitors (Bleaching Agents)
Injury Symptoms:
Effected plants either do not emerge or emerge white or bleached and eventually die.
Older leaf tissue is affected first.
Phosphorylated Amino Acid (Nitrogen Metabolism) Disrupter
Injury Symptoms:
Injury is similar to that of the cell membrane disrupter herbicides.
Unknown Herbicides
Injury Symptoms:
Injury is almost similar to that of the cell membrane disrupter herbicides.
f.
Herbicide Selectivity
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Selectivity is accomplished primarily by 3 methods:
1. Selectivity by Placement
 Selectivity avoiding or minimizing contact between the herbicide
and the desired crop is called selectivity by placement. Selectivity
mainly for non-selective herbicides.
 Example: apply glyphosate directly to the weed, not exposing to
the crop. Suitable especially for big and tall crops i.e. orchard,
plantation and landscape areas.
2. Selectivity by Differences in Crop and Weed Growth Stage
 Selectivity as a result of some morphological and physiological
between crop and weed. Selectivity mainly for non-selective
herbicides.
 When crop is older than the weed, crop will have an advantage on
things such as:
- thicker cuticle and leaf wax: less herbicide penetration.
- more leaves: many angles - difficult to get a good coverage;
insufficient translocation of herbicide.
- higher growing point (meristem): growing point is protected,
less likely for foliar herbicides to reach the growing point.
- deeper root: less absorption: insufficient translocation.
3. True Selectivity
 Selectivity because of the differences in the biochemical process in
plants. Selectivity in selective herbicides.
 Broadleaf weeds and crops/plants are not affected by ACCase
herbicides because their ACCase is less sensitive to these
herbicides.
 Grass crops (corn, rice, wheat…) are also not affected by ACCase
herbicides because they can rapidly metabolise these herbicides
into inactive product (difference in metabolism rate between grass
crops and grass weeds).
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 As for the Synthetic Auxin (PGRs), broadleaf plants are more
sensitive to the changes in the auxin level than the grass plants.
 Difference in absorption of dinitroaniline herbicides between
tolerance crops and susceptible weeds. And of course, herbicide
placement plays an important part too…..
g.
Evolution of Herbicide Resistance in Weed Species

Herbicides have revolutionized weed control practices all over the world.

Farmers preferred herbicides over cultural and mechanical control practices
because of time and cost efficiency, easy to apply, and, fast + effective weed
control.

However, good things won’t last long..... weed populations have developed
resistance to herbicides all over the world, which has threatened the efficacy
and sustainability of herbicides for weed control.

What is herbicide resistance (HR)???
“The inherited ability of a plant to survive and reproduce following exposure
to a dose of herbicide that would normally inhibit and kill wild type
individuals of the same population.”

What is herbicide susceptibility???
“The degree to which a plant is subject to injury or killed by a particular
herbicide.”

What is herbicide tolerance???
“The inherited ability of a species to survive and reproduce following a
herbicide treatment.”

So is there any difference between HR and herbicide tolerance???
“There is. No selection to make the plants tolerant; those plants simply possess
a natural tolerance. Resistant (R) plants were originally susceptible (S), but
through continuous exposure to herbicide, they become R.”
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How plants evolved resistance???
By continuous selection pressure from the HERBICIDE, naturally or induced
via genetic engineering by human (see picture below).

Types of Herbicide Resistance”
1. Cross resistance
 Plant is resistant to two or more herbicides having the same MOA.
 i.e. Lolium rigidum is resistant to diclofop (WSSA Group 1/HRAC
Group A; Chemical Family APP; MOA ACCase biosynthesis
inhibitors) and clethodim (also WSSA Group 1/HRAC Group A;
Chemical Family CHD; MOA ACCase biosynthesis inhibitors).
2. Multiple resistance
 Plant is resistant to two or more herbicides having different MOA.
 i.e. Eleucine indica is resistant to glyphosate (WSSA Group 9/HRAC
Group G; Chemical Family Glycines; MOA EPSPS inhibitor) and
fluazifop-P-butyl (WSSA Group 1/HRAC Group A; Chemical Family
APP; MOA ACCase biosynthesis inhibitors).
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3. Resistance mechanisms
 There are several mechanisms endowing resistance to herbicides in
weed species have been suggested and proven so far.
 Resistance mechanism can be divided into 2 major categories: target
site resistance and non-target site resistance mechanisms (see picture
below).
RESISTANCE
MECHANISMS
TARGET-SITE
RESISTANCE
NON-TARGETSITE RESISTANCE
GENE MUTATIONS
REDUCED
UPTAKE
OVEREXPRESSION/
OVERPRODUCTION
OF GENES
REDUCED
TRANSLOCATION
ENHANCED
METABOLISM
SEQUESTRATION

Herbicide resistance management
Divided into 2 types:
1.
Prevention and Delaying Resistance
 Use herbicides wisely
Where available, herbicide applications should be based on
economic thresholds – control when necessary (critical
control period). And don’t cut the rates.
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 Herbicide Rotation
Don’t stick to a single herbicide or herbicides with a single MOA.
 Crop Rotation
Can manipulate planting time, spectrum of weed infestation,
cultivation techniques, choice of herbicide with different mode of
action, different stage and different way of application.
 Integrated Weed Control Methods
Mechanical weeding eliminates the weed plants before the seed
set.
Mulching will simultaneously cover weed seeds and stop from
germinating.
2.
Post-evolution
 Crop rotation and herbicide rotation will keep the resistant
population down.
 Maximize crop competition (row spacing, proper fertility,
optimum planting dates, water management etc.).
 Ensure clean and certified seed is planted each season.
 Clean farm machinery is important to avoid movement of seeds
from HR weed spp.
h.
Herbicides Activity, Persistence and Residues in Soil

Herbicides (pre-emergence or post-emergence that reach the soil), upon
contact with the soil are subject to various processes.

Important because they can affect the activity, efficacy and behaviour of the
herbicide.

Also determine the persistence of herbicides in the soil.

Herbicide soil persistence or residual life is the length of time a herbicide
remains active in soil.

Often stated as herbicide half-life: a measure of how long it takes for 50% of a
chemical to degrade.
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These processes is regarded as “environmental fate” of herbicides.
Environmental Fate of Herbicides
1.
Micro-organism Decomposition

The principal micro-organisms in the soil are algae, fungi, and
bacteria.

Micro-organisms use all types of organic matter, including
organic herbicides. Some chemicals are easily decomposed and
some not.
2.
Chemical Decomposition

Chemical decomposition destroys herbicides through interaction
with the soil constituents of oxygen, hydrogen or water (i.e. via
oxidation or hydrolysis).
3.
Adsorption by the Soil

Herbicides adsorbed by clay and organic matter (OM) particles
making the herbicide unavailable for uptake by the weed.

High adsorption in soils high in OM and clay than sands.
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The adsorbed herbicide may be released so slowly that the
chemical is not effective as a herbicide.
4.
Leaching

The movement of herbicides through soil by water.

Rain or irrigation water following herbicide application will
usually leach the herbicide.

Water-soluble herbicides are most easily leached.

Important to weed control effectiveness, crop injury, herbicide
carryover, and the potential for environmental problems.

Herbicide carryover problem is lessened when leaching happens,
but problem with underground water pollution will occur.
5.
Volatilization

Herbicides may evaporate and be lost to the atmosphere as
volatile gases.

Herbicides vary widely in volatility and loss to the atmosphere as
a gas.

6.
Herbicide volatilization increases as the temperature rises.
Photodecomposition

Degradation of herbicides by sunlight.

In the photodecomposition process, the herbicide molecule
absorbs energy from sunlight, causing chemical reactions that
result in herbicide inactivation.

7.
Many soil incorporated herbicides are light sensitive to sunlight.
Plant Uptake

The uptake of herbicides by plant (crops or weeds) roots results
in their removal from the environment; hence, reduced
concentrations in the soil.
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Surface Runoff

A herbicide may leave the application site in surface water
running off the site.

The herbicide may be dissolved or suspended in the runoff water
or it may be adsorbed to soil carried by runoff water.

A serious concern because runoff water ultimately makes it way
into rivers and surface reservoirs used for drinking water.

Herbicides Carryover:
 The concern is that herbicides applied this year/season may carry over and
injure small plants lined out the following year caused by the high
persistence in soil.
 Herbicides with residual activity/high soil persistence are like a doubleedged sword.
 We want them to persist for several months in order to suppress weeds.
 Yet, we don’t want them to persist too long so that they interfere or injure
crops planted the following year/season.
 Great if we can have herbicides that can last throughout crop growing
period, then self destruct instantly before the next planting season, but not
likely to happen.

Guidelines To Avoid Carryover Problems:
 Calibrate the sprayer and apply herbicide uniformly: avoid overspray.
 If incorporating, make sure it is done thoroughly and uniformly.
 Consider applying reduced rates of a persistent herbicide in combination
with a less persistent herbicide.
 Apply herbicide as early as possible and delay planting of the follow crop
if carryover is suspected.
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Conclusion to Unit

Herbicides need to enter plant cell/tissue to cause phytotoxicity.

There are several entry sites depending upon types of herbicide and weed
species.

To enter/penetrate plant site-of-action, herbicide must be absorbed first.

Herbicides, especially systemic must be translocated to the site of action
within the plant to give toxic.

Herbicides can be translocated via two (2) major modes: apoplastic and
symplastic.

Herbicide selectivity is the basis for successful chemical weed management in
crops.

Repeated use and strong reliance on herbicides led to evolution of herbicide
resistance in weed populations, especially for herbicides with a single MOA.

The persistence and residue of herbicides in soil are influenced by various
processes known as “environmental fate of herbicide”
Exercise / Activity
1. Draw and label a leaf of a plant. Using 2 different colours of pen show how a
herbicide is taken up by the leaf and where it will go once it is inside the leaf.
2. Why can particular herbicides kill broadleaf plants but not grasses?
3. What are two ways that a herbicide can be taken up into a plant?
4. Translocation of a herbicide depends on the movement of the chemical through
the plant. Name two plant parts involved in moving herbicides throughout the
plant.
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Unit 8
HERBICIDES SAFETY GUIDELINES
Introduction to Unit
This topic will discuss the safety information and guidelines that must be followed
while handing and applying herbicides.
Objectives to Unit
1. Explain the safety guidelines in handling and using herbicides.
Topic 8: Herbicide Safety Guidelines
a.
Herbicide Labels and Safety Guidelines

Herbicide label contains all information on the herbicide.

Label and labelling is important for effective use from herbicides and safety to
environment.

Info provided on the label:
1. Product information: contents of herbicides etc…
2. Use information: how to apply, how much, when to apply etc…
3. Safety information

Child Hazard Warning: All pesticide labels must bear the statement
“KEEP OUT OF REACH OF CHILDREN”.

Signal Words: DANGER, POISON, WARNING, CAUTION.

Hazards to Humans and Domestic Animals.

Personal Protective Equipment (PPE) Statements: wearing personal
protective equipment listed on the label: minimize your exposure.
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4. Statement of Practical Treatment

These statements tell you the first aid treatments recommended in case
of exposure or poisoning.
5. Typical statements include:

In case of contact with skin, wash
immediately with plenty of soap
and water.

In case of contact with eyes, flush with water for x minutes and get
medical attention.

In case of inhalation exposure, move from
contaminated area and get
medical attention.

If swallowed, drink large quantities of milk,
egg white, or water -- do
not induce vomiting.

If swallowed, induce vomiting.
6. Physical or Chemical Hazards

These statements will tell you of any special fire, explosion, or chemical
hazards the product may pose.

For example:
o Flammable: Do not use, pour, spill, or store near heat or open
flame. Do not cut or weld container.
o Corrosive: Store only in a corrosion-resistant tank.
7. Environmental Information

Herbicides may be harmful to the environment.

Some products are classified as RESTRICTED USE HERBICIDE
because of a potential environmental hazard.

Watch for special warning statements on the label concerning hazards to
the environment.

For example:
o This product is highly toxic to bees.
o This product is toxic to fish.
o This product is toxic to birds and other wildlife.
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Summary:

Read the label before purchasing the herbicide, to determine:
o whether this is the herbicide you need for the job.
o whether the herbicide can be applied using the application equipment
available.

Read the label before you mix the herbicide to determine:
o necessary protective equipment for safe handling.
o what you can mix with the product (compatibility).
o how much product is required.
o the proper mixing procedure.

Read the label before applying the herbicide to determine:
o safety measures necessary.
o how and when to apply.
o where the herbicide can be used.
o restrictions of use.

Read the label before storing or disposing of the herbicide and container,
to determine:
o where and how to store.
o how to properly clean and dispose of the container.
Conclusion to Unit
Handle Herbicides With Great Care:
•Care about yourself
•Care about people working with you
•Care about the crops
•Care about other organisms
•Care about your environment
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