Proceedings of the First International Weed Control Congress, Melbourne 1992
173
Effect of environmental factors on herbicide performance
P. Kudsk and J.L. Kristensen
Department of Weed Control, Research Centre for
Plant Protection, Flakkebjerg, DK -4200 Slagelse,
Denmark
Summary
The effect of light, temperature, humidity,
soil moisture, wind and precipitation on the
performance of foliage- applied herbicides is
reviewed. Furthermore, the inherent
difficulties in extrapolating from controlled
conditions to the field are discussed.
Introduction
Practical experience shows that herbicide
performance is affected by environmental
conditions before, at and after herbicide
application. The environment influences the
growth and physiological status of the plant,
and of the herbicide, as well as the interaction
between plant and herbicide.
Generally, the performance of foliage
applied herbicides is influenced more by
environment than is the performance of soilapplied herbicides; the effects of the latter
being primarily affected by soil - moisture. In
this review only the influence on foliage applied herbicides will be discussed. Besides
reviewing some of the relevant literature, the
inherent problems in extrapolating results from
studies under controlled conditions to the field,
and possible further improvements, will be
discussed.
Effect of climatic parameters on
herbicide performance
Light
Generally, cuticle development is positively
correlated with light intensity (50). However,
the importance of cuticle development on
herbicide activity is obscure (95). Light also
influences plant growth. Decreasing light
intensity increased the ratio of shoots to
rhizome nodes in common couch (Elymus
repens) (16,104). A high shoot:rhizome node
ratio is believed to promote glyphosate activity
because more herbicide can be intercepted per
rhizome node (88). Pre- treatment light conditions did not influence the response of peas
(Pisum sativum) and velvetleaf (Abutilon
theophrasti) to 2,4 -D (30).
Light is required for the activity of
photosystem II inhibitors, bipyridyliums and
diphenylethers (39). More bentazone penetrated white mustard (Sinapis alba) (96) and
symptoms developed faster on common
cockleburr (Xanthium pensylvanicum) leaves
treated with bentazone at high rather than low
light intensity (92). However, in glasshouse
experiments phytotoxicity of bentazone was
found to be greater at 25 and 50% daylight
than at full daylight (105). Similarly, ioxynil
activity on chickweed (Stellaria media) was
found to be inversely correlated with all but
the highest light intensity (full daylight) (77).
In these experiments low light intensities were
achieved by shading the plants and temperature and humidity were not altered compared
to the non- shaded plants. Savory et al. (102)
also concluded that ioxynil and bromoxynil
activity was inversely correlated with light
intensity. In their outdoor pot experiments,
however, humidity was also inversely correlated to light intensity making it difficult to
distinguish between the influence of light
intensity and humidity. The effect of
acifluorfen against anoda (Anoda cristata) and
velvetleaf was unaffected by light intensity
(76).
These results indicate that although light is
required to activate these herbicides, very high
light intensities may have an adverse influence. Applied to the foliage, they act as
contact herbicides and therefore axillary buds
which do not intercept the spray are often
viable. Although symptoms develop more
slowly at low light intensities, the ability of
the unaffected parts of the plants to regrow
may also be reduced due to the reduced rate of
photosynthesis. This may explain the improved overall activity at low light intensity.
Applied to the soil, the activity of photosystem
174
Proceedings of the First International Weed Control Congress, Melbourne 1992
II inhibiting herbicides increases with increasing light intensity (114). This difference can
be attributed to the systemic nature of such
herbicides when soil- applied and therefore
they are distributed to all transpiring plant
parts.
The rate of phloem translocation of assimilates depends on the rate of photosynthesis
(35). Thus, light intensity may be expected to
increase the activity of foliarly- applied
systemic herbicides. More glyphosate was
translocated to the rhizomes of common couch
24 hours after application at high rather than at
low light intensity but after 48 hours no
differences were observed (22)." This result
agrees with the general experience that light
intensity does not influence the long term
effect of glyphosate (15,47). If common couch
plants were fragmented after 24 h, fluazifopbutyl was more active at high light intensity
(24). The long term effect of fluazifop -butyl
and sethoxydim on common couch, however,
was inversely correlated with light intensity
(24,25). It was suggested that the inverse
correlation was due to a delayed and reduced
chlorosis of the foliage allowing more herbicide to be translocated.
Temperature
Temperature has a profound influence on the
growth and development of plants in the pre spraying period. Besides affecting cuticle
development (4,50,120), temperature may also
change plant morphology and physiological
processes. For example, common couch plants
raised at high temperatures retained three
times more spray than plants raised at low
temperatures (27). Wild oat (Avena fatua)
plants grown at low temperatures were less
susceptible to difenzoquat than plants grown at
high temperatures (56). On the other hand,
pre -spray temperature did not influence the
effects of 2,4 -D on peas, whereas a slightly
lower effect was found on velvetleaf raised at
low temperature (30).
In general, uptake and translocation of
foliage applied herbicides increase with
increasing temperature (35,118). For example,
improved performance with increasing temperatures has been found with bromoxynil
(84), ioxynil (77), thifensulfuron (58,82),
acifluorfen (76,98), difenzoquat (80) and
sethoxydim (97). With chlorsulfuron (85) and
bentazone (83) less conclusive results were
reported.
Plants may recover better at higher rather
than lower temperatures which may explain
inconsistent results with contact herbicides.
For example, optimum activity of three
diphenylethers was achieved if application was
made under warm conditions followed by a
period of lower temperatures 10 days after
spraying, as the cooler conditions suppressed
regrowth (121). In many studies the relative
humidity (RH) is kept constant when varying
temperature. Vapour pressure deficit indicates
the drying power of the air (73) and therefore
it, not relative humidity, should be kept
constant. If relative humidity is kept constant,
vapour pressure deficit increases with increasing temperature, i.e., not only one but two
climatic parameters are varied at a_time.
Therefore, relative humidity may give rise to
inconclusive results when assessing the
influence of temperature on herbicide performance.
Glyphosate is generally more efficacious at
low temperatures (16,75). There seems to be
several reasons for this effect. Firstly,
translocation was only affected initially by
temperature, whereas the final amount, was
independent of temperature (37). Secondly,
metabolism was greater at high temperatures
(23). Thirdly, growth was faster at high
temperatures leading to a dilution of the
herbicide within the plarit.(27)'. An inverse
correlation between temperature and activity
was also noted with fluazifop -butyl against
common couch (24) but not against johnson
grass (Sorghum halepense) and bermuda grass
(Cynodon dactylon) (124). In the latter study,
plants were only kept for four days at the
contrasting temperatures and then moved to a
glasshouse, while in the former study tempera:
tures were maintained for four weeks.
In the cooler regions of the world frost can
alter the effectiveness of herbicides used for
weed control and their phytotoxic effects on
crops. A mild frost did not influence the effect
of glyphosate on common couch (32). Subsequent studies revealed that translocation was
not affected by a mild frost (33,36). However,
a severe frost, which damaged the foliage,
reduced translocation to the rhizomes (36).
When applied during a frost the performance
of dinoseb, bentazone, 2,4 -D and chlorsulfuron
on white mustard (Sinapis alba) was reduced
(53). When applied before or after the frost,
Proceedings of the First International Weed Control Congress, Melbourne 1992
only the performance of 2,4 -D and
chlorsulfuron was reduced. The difference in
response to frost may reflect reduced phloem
translocation of assimilates which reduces the
effectiveness of the systemic herbicides 2,4 -D
and chlorsulfuron. However, the effectiveness
of five systemic wild oat herbicides was not
reduced by a mild frost (123). Field experiments in cereals have revealed little effect of
frost on weed control and crop injury when
herbicides are used (42,116). Recommended
doses were applied, resulting in a very high
level of weed control, and this may have
masked any adverse effects of frost.
In most reports temperature was held
constant from spraying to harvest or the plants
were moved to the glasshouse and placed
under often unspecified climatic conditions.
Few reports have examined the effect of
changing temperatures in the post- spraying
period. Three days of warm conditions following the application of difenzoquat to wild oat
resulted in maximum effect irrespective of
temperatures on the following 11 days.
Similarly, three days of cold conditions
following application resulted in a reduced
effect, even when followed by 11 days with
high temperatures (81). In contrast, although
the visual effect of fenoxaprop -ethyl on three
grass weeds was significantly less at low
compared to high temperatures after four
weeks, the effect after six weeks did not differ
if temperature was increased for the last two
weeks (70). These two examples illustrate that
temperature changes in the post- spraying
period may affect herbicides differently.
Humidity
Plants grown at high humidity tend to have
smaller leaves and increased numbers of
stomata and trichomes (45). The quantity of
epidermal wax on Brussels sprouts (Brassica
oleracea var. gemmifera) was more than
halved when plants were grown at 90 %
compared to 40% RH (4). How these changes
influence herbicide performance is difficult to
predict. Velvetleaf grown at high humidity
was somewhat more affected by 2,4 -D than
plants grown at low humidity, whereas the
response. of peas was not affected by pre -spray
humidity (30). Greater uptake and
translocation of dalapon was found in beans ,
(Phaseolus vulgaris) grown at high compared
to low humidity (93).
175
High humidity can be expected to increase
uptake of herbicides (17). Accordinglj' ;;the
activity of chlorsulfuron (85), thifensulfuron
(68,82), acifluorfen (76,98), sethoxydim (25),
fluazifop -butyl (24,124) and glyphosate (27)
was improved by increasing humidity. Contrasting results have been reported with
bentazone. High humidity improved activity
on redroot pigweed (Amaranthus retroflexus)
(83), whereas a.reduced effect was found on
fat -hen (Chenopodium album) (31). It was
suggested that stomatal behaviour was respon -.
sible for the reduced effect on fat -hen at high
humidity. This result may reflect a specific
herbicide /plant/environment interaction, as
increased uptake of bentazone at 80 compared
to 40 % RH was found in seven out of eight
plant species (29).
The literature indicates that the performance of water - soluble herbicides is more
affected by humidity than lipophilic herbicides. This is particularly evident when
comparing the influence of humidity on the
activity of various formulations of the same
active ingredient. For example, the activity of
the water- soluble sodium salt of ioxynil on
chickweed was improved with increasing
humidity, whereas its lipophilic octanoate
ester was unaffected (77). Similar results were
found with a salt and ester formulation of
bromoxynil (102). In our own experiments, the
activity of salt formulations of the
phenoxyalkanoic acids was generally more
affected by humidity than ester formulations
(unpublished results). With the sulfonylureas,
a positive correlation has been found between
water - solubility and the influence of humidity
on performance (68).
The influence of humidity on particular
water - soluble herbicides may be due to several
factors. At high humidity the widely believéd
aqueous pathway through the cuticle is
probably more permeable to polar herbicides
than at low humidity. However, the importance of humidity compared to soil moisture
on cuticle hydration has been questioned (49).
High humidity also extends droplet evaporation time. This is likely to affect the uptake of
water- soluble herbicides as they will not
partition into the lipophilic epicuticular wax.
The effect of temperature on herbicide
performance involves uptake, translocation,
metabolic activity and the potential of the
weed to recover. Apparently, the effect of
.
176
Proceedings of the First International Weed Control Congress, Melbourne 1992
humidity is primarily associated with herbicide
uptake, although an effect on translocation in
perennial weeds has been reported (20). If
humidity mainly affects herbicide uptake, then
it is postulated to be more important at the
time of spraying than in the post-sprayingperiod. In a study on acifluorfen, humidity at
the time of application affected performance.
However, a shift from <30 to >95% RH and
trice versa 24 hours after application did not
affect activity.(76). If dew forms on the leaf
surface, then high humidity, even several days
after application, can affect herbicide activity
(see section on precipitation).
As most effects of humidity uptake is
related to herbicide uptake, it is not surprising
that the adverse effects can often be overcome
by the addition of adjuvants. For example,
enhancement of bentazone uptake by
adjuvants was more pronounced at 40 than at
80 % RH (29) and the addition of oil adjuvants
significantly reduced the adverse effects of
low humidity on bentazone activity on redroot
pigwecd (83). Theoretically, increased droplet
spread, as often occurs when adjuvants are
added, should increase their evaporation rate
(73). However, many surfactants are
hygroscopic (94) and their ability to bind
water and extend the droplet drying period
may partly explain how they overcome the
effect of low humidity on herbicide performance.
A problem when assessing adjuvants is
whether they actually reduce the detrimental
effect of low humidity or just increase activity,
thus masking its influence. Measurement of
such effects necessitates the use of different
experimental techniques. Those that assess the
horizontal distance between dose response
lines are preferred (69,109). By applying this
principle, it was found that addition of a nonionic surfactant significantly reduced, but did
not eliminate, the influence of humidity on the
performance of chlorsulfuron (Table 1).
Soil moisture
Herbicide performance is generally reduced on
moisture- stressed plants. This has been
reported for ioxynil (77), chlorsulfuron (85),
thifensulfuron (82), bentazone, phenmedipham
(60), diclofop methyl, difenzoquat, flampropmethyl (122), glyphosate (51), sethoxydim
(25) and fluazifop -butyl (24,54).
In general, plants growing under soil
moisture stress have smaller leaves, develop a
thicker cuticle and deposit more wax than
plants grown under adequate moisture conditions (49). Such changes in size and surface
characteristics may influence both retention
and uptake. Furthermore, water - stressed plants
Will gradually close their stomata, leading to a
decline in photosynthesis (113) which will
reduce phloem translocation. Evidently,
moisture stress can affect herbicide performance in several ways.
Most studies on2,4 -D have shown that
translocation, but not uptake, is reduced in
moisture stressed plants (6,91). The reduced
activity of glyphosate on moisture- stressed
common couch and common milkweed
(Asclepias syriaca) plants was caused by both
reduced uptake and translocation (55,115). No
difference in uptake and translocation of
fluazifop -butyl was found between water stressed and non - stressed common couch
plants (24,54). However, retention and deesterification of fluazifop -butyl to the active
acid, fluazifop, was reduced (26). The reduced
performance of ioxynil on moisture stressed
chickweed plants was primarily related to
reduced uptake (78). It was suggested that a
higher content of chlorophyll a, carotene and
lutein per gram fresh weight in the leaves of
the moisture stressed plants could have
contributed to the difference in susceptibility.
Recovery of translocated 2,4 -D in stressed
plants was about half of that of non - stressed
plants 24 hours after watering moisture-
Table I
Effect of 0.1% non -ionic surfactant on the influence of humidity on the performance of
chlorsulfuron. Figures in parenthesis are approximate 95% confidence intervals. After (69)
Rel. humidity
35 %
85 %
Relative potency'
Without adjuvant
With adjuvant
1.00
5.42 (4.34 -6.59)
1.00
1.62
(1.31-1.94)
The relative potency indicates the ratio between the doses giving the same effect at 35 and 85 % rh
irrespective of response level.
Proceedings of the First International Weed Control Congress, Melbourne 1992
stressed bean plants (6). This result agrees
with those from our own research showing that
if moisture - stressed plants were watered 24
hours before treatment, then bentazone activity
was restored (Kristensen, unpublished results).
These findings indicate that plants can recover
quickly from moisture stress and it may
therefore be more favourable to delay herbicide application, if rain is forecast, than to
spray moisture - stressed plants.
Wind
Wind is known to cause drift and consequently
reduce spray application efficiency (21).
Likewise, wind influences collection efficiency on both vertical and horizontal targets
(87), indicating that wind may affect deposition of, particularly, the small droplets on the
plants. The biological significance of such
qualitative differences in deposition is not
known.
The leaf surface is damaged by wind due to
leaf collisions (110) and abrasions by soil
particles. It can be envisaged that this may
result in an increased retention and uptake of
herbicide. An artificial wind speed of 32 km
h-' reduced the tolerance of peas to dinoseb
applied 24 hours later (38). From practice it is
well known that other crops, e.g., sugarbeets,
tend to be more susceptible to herbicide
application after a windy period. However,
very little is known about the influence of
wind on the activity of herbicides at the time
of application. The effect of potassium
cyanate on purslane ( Portulaca oleracea) was
reduced when wind speed increased (2). The
detrimental effect of wind was more significant at low than at high humidity, indicating
that the adverse effect of wind was due to
faster droplet drying. The salt formulations of
ioxynil and bromoxynil produced less effect in
a growth chamber, resulting in a much faster
drying of the droplets than in a glasshouse and
outdoors. If plants in the glasshouse were
subjected to wind, then no differences in
activity were observed. Conversely, only
minor effects due to wind, before and/or after
spraying, were found in experiments with
different herbicides, wind speeds and plant
species (86).
Very little is known about wind speeds in
crops 1 - 2 metres in height and close to the
soil surface. It can be anticipated that wind
speeds close to the ground will be lower. It
.
177
seems reasonable to expect that wind, corn -.
pared to the other climatic parameters »ill .'
have a less pronounced influence on herbicide.
Precipitation
Dew
Dew is often present when herbicides are
applied in the early morning or late evening.
The leaf surface may also be wet due to rain,
fog or irrigation.
Little is known about the performance of
herbicides applied to wet plants, although the
label often recommends to spray dry plants.
The performance of glyphosate was not
reduced on wet common couch plants when
applied by either a laboratory pot sprayer (19)
or an aerosol sprayer (27). Conversely,
variable results were found when evaluating
the influence of dew, produced by a condensation method, on the performance of 10 herbicides (7). Results from our own experiments
showed that dew could reduce herbicide
activity if high volume rates were used (66).
Wet leaf surfaces will increase the tendency of droplets to bounce off (48,107). In
two of the studies (19,66) retention was
reduced on wet plants, indicating increased
droplet bounce and/or run off of herbicide.
Despite this, herbicide performance was not
always reduced. Increased uptake due to the
cuticle being fully hydrated, the herbicide
being kept in solution and redistribution of
herbicide from the lamina to other parts of the
plant, may be responsible for the lack of
correlation between retention and biological
activity (19). Applying herbicides to the leaf
sheaths instead of the lamina has been shown
to increase activity (28,117).
Rewetting the leaves, i.e., simulating dew,
after application, without causing run -off, has
also been reported to both increase and
decrease herbicide performance (7,19,80). The
herbicide being redissolved and redistributed
could explain an improvement in effect, while
run -off is the most obvious reason for a
reduced effect from dew after application.
Rain
Rain affects the susceptibility of plants by
damaging the leaf surface (5). A combination
of wind and rain was found to increase the
susceptibility of peas to dinoseb more than
wind alone (38).
However, rain is most important when it
178
Proceedings of the First International Weed Control Congress, Melbourne 1992
occurs shortly after herbicide application.
Intensity and duration determine rain volume.
In many studies rain was simply applied by a
moving nozzle; the númber of passes determining rain volume. This does not simulate
natural rain, as drop size and velocity will be
particularly unrealistic. Therefore, a rain
simulator should be used when examining
rainfastness of herbicides (57,103).
Table 2
Interval in hours between herbicide
application and rainfall of 2 to 4 mm for herbicides
to attain maximum activity. White mustard and
barley /oat were used as test plants.
0-2
2 -6
Cycloxydim"
Fenoxaprop- ethyl'
Chlorsulfuron' Bentazone
Metsulfuron'
Difenzoquat
Phenmedipham Glufosinatammonium
Triasulfuron'
Glyphosate
Tribenuron'
Mecoprop
potassium
salt
Flamprop- isopropyl
Fluazifop -butyl'
l-laloxyfop- ethozyethyl
Mecoprop
ethylene-glycyl-diester
Sethoxydim"
Tralkoxydim"
>6 Hours
recommended surfactant added "recommended
oil adjuvant added
While light rain after application may
improve efficacy, despite some wash off of
herbicide (18,89,106), higher rain fall can
result in a substantial loss of herbicidal
activity. Rainfastness of a herbicide is related
to the susceptibility of the deposit to be
redissolved and to uptake rate (74). Therefore,
it is not surprising that water - soluble herbicide are generally móre vulnerable to rain
than lipophilic herbicides. See Table 2,
summarizing our own research on rainfastness.
Herbicides which are rainfast within two hours
are all lipophilic compounds, whereas the
herbicides in the group requiring a rain -free
period of more than six hours are all watersoluble. The intermediate rainfast sulfonÿlurea
herbicides are all moderately water- soluble.
Other studies have also confirmed the poor
rain fastness of water - soluble herbicides
(13,14,27,40,52,71,83) and the improved
rainfastness of lipophilic herbicides
(13,14,24,25,70). Our results with mecoprop
showed the ester to be more rainfast than the
salt (Table 2). A similar result for 2,4 -D has
been reported (8). Paraquat and diquat are an
exception. They were found to be rainfast
shortly after application. This was attributed to
an ion -ion interaction between the cationic
herbicides and the anionic leaf surface (12).
Climatic conditions in the rain -free period
affect uptake rate and hence rainfastness.
Glyphosate was more rainfast on common
couch when plants were kept at 90 than at 50%
RH (19). Rainfastness of tribenuron was
erratically affected by climatic conditions in
the rain -free period (67).
Most studies on rainfastness vary rain
volume but not intensity. Hankins reported
that 5 mm of rain applied at an intensity of 0.5
or 2 mm h-' did not reduce bentazone activity
but at 5 mm h-' herbicide activity was reduced
by 40% (17). In our own work, we have
observed only minor effects from rain intensity, i.e., rain duration (68) (Table 3). The
most important factor determining rainfastness
is the time interval between spraying and the
onset of rain and rain volume.
Table 3
Effect of 2.5 mm of rain on the activity
of tribenuron applied alone or with 0.1% non -ionic
surfactant on white mustard (Sinapis alba). Figures
in parenthesis are approximate 95% confidence
intervals. Average of 2 experiments.
Time
interval
1 hour
Rain
intensity
5 mm Iv'
50 nun IF'
4 hours
5 mm h'
50 mm h''
No rain
Relative potency'
- Surfactant
+Surfactant
0.22 (0.12-0.31)
0.21 (0.13-0.30)
0.54 (0.41-0.66)
0.38 (0.24 -0.53)
0.32 (0.20 -0.44)
0.81 (0.63 -1.00)
0.51 (0.38-0.63)
0.69 (0.53 -0.85)
1.00
The relative potency indicates the ratio between
doses giving the same effect without and with
rain irrespective of response level.
Increasing rain volume washes off more
herbicide, but at a certain level no further
reductions in activity occur. In the experiments
summarized in Table 2, increasing rain volume
beyond 3 to 5 mm only marginally reduced
activity. Other studies have also shown that
applying more than a few mm of rain will have
only a minor impact on herbicide activity
(1,80,82,85). The amount of rain required to
Proceedings of the First International Weed Control Congress, Melbourne 1992
achieve maximum reduction in 2,4 -D activity
varied from 1 to 15 mm depending upon plant
species and herbicide formulations (8); plants
with hairy leaves required the most rain.
However, in these tests rain commenced less
than one minute after herbicide application,
i.e., before the herbicide deposit was dry.
Differences in rainfastness between plant
species were also found with chlorsulfuron
(85). Similarly, rainfastness also differs
between indoor and outdoor grown plants
(8,61).
Other studies have reported differences in
rainfastness between plant species, but in
many of these studies the same dose was
applied to the different species. If the species
does not respond similarly to the herbicide, it
is likely that the herbicide will be more
rainfast on the more susceptible than the more
tolerant species, because on the former the
dose may be higher than required to attain
maximum activity and consequently rain will
have a less adverse effect. In several of the
reports it is impossible to decide whether
differences in rainfastness are due to different
response levels or are true differences in
rainfastness between the species. Conclusions
from such studies are of limited value as they
are only valid at the response level obtained in
the experiment.
A similar problem exists when reviewing
the influence of adjuvants on herbicide
rainfastness. Is improved rainfastness simply
due to increased herbicide activity which
masks the effect of the rain? Many studies are
not designed to answer this question, because
only one or two doses are included and
because the doses with and without adjuvant
do not produce equivalent effects.
This problem can be overcome by including more doses and examining the displacement of the dose - response curves
(61,62,64,65,67,68,90) or by using doses
which produce equivalent effects on the
various weed species. This was done by .
Behrens and Elakkad (8) in their excellent
glasshouse and field studies on the rainfastness
of two formulations of 2,4 -D.
Despite the inherent problems in assessing
the influence of adjuvants on herbicide
activity, some studies have clearly shown that
adjuvants can improve rainfastness. Adjuvants
mainly act by increasing uptake rate, although
some are claimed to provide physical protec-
179
tion of the herbicide deposit against wash -off.
The results in Table 3 reveal a significant',' r
effect of an adjuvant on the rainfastness of
tribenuron and similar results have been found
with other sulfonylurea herbicides (62,65). The
rainfastness of bentazone was greatly improved by the addition of mineral and vegetable oil adjuvants (40,83,90). Addition of a
non -ionic surfactant to the alkanolamine salt
of 2,4 -D improved rainfastness on some plant
species (8). Adjuvants have also been shown to
improve rainfastness of glyphosate (111,112)
and glufosinat- ammonium (71), probably the
two herbicides most susceptible to rain after
application.
Recently there has been a lot of interest in
the use of the organosilicone surfactants
(10,125). These promote stomatal infiltration
due to an extremely low aqueous surface
tension (108). If herbicide enters the stomatal
cavity, it will not be washed off by rain.
Accordingly, it has been found that the rain free period required to attain maximum effect
can be significantly reduced (43,99,100).
Besides being specific to herbicides, the
organosilicones have pronounced specificity
for plant species, in both efficacy and
rainfastness (44,99,100). Consequently, when
controlling a mixed population of weeds, the
organosilicones may not be the optimum
adjuvant.
Problems in extrapolating results from
controlled conditions to the field
Climatic conditions around the time of application are generally considered one of the key
factors affecting herbicide activity and the
cause of much of the variation in herbicide
performance (34). Numerous reports have been
published on the influence of the environment
on herbicide performance but most examined
one climatic parameter while keeping the
others constant. Furthermore, it has normally
been examined at a fixed level, although
temperature and humidity fluctuate. Such
studies allow for a ranking of the individual
parameters. However, it is difficult to transfer
the results into farmer recommendations
because natural conditions were not mimicked
nor interactions between the individual
parameters examined.
To reduce farmers' costs and to meet the
political demand for a reduction in the use of
herbicides, it is important that we can predict,
180
Proceedings of the First International Weed Control Congress, Melbourne 1992
and improve, herbicide performance under
varying climatic conditions (63).
If results from controlled conditions are to
be extrapolated to the field, it is important that
climate simulators can simulate natural
conditions-realistically. This is highlighted by
the results öf Bethlenfalvay and Norris (9).
They found that time of day had no influence
on sugarbeet injury by desmedipham when
day /night temperatures and light were held
constant. When temperatures were cycled and
light intensity varied, morning applications
damaged sugarbeets more than late afternoon
applications, similar to field observations.
Although the need to simulate climate conditions is obvious, few climate simulators are
suitable. Besides the climate simulators
developed at our department (59), we only
know of one other suitable example (79).
The units at our department can accurately
simulate natural diurnal fluctuations in temperature and humidity. The temperature and
humidity data can either be programmed or a
sensor can be placed outside and the climatic
conditions outdoors simulated. Furthermore,
soil moisture content can be accurately
controlled, as each pot can be automatically
watered several times daily. This is important,
as soil moisture can interact with humidity
(77). Light, of course, cannot be simulated as
closely as other parameters, however light
intensity is well above photosynthetic light
saturation. Further technical details about the
climatic simulators can be found elsewhere
(58).
In climate simulators the micro- and
macroclimate are more or less identical, due to
constant air flow and the lack of a crop.
canopy. In the field micro- and macroclimatic
conditions can differ significantly (73).
Therefore, it would be more correct to simulate the microclimatic conditions and extrapolate the results to macroclimatic conditions.
However, relatively little is known about the
relation between macroclimate and the
microclimate conditions in the crop canopy.
Anyhow, most herbicides are applied at an
early growth stage of the crop, where differences in micro- and macroclimate can be
anticipated to be less pronounced.
Plants grown in climate simulators often do
not resemble field grown plants. Simulating
natural climatic conditions and increasing light
intensity will make the plants look more
natural, but still, differences occur due tothe
lack of wind and rain. Such plants will often
respond to herbicides differently than plants in
the field (46). To simulate field conditions as
closely as possible, it is preferable that plants
are raised outdoors.
To simulate natural climatic conditions,
one has to select those climates that represent
the natural climatic variation. A Danish survey
revealed that, over short periods without rain,
the vapour pressure is rather constant
(Kristensen, unpublished data). Thus, an
inverse relationship exists between temperature and humidity. Therefore, we selected nine
climates by combining three daily average
temperatures with three levels of diurnal
fluctuations, while keeping the vapour pressure constant (Table 4). These selections
represent the natural variation in temperature
and humidity met under Danish conditions.
The high and low humidity represent cloudy
and sunny days, respectively.
A range of herbicides has been tested at the
nine climates and the results have revealed
smaller differences in activity than reported in
previous studies (59). There appears to be
several reasons for this. Firstly, in our studies
large temperature and humidity differences
occur only for a few hours, whereas if temperature and humidity are kept constant,
differences prevail for 24 hours. Secondly, as
vapour pressure is constant, increasing the
temperature will increase the vapour pressure
deficit. Any benefits of increasing temperature
may, therefore, be counteracted by an adverse
effect of the increased vapour pressure deficit.
Although it is well known that temperature
and humidity are inversely correlated, very
often this interaction has not been addressed.
Instead the effect of temperature has been
examined at a fixed humidity level and vice
versa. Thirdly, as the objective of our experiments is to adjust dose recommendations
according to post -spray climatic conditions
and as weather conditions can only be reliably
forecast about five days ahead the plants are
kept in the simulators for only a few days and
then placed under identical climatic conditions. If the long -term weather conditions are
more important than short-term, there is little
point in conducting such studies.
Proceedings of the First International Weed Control Congress, Melbourne 1992
Table 4
Natural climates in climate simulators.
181
5.0°C
4.0-6.0°C
2.0-8.0°C
0.0-10.0°C
87-100% RH 66-100% RH 50-100% RH
computer system applying factor- adjusted
doses. Brighton Crop ProtectióüÇonferénce Weeds, pp. 555 -60.
4. Baker, E.A. (1974). The influence of environment on leaf wax development in Brassica
oleracea var. gemmifera. New Phytologist 73,
12.5°C
10.5-14.5°C 8.0-17.0°C 5.5-19.5°C
77-100% RH 55-100% RH 40-100% RH
5. Baker, E.A. and Hunt, G. (1986). Erosion of
waxes from leaf surfaces by simulated rain.
20.0°C
16.0-24.0°C 14.0-26.0°C 12.0-28.0°C
61-100% RH 48-100% RH 37-100% RH
6. Basler, E., Todd, G.W. and Meyer, R.E.
Average daily Humidity
temperature High
.
Medium
Low
Another factor which should be studied is
the influence of the time of day when applied.
Field studies have time of day as important for
the performance of bentazone (41), whereas
for MCPA and acifluorfen it was less so
(72,119). If high humidity is important to
herbicide activity, then the time of day when it
is applied may be more critical thin the
weather conditions in the days before and after
application.
The results from our studies will be
incorporated in a computer -based advisory
program (3). Based on inputs of weed species,
their growth stage, prevailing climatic conditions and, in cereals, the variety, the program
selects a herbicide and suggests an adjusted
dose (`factor- adjusted dose').
Conclusion
Despite numerous reports on the influence of
the environment on herbicide performance, it
is difficult to use this information to refine
herbicide recommendations. The development
of climate simulators will help to ensure more
realistic results. Even so, problems will still
exist when extrapolating to field conditions.
Simulating climatic conditions in climate
simulators is a more rational and cost - effective
approach than conducting field experiments.
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