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Module # 5 – Component # 2
Nutritional Value of Plants
Introduction
Ungulates live off plants. However, plant material is difficult for ungulates
to utilise, because the plant has many refractory and inhibitory substances to
defend itself against herbivores. The most successful refractory substance is
fibre. Many of the evolutionary and ecological patterns in ungulates can be
explained in terms of the interaction with the fibre and toxin content of
plants.
One of the most important aspects of ungulate/vegetation interactions is the
division of plant material into cell walls and cell content. The cell content is
referred to as the soluble component while the cell wall is made up of dietary
fibre. Plant cells also consist of other components which are important for the
ungulates, but these are not discussed here.
Nutritional Value of Plants
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Energy and Protein from Plants
Energy
A meaningful distinction between soluble substances and fibre is the
degree of digestibility by herbivores. The sugars, starches and most of the
proteins in the cytoplasm (the content of the cell) of plant cells are relatively
digestible by enzymes secreted by most mammals, including ungulates. In
contrast, the components of the cell walls are difficult to digest. Cellulose
and hemicellulose (fibre) is a large source of energy for herbivores.
Although mammalian enzymes cannot digest fibre, there are a number of
bacteria, protozoa and fungi that can digest cellulose.
Consequently, the digestion of cellulose and hemicellulose by ungulates has
been made possible by the evolutionary symbiosis with microbes that break
the beta-glycoside bonds by anaerobic (not in the presence of oxygen)
fermentation in the ungulate’s gastro intestinal tract (GIT). The ungulate
provides a favourable environment and a relatively constant source of
food for the microbes. Since fermentation is anaerobic, the products of
microbial degradation possess much of the original energy of the fibre that
can be used by the ungulates.
Furthermore, the microbial cells can be digested to a certain extent by the
host. The microbes can also ferment the soluble components reasonably quickly.
Such fermenting microbes occur in the GIT of many mammals, including
humans. Digestion of fibre, however, occurs only in mammals that retain the
fibrous material long enough for fermentation to occur.
Protein
Protein is more critical than energy in many situations. The protein/fibre
relationship is the most critical determination of the nutrient value of
vegetation for ungulates. The protein content of plant cells differs, as well
as the distribution between the cell wall and the cytoplasm. Generally, as the
cell wall components increase in plants, the protein content tends to
decrease.
In grasses, the
proportional.
cell
wall
and
protein
content
are
inversely
In leaf material, the total amount of protein tends to remain
constant even if the fibrousness of the vegetation increases.
Nutritional Value of Plants
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Veld types
For grazing purposes, the natural veld in South Africa can be divided into
three broad veld types namely:
Sweetveld
Sourveld
Mixed veld
(Figure # 18).
The ratio between sweet and sour grasses determines the veld type.
Sweetveld, sourveld and mixed veld originated primarily through the
adaptation of plant species to climatic factors such as rainfall and
temperature as well as edaphic and management conditions.
Figure # 18 - The distribution of sweet, sour and mixed veld
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Sourveld
As a result of relatively high rainfall in sourveld areas, the soil is
constantly subjected to the leaching out of plant nutrients.
This leaching results in a decrease in pH and the fertility of the soil.
Leaching of plant nutrients has a negative influence on the nutritive
value and palatability of grasses.
Sourveld is only palatable and nutritious during the growing
season. Grasses have very low nutrient value for grazing during
the winter.
Sourveld occurs mainly as fire climax grassveld in high
rainfall areas (>800 mm per annum [31 ½ in]) or at lower
rainfall (>600 mm per annum
[23 ½ in) on acid, sandy soils.
Occurs at higher altitudes (over 1 000 m [3280 ft]) and at lower
temperatures than sweetveld.
Can tolerate overgrazing but this will lead to lower production.
Sweetveld
In sweetveld areas there is very little leaching of the soil
resulting in higher plant nutrient levels in the soil. The higher
nutrient levels have a positive influence on the nutritive value and
palatability of grasses.
Sweetveld is palatable and nutritious throughout the year.
Sweetveld occurs mainly in lower rainfall areas (250 - 500 mm per
annum
[10 in – 19 ½ in]).
Occurs in lower altitude and higher temperature (nearly frostfree) areas than sourveld.
Sweetveld is very sensitive to overgrazing, especially during the
growing season.
Mixed veld
Mixed veld represents an intermediate form between sweetveld and
sourveld.
Mixed veld of which the characteristics correspond to those of sweetveld is
known as sweet mixed veld and vice versa as sour mixed veld.
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These three terms for South African Veld Types are merely a quick and
dirty layman’s guide. South Africa has in fact been thoroughly described
in terms of its various veld types. The veld types are first divided up into
biomes:
Biome Types
Forest
Fynbos
Grassland
Nama Karoo
Savanna
Succulent
Thicket
Number of Veld Types
3
10
15
6
25
4
5
Table # 5 - South African Biomes
The following table gives a detailed look at the Savanna Biome and how it
has been divided up into 25 distinct veld types. This type of information is
important to land owners and prospective land buyers – as an indication
of the agricultural (including game farming) potential a piece of
property has.
Savanna Biome Veld Types
Clay thorn Bushveld
Coast – Hinterland Bushveld
Coastal Bushveld
Eastern Thorn Bushveld
Kalahari Mountain Bushveld
Kalahari Plateau Bushveld
Karroid Kalahari Bushveld
Kimberly Thorn Bushveld
Lebombo Arid Mountain Bushveld
Mixed Bushveld
Mixed Lowveld Bushveld
Mopane Bushveld
Mopane Shrubveld
Natal Central Bushveld
Natal Lowveld Bushveld
Shrubby Kalahari Dune Bushveld
Sour Lowveld Bushveld
Soutpansberg Arid Mountain Bushveld
Sub-arid Thorn Bushveld
Sub-humid Lowveld Bushveld
Sweet Bushveld
Thorny Kalahari Dune Bushveld
Waterberg Moist Mountain Bushveld
Table # 6 - The Veld Types of the Savanna Biome
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The Effects of Fire on Herbage Quality
Short term effects
The crude protein content of the recovery growth of burned veld is higher
than that of the new growth of plants in an unburned veld. By burning the
veld, the protein content in the plants can be increased.
Recovery growth that appears after burning is more palatable than the
older growth before the burn.
After burning there is an abundance of nutrients released by the ashes,
this abundance of nutrients causes a stimulation of plant growth.
Fire removes all the old and dead plant material that can not be utilised
by wildlife.
Long term effects
Fire improves the quality of the grazing by assisting the maintenance of
fire climax grasslands.
Fires in Savannas
Fire in Savanna plays an important role in the development and
maintenance of productive and stable Savanna communities.
Fire in Savanna decreases bush encroachment.
Fire in Savanna brings the browse down to a level on which it can be
utilised by wildlife.
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Feeding Spectrum / Gradient
The chemical composition of plants as possible food for ungulates forms a
spectrum or gradient from low fibre/high soluble substances (usually high
protein) to high fibre/low soluble substances (usually low protein) (Figure #
7).
The quality of the vegetation for ungulate feeding is mainly determined by its
fibre content. Quantity is, however, also important. There is a positive
association between the fibre content and biomass of plant material. Plants of a
higher quality tend to be available in lower quantities.
The quantity and quality of the fibre content of plant material differs seasonally.
The quality of leaves, which have a moderate amount of fibre, and are relatively
proteinaceous, remain constant throughout the year when they are available.
The quantity, however, varies a lot. The biomass of grass does not vary as
much, but the quality decreases greatly as it becomes physiologically mature. If
the patterns of quality and quantity are combined, then a gradient of the
nutrient value of plant material can be determined.
Woody
Structure
H
ig
h
Mature
Grass
A
B
U
N
D
A
N
C
E
Growing
grass
Dicot Foliage
Meristems
Tubers
L
o
w
Fruits
Low
High
FIBROSITY
Figure # 7 - The fibre content of plant material
Nutritional Value of Plants
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