- Factors influencing the palatability of herbage and species selection by the animal
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Factors influencing the palatability of herbage and species selection by the animal
ONE of the main problems in veld management is to obtain a high standard of veld production and to maintain this veld without the invasion of undesirable plants. Another problem is the control of undesirable plants by grazing with specific types of animals. Preferences for a specific plant differ from animal type to animal type, as well as between individual animals of the same type. An indicator of the plant-animal relation is the well-known fact that animals select their diet from the total available herbage.
Before considering the factors influencing the palatability of herbage and species selection by the animal, certain terms should be considered (Heady, 1964).
1. Palatability: Palatability can be defined as plant characteristics or plant conditions that stimulate the animal to graze the plant.
2. Palatable: This term has been defined as pleasing to the taste and therefore pleasing to the mind.
3. Preference: Preference is reserved for selection by the animal and is essentially behaviourism.
4. Relative preference: This indicates the selection of a plant species in proportion to another species.
Species selection is the result of a stimulation reaction relationship between the plant and the animal. The palatability of the plant stimulates the animal to select the plant as a constituent of its diet. In other words the reaction of the animal to the stimulation is to graze the plant. The stimulation-reaction relationship in food selection and acceptance is controlled by a complex chain of events. It has been suggested that this chain of events consists of three interrelated systems (Young, 1948):
1. The first system is within the body of the animal and includes items such as nerve stimuli initiated by energy release, blood sugar concentration, body temperature, movement of the digestive tract, the senses as well as tiredness of the mouth parts. This system is. However more associated with the stopping of intake than with the beginning.
2. The second system includes the adaptation of the animal to the available herbage. However, there has not been much research done on the effect of a change in available herbage on the selection pattern of the animal.
3. The third system that affects the acceptance of plants is the nutritive value of the herbage and the physical environment of the animal. All three systems are interrelated in a stimulation reaction chain of events that can include the following: recognition of food, movement to the food, appraisal, initial intake and cessation of intake.
Species selection may take place in any stage of the chain and can be controlled by any number of factors. This situation is better described by selective reaction rather than taste reaction in a definition of palatability, as the animal does not react on all the stimuli of all the plants, but only on certain stimuli coming from certain plants.
Hardison, Reid and Woolfalk (1954) divided the factors that influence species selection, into two groups: plant characteristics and animal characteristics. On the other hand, Heady (1964), mentions the following five groups of factors that can influence selection:
. Associated species;
. Climate, soil and topography;
. Type of animal; and
. Physiological status of the animal.
(a) Chemical composition of the plant
Jones (1933) as cited by Hardison et al (1954) found in an experiment with cattle that prostrate growing plants did not have such a high preference rating as erect growing plants. Jones concluded that prostrate growing plants were the less palatable part of the available forage. Johnstone-Wallace and Kennedy (1944), however, had results showing cattle grazing young growth not higher than 10-12 centimetre, even when taller plants were readily available. As taller plants have a higher fibre content than prostrate plants, it can be assumed that not the height but the chemical composition of the plant affects the palatability of the plant.
Extensive work has been done to find a correlation between the chemical composition and the palatability of the plant. This correlation has been established by various workers.
High positive correlation has been found between the palatability of plants and the following chemical compounds:
(3) Acetic acid;
(4) Linolenic acid; and
(5) Butyric acid.
Hardison et al (1954) has found that plants high in ether extract are very palatable to animals. It has been found that an increase in fats resulted in greater preference. As protein, fats, sugars and preferred components of ether extract increase in percentage composition, lignin and cellulose decrease. The opposite of this statement is also true, therefore negative correlations of lignin and cellulose with increased palatability is shown.
Contrary to this, Louw, Steenkamp & Steenkamp (1967), have found that plants with a high ether extract are highly unpalatable in the Karoo. This apparent conflicting result can be ascribed to the fact that the plants used by American researchers were high in fats and low in resin, while in the Karoo the plants with the highest ether extract were high in resin and low in fats. When considering ether extract content as a criteria of palatability, it should be borne in mind that this method determines all the ether soluble compounds including fat and resin.
As shown in the table below the palatable plants in the Karoo have a very low percentage of ether extract (Louw et al 1967).
palatability Pentzia incana Ankerkaroo 3,7
Salsola tuberculata Ganna 1,3
Plinthus karooicus Silwerkaroo 1,8
Chrysocoma tenuifolia Bitterbush 12,8
Euryops spp Harpuisbos 15,9
Elytropappus rhinocerotis renosterbos 23,7
Several compounds, including nitrates and tannin, are believed to decrease the palatability of plants. With the addition of sugar or saccharine to plants high in protein, potassium, calcium, iron, fat, nitrates and vitamins, the palatability of these plants increase (Wagnon and Goss, 1961). The same principle applies when spraying molasses on grass with a low palatability.
Many conflicting results have been reported in studies conducted to determine what chemical compounds influence herbage selection. As shown, a positive correlation was reported for protein, sugar, acetic acid, linolenic acid and butyric acid content and palatability. On the other hand it was reported by various research workers that no consistent correlation exists between palatability and chemical composition (Johnstone-Wallace and Kennedy, 1944, and Hardison et al, 1954). Perhaps more significant than the amount of any chemical compound is the combination of chemical compounds. Therefore nutritive value is more generally used to describe the palatability of plants than individual chemical compounds.
Published results of work done on the palatability as related to chemical composition usually give a list of compounds that increase as preference increases and another list of compounds that decreases as palatability decreases. Unfortunately little work has been done to determine the effect of individual compounds on the palatability; this could have been done by changing the composition of one compound and keeping the other compounds constant.
The actual chemical composition of the plant differs in the various parts of the plant. Leaves usually contain a higher ether extract and crude protein than the stem, but are lower in lignin, cellulose and crude fibre content. The chemical composition of fruits and seeds vary between species but usually they contain a high content of crude protein, fat and carbohydrates.
It has been determined that sheep and cattle prefer fruits and leaves to stems
(Heady and Torell, 1959; Van Dyne, 1963 as cited by Heady, 1964). Whether this is the result of the chemical composition or other factors, is not known. There is no definite explanation of this phenomenon as fruits and leaves can be grazed separately but it is difficult to graze stems without grazing fruits and leaves as well.
(b) Growth stage of the plant
The growth stage of the plant has been mentioned as a factor that can influence the palatability of the plant to a great extent. The older the plant the less palatable it becomes. This can be due to the fact that older plants are woodier than younger plants, with younger plants having a higher moisture content and less harsh foliage. Davies (1925) as cited by Hardison et al (1954) considered the fact that sheep preferred young growth as incidental. Grasses and broad-leaved bushes show an increase in crude fibre, lignin, cellulose and carbohydrate content with increasing age and a decrease in crude protein. These are actual changes in the plant itself and are further affected by a change in the leave stem ratio (Heady, 1954). In a mixed grass-bush veld the selection pattern changes as all the plants do not reach maturity simultaneously.
(c) Environmental conditions
Climate, topography and soil moisture have been listed as affecting palatability. These environmental factors affect palatability aspects of plants such as chemical composition and harshness of foliage (Cook, 1959).
(d) Morphology of the plant
The external structure (morphology) of the plant can also affect the palatability or acceptability of the plant. Plant selection is affected by the presence or absence of thorns, hair, position of the leaves, stickiness and texture of the leaves and the stems (Heady, 1964).
2. Associated species
The selection of a specific plant species is dependent on the availability of other choices. The selection for certain plant species varies from veld-type to veld-type. Observations in the U.S.A. (Hurd and Pond, 1958), have shown that certain plants are readily eaten when available in small amounts. When the same plants were available in abundance, the position in the selection pattern was markedly changed. Apart from observations, not much work has been done regarding the role of associated species in the selection of herbage by animals.
3. Climate. Soil and Topography
These factors influence the animal self rather than the plant. The grazing habits of the animal is changed by, for example, temperature variations, rainfall, and movement of the animal is affected by the drought cycle, soil texture and slope of the camp and weather as a whole (Heady, 1964).
4. Kind of animal
The selection by various kinds of animals differs markedly, with each specie showing innate preference for plant specials, plant parts and plants in specific growth stages. The selection within a kind varies from animal to animal, from area to area, season to season, day to day and even within the same day. A decrease in the availability of preferred species can also lead to a change in the selection pattern of the animal.
Hesselgren (1749) as cited by Hardison et al (1954) fed hundreds of plant species to cattle, sheep, goats, horses and pigs. The species were fed separately and in mixtures. The goats and sheep, in this experiment, were less selective and the pigs more selective in their grazing habits than the other animals. Hesselgren found that plants that was considered to be aromatic were not readily selected by the animals and that the sheep were not selective in respect of the poisonous and non-poisonous species.
The grazing habits of the four main animal types can generally be described as follows:
(i) Horse - short grazer;
(ii) Sheep - low stratum grazer;
(iii) Goat - multi stratum grazer; and
(iv) Cattle - deep (high) stratum grazer (Roux, unpubl. data).
5. Physiological status of the animal
The selection pattern of the animal is closely associated with the physiological status of the animal, in other words, whether the animal is pregnant, lactating, fat, hungry or ill. The sense of sight, smell, touch and taste as well as instinct and experience of the type of grazing can also have an effect on the selection pattern of the animal. In an experiment where the effect of the sense of taste, smell, sight and touch on the selection pattern of the animal was determined, it was found that taste had the greatest effect and that the other senses were supplementary to taste. Smell did not have such an important effect on the selection pattern. Sight and touch were associated more with specific plant structure such as succulence and growth form. When all four senses were hampered, this did not lead to a completely random selection, although the percentage unpalatable plants in the diet were increased (Krueger, Laycock and Price, 1974).
Stapleton (1948) as cited by Hardison et al (1954) explained the selective grazing pattern of animals on the base that the animals can "feel" the need for a certain nutrient. Hancock (1950) as cited by Hardison et al (1954) observed a great similarity in the selection pattern of twins. These observations show that the selection pattern of animals is affected by certain inherent factors in the animal.
All the factors that influence the selection pattern of animals can be divided in four groups:
(a) Palatability - This include$ all characteristics of the plant that can be recognised by the animal, in other words, mainly morphological characteristics.
(b) Condition of available plant material –This group includes the microclimate, soil condition, relative abundance, contamination, associated species and phenology of the plant (growth stage).
(c) History of the animal - This factor includes the evolution of the grazing habits as well as the previous experience of the animal as far as the specific plant species is concerned.
(d) Physiological status of the animal – This group includes all the possible physiological stadia of an animal.
The question of what an animal eats and what influences the animal to select a certain plant or plant parts has been asked for many years. As species selection is influenced by all four groups of factors in an interrelated manner it is difficult to answer all the questions related to species selection. From the fact that animals select its diet from the total available herbage evolves the problem to obtain and maintain a high standard of veld production.
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