Grazing animals affect pastures by defoliation, by treading and by excretion. Each of these factors affects forage production, forage quality and botanical composition, and each factor in turn is affected by these pasture characteristics. There is, therefore, no simple cause and effect relationship. In addition, no simple cause and effects and pasture characteristics are influenced directly or indirectly by environment.
Despite the complexity of the animal / plant interaction, it is useful to consider some of the main effects of defoliation, treading and excretion to gain a better understanding of some of the principles of grazing management.
Defoliation
The general effect of defoliation on grasses has already been discussed. Defoliation by animals differs from burning or cutting in that it is both selective and gradual. Two types of selective grazing take place – species selective grazing and area selective grazing. In addition, different species and classes of animals differ in their manner of selecting herbage.
Species Selective grazing
Species selective grazing is the habit of grazing animals to graze certain species in the veld in preference to others. Danckwerts, Aucamp& Barnard (1983) monitored species selective grazing in sweet grassveld in some detail. They concluded that animals do graze selectively, even in sweet grassveld, and that Themedatriandra was by far the most preferred, commonly-occurring species. They did however, show that the degree of preference can change with time of year, and that species selective grazing continues even when grazing pressures are very high.
The way grass plants are defoliated has also been investigated. Dancwerts (1984) found that when cattle defoliate tillers in sweet grassveld, they remove virtually all leaf material at the first bite. Cattle graze grass plants from the top downwards in sweet grassveld. The same pattern exists with sheep grazing in the central grassveld of the Orange Free State. Sheep utilise shorter grass and can literally dig for grass below ground level.
In the DohneSourveld, Daines (1976) monitored mean percentage utilisation of species with increasing grazing pressure. As grazing pressure increased, so percentage utilisation per species increased, but this does not necessarily imply that plants were being grazed progressively from the top downwards. The same phenomenon was observed by Danckwerts (1984), when he also found an increasing percentage utilization due to the result of an increasing number of defoliated tillers in a tuft (grass bundle).It is probably true to say that when swards are short; animals remove virtually all the leaf material on grazed tillers at the first visit.
When tufts have been visited once, animals will initially avoid the partially grazed tufts in favour of ungrazed plants but will return to the ungrazed portion of the partially grazed preferred species before even partial grazing of all less–preferred species.
After being grazed, tillers and tufts will re-grow and, if grazing animals are present for long enough, they may return to graze re-growth. In this way, animals may graze regrowth of preferred species fairly frequently, while less-preferred species are either left ungrazed or infrequently grazed. This process results in over-utilisation of preferred species and complete avoidance of unprefrerred species. It gives selectively grazed veld its distinctive “patterned” appearance and results in a change in species composition and veld deterioration.
It does appear, however, that at least some veld species may become overgrown and later die if they are left ungrazed for a number of years; and if they are not rejuvenated by removal of top growth by some other means such as fire (Tainton, 1981). Thus, selective grazing can result in veld deterioration in two ways – overgrazing of some plants and undergrazing of others.
Tall tillers may be re-grazed before significant re-growth has commenced and, in this instance, the “second bites” do not involve removal of re-growth. Danckwerts (1984) found that in sweetveld, grazing of re-growth did not take place within a 2-week period, implying that at most times, animals can be left in paddocks for up to 2 weeks without over-utilising the preferred species. It is possible that during this 2-week period, carbohydrate transfer could take place from the ungrazed tillers, when partially grazed in the same tuft of grass. When these tillers themselves are grazed within the 2–week period, their vigour might be diminished. Despite this, the fact that individual tillers were not re-grazed within 2 weeks, even when significant growth was taking place, is considered sufficient justification for recommending a period of occupation of 2 weeks to avoid the ill-effects of species selective grazing. This is necessary, since shorter periods of occupation, also implies that the animals return to the paddocks in a shorter period of time to proceed with grazing, before the grazed plants had time to recover, and the ill-effects of selective grazing will be aggravated. When swards are dormant in winter or during droughts, animals can be kept in paddocks for considerably longer than 2 weeks without grazing re-growth.
Area selective grazing
Area selective grazing is the habit of grazing animals to graze certain areas in preference to others. It often results in over–utilisation of preferred areas, which may lead to veld deterioration.
Veld is invariably extremely heterogeneous, with each unit of veld different from its neighbour. The species composition, slope, rainfall, temperature, soil, aspect and a host of other factors may differ from one part of a form to another, giving rise to the heterogeneity. Heterogenicity, in turn, leads to selective grazing. To achieve satisfactory and uniform utilisation of the grazing, it is necessary to recognise and to separate (by fences) each uniform unit.
The following aspects may be of particular importance in defining homogeneous units:
a. Land form: terrain morphology, aspect, slope, surface condition and erosion. Of particular relevance, with regard to area selective grazing, is aspect. In the southern hemisphere, northern aspects are drier and may therefore produce less forage than southern aspects. In contrast, northern aspects are more palatable than southern aspects, and overgrazing of northern aspects is very obvious when both aspects are fenced into the same paddock. Their separation is considered essential to avoid area selective grazing.
b. Soil and geology: situation, texture, intensity of leaching, soil depth and parent rock. Parent rock material is particularly important in the Eastern Cape Region. Soils derived from volcanic rock, basalt and dolerite in particular, generally accommodate more palatable vegetation, which is more resistant to overgrazing than soils derived from sedimentary rock.
c. Vegetation: formation (general type), basal cover, veld type, species composition and feeding value of species.
d. Climate: micro–climatic variations in factors such as snow, frost, mist and exposure to wind
e. Other features: such as previous utilisation patterns.
It is the task of the operator to select those features of particular importance in each situation and to fence accordingly. The concept of uniform grazing areas, even without being consistentlydescribed, is vital in grazing management.
Difference between Animals in diet selection
Within reasonably confined limits, all mammals have similar nutrient requirements. Although there are differences between animals of different age, sex and between active animals and animals at rest, the mean standard basal metabolism of animals is similar.
To meet this high relative requirement, the small animal, in comparison with a large animal, may (a) have a high intake, (b) select a diet of high nutritive value and (c) have a high digestive efficiency. In general terms, this means that the potential for selective defoliation varies inversely to animal size.
In the context of commercial veld management, the implication is that domestic goats and sheep are likely to be more selective than cattle. The proportional mix of these animals is therefore an important consideration. Goats are generally run to utilise browsable material and, although there is overlap in their diet, their impact as grazers is generally given little consideration. However, with the current attractive financial remuneration from goats and Angora goats in particular, an increasing number of producers are running goats as grazers. In view of their high relative energy requirement (even higher than sheep), one expects that they could be highly selective grazers. Therefore, although the impact of goats as grazers has not as yet been quantitatively assessed, it may well result in veld deterioration. This aspect is currently under investigation.
Turning to the cattle-sheep ratio, Heady (1975) refers to a ratio of 1:15 being the most commonly practiced worldwide. In the Eastern Cape, the differences between the effects of sheep and cattle grazing have been compared only in the DohneSourveld. This work has resulted in the general recommendation that the cattle: sheep ratio should not be more than 1:6 in sour grassveld. In sweet grassveld, Du Toit (1972) found sheep grazing alone to be very selective and detrimental in terms of species composition. In view of the vulnerability of sweet grassveld to overgrazing, it is currently recommended that sheep be eliminated where possible from sweet grassveld. Further information is required with regard to optimal species mixes in the KwaZulu Natal Region.
In conclusion, it can be said that among grazing animals, the need for managerial skill increases as the size of the animal decreases, to compensate for increasing selective defoliation.
Trampling
Trampling by animals may influence pastures both directly and indirectly in a number of ways. Firstly, trampling may physically damage plants; secondly, it may affect the soil and thus indirectly plant growth; thirdly, it may assist in dissemination and germination of seed, and finally, a combination of these factors can cause changes in species composition.
Movement of animals may have several influences on soil. Firstly, soil may be physically moved by walking animals and may cover short plants. Secondly, trampling of litter promotes its decomposition and accelerates its incorporation into the soil. Thirdly, animals alter the structure of the soil by compaction or loosening, depending on soil type and its moisture content. Compaction and loss of structure occurs more readily when soils are moist (mentis, 1981). Many, but not all, studies have increased soil bulk density with grazing pressure. Loosening (or chipping), rather than compaction, occurs most readily when the soil is dry. This can have an advantageous effect by increasing water infiltration and seed germination in sealed or “capped” soils but can also have the disadvantage of increasing soil loss to wind.
With regard to trampling effects on seed dissemination, animals often assist in this process. Seeds, rhizomes and stolons attach to hooves and hides and may be transported considerable distances. Ingestion may also aid germination. Germination may also be promoted by burial of seeds as a result of hoof action.
The effects of trampling are therefore neither universally good nor universally bad. Continual heavy trampling does; however, tend to cause veld degeneration through changes in species composition. This is particularly evident in sourveld where Eragrostis plana and Cynodondactylon, both undesirable species, often dominate heavily trampled veld.
Excretion
The effect of excreta on pastures has been well-documented and is reviewed by Heady (1975). However, the overall effect of animal excretion is that it speeds up recycling of nutrients. The rate of this recycling depends on a number of factors, but perhaps the most important is stocking rate.
A further effect of excreta is that fouled plants become temporarily unacceptable to animals. In this regard, animal movement within a paddock may redistribute nutrients across space. Heavy concentrations of excreta may result in a change in plant species composition, although this effect is often prevented by trampling.