In the above section, the primary effects of defoliation were discussed. The way by which defoliation is done introduces some secondary (effects) factors which can be very important. Of the different ways of defoliation, cutting, grazing and burning are the most important.
The secondary effects of cutting
As mentioned earlier, mechanical cutting is non-selective and man has almost total control over the frequency and intensity of cutting. This makes the secondary effects less and makes experimentation and interpretation easier.
The two most important secondary effects are:
- The influence of the treading by machinery on the vegetation and the soil. Although this may be an important factor it is usually much smaller than the reading effect of animals.
- With mechanical cutting, no fertiliser in the form of urine and dung reaches the soil.
The secondary effects of grazing
Selective grazing
Example of selective grazing where the more mature grass is avoided.
Selective grazing is the phenomenon whereby all plants are not defoliated to the same degree by the animals.
When an animal is allowed to select, it will select the more palatable plants and parts of plants and it will normally select a diet with a higher nutritive value than the average of the grazing on offer. Selection usually also causes higher intakes. These two factors normally lead to a (very) high production per animal.
On the other hand, selective grazing is (usually) detrimental to vegetation. If the palatable plants are grazed severely over long periods, their competitive ability is weakened. This helps the less palatable plants to increase the cost of the palatable ones.
The fact that the whole (entire) plant is seldom defoliated at once by grazing can have certain advantages compared with cutting. The different parts of the plant are usually connected through vascular tissues. If only a part of the plant is defoliated the undefoliated part can support the defoliated part.
The detrimental effects of selective grazing are usually much greater than the beneficial effects. Selective grazing and overgrazing are often regarded as two equally important causes of veld deterioration in South Africa.
Trampling or treading
The effects of trampling by animals are often overlooked.
The effects of trampling work in two ways:
- The plant can be directly damaged by trampling;
- The chemical and/or physical characteristics of the soil may be changed by trampling which may then influence the vegetation. The physical properties of the soil are more affected by trampling than the chemical properties.
Effect of trampling on the vegetative growth of vegetation.
Under dry conditions, the direct damage to plants by trampling is more important than the detrimental effects on the soil. Small low-growing plants (like opslag) are more subjected to trampling damage than bigger plants. Apart from injuring the plants, trampling leads to big losses in edible material. These losses are usually much bigger than often realised.
Trampling of wet soil has two detrimental effects:
- Destruction of the structure of the topsoil; and
- Compaction of topsoil. These two factors usually cause a lower infiltration rate which leads to drier soil, more run-off and soil erosion and poorly aerated soil. The drier and more compact topsoil also makes it more difficult for seedlings to become established.
Trampling of soil around water point.
An important result of trampling under dry conditions is the development of footpaths. Footpaths are the result of concentrated trampling because it is impossible to get an even distribution of hoofprints. These footpaths can be an important factor in water and wind erosion.
Although not scientifically proven yet, it is assumed that the trampling of dry soil may have certain advantages. It can help to break the crust of the topsoil which helps the covering of seeds in the soil and the establishment of seedlings. The most trampled areas are often the areas with the most seedlings after the rain.
Although the effects of trampling are more important in wet areas with high carrying capacities, their effects in dry areas cannot be ignored.
Recycling and distribution of plant nutrients
The main elements involved in recycling are nitrogen, potassium, phosphorous, sodium, sulphur and calcium.
A growing animal retains about 15% to 20% of the nitrogen it takes in. The retention of minerals is usually a little higher. The nitrogen and minerals excreted by the animal reach the soil in the form of urine and dung.
These urine and dung patches are well-fertilized patches and usually give a much higher production than the surrounding soil. Unfortunately, animals tend to avoid these patches for some time, probably because of the smell. This is especially true if the dung and urine are of the same animal type. Fouling can therefore cause big losses in edible material where cattle graze on intensive pastures.
Unfortunately, animals are poor fertiliser distributors. The urine and dung patches are usually localised and are not evenly distributed over the camp, but concentrated near gates, watering points and sleeping places.
As in the case of trampling, fouling and the recirculation and distribution of plant nutrients are more important in intensive humid areas than in extensive dry areas. Very dry hot weather may, for example, lead to big N losses from urine and dung patches in the form of NH3.
Under intensive conditions cutting or grazing can affect soil fertility. Grazing is unlike fertilisation with regard to the addition of plant nutrients to the environment, but at least a large percentage of the plant nutrients go back into the soil whereas almost all nutrients are removed by a cutting treatment.
Distribution of seeds
The animals distribute seeds which cling or stick to their wool or hair and also those seeds that can pass undamaged through the digestive tract. The distribution of seeds by animals is more important in extensive than in intensive areas. Seeds can further also be disturbed by wind and water.