The interaction between grazing animals and the grass plant
Prof Chris S Dannhauser
2.1 Selection of grass species by the grazing animal
In Part 1 of this series the characteristics of different grass species were discussed.
Animals are able to “sense” the differences between different grass species and different plant communities.
When they enter a well-rested camp where all the grasses (palatable and un-palatable) are grown out, as shown in Figure 1.1, they will start selecting the more palatable species and the more palatable areas in the camp first .
During the first few days, the palatable species will be grazed and later on the animals will gradually start grazing more semi-palatable species, because the leaves of the palatable ones becomes less. This grazing pattern is shown in Figure 1.2 below.
If animals stay longer in the camp they will start grazing the unpalatable species too, as shown in Figure 1.3, and that can go over to overgrazing.
On every farm and in every camp, different grasses grow, which can be classified as palatable, semi-palatable and unpalatable and the grazing pattern animals will follow, with these different species, was shown in Figures 1.1, 1.2 & 1.3. If the animals stay too long in a camp they will over-graze the veld as shown in Figure 1.3. That will damage the palatable species, because they will be graze too short.
If the animals are taken out before they start grazing the unpalatable species as shown in Figure 1.2 less harm will be done. This is called the take half, leave half approach.
2.2 The take half, leave half approach
The theory behind the take half, leave half approach is that palatable grasses are selected before and at a bigger scale than semi-palatable and unpalatable grasses. Figure 2 shows to what level the different grasses should be grazed. Only ½ of semi-palatable and ⅔ of palatable grasses should be removed and the unpalatable ones should not be grazed The palatable grasses should not be grazed down to ground level, at least 10 cm of the tuft should be left.
Figure 2: The end result of grazing according to the take half, leave half approach.
If animals stay longer in the camp, over-grazing of the palatable and semi-palatable grasses can cause permanent damage to the veld. The following four photos give an indication of the difference between un-grazed veld (Photo 1), the take half/leave half concept (Photo 2), overgrazing (Photo 3) and severe overgrazing (Photo 4).
2.3 What is the effect of overgrazing on the grass plant?
The leaves of the grass plant act as a “food factories”, they produce sugar and starch and that is the energy source that is used for the growth of the grass tuft. When plants are grazed moderately, enough sugar and starch can be produced to feed the whole grass tuft. The excess energy will then be stored in the roots and in the base of the tufts, mainly during late summer and autumn. This stored energy is essential for over wintering (when there is no production of energy) and also for re-growth during the start of the following growing season.
When an area is over-stocked with grazing animals, it will lead to over-grazing of the grass plants. If animals defoliate the same tuft repeatedly, the leaves will be grazed too frequent and too short and that will decline the sugar and starch (energy) production. In such a situation the energy reserves, that was stored in the roots, during the previous autumn, will be utilized. Then the roots might die off because of energy depletion. If the over-grazing continues, the complete grass plant will die. That is the main reason why some parts of South African veld is in such a bad condition.
Figure 3 shows what happened to the roots of plants under different grazing intensities.
The plant on the left hand side of the figure was defoliated (grazed) every 3 weeks, to a height of 14 cm, and the results showed that a strong root system was left.
On the right hand side the plant was also defoliated every 3 weeks, to a height of 2 cm. It is clear this plant was over utilized, which resulted in a decline of the root system and poor growth will follow in future. The root system in the middle is that of a plant grazed to a height of 8 cm), that was not so harmful to the root system.
|Graze to 14 cm|
|Rest 3 weeks|
|Graze to 8 cm|
|Rest 3 weeks|
|Graze to 2 cm|
|Rest 3 weeks|
Figure 3: An indication of the impact of grazing on grass roots
2.4. Resting of veld
The very important management principle that was mentioned earlier is resting of the veld. The consequences of over-grazing or over-utilization, which was explained earlier, emphasized the need of regular periods of rest to keep the grass tufts vigorous. It is also to the benefit of the animal, because they can graze young, nutritious plant material in the form of regrowth after a short rest. This is only applicable if the stocking rate is realistic because then the palatable grasses are not grazed too severe. Proper resting of the veld is possible by using the take half, leave half approach and to do this a good grazing camp system is important.
Camp systems will be explained later in this series
Prof Chris S Dannhauser
Cell no (+27) 082 873 4736