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PCV du Toit

Grootfontein Agricultural Development Institute

Private Bag X529





The Ecological Index Method (ElM) with its disjunct series of index values, 10, 7, 4 and 1, and the improper marriage between palatability -and ecological values, make estimates of current grazing capacities of an area unreliable. Consequently a model was developed to estimate these index values objectively, independent of the ElM. Properties describing the agronomic value of the plant species; size, animal available dry matter and chemical composition, were employed in the construction of the model.


Method and Results


Canopy spread cover, the amount of grazeable forage, the nutrients Ca, K, Mg, Na and P, acid detergent fibre, crude protein, total digestible nutrients, and percentage ether extract of a number of grasses and karoo bushes were measured. High ether extracts and aromatic oil contents render plants unacceptable to ruminants and cause the loss of energy-rich esters, ethers and aldehydes through their urine. Although the TON content of plants may therefore be high, from a nutritional point of view, they may be inferior. Compare Chrysocoma ciliata, TON content 53.9 and ether extract values of 8.9 in summer and 9.2 in winter, to Pentzia incana, TDN content 50.6 and ether extract values of 2.3 in summer and 3.9 in winter. From a nutritional point of view C. ciliata is inferior as a forage plant, ingestion of large amounts of this plant leads to alopecia in Iambs and photosensitivity and laxation in goats and sheep. The ether extract value is used as a divisionary element in the karoobush model. In grasses on the other hand, high ether extract values are associated with high carotene contents, favourable with regard to vitamins A, B, E and D content. Themeda triandra , TDN content 55.4 and ether extract values of 2.02 in summer and 1.92 in winter, opposed to Aristida congesta, TDN content 53.5 and ether extract values of 1.34 in both summer and winter. In the grass model the ether extract value is used as a multiplicative element. Furthermore, unfavourable K/(Ca + Mg) ratios may lead to tetany.




Final models: Grazing index values of karoo bushes = {(canopy spread cover + grazeable forage + TDN + [K / (Ca + Mg)]) / ether extract} 1100. Grazing index values of grasses = {(canopy spread cover + grazeable forage + TDN + [K / (Ca + Mg)]) x ether extract} /100 (Du Toit 1996). Grazing index values of species vary throughout the year, from year to year, and also depend on soil type, rainfall and temperature. These facts shed light on palatable vs. unpalatable karoobushes and have direct effects on the estimation of current grazing capacities for a particular area, where the estimation of differing seasonal grazing capacities will have to receive attention.




Arid Zone Ecology forum 1997: 16 -17