- A study of diets selected in three arid veld types
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A STUDY OF DIETS SELECTED IN THREE ARID VELD TYPES
PCV du Toit
Grootfontein Agricultural Development Institute
Private Bag X529
The ability of two sheep breeds (Merinos and Dorpers) to adapt to the herbage on offer was studied in three arid veld types. Carnarvon, Arid Karoo, mean annual rainfall 203.5 mm. Grootfontein, False Upper Karoo, mean annual rainfall 359 mm. Jansenville, Noorsveld, mean annual rainfall 264 mm, in rainshadows as low as 170 mm. By definition an arid area receives less than 500 mm rain per annum, therefore, all three stations fall well within the limit circumscribing such an area.
Oesophageal fistula extrusa samples were collected in the winter, spring, summer and autumn of the 1984/85 to 1986/87 seasons. 500 points per sample were analyzed for the percentage contribution by weight, of five functional species groups, karoobushes, grasses, annual plants, trees and shrubs and, noors to the diet. The d-index of Willmott (1981) was used to compare the diets.
Merinos selected more grass, while the Dorpers selected more karoo bushes overall, as well as more of the woody component in the Noorsveld. In the Arid Karoo, while most of the differences were noted during the plant reserve storage and dormant seasons, there was a 6 to 8 % difference in the diet selected, however, these two breeds select very similarly during the growing season. In the False Upper Karoo there was an 11 to 25 % difference in the diet selected by the Merinos and the Dorpers during the growing season. In the Noorsveld there was a 2 to 3 % difference in the diet of the softer grass and karoobush, selected by the Merinos and the Dorpers during the growing season.
The overlap in diets selected by different sheep breeds, is not large enough to consider combining different breeds to better utilise the vegetation of the Arid Karoo (Du Toit et al. 1995a). The large differences in the diets selected by the two breeds in the False Upper Karoo, indicate the existence of special niches in the veld where these breeds will graze preferentially in order to satisfy their requirements, through grazing different proportions of the vegetation on offer, selectively. It is possible to combine different breeds to make better grazing use of this veld (Botha et al. 1983). During the growing season in the Noorsveld there is a large overlap in the selected diets, which indicates that both breeds are well adapted to the vegetation stratum consisting of karoobush and grass where this veld has not become too degraded. Indications are that Merinos are particularly ill adapted to graze good condition Noorsveld advantageously on their own. Dorpers can be combined with Boergoats to advantage (Du Toit et al. 1995b).
Arid sone ecology forum 1997: 15 -16