- Diet selection by different small stock species in the Arid Karoo
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DIET SELECTION BY DIFFERENT SMALL-STOCK SPECIES IN THE ARID KAROO
P.C.V . du Toit, C.D. Blom
Grootfontein Agricultural Development Institute, Private Bag X529,
Middelburg CP 5900
and W .F. Immelman
Carnarvon Experimental Station, P.O. Box 98, Carnarvon 7600
Methods to increase meat and fibre production off Karoo veld are continually being investigated. To this end the diet selection of various small-stock species were studied on the Arid Karoo veld (Veldtype no. 29, Acocks 1988) at Carnarvon Experimental Station.
A hypothesis was postulated and tested, which states that different small-stock species utilise different components of the veld, therefore, the degree to which the selected diets differ, can be exploited to advantage, i.e. a higher grazing capacity can be recommended (Botha 1984). A definite grazing capacity is laid down for a specific small-stock species in a specified veld type. Botha's (1984) reasoning behind the hypothesis is that, by combining different small-stock species and by incorporating the degree to which their selected diets differ, it is possible to apply a heavier stocking rate to the veld, without the vegetation deteriorating. Meat and fibre production can then be increased per unit land area.
Dry ewes and wethers/kapaters of Afrino, Dorper and Merino sheep and Angora goats were fistulated at the oesophagus (Bredon et at. 1967). They were allowed to freely graze the veld on the Carnarvon Experimental Station and to become accustomed to it. Oesophageal fistula samplings were carried out during the winter, spring, summer and autumn periods of three consecutive seasons, 1984/85 to 1986/87. The collected samples were examined microscopically. The percentage contribution that the palatable, less palatable and unpalatable karoo bushes, grass and ephemerals (opslag) made to the diet, was established for each species, sex, and for each sampling period. Botanical surveys were carried out simultaneously, in order to compare the selected diet to that which was actually available.
The d-index of Willmott (1982) was used in the comparisons. The closer the index value approaches to 1,0, the better the agreement between the two variables compared.
The botanical surveys differed by less than 3 % between the seasons (Table 1), indicating a high degree of agreement in the recorded botanical composition between seasons, and revealing the surprising fact that no component of the vegetation occupied a very prominent place at any time.
Seasonal diet selection
The selected diets were compared to the results of the botanical surveys. Owing to the small differences between the wethers/kapaters and the ewes, the data from the different sexes were combined prior to further analysis (Table 2).
On average it can be said that during winter the available forage is grazed. The main components grazed include Eberlanzia ferox ( doringvygie), Pentzia spinescens (doringkaroo) and grass (Aristida congesta and Enneapogon desvauxii). Willmott's index of agreement (Willmott 1982) between the selected diet and the available forage (as established by the botanical survey) varied between 0,82 and 0,93 (Table 2 and Figs. 1 and 2, where UPB refers to unpalatable karoo bushes, LPB refers to less palatable karoo bushes while PB refers to palatable karoo bushes).
During spring the animals concentrate on ephemerals such as Anchusa capensis (ossetongblaar), Lepidium africanum (peperbossie) and Mesembryanthemum nodiflorum (brakslaai), with a d-index that varied between 0,13 and 0,19. During summer the animals concentrate on grass, mainly annual grasses of the species Aristida congesta (steekgras) and Enneapogon desvauxii (agtdae- gras), with a d-index that varied between 0,47 and 0,49.
In autumn the animals concentrate on the less palatable karoo bushes, notably Pentzia spinescens (doringkaroo), while still taking in fair amounts of the rest of the available forage, as indicated by the d-index that varies between 0,83 and 0,85.
The low indices of agreement calculated between the botanical survey and the selected diet during spring and summer, indicate the very pronounced selective grazing habit of the animals. This can also be seen in Figs. 1 and 2.
Differences between small-stock species
During the vegetation's growing season (spring and summer) (fable 2), the mean index of agreement in the selected diet. between the different small-stock species, was d = 0,95 to 0,97. During autumn, d was 0,86, while during winter, d was 0,79. The 3 to 5 % difference between the selected diets of the different small-stock species and physiological classes during the growing season is too small to consider combining small-stock species in order to increase the stocking rate.
In the Arid Karoo, there is a 95 to 97 % overlap in the selected diets of different small-stock species during the growing season. The difference between the selected diets of the different species is too insignificant to consider making use of different species-combinations. The slight difference in the selected diet cannot be used as the basis for increasing the grazing capacity in an effort to utilise the vegetation more effectively, in order to raise the meat and fibre production potential of the area.
ACOCKS, J.P.H. 1988. Veld Types of South Africa. Memoirs of the Botanical survey of South Africa no. 57. Government Printer, Pretoria.
BOTHA, P. 1984. Veld: Determination of animal ratio on a mixed dwarf shrub veld in the Karoo and arid areas based on diet selection by Afrino, Dorper and Merino sheep and Angora goats. Unpublished research facet. Grootfontein Agricultural Development Institute.
BREDON, R.M., TORELL, D.T. & MARSHALL, B. 1967. Measurement of selective grazing of tropical pastures using oesophageal fistulated steers. Journal of Rangeland Management, 20: 317-320.
WILLMOTT, C.J., 1982. Some comments on the evaluation of model performance. Bulletin of the American Meteorological Society, 63:1309-1313.
Karoo Agric, Vol. 6, No 1, 1994 (25-27)