- Diet selection by Dorpers in the Noorsveld
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Diet selection by Dorpers in the Noorsveld
PCV du Toit
Grootfontein Agricultural Development Institute
Private Bag X 529
The diet selection of various sheep and goat breeds was studied on the Noorsveld at the Jansenville Experimental Station. The reason for this study was that a definite grazing capacity in a specific veld type is laid down for any specific small stock breed. However, it was argued that, by combining different small stock breeds and by incorporating the degree to which their selected diets differ, a heavier stocking rate can be applied to the veld, without causing degradation to the vegetation. Meat and fibre production can then be raised per unit land area.
Noorsveld occurs on the undulating plains and in the valleys around Jansenville, at an elevation of 300 to 700 m. On steeper slopes, Spekboomveld occurs intermingled with the Noorsveld. Typical Noorsveld is a monotonous vegetation type consisting of Euphorbia coerulescens, noors. The Noorsveld is not very large, about 200000 ha in extent and owing to its proximity to the Valley Bushveld and the Spekboomveld, it shares a large percentage of trees and shrubs in common with these veld types. The mean minimum and maximum temperatures are 11.5°C and 26.5°C respectively. The area is largely frost-free. The average long-term precipitation is 264 mm/annum, while it is as low as 170 mm/annum in rain shadows.
Ten dry ewes and ten neutered males for each of the breeds were fistulated at the oesophagus. Sampling of the vegetation was carried out during the winter-, spring-, summer- and autumn seasons of three consecutive years, 1984/85, 1985/86 and 1986/87. Precipitation during two of the years was below average with one above (Fig. 1). Sampling was conducted over four days and took place in the early morning. The area sampled was 46 ha in extent. Botanical surveys and the collection of fistula samples were carried out simultaneously, in order to compare the botanical composition of the selected diet to the available forage. An estimate of the botanical species composition and hence the presumed available forage, was obtained by recording the canopy spread cover.
In the botanical surveys all the plants were identified to species level. Plant fragments from the fistula extrusa samples were identified to species level as far as possible, but in the case of the grasses this was extremely difficult. In order to facilitate direct comparisons between the selected diets and the botanical surveys, the plant species were grouped. These groups are well known, they are: palatable Karoobushes, less palatable Karoobushes, unpalatable Karoobushes, grass, trees and shrubs, and noors. The percentage contribution of the palatable, less palatable and unpalatable Karoo bushes, grass, noors and trees and shrubs in the diet, was established for each breed, sex, sampling period and for the botanical survey conducted in each season.
The selected diets were compared to the results of the botanical surveys, on a season by season basis.
The d-statistic (the index of agreement) described by Willmott, was used in the comparisons between the botanical surveys and the diets selected. According to this statistic, the closer the value approaches one, the better the agreement between the two variables being compared and conversely. The closer the index value approaches zero, the poorer the correspondence between the compared variables. This means in effect that non-selective grazing is taking place the closer the value approaches one. As the value approaches zero, grazing becomes more and more selective, the selected diet then differs to a large degree from the diet on offer as established by the botanical survey.
There was a high degree of agreement between the results of the botanical surveys carried out on the Noorsveld in the different seasons (Fig. 2).
During any of the seasons, the Dorpers largely selected those plants which were actively growing (Figs. 3&4). From Figures 3 and 4 it is evident that on the whole, the Dorper ewes grazed less selectively than the Dorper wethers.
On average it can be said that during winter, the Dorpers grazed the available forage, to a greater or lesser degree, the d index varies from 0.48 to 0.76.
During spring the Dorpers largely concentrated on grass and mainly selected two of the climax grasses, Cenchrus ciliaris (blue buffalo grass) and Digitaria eriantha (finger grass). A large degree of selective grazing took place in spring, when the d index which varies from 0.17 to 0.27 is referred to.
During summer, the Dorpers tended to concentrate on grass. A large degree of selective grazing took place during the wetter summers, the d index varied from 0.14 to 0.61. Irrespective of their ecological importance, grass in this area seems to be very palatable and is well-grazed whenever it is available. During the dry summer of 1987, the Dorpers tended to graze the available forage, with the d index varying from 0.50 to 0.76. In this instance, when reference is made to Figures 3 and 4 and the d indices, it will be noticed that the ewes grazed more selectively than the wethers.
In autumn the Dorpers concentrated on the less palatable Karoo bushes, in this instance mainly Pentzia incana (ankerkaroo), but consumed the available forage to a large degree as the d index indicates, it varies from 0.58 to 0.93. In this instance the ewes are the less selective grazers. The wethers graze from 5 to 20% more selectively than the ewes.
It is well known that grazing during the active growing seasons has a tremendous impact on the vegetation. Grazing of the grasses during their active growing season leads to their elimination and the development of a virtually pure stand of karoobushes. This grazing impact is not nearly as severe during the dormant season of the grasses and since the Karoobushes have two distinct growth peaks, the second growth peak occurring in autumn to early winter, Karoobushes tend to be grazed in preference to the grasses during autumn and early winter, with the consequent development of a mixed stand of grass and Karoobushes. When deciding to adjust grazing capacities, differences in diet selection during the dormant season can largely be ignored, since grazing during this time is not as deleterious to the vegetation as during the active growth season. In order to calculate differences in the selected diets between small stock species and breeds, with a view to raising the stocking rate, the grazing impact of the animals should be considered on the actively growing vegetation.
The large role that precipitation plays in the production of available grazing cannot be underestimated. Following the "drought" at the start of 1986, dry matter became limiting and the Dorpers were forced to graze more aggressively. This grazing behaviour led to the grazing of large amounts of noors during the winter of 1986 (refer to Figs. 3 & 4). When it is considered to combine animal breeds and to adjust the grazing capacities in order to make better use of the available grazing, the vagaries of the climate should be remembered.
Meat production can be raised per unit land area. It remains questionable, however, whether the gain realised in meat and fibre production will offset the greater expenditure in materials, especially fencing and managerial time. It can, however, be suggested that farmers in a favourable position, buy in wethers to finish for the market. This strategy will least strain the management of the farming enterprise and, the wethers can be off-loaded at the first signs of drought or a deterioration in the vegetation taking place.
Acocks JPH 1988. Veld Types of South Africa. Memoirs of the Botanical Survey of South Africa, no 57. Govt Printer, Pretoria.
Anonymous 1991. Landbou-ontwikkelingsprogram, Karoostreek. Grootfontein Agricultural Development Institute.
Blom CD 1981. Group Classification of Karooplants. Unpublished report. Grootfontein Agricultural Development Institute.
Botha P 1983. A comparison between the diets of small and large stock on mixed Karoo veld. Proceedings of the Grassland Society of southern Africa, 18: 101-105. Botha P 1986. Weigewoontes van merinoskape. Karoo Agric 3(7):10-12.
Meissner HH, Hofmeyr HS, Van Rensburg WJJ & Pienaar JP 1983. Classification of livestock for realistic prediction of substitution values in terms of a biologically defined large stock unit. Technical Communication no. 175. Govt Printer, Pretoria.
Willmott CJ 1981. On the validation of models. Phys. Geogr., 2:184-194.
Table 1. Species composition of the noorsveld as identified in the selected diet of the dorpers, with their common names and grazing values
Unpalatable karoo bushes
Less palatable karoo bushes
Palatable karoo bushes
Trees and shrubs
May tenus heterophylla
Dorper news, 55 p 11 -14