- Hardiness in Small Stock
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HARDINESS IN SMALL STOCK
Grootfontein College of Agriculture, Middelburg, CP 5900
Although South Africa is mainly an arid country, with large areas classified as semi-desert, it does not mean that there cannot be farmed under these conditions. Several indigenous sheep breeds are excellently adapted to survive, produce and reproduce under these harsh environmental conditions. The hardiness of an animal refers therefore to the ability of an animal to survive, produce and to reproduce under extreme environmental conditions without supplementary feed.
It is generally known that the glucocortico-steroid hormones of the adrenal gland, especially cortisol, play an important role in the ability of animals to cope with various kinds of stress. Insufficient synthesis and/or secretion of these hormones result in a condition known as hypo-adrenocorticism or Addisons's diseases in humans. The symptoms of hypo-adrenocorticism involve virtually all organ systems and can be summarised as follow (Smith et al, 1983):
- Hypoglycemia under stressful conditions
- Decreased protein mobilisation
- Decreased fat mobilisation
- Disturbed gastrointestinal function
- Loss of appetite
- Marked susceptibility to even minor stresses
- Low glycogen concentration
- General weakness
2. WOOL PRODUCTION VS. REPRODUCTION AND GROWTH
From observations in practice it is known that the growth and reproduction of wool producing sheep are usually lower than that of mutton breeds in extensive areas. In an experiment, carried out at Grootfontein, the effect of wool production potential on other production characteristics were investigated in woolled sheep with varying wool production potentials under veld conditions over two successive years. No supplementary feed was provided. The result of this experiment are given in Table 1. These results are in accordance with the general trend of an inverse relationship between wool production and reproduction and growth.
3. WOOL PRODUCTION POTENTIAL VS. BLOOD GLUCOSE CONCENTRATION IN STRESSED ANIMALS
An experiment was carried out at Grootfontein with five sheep breeds, namely, the Merino. Dohne-merino, Afrino, Dorper and Namakwa-Afrikaner. The wool production for these breeds was 0.258, 0.194, 0.107, 0.043 and 0.057 kg/W0-75 respectively.
Blood glucose concentration was measured under cold stress and during fasting. From the results, as displayed in Figures 1 and 2, it was evident that breeds with a high wool production function were not able to maintain their blood glucose levels to the same extent as the mutton breeds.
4. WOOL PRODUCTION VS. HARDINESS
From the results obtained in the present trials, as well as from several other experiments and from experience in practice, the characteristics of high fibre producing small stock can be summarised as follow:
- Lower reproduction rate
- Higher mortality rate
- Lower growth rate
- Lower resistance to cold stress
- Lower resistance to starvation
- Blood glucose more sensitive to stress
- Lower voluntary feed intake.
These facts suggest that the animals with a lower fibre production function are more hardy as compared to the high fibre producers. The precise mechanism which causes this negative correlation between wool production and hardiness is not yet known. The large resemblance between the symptoms of hypo-adrenocorticism, mentioned earlier, and the characteristics of high fibre producing animals, however, has led to the hypothesis that fibre production in small stock is in some way related to the activity of the adrenal gland. It is postulated that selection for increased fibre production actually decreases the activity of the adrenal gland. This in turn causes a change in the intermediatory metabolism in order to increase fibre production (protein). Unfortunately, this metabolic adjustment apparently occurs to the detriment of hardiness (glucose metabolism). In other words, the shift in the metabolism to increase protein synthesis simultaneously impairs the glucose metabolism causing the animal to be more sensitive to stress (Herselman, 1990). Work done in Australia clearly illustrates that the artificial administration of cortisone acetate decreases wool production in adrenalectomized Merino sheep. The effect of cortisone on the shift in the intermediatory metabolism is also evident from an experiment carried out at Grootfontein in which Angora goats were treated with cortisone acetate. In this experiment hair production was decreased by 25% and muscle growth by 40%. Fat synthesis, on the other hand was increased considerably. This is clearly demonstrated by the parameters used, namely, intestinal fat and fat thickness on the back which were increased by 64% and 360% respectively.
In order to establish any differences in adrenal activity in different breeds of small stock, the insulin stimulation test was carried out on Merino, Dohne-merino, Afrino, Dorper and Namakwa-Afrikaner sheep. The insulin stimulation tests is the routine clinical test used to identify adrenal malfunction in humans.
This test is based on the principle that insulin, when administered intravenously, causes a rapid drop in blood glucose concentration. In normal subjects the drop in blood glucose causes a sharp increase in the blood cortisol level.
The results of this test, given in Figures 3 and 4, are in general accordance with the hypothesis, put forward earlier, and illustrates definite differences in adrenal activity between small stock breeds. Whether this impaired adrenal activity, as found in high fibre producing animals, is the primary or only cause of the lack in hardiness, generally associated with these animals, needs further clarification.
5. IMPLICATIONS OF SUPPLEMENTARY FEEDING
Because many of the problems in respect of hardiness in high fibre producing animals can be ascribed with a great deal of certainty to an impaired glucose metabolism, the implications of supplementary feeding affecting the glucose metabolism should be duly considered in breeding animals. Many of the problems related to Angora goat farming are also caused by a drop in the blood glucose concentration. It was also found that the supplementation of grain causes a sharp rise in blood glucose concentration of ruminants. This observation led to the development of the "chocolate mealie" at Grootfontein in 1982 for the purpose of counteracting environmentally induced decreases in blood glucose levels. This enabled the Angora goat industry to overcome some serious problems such as abortion and losses during cold spell, associated with blood glucose insufficiency.
Most ram breeders in South Africa supplement their animals with grain to allow optimal growth. Obviously, the primary underlying cause for differences in hardiness between animals is thereby eliminated. By following such a practise the danger of selecting animals with inferior hardiness exists. When such an animal is then used by a commercial farmer in an extensive area, especially where no supplementary feeding is provided, the offspring of this animal cannot adapt to these harsh conditions. The first problem, which are usually observed in such adapted animals, are unsatisfactory reproduction rates and high mortality rates.
From the foregoing it is evident that the "best" animal for a specific environment is not necessarily also the "best" animal for another environment. In order to eliminate this problem of identifying superior breeding animals for a specific environment, selection of these animals should occur under similar conditions as those in which there offspring will have to produce and reproduce. In other words, it is preferable that a commercial farmer should buy rams from a breeder whose rams are raised under conditions similar to his own.
Proc. S.A. Soc. Anim. Prod. Highveld Branch, 1992South African Federation of Group Breeders Newsletter, 1992