- Physio-endocrine responses to genetic change in the Angora goat
|Last update: August 17, 2011 03:23:00 PM|
PHYSIO-ENDOCRINE RESPONSES TO GENETIC CHANGE IN THE ANGORA GOAT
D Wentzel and M J Herselman
Grootfontein College of Agriculture
Middelburg C P, 5900
Evidence of some interrelationships between fibre production potential and physio-endocrine mechanisms underlying hardiness is represented and discussed in relation to the characteristic problems found in the Angora goat. As a result of this information reassessment of present breeding objectives and practices seems to be indicated.
Angora goats are raised primarily for the production of mohair which is highly cherished for its super textile qualities. In man’s endeavours to improve the production of this highly sought-after fibre, the genetic pathway was adopted as one of the means of increasing mohair production per animal, while retaining the desirable characteristics of the fibre.
In this respect, tremendous progress has been made over the years. This is evident from the unsurpassed efficiency of fibre production by the Angora goat compared with all other fibre-producing small stock breeds (Gallagher & Shelton, 1972). In addition, breeders have succeeded in decreasing undesirable kempy fibres in the fleece to a negligible amount.
The Angora goat is, however, also well-known for its characteristic reproduction problems, such as extremely poor kidding rates especially among young animals in sub-optimal environments. Besided poor conception rates, the incidence of disastrously high abortion rates, afflicting up to 80% of the breeding animals in some flocks (Van Heerden, 1963), has presented for many years a serious problem to the industry. Furthermore, the frequent losses of varying numbers of animals during cold spells and the poor growth rate of young animals, coupled to an exceptionally high mortality rate (Terblanche, 1987), add to these serious constraints.
Table 1 : Mortality rate in young Angora goats
|Birth to weaning||13.9|
|Weaning to 18 months age||12.5|
As a result of the mentioned problems several comprehensive investigations of physio-endocrine mechanisms in the Angora goat have been undertaken in an at- tempt to establish the aetiology underlying an apparent innate physiological weakness. This research has certainly contributed to a better understanding of the complex interrelationships between the various physio-endocrine mechanisms involved in the processes of fibre production, tissue growth, reproduction and the maintenance of other body functions. In addition, the results obtained stress the necessity of close liaison between scientists in the various disciplines, e.g. nutritionists, physiologists and geneticists, if any real progress is to be achieved in the quest to improve production. This is the emphatic message of this review.
Initial investigation of the abortion syndrome in the Angora goat by Van Heerden (1963) excluded the possibility of any mineral or vitamin deficiency as a possible cause of reproductive failure. The outcome of this research indicated an endocrine deficiency leading to the failure of the maintenance of pregnancy. Subsequent research by Van Rensburg (1971) supported - this conclusion by providing ample evidence of the involvement of the maternal adrenal in the defect. This was supported by further research of the problem by Wentzel (1973) who actually identified two distinct types of abortion with different endocrine bases. The salient feature arising from these trials, however, was the finding that a drop in blood glucose concentration; following various kinds of nutritional stress or any other kind of stress increasing the demand for energy, is the primary trigger for the onset of the encodrine mechanism leading to most abortions (Wentzel, Le Roux & Botha, 1976).
This has been proved by the fact that earlier disastrous abortion rates had been curtailed to negligible rates by energy supplementation, especially of starch-containing supplements capable of increasing blood glucose concentration.
On account of these findings the physiological responses to cold stress were investigated. This revealed that a drop in blood glucose concentration precedes the collapse of the cold-stressed animal - again indicating an energy deficiency (Wentzel, Botha & Viljoen, 1979).
In accordance with the foregoing, the problem of retarded growth in kids was also greatly alleviated by similar supplementation of energy (Van der Westhuysen, Wentzel & Grobler, 1988).
Similarly, remarkable increases in kidding rates of up to 60% were obtained by flushing breeding animals prior to and during the mating period (Wentzel, 1983 - unpublished results). For these purposes, the use of alkali-ionofore-treated whole grain has proved to be unexcelled, obviously owing to its excellent ability to maximise the increase in blood glucose concentration (Wentzel, 1982).
In view of the foregoing, it is apparent that most of the problems experienced by mohair producers can be solved by supplementation with starch containing feeds capable of increasing blood glucose concentration. This has been proved experimentally as well as in practice, and lends substantial support to the hypothesis put forward by Van Rensburg (1971), namely that selection for mohair production results in a reduction of adrenal function - thus the secretion of adrenocorticosteroids, thereby reducing the constraints limiting fibre production. This condition of hypo-adrenocorticism, representing a correlated response to selection for increased mohair production potential, will, as summarized by Herselman (1990), lead to the following metabolic adjustments :
- Decreased ability to maintain blood glucose concentration under stressful conditions
- Increased gluconeogenesis
- Decreased fat deposition
- Decreased ability to mobilize fat reserves
- Decreased voluntary feed intake
- Decreased resistance to cold stress
- Increased fibre production
- Increased fibre diameter
Furthermore, Herselman (1990) estimated the energy cost of fibre production to be approximately four times higher than the requirement for body growth. In view of these findings and the existing knowledge on physio-endocrine mechanisms in the Angora goat, he attributed the typical problems associated with this breed, and which are without exception related to an energy deficiency, to a condition of adrenocortical insufficiency. This hypo-adrenocorticotrophic condition obviously leads to a weakened intermediate metabolism, which is displayed in reduced blood glucose levels, the latter being the underlying cause of the various conditions mentioned earlier. In addition, the previously mentioned metabolic adjustments associated with reduced adrenal function are displayed typically by the Angora goat of today.
It is therefore reasonable to assume that the progress achieved in breeding an animal with a higher fibre production potential was accompanied with a weakening of the physio-endocrine mechanisms underlying "fitness" or "hardiness" or "survivability".
Obviously, the mentioned defects, which had become worse over time as a result of continuous selection pressure on mohair production, have been reduced to a large extent by efficient supplementation practices. In other words, sub-optimum environments were supplemented in order to meet the requirements of the "high fibre-producing" goat. Especially during the past 15 years this approach was followed with great success in practice because of a favourable cost-effective situation. At present, however, circumstances have changed drastically. Present economic considerations call for a "low input" goat, capable of producing and reproducing under extensive grazing conditions without any feed supplementation. In this respect it is important to note that the high fibre production potential of the Angora goat does not present a problem in favourable environments where the nutritional requirements of the animal are met, whereas in environments where these requirements are not satisfied, the high potential for fibre production becomes a serious constraint for survival. From this it follows that the higher the fibre production potential of the animal is, the higher the nutritional requirements will be and the better the environment will have to be to allow "normal" production and reproduction. On account of this genotype x environment interaction, the principle for optimum production, with minimum or no supplementation, will involve the matching of the correct type of animal (referring to fibre production potential) to a specific environment.
In view of the fore-going information and thus a better understanding of the physio-endocrine processes underlying hardiness or fitness of Angora goats, some pertinent questions arise about current breeding practices and selection objectives.
Firstly, the question should be asked whether present breeding policies aiming at increased fibre production (fleece mass), are warranted. Considering the Karoo and Valley Bushveld, which are the natural habitats of the Angora goat in South Africa, nutritional stress is a general and regular occurrence and a more robust animal, which produces less fibre, would conceivably be a better adapted animal.
Secondly, the use of grain in the diet of breeding animals, obviously has an extremely detrimental effect on the efficiency of selection, especially for fitness traits. Where stud animals receive grain in their diet as a normal practice, it means that those animals which could be better producers in sub-optimum environments, cannot be identified. The stud breeder is unfortunately forced to feed (or overfeed) animals since many flock farmers still appear to prefer well-fed rams. The flock farmer will in future have to realize that he can achieve effective genetic progress in his flock only by purchasing the best performers out of a similar environment. This will also probably be the only way to ensure that his animals are adapted to his specific conditions.
In this regard, the principle of the so-called veld ram clubs, where breeding animals are raised on natural pastures in order to allow the identification of superior performers, should be supported, on condition that the provision of supplementary feed, especially grain, be restricted to the absolute minimum. Supplementation with excessive amounts of grain to promote growth of fibre production will once again render the identification of the best performers under less favourable conditions virtually impossible.
Taking the present market indicators and the demand for minimum input costs into account, selection pressure for lower fibre diameter is at present a more appropriate guideline, rather than for higher fibre production. Obviously, genetic progress towards the production of finer fibres would be enhanced if production were allowed to decrease, owing to the negative relationship between these two traits. In fact, according to the earlier evidence presented, such an approach might lead to the breeding of a more robust animal, better adapted to extensive environments and producing a higher quality fibre.
In conclusion, this review was an attempt to shed some light on the complex interrelationships between physio-endocrine mechanisms affecting fibre production and fitness traits in the Angora goat. The better understanding of the various links between genetics, nutrition and physiology stresses the fact that closer liaison between these subject disciplines is imperative for any improvement in the efficiency of mohair production under extensive farming conditions.
Proceedings 30th SASAP congres