- Fibre production potential versus fitness in fibre-producing animals
|Last update: August 17, 2011 03:05:02 PM|
FIBRE PRODUCTION POTENTIAL VERSUS FITNESS IN FIBRE PRODUCING ANIMALS
Grootfontein College of Agriculture, Middelburg CP, 5900.
Research during the past two decades has provided ample evidence of some inter-relationships between fibre production potential and physiohormonal mechanisms underlying hardiness in fibre producing animals. Broadly speaking, these studies suggest a negative relationship between fibre production potential and fitness or hardiness. If this is true, a reassessment of present breeding objectives and practices will obviously be necessary.
The purpose of this paper is to discuss briefly the findings of various investigations which have dealt with this issue and, in view of the available information, to reconsider certain breeding strategies.
If the efficiency of fibre production (or fibre production potential) is expressed as the amount of fibre produced per unit body mass, the sequence from top to bottom of efficiency in fibre producing small stock breeds in the RSA will roughly be as follows: Angora goat, strong wool Merino, fine wool Merino and Dohne Merino, S.A. Mutton Merino and Afrino, Dorper and the fat tailed breeds (Namaqua, Ronderib Afrikaner and Blackhead Persian). The Angora goat with its unsurpassed efficiency of fibre production is, however, also well known for its characteristic reproduction problems, such as extremely poor kidding rates especially among young animals in sub optimal environments. Besides poor conception rates, the incidence of disastrously high abortion rates, afflicting up to 80% of the breeding animals in some flocks, 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 - Table 1) add to these serious constraints.
As a result of the mentioned problems several comprehensive investigations of physio-endocrine mechanisms in the Angora goat have been undertaken in an attempt to establish the aetiology underlying an apparent innate physiological weakness. This research has certainly contributed to a better understanding of the complex inter-relationships between the various physio-endocrine mechanisms involved in the processes of fibre production, tissue growth, reproduction and the maintenance of other body functions. In this respect it is therefore appropriate to give a short survey of the most important findings.
The salient feature arising from these trials 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 endocrine mechanism leading to most abortions.
This has been proved in practice by the fact that earlier disastrous abortion rates were 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 research revealed that a drop of blood glucose concentration precedes the collapse of the cold-stressed animal -again indicating an energy deficiency.
In accordance with the foregoing, the problem of retarded growth in kids was also greatly alleviated by similar supplementation of energy.
TABLE 1 - Mortality rate in young Angora goats
|Birth to weaning||13.9|
|Weaning to 18 months of age||12.5|
Similarly, remarkable increases in kidding rates of up to 60% were obtained by flushing breeding animals prior to and during the mating period. For these purposes, the use of alkali-ionophore-treated whole grain (chocolate grain) has proved to be unexcelled, obviously owing to its excellent ability to maximise the increase of blood glucose concentration.
In view of the forgoing, 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 that selection for mohair production results in a reduction of adrenal function - thus the secretion of adreno-corticosteroids, thereby reducing the constraints limiting fibre production. This condition of hypo-adrenocorticism, representing a correlated response to selection for increased mohair production potential, will lead to the following metabolic adjustments:
- Decreased ability to maintain blood glucose concentration under stressful conditions
- Decreased gluconeogenesis
- Decreased fat deposition
- Decreased ability to mobilise fat reserves
- Decrease voluntary feed intake
- Decrease resistance to cold stress
- Increased fibre production
- Increased fibre diameter
Furthermore, the energy cost of fibre production has been estimated 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, the typical problems associated with this breed, and which are without exception related to any energy deficiency, have been attributed to a condition of adrenocortical insufficiency. This hypo- 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.
Although not to the same extent, a similar situation prevails in the wool industry where those animals with the highest fibre production potential suffer most in sub-optimal environments. In fact, in some of the more harsh environments of the RSA, sheep with a high wool production have virtually disappeared and have been replaced by animals with a lower fibre production potential. In accordance with this situation in practice, recent research at Grootfontein has decisively proved the inverse relationship between wool production potential and fitness traits.
In view of the foregoing evidence, it is therefore reasonable to assume that the progress achieved in breeding animals with a higher fibre production potential has been accompanied by a weakening of the physio-endocrine mechanisms underlying "fitness" or "hardiness" or "survivability".
Obviously, the magnitude of the mentioned defects, which has become worse over time as a result of continuous selection pressure on fibre production, has been reduced considerably by efficient supplementation practices. In other words, sub-optimum environments were supplemented in order to meet the requirements of the "high fibre-producing" animal. 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 in-put" animal, 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 any animal 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. For this it follows that the higher the fibre production potential of the animal, 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 of 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 afore going information and thus a better understanding of the physio-endocrine processes underlying hardiness or fitness or fibre producing animals, 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 semi-arid extensive areas where most of the fibre-producing small stock in South Africa is found, 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, however, 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 feeds, especially grain, be restricted to the absolute minimum. Supplementation with excessive amounts of grain to promote growth or 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 that 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 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 inter-relationships between physio-endocrine mechanisms affecting fibre production and fitness traits in fibre producing animals. 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 animal fibre production under extensive farming conditions.
Journal, Dohne Merino Breeders Society, 25 th Anniversary Edition, Vol 15 No 1