- Application of physiological principles in small stock production
|Last update: August 12, 2011 02:43:41 PM|
APPLICATION OF PHYSIOLOGICAL PRINCIPLES IN SMALL STOCK PRODUCTION
D Wentzel, M.J. Herselman, P.R. King & J.J. Olivier
Grootfontein Agricultural Development Institute, Middelburg 5900
In a dictionary sense, physiology refers to characteristics associated with the healthy or normal functioning of an organism, thus dealing with differences in reactions or functional properties, rather than in morphological features. Furthermore the study of physiology is described as to enquire on natural phenomena in order to explain them in terms of natural laws. The majority of research efforts aimed at understanding the various physiological mechanisms have been predominantly prescriptive, with a few focusing on acute physiological disorders. Remedial therapy of these disorders is usually based on understanding both the "normal physiology" and the perturbation or disorder involved. On account of this information several therapeutic treatments have been developed to restore the "normal" physiology of the animal in its specific environment. Many examples of successful achievements to alleviate even acute physiological disorders exist in practice. It is perhaps appropriate to refer to a few of these outstanding achievements.
Nutritionists have developed excellent diets, balanced up to trace element level, as well as very efficient supplementary feeding practices. In addition, the effective buffering of rumen pH with alkalis together with the manipulation of Volatile Fatty Acids (VFA) production by means of antibiotic ionophores (reducing lactic acid production and enhancing propionic acid production) has lead to a situation where energy intake could be maximised without the ill- effects of acidosis. Apparently the same principle is used to reduce the incidence of bloat (Prussic acid poisoning) in animals grazing legume pastures. In semi-arid areas American aloe is used extensively as a drought feed, but has the disadvantage of causing lameness in stock consuming large amounts of the, fodder. Research revealed that this problem related to a condition of acidosis and could be prevented by neutralising the excessive acids in the fodder with lime. The use of whole grain high-energy concentrates has also proved to be very effective to counteract the physiological process leading to pregnancy enterotoxomnia by reversing a negative energy metabolism.
Reproduction physiologists have provided the essential principles to maximise reproduction and, if necessary, even to make use of artificial hormones to increase reproductive rate dramatically to supra-normal levels. The most common procedure used for this purpose is the control of oestrus by means of progestagen-impregnated sponges or devices with or without the administration of gonadotrophin. Obviously, this development found a useful application in embryo collection programmes, allowing the collection of up to twenty or more embryos with one flushing. Successful breeding of Angora does beyond the natural breeding season has been accomplished by means of heterosexual contact with sexually active rams. In addition a significant increase in reproductive rate of anestrous does was obtained with Melatonin implants before introducing the sexually active rams. A consistent and significant increase of about 20% in reproductive rate as a result of sexual behaviour of the ram has been demonstrated. By exploiting this "ram-effect", lambing percentage by using Ronderib Afrikaner rams as teasers following artificial insemination was successfully increased. Subsequently it was also found that this effect relates to increased oxytocin secretion.
Geneticists have made a major contribution by manipulating the genetics of animals and, with the aid of livestock improvement schemes, succeeded to increase both quantity and quality of production rather dramatically. The implementation of modern technology such as BLUP (Best Linear Unbiased Prediction) and MOET (Multiple Ovulation and Embryo Transfer), adds a further dimension to "improved" production.
The interrelationship between fibre production and fitness in the Angora goat has been discussed in detail. Essentially, tremendous progress has been made over the years to increase mohair production in Angora goats up to the stage where this breed is at present the most efficient fibre-producing breed. The Angora goat is, however, also well known for its characteristic problems generally associated with poor fitness e.g. poor conception rates, high abortion rates, exceptionally high mortality rates, especially in kids and young goats, frequent losses during cold spells, poor growth in young animals and in general a poor performance in sub-optimum environments. Several comprehensive investigations of the physiological mechanisms underlying most of these problems revealed an energy deficiency, resulting from a weakened energy metabolism, as the primary cause of the poor fitness. The latter is apparently related to adrenal malfunction following continuous selection for mohair production.
Follow-up trials clearly demonstrated major differences between breeds in their physio-endocrine responses to various kinds of stress (e.g. cold stress and starvation) usually characterised by a better "resistance" to stress in those animals producing less fibre. In accordance with the foregoing results on hormone profiles throughout the year, revealed that the Angora, in comparison with other small stock breeds, maintains a consistently and decisively lower level of those hormones involved in the control of metabolism. Recently, evidence suggesting that in addition to adrenal malfunction, dysfunction at pituitary level of the hypothalamus-pituitary-adrenal-axis could also be involved in the poor response of Angora goats to stress, was obtained.
SHEEP AND WOOL INDUSTRY
Although not of the same magnitude, a similar situation prevails in this industry where complaints about poor conception, high mortality in lambs, poor growth rate and in general a poor constitution are quite common. Once again these symptoms are much more prevalent amongst sheep with a high fibre production potential, when kept in a sub-optimum environment.
In an attempt to investigate this aspect in sheep, genotypes with varying potentials for fibre production were obtained by crossbreeding of various woolled breeds found in South Africa. Evaluation of the progeny of these types revealed a close inverse relationship between wool production potential and fitness traits such as age of puberty, growth rate, lambing percentage, etc.
Analysis of data sets in Merino and Afrino sheep revealed similar inverse relationships between wool production potential and ewe productivity. In another trial, conclusive evidence of a genotype x environment interaction in Merino sheep selected for wool production in a favourable environment, compared to an unselected control group was obtained. Although the group with the higher fibre production potential produced more wool in a favourable environment, they produced less wool than the control group, in a sub-optimum environment. These results and follow-up trials once again elicited the physiological significance of varying fibre production potential in relation to fitness traits of animals kept in sub-optimum environments.
MUTTON SHEEP INDUSTRY
In mutton sheep, selection for body size is generally accepted to be of great importance. Recently it has been shown that the efficiency of "too" big animals tend to be lower than those with an "optimum" size for the breed. Results clearly demonstrate that body size is not a prerequisite for efficiency.
Another aspect of importance is that animals with larger fat depots are perceivably better adapted to the harsher semi-arid regions. In this regard, research at Carnarvon Experimental Station clearly demonstrated that during a drought, Afrino sheep had to be supplemented for a considerable period before Dorper sheep needed supplementation, whereas Namaqua Afrikaner sheep (indigenous fat tail) survived without any supplements. If the scanty evidence available on this relationship proves to be true, the concept of selecting for "lean meat" will certainly not be conducive to improved hardiness. Obviously, research aimed at elucidating all the implications of shifting the metabolism in favour of either protein or energy production, its' significance in various environments and its effect on the various physiological processes (e.g. lactation, growth, pregnancy, fibre production, etc) should be regarded as a high priority.
Although much more research is needed to elucidate all aspects of the mechanisms underlying these complex interrelationships, even the limited evidence available has implications for breeding policies. In the first place, the genotype x environment interaction, as displayed in these trials and in practice, emphasizes the importance of performing selection and breeding under conditions similar to those under which commercial production takes place. This concept can certainly no longer be played down, as is already evident in practice by the rapidly growing interest and support of veld ram clubs and sales. Secondly, the question is raised whether the fibre production potential of some of our breeds and types does not exceed that which can be sustained by the natural environment. Thirdly, the supplementation of breeding stock, especially with starch containing feeds, will eventually lead to the breeding of animals which are mal-adapted to the natural environment - a situation which is at present prevalent in South Africa and of great concern. Fourthly, breeders need to change their perception of an universal "ideal" type of animal which is the best producer in all environments. Obviously, in fibre producing breeds the ideal situation should allow for the breeding of animals with various potentials for fibre production in order to provide the required type of animal for a specific environment. The role of shows and championships, again promoting a specific type (in an optimum environment) without specifying the environment in which the animal has to perform, should also be questioned. This is obviously of greater importance in those breeds usually kept in harsh environments (Dorper, Boer goat, etc.).
Another aspect which needs some clarification is the apparent relationship between peri-natal mortality and fibre production function. Poor viability of newborn kids/lambs in high fibre producing breeds has lead to the development of several artificial management practices to alleviate the problem but, once again, the underlying physiological basis of the problem remains to be elucidated. Although relatively little conclusive evidence exists on this issue, it seems reasonable to contemplate involvement of some of the previously mentioned physio-endocrine mechanisms. Considering both the preparturient decline in maternal corticosteroids and the abrupt and rather dramatic increase in foetal corticosteroids generally associated with the induction of birth or labour - following decreased progesterone levels - and the apparent inverse relationships between adrenal function and fibre production potential, the question arises whether the condition of "hypo-adrenocorticism" associated with efficient fibre producers is not detrimental to viability of the newborn kid /lamb. In this respect the condition of hypo glycaemia, underlying several of the conditions (abortion, coldstress, etc.) related to poor fitness in Angora goats and which is invariably linked to hypo-adrenal function, is a major cause of failure of sustaining body temperature in newborn kids/lambs. Furthermore, the role of corticosteroids in the development of lung surfactant as well as its generally accepted anti-inflammatory action are obviously important functions to ensure survival of the newborn kid/lamb.
An issue, which has been the subject of several investigations, is what is responsible for the difference in feed intake (including grazing or feeding characteristics) of various animals. It is generally accepted that animals with a higher fibre production function are more selective grazers and relatively poor utilisers of poor quality feeds. Although some of the factors affecting intake are known, the physiological basis thereof still remains rather obscure. Obviously, this issue is of special importance to geneticists, as it has far reaching implications in respect of the genotype x environment concept, discussed earlier.
From a research point of view, investigation of the physio-endocrine mechanisms controlling the energy metabolism, fat deposition and mobilisation, fat consistency, reproduction, fibre production and general fitness, as well as interactions between them, is indicated. Furthermore, considering the vast differences between genotypes and environments found in the RSA, estimation of variance components for genotype x environment interactions are of paramount importance for improved accuracy of selection. For this purpose a multi-disciplinary effort, including geneticists, nutritionists and physiologists, appears to be a prerequisite if any progress in increasing the efficiency of production is to be achieved. In conclusion it is encouraging to learn that appropriate research projects to address some of these aspects are already underway. While it is accepted that this kind of research is expensive and difficult, it is believed that a better understanding of the basic physiological principles will contribute to improved small stock production in practice.
Proc. S.A. Soc. Anim. Prod. 34, 75