Last update: March 27, 2012 11:28:59 AM E-mail Print

 

MUTTON SHEEP: CHANGES AND CHALLENGES

P. G. Marais & E.M. van Tonder

Grootfontein Agricultural College

Middelburg C.P.

5900

 

INTRODUCTION

During the past 25 years the major changes in breed characteristics, which occurred in Dorper sheep, have been a decrease in fatness at an acceptable slaughter mass and an increase in growth rate accompanied by larger mature sizes. Changes in conformation have accompanied the changes in growth rate and carcass composition. It is clear that growth rate, size and concomitant changes in body composition are of great economic importance in mutton production and therefore reflect to some extent industry adjustments towards more efficient breeding of Dorper sheep. Available evidence indicates that fast growing, lean animals are more efficient than slower growing fatter breeds in converting feed energy to lean tissue.

A great deal of research in growth and development of mutton producing animals was aimed at elucidating patterns of development of the major tissues (muscle, bone and fat) and the distribution of these tissues in the carcass. This research identified many factors, which exert an influence on growth and tissue distribution patterns, thus affording opportunities to exert more control on these properties and ultimately on carcass composition.

A superior carcass is characterized by a high proportion of muscle tissue, a low proportion of bone and an optimal level of fatness. As the amount of fat approaches a more moderate and desirable level, greater emphasis can be placed on increasing the proportion of muscle.

Accomplishing substantial changes in carcass composition always runs the risk of upsetting nature's balance in respect of functional requirements developed in each species over its evolutionary history.

 

FACTORS INFLUENCING PROPORTION OF FAT

Since the major change in carcass composition has been the reduction of the amount of fat, it may be worthwhile to review the factors, which have been manipulated or can be manipulated to bring about this change. Fig. 1 a represents tissue growth relative to live mass in sheep. Muscle mass in proportion to bone mass in the carcass of a normal Iamb at birth may be in the order of 2:1, whereas at a slaughter mass of approximately 16 kg the ratio may be 4:1. Thus, muscle has a much faster relative growth rate than bone. Fat growth starts relatively slowly and increases geometrically as the animal matures. This pattern of growth pertains to all meat-producing animals.

In Fig. 1 b the influence of sex on the development of fat is illustrated. Wethers and ewes fatten earlier and faster in relation to their live mass than rams.

Genetic differences are illustrated as maturity type in Fig. 1c. Some breeds reach maturity and fatten earlier than others.

Plane of nutrition has a well-known effect on the fattening pattern. The classic work on pigs proved that a high plane of nutrition promoted earlier fattening while a low plane resulted in a delayed or slower fattening rate. These results have since been confirmed several times for nearly all the animal species.

 

FACTORS INFLUENCING MUSCLE DEVELOPMENT

The proportion of muscle in relation to live mass seems to be hardly influenced by fatness or live mass. In meat producing animals the amount of muscle tissue expressed as a percentage of live mass, may be a valuable index of meat production since genetic differences appear to play an important role. These differences as are illustrated in Fig. 1 d represent breeds classified from light to heavily muscled. Marked differences in the proportions of muscle mass to live mass seem to be a reflection of the muscle: bone ratio. Considerable differences in muscle: bone ratio has been reported among breeds. However, there seems to be scope for considerable increases in muscle: bone ratio, with a resultant improvement of muscle yield in proportion to live mass.

 

 

FAT PARTITIONING AND DISTRIBUTION

Fat distribution among the depots throughout the body is of major importance to the commercial value of meat animal carcasses.

Partitioning and distribution of fat in the carcasses of cattle, sheep and pigs have recently been reviewed. Growth patterns of the fat deposits of cattle, pigs and sheep are given in Table 1. The results for sheep indicate that subcutaneous fat (SCF) had a higher growth rate than kidney knob and channel fat (KKCF), which in itself was high, and that the growth rate for intramuscular fat was slower.

 

 

Important breed differences in fat partioning were reported. Breeds specifically improved for fat Iamb production appear to have a higher subcutaneous fat proportion than those bred for adaptability and maternal performance. It seems that either genetic or environmental factors responsible for increased fattening characteristics tend to result in a greater proportion of  subcutaneous to intramuscular fat. Such results are compatible with the fact that subcutaneous fat develops later than intramuscular fat. Decreasing total fat by either genetic or environmental means would have the opposite effect.

Deposition of intramuscular fat (marbling) is still of economic importance in the USA although its contribution to meat quality has often been questioned. There is evidence that intramuscular fat increases in proportion to total fat, and at a similar relative rate. One would be inclined to conclude from this relationship that marbling would be difficult to achieve without a comparable response in general fatness.

Berg & Butterfield (1976) suggested that fat partitioning and distribution might be related to local pressures that develop with growth. Thus, the body cavity and the IMF depots find little resistance initially, but, as they expand, increasing resistance is encountered, causing more of the surplus energy to be stored under the skin as subcutaneous fat. Muscle and body shape create variable pressures, with large fleshy muscles of the hindquarters causing more resistance to the IMF deposits than the flatter, looser muscles of the thorax and neck. This results in shifting the IMF proportion forward with increasing fattening. SCF depots gradually expand under the skin in less resistant areas (e.g. flank, twist, brisket, and in front of and behind the shoulder) resulting in overall smoothness and roundness of very fat animals.

It would appear that fat partitioning among the depots has both genetic and environmental determinators. Recently, selection aimed to decrease subcutaneous fat (back fat) resulted in variable amounts of fat in the other major depots in different populations of pigs. More investigation into the types of correlated changes which occur in each of the fat depots when one of the major depots (e.g. SCF) is altered by selection is needed.

 

MUSCLE GROWTH AND DEVELOPMENT

During the past 25 years much research has been done to clarify growth patterns and muscle distribution as well as factors influencing it. From this research it was concluded that the proportions of the major tissues were affected by level of nutrition and that the distribution of these tissues, particularly muscle, between superior and inferior parts, was also affected.

However, the influence of nutritional level on the distribution of the muscle mass is not entirely clear notwithstanding evidence to 1hB.-effect that in the early stage of growth, the influence of nutrition is related to total muscle growth.

The similarity of muscle distribution among breeds of cattle, led to extensive research comparing breeds and types within various domestic and other species of animals. Although some statistically significant differences were found between breeds, these differences were small and generally considered to be of little commercial importance. Some authors nevertheless concluded that differences which exist are probably of genetic origin and could possible be exploited. It is, however, clear that any form of selection employed in breed development and improvement in any of the species has little influence on muscle distribution.

 

BONE GROWTH AND DISTRIBUTION

In his expose, Hammond (1932) demonstrated differential growth in bones of sheep and suggested a wave of growth, starting at the head, proceeding backwards along the trunk, while secondary waves start at the extremities of the limbs and spread upwards. All these waves meet at the junction of the loin with the last rib. The findings of subsequent studies on the relative growth of bone in cattle and pigs failed to agree with Hammond's suggested growth gradients. Differences in relative growth of bones between species could have resulted from comparisons made at different physiological stages of development. Breed differences in bone distribution at a constant total bone content were reported in cattle and pigs. It is possible that these differences are to some extent a reflection of mature size; the larger breeds having more of their bone growth in the earlier developing leg joints at a constant total mass. However, there may still be some unexplained genetic differences among breeds but inconsistencies which may exist in the distribution of bone are unlikely to be of much commercial interest.

 

LIVE ANIMAL EVALUATION

Historically, live animal judgement followed various guidelines and many objectives, In the context of meat animal production, the purpose of live animal evaluation will be restricted to the estimation of carcass composition and tissue distribution and factors determining the value of the meat animal carcass.

From the available evidence it is clear that muscle and bone distribution are quite uniform among animals of the same breed at comparable stages of maturity. Estimation of the proportion of muscle in high price cuts by live animal evaluation would therefore seem to be futile since reliable methods for measuring muscle in any part of the body are in any case nonexistent.

On the other hand it is also equally unlikely that live animal evaluation, employing presently available techniques, would be accurate enough to observe differences in fat partitioning which at least has been shown to differ among breeds: thus affording the possibility of breeding animals with more desirable fat proportions.

Slaughtering and carcass evaluation, particularly for fat distribution would probably have to be used in order to characterize different genetic groups which could then be used for breeding purposes. Thus, practical estimation of carcass composition would involve assessment of the proportion of fat, at a fixed level of fatness, and the muscle: bone ratio. Sustained efforts to reduce fatness at an acceptable marketing mass is envisaged. By increasing the muscle: bone ratio at an optimum level of fatness meat yield on the basis of both the live animal and the carcass could be increased materially.

Although accuracy in evaluating fat proportions and indirectly the lean tissue content of a carcass as the other major variable have been promising, methods for estimating lean (muscle tissue) in carcasses of equal fatness have been less successful. In the meantime subjective live animal evaluation, which has a strong historical and traditional base in animal science may be of some use.

Measurements on the live animal body such as length, height and circumference have shown very little relation to carcass composition. Linear measurements are, of course, correlated with mass and therefore, in turn, would be correlated to some extent with characteristics of which mass is a predictor. Research concluded that the modern beef/mutton animal is generally smaller and more compact than its predecessors, but this trend has not increased the proportion of lean in the carcass or changed its distribution. It should be reflected that a reversal of this trend i.e. from smaller and more compact to taller and longer carcasses, may also not result in any change in composition or distribution of tissues.

 

FUTURE PROSPECTS IN MEAT PRODUCTION

It is expected that some of the trends in meat production of the past 25 years will proceed and that animals growing rapidly and more efficiently will continue to be favoured.

A further reduction in the proportion of fat in marketed carcasses is likely to continue and researchers will have to study possible functional impairments likely to occur if and when physiological limits are encroached upon.

When the minimum acceptable level of fat is approached, more emphasis will probably be placed on increasing the proportion of lean (muscle tissue) which will manifest in high lean: bone and lean: live mass ratios. The possible functional implications brought about by extreme proportions will generate a need for research aimed at exploiting the advantages of animal potentials. Methods for estimating muscle development, which are independent of those used for determining fatness, will need to be improved. The estimation of conformation on a live basis, after the necessary adjustments for fat content have been made, may act as an interim approach.

 

LITERATURE

BERG, T.Y. & BUTTERFIELD, R.M., 1976. New Concepts of cattle growth, Sydney University Press.

HAMMOND, J., 1932. Growth and development of mutton qualities in the sheep. Oliver & Boyd. Edinburgh.

 

Published

Dorper news 36