Effects of the plane of nutrition on growth and the development of carcass quality in lambs Part II. Effects on lambs of 30 lb. carcass weight

Extract

1. Four groups of half-brother-sister lambs of eight individuals, each of balanced sexes, have been reared on quantitatively different planes of nutrition so as to grow along predetermined growth curves until they reached 30 lb. dressed carcass weight when killed. Treatment differences started at the end of the third month of foetal life. One group was reared on a High Plane of nutrition throughout (High – High), another on a High Plane up to 6 weeks of post-natal life followed by a Low Plane (High-Low), the third on a Low Plane up to 6 weeks after birth, followed by a High Plane (Low-High), and the fourth on a Low Plane throughout (Low-Low). The High-High lambs were killed at 9 weeks old, the High-Low and Low-High at 15 weeks and the Low-Low at 41 weeks old. This allowed for comparison of animals of the same carcass weight but different age and of animals of the same age and carcass weight, but having differently shaped growth curves. The relative effects of the treatments on the development of the body proportions and carcass quality have been studied.

2. The proportion of dressed carcass to organs and offals was highest (48·9% of live weight) in the High-High group, lowest (42·9%) in the Low-Low, and intermediate in the High-Low (45·6%) and Low-High (46·3%). Variation in carcass percentage due to sex was not significant, though 4·8% higher in the females than in the wethers of the High-High group.

3. The weight of the head is more affected by the age of the animal than by the plane of nutrition or the carcass weight, it being of the same weight in the Low-High and High-Low lambs, but significantly lighter in the High-High and significantly heavier in the Low-Low groups. The skin, with wool, was much heavier in the Low-Low than in the other groups, while the weight of the blood was directly affected by the plane of nutrition prior to slaughter, it being significantly heavier in the High-High and Low-High groups than in the High-Low and Low-Low ones.

4. The development of the brain and the eyes was found to be primarily a function of age, these organs being lightest in the High-High and heaviest in the Low-Low group. The brain (as in Part I) appeared to be earlier maturing than the eyes, the differences between the age groups all being significant in the case of the latter, but only the High-High lambs had significantly lighter brain than the other groups.

5. Marked differential effects due to the plane of nutrition and/or age on some of the internal organs were observed, though others like the heart appear to be a function of body weight, being equally well developed in all treatment groups.

6. Organs like the thymus glands, the liver, the spleen, the kidneys and the small intestines are for their development, like the blood, found to be more dependent on the level of nutrition at the time of slaughter than on age or carcass weight, being heavier in the High-High and the Low-High groups than in either the High-Low or the Low-Low. These differences are in most cases highly significant.

7. The development of organs like the oesophagus, the rumen, the reticulum, the omasum and the genital organs, appears to be more influenced by age than the plane of nutrition, while that of the abomasum, the caecum, the large intestines, the rectum and the pancreas is influenced by the plane of nutrition as well as age.

8. Apart from the urinogenital organs, the pancreas is the only organ which exhibits a significant difference due to sex, being 14% heavier in the males.

9. In general, the organs appear to have priority claim for the available nutrients in the blood stream to allow them to develop to such an extent, which enables them to cope with their function at any age. When the nutritive supply is low, organs, whose function is directly associated with growth, suffer proportionately more than those which are of primary importance for life, like the brain, the eyes and the heart.

10. The four treatments produced two distinct types of carcasses. The High-High and the Low-High carcasses were of very similar conformation, short in the leg, blocky and well covered with subcutaneous fat, with the late maturing trunk joints proportionately better developed compared with the HighLow and Low-Low carcasses, which in conformation resembled each other, and were leggy, lank and badly covered with subcutaneous fat. In the latter two groups the early developing legs and neck were proportionately better developed than the later maturing trunk joints.

11. Marked differential effects were produced on the carcass composition. Bone in the dressed carcass was significantly lighter in the Low-Low than in the other groups, due to the inhibiting effects of continuous undernutrition on growth in thickness of bone. The difference in bone weight between the other groups was insignificant, though the High-Low had the heaviest bone in the carcass. Muscle was better developed in the High-Low and the Low-Low groups than in the Low-High and High-High groups, while fat, especially the subcutaneous fat, was better developed in the two latter groups.

12. A High Plane of nutrition prior to slaughter appears to be essential for good development of the latest maturing tissue, the subcutaneous fat. Under limited food supply the earlier maturing tissues, muscle and bone, especially for growth in length, have priority claim for the available nutrients.

13. The ewes had significantly lighter bone, slightly lighter muscle, though insignificantly so, and significantly heavier fat than the wethers.

14. As between the tissues, so within the anatomical units of each, marked differential effects were produced. Limited nutritive supply at any age causes greatest inhibiting effects on the tissues or those parts of any one tissue, which have the highest growth intensity at that age. Similarly, good nutrition at any age benefits most the parts of highest growth intensity at that time.

15. All tissues show great recuperative capacity if provided with adequate food supply after restricted nutrition, unless at that time they have passed their age of high growth intensity.

16. A Low Plane of nutrition from 6 to 15 weeks following high feeding, in the High-Low lambs, had the effect that not only was the subcutaneous fat, the tissue of highest growth intensity at that age, more retarded in development than the other earlier maturing tissues, but all the tissues were relatively more affected in the later maturing regions of the body than in the earlier maturing parts. On the contrary, a High Plane of nutrition from 6 to 15 weeks following severe undernutrition, in the Low-High lambs, resulted in proportionately better development of the subcutaneous fat than the other tissues, and within each tissue a proportionately better development in the late maturing regions of the body, as compared with the earlier developing parts. This is because of the relatively greater effect of the poor nutrition earlier on, on the parts of highest growth intensity during that period, and greater beneficial effects of the good nutrition afterwards on the parts which then had relatively higher growth intensity. Thus the bones of the leg (early maturing) were heavier in the High-Low than in the Low-High, while the ribs (late maturing) were heavier in the Low-High group. Muscle in the Low-High was heavier in the loin but lighter in the limbs and neck than in the High-Low, and the difference in fat in favour of the Low-High was much greater in the trunk joints than in the limbs, while the High-Low had even more fat in the neck.

17. Comparison of the development of the different anatomical units of the major tissues in the High-High and Low-Low carcasses showed that within each tissue the latest maturing parts suffered proportionately more from the continuous undernutrition, with the exception of the ribs, which, though the latest maturing part of the skeleton, were of the same weight in the Low-Low as the High-High, though total bone in the carcass was heavier in the latter. This is accounted for by the so late onset of high growth intensity in the ribs that the High-High lambs were killed before the ribs could benefit from the good nutrition as much as earlier maturing parts of the skeleton, while in the Low-Low, the ribs, having high growth intensity for a long time after the growth rate of earlier developing parts of the skeleton had declined, could compete for nutrients with considerable success with still later maturing tissues, muscle and fat.

18. In the skeleton the shape of individual bones was much more affected by the different treatments than their weight. This accounted for the great treatment differences in carcass conformation. The earlier maturing growth in length is much less affected than the later maturing growth in thickness, and the earlier maturing bones like the cannons are less affected than the later maturing femur or the pelvis.

19. The percentage of marbling fat in the longissimus dorsi muscle appeared to be more dependent on age of the animal than on the plane of nutrition or the state of fatness of the animal, being lowest in the High-High and highest in the Low-Low carcasses.

20. The iodine number of the marbling fat was also found to be more affected by the age of the animal than by the state of fatness or the plane of nutrition, it being highest in the High-High and lowest in the Low-Low carcasses.

21. The water content of the muscle was highest 78%) in the High-High lambs, but lowest in the Low-Low ones (74·9%).

22. The four treatments caused great and highly significant differential effects on the various carcass measurements. As some of these measurements have been found by previous workers to be even more important indices of meat quality than is the actual composition of the carcass, it may be claimed that the different treatments have affected carcass quality even more than carcass composition.

23. The treatment differences were proportionately much smaller in the external measurements, which are primarily indices of skeletal development, than in the internal muscle measurements and by far greatest in the fat measurements.

24. External measurements which should be small for desirable conformation like the leg length, F, the tibia plus tarsus, T, and the length of the cannon, M, were significantly greater in the Low-Low and the High-Low than in the other groups, being smallest in the High-High.

25. The most important muscle measurements, such as the depth of the eye muscle, B, and the shape index, B/A × 100, were best developed in the High-High, followed in a downward order by Low-High, High-Low and Low-Low groups.

26. The fat measurements were much better developed in the High-High and the Low-High groups than in the Low-Low and the High-Low ones.

27. The females had significantly better developed fat measurement than the wethers, while no significant sex differences were observed in the other measurements.

28. Considering the several factors studied which affect carcass quality, the High-High lambs yielded carcasses of the best quality, especially the females, closely followed by the Low-High ones, while the Low-Low carcasses were of much inferior quality and the High-Low still poorer. Those of the latter two groups were more typical of an unimproved late maturing breed than being of the same breed as the High-High and Low-High carcasses.