Published online by Cambridge University Press: 01 June 1997
At Mount Derrimut Field Station from 1982 to 1985, growth and body composition was studied in three groups of wethers from the same flock. A control group (C) and a restricted group (R) were housed from the age of 1 day until slaughtered. The first slaughter was at the age of c. 1 year (reported elsewhere) and the second at the age of c. 3 years. Apart from the first 36 days, when group R was fed a severely restricted ration, both groups were fed a high quality diet ad libitum throughout the experiment. At the end of the second year, a group of 2-year-old wethers (G) was introduced, to compare the growth of these sheep with those in groups C and R. Group G sheep were reared on their dams, kept as grazing animals and killed at the second slaughter.
At the age of c. 3 years there was no significant difference between groups C and R for mean fleece-free body weights, but the sheep in group G were significantly lighter (P<0·01) in spite of reaching mature size for grazing sheep in the general flock.
After the age of 7 months there was no significant difference between groups C and R in rate of wool production. Rate of wool production in group G sheep was not significantly different from that of sheep in the other groups except during the last 6 months, when they produced significantly less wool (P<0·05).
Regression analyses for surface and X-ray measurements against fleece-free body weight (independent variable) showed no differences between groups C and R, except for width of metacarpal (X-ray measurement). In this case, the regression coefficients were different (P<0·01) but, at slaughter, actual bone measurements were the same. Both X-ray measurements and those at dissection showed that the metacarpals of group G were significantly (P<0·05) longer and narrower, and had a narrower cortex, than those in the other two groups (P<0·05).
There were no significant differences between groups C and R in mean weight and chemical composition of the parts and tissues examined; however, the carcass fat content of sheep in group G was significantly less (P<0·01) than in the other two groups. The sheep in group G had higher weights (P<0·01) for the brain, the digestive tract (DT), kidneys and liver than those in groups C and R.
Cell size in the semitendinosus muscle, as indicated by the protein[ratio ]DNA ratio, increased (P<0·01) between the first and second slaughters. There were no differences between groups in protein[ratio ]DNA ratios, but the DNA and protein contents were significantly higher (P<0·05) in the kidneys and liver in group G than in the other groups. Although the weights of the semitendinosus muscle were heavier (P<0·01) in groups C and R than in group G, on a fat-free basis, there were no significant differences between treatments in muscle weight or in DNA and protein contents.
It is concluded that sheep will make a complete recovery after a severe nutritional check in early post-natal life if given the opportunity to do so. Even though large differences in body weight may develop between sheep, this appears to have little effect on mature skeletal dimensions. However, at maturity, with grazing sheep that grow relatively slowly, the metacarpals are likely to be narrower and longer than those of the same genotype fed to grow faster. Even with sheep fed ad libitum throughout life, the use of X-ray and surface measurements in young sheep at a particular body weight are unlikely to be useful for the accurate prediction of mature body weight.