GROWTH OF SHEEP TO THE AGE OF 3 YEARS AFTER A SEVERE NUTRITIONAL CHECK IN EARLY POSTNATAL LIFE

At Mount Derrimut Field Station from 1982 to 1985, growth and body com position 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. A part from the first 36 days, when group R was fed a severely restrict ed ration, both groups were fed a high quality diet ad libitum through out the experiment. At the end of the second year, a group of 2-year-o ld wethers (G) was introduced, to compare the growth of these sheep wi th those in groups C and R. Group G sheep were reared on their darns, 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 an d R for mean fleece-free body weights, but the sheep in group G were s ignificantly lighter (P < 0.01) in spite of reaching mature size for g razing 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 prod uction. Rate of wool production in group G sheep was not significantly different from that of sheep in the other groups except during the la st 6 months, when they produced significantly less wool (P < 0.05).Reg ression analyses for surface and X-ray measurements against fleece-fre e body weight (independent variable) showed no differences between gro ups C and R, except for width of metacarpal (X-ray measurement). In th is case, the regression coefficients were different (P < 0.01) but, at slaughter, actual bone measurements were the same. Both X-ray measure ments and those at dissection showed that the metacarpals of group G w ere significantly (P < 0.05) longer and narrower, and had a narrower c ortex, than those in the other two groups (P < 0.05).There were no sig nificant differences between groups C and R in mean weight and chemica l 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 i ndicated by the protein:DNA ratio, increased (P < 0.01) between the fi rst and second slaughters. There were no differences between groups in protein:DNA ratios, but the DNA and protein contents were significant ly higher (P < 0.05) in the kidneys and liver in group G than in the o ther groups. Although the weights of the semitendinosus muscle were he avier (P < 0.01) in groups C and R than in group G, on a fat-free basi s, 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 po st-natal life if given the opportunity to do so. Even though large dif ferences in body weight may develop between sheep, this appears to hav e little effect on mature skeletal dimensions. However, at maturity, w ith grazing sheep that grow relatively slowly, the metacarpals are lik ely to be narrower and longer than those of the same genotype fed to g row faster. Even with sheep fed nd libitum throughout life, the use of X-ray and surface measurements in young sheep at a particular body we ight are unlikely to be useful for the accurate prediction of mature b ody weight.