Effects of gender, time of castration, genotype and feeding regimen on lamb growth and carcass fatness.

Two concurrent trials were conducted to examine the effects of gender, time of castration, genotype and feeding regimen on lamb growth rate and carcass fatness. Trial 1 compared Polypay and Coopworth x Polypay male lambs either left intact or castrated early, mid or late in growth. Trial 2 compared Hampshire-sired lambs (females, early castrate wethers and late castrate wethers) from Suffolk x Coopworth dams and whiteface x Coopworth dams. Gender and time of castration significantly affected growth rate in Trial 1 but not in Trial 2. Rams were more efficient than wethers, but no other gender or time of castration effects on feed efficiency were observed. Delaying castration and pasture grazing both reduced fatness. Lambs grazed on irrigated pastures until they weighed 41 kg then finished in drylot had a lower dressing percentage than lambs fed in the drylot throughout, but USDA quality grades were similar. Substitution of Polypay genes by Coopworth genes in crossbred lambs did not significantly alter any of the fatness traits measured but increased ribeye area. Hampshire-sired lambs grew faster than whiteface lambs, particularly in drylot. They had a higher dressing percentage without increased fatness, apparently because of greater muscling in ribeye and leg.