Growth and carcass characteristics of male crossbred lambs from high fecundity Booroola x Polwarth ewes: effects of litter size, castration and age

Growth and carcass characteristics were obtained for 154 Border Leicester-sired male lambs, from highly fecund Booroola x Polwarth ewes, grazed on improved Tasmanian pasture and either left entire or castrated, followed by slaughter at 134, 192 or 288 days of age. Of all lambs, 15.6% were single born and 84.4% were multiple born (54.5% twins, 19.5% triplets, 8.4% quadruplets and 1.9% quintuplets). Multiples were lighter than singles at birth (3.6 v. 4.4 kg), at weaning at 78 days (21.0 v. 25.8 kg), and before slaughter at 134 days (27.5 v. 31.1 kg), 192 days (33.9 v. 36.1 kg) and 288 days (37.5 v. 42.5 kg). Similarly, carcasses were lighter and leaner, and dressing percentages were lower for multiples than for singles, but when carcass weight was included as the covariate in further analysis of carcass characteristics, there was no effect of birth type. There were no significant interactions between birth type and castration. The growth rate of ram lambs became progressively faster than wethers from weaning onwards. Liveweight differences could be detected from 134 days of age (28.8 v. 27.2 kg), but ram lamb carcasses were not heavier until 288 days (18.1 v. 16.0 kg). At 192 days, rams were leaner than wethers (condition score, 2.2 v. 2.5; GR tissue depth, 8.5 v. 11.5 mm; C-site fat depth, 3.7 v. 5.1 mm), and rams had lower dressing percentages (41.8 v. 44.0%) and larger eye muscle areas (1097 v. 1010 mm2), but carcass weights were similar (14.8 v. 14.6 kg). We conclude that the only effect of multiple birth was to delay maturation and development of lambs to reach the suitable market weight. Leaving males entire did not improve carcass weights until 288 days, which is of little benefit to Tasmanian producers where lambs are normally sold at a much younger age.