Genotype and age effects on sheep meat production 1. Production and growth

The growth, wool production and teeth eruption of animals from five genotypes [ Poll Dorset(growth) ( PDg) x Border Leicester x Merino ( BLM); PDg x Merino ( M); Poll Dorset(muscling) ( PDm) x M; M x M; BL x M] were evaluated over the age span of 4 to 22 months of age. In total, 16 sires were used to generate the animals and these were selected for growth based on estimated breeding values ( EBVs), apart from one group ( PDm) which was selected for muscling based on EBVs. In total, 595 animals were slaughtered at 4, 8, 14 or 22 months of age. The birthweight of lambs born to BLM ewes was 0.6 kg heavier ( P < 0.001) than those born to Merino ewes, and wether lambs were 0.2 kg heavier than ewe lambs ( P < 0.001). The yearling weight ( YWT) EBV had no effect on birthweight. There were significant ( P < 0.001) differences between genotypes for predicted weaning weight at 4 months of age, with a range of 10.6 kg among the groups. The PDg x BLM were the heaviest ( 37.8 +/- 0.7 kg) and the Merino the lightest ( 27.3 +/- 0.8 kg). Wether lambs were 2.0 kg heavier ( P < 0.001) than ewe lambs at weaning. The YWT EBV had no effect on weaning weight. There was a significant ( P < 0.001) interaction between the age at slaughter and genotype for final liveweight of each age group. The PDg x BLM were the heaviest at each age and the Merinos the lightest, and for the oldest animals, the BLM were significantly ( P < 0.05) heavier than PDm x M, whereas at younger ages this was not the case. The significant interaction ( P < 0.001) between sex and age at slaughter reflected the increasing difference in liveweight between the sexes as age at slaughter increased. There was also a significant ( P < 0.05) interaction between the YWT EBV and age at slaughter. The overall coefficients were - 0.01, 0.14, 0.34 and 0.65 for slaughter ages 4, 8, 14 and 22 months, respectively, indicating that in young animals there was no effect of sire YWT EBV on liveweight, but as the animals aged there was an increasing effect such that progeny of sires with high sire YWT EBV were heavier. This effect was evident within all genotypes. There was a sire breed effect on fleece weight and total wool weight ( P < 0.001) with Merino- sired hoggets producing 0.4 kg more wool than Border Leicester- sired hoggets and 1.5 kg more than Poll Dorset- sired hoggets. There was a weak genotype effect on time to erupt the first permanent incisor, which if modelled as a proportion of Border Leicester ( 0, 1/ 4, 1/ 2) was significant ( - 34 +/- 12 days, P < 0.05), Border Leicester earlier than not Border Leicester. Ewe lambs took 9 days longer ( P < 0.05) to erupt their first permanent incisor compared with wether lambs.