Lamb production from diverse genotypes 1. Lamb growth and survival and eweperformance

Growth and survival of 3673 female and cryptorchid lambs representing the r ange of types (second cross, first cross and Merino) produced in the Austra lian lamb industry are reported. The lambs were sired by a selection of Pol l Dorset (D; no. = 7), Texel (T; no. = 10), Border Leicester (BL; no. = 12) and Merino (M; no. = 12) rams and born to Border Leicester X Merino (BLM) and Merino (M) dams, which resulted in six lamb genotypes (DXBLM, TXBLM, DX M, TXM, BLXM and MXM). The second cross lambs (DXBLM, TXBLM) were heavier a t birth, weaning and post-weaning and had higher growth rates, than first c ross lambs (DXM, TXM, BLXM), which were higher than MXM lambs (P < 0.01). F or post-weaning weight (cryptorchids at 156 days) second cross lambs were 4 .2 (s.e. 0.3) kg heavier than comparable sired first cross lambs, with the BLXM 2.6 (s.e. 0.5) kg lighter than other first cross types and the MXM a f urther 4.2 (s.e. 0.6) kg lighter. The average advantage of D over T crosses was 1.0 kg for post-weaning weight, with greater variation amongst individ ual sires. Lamb survival to weaning was 76% and it was affected by birth weight and bi rth type (P < 0.01). Sire breed had no significant effect on lamb survival, whereas lambs from BLM ewes had higher survival than those from M ewes (P < 0.01). Overall the proportion of ewes lambing was 84% with a mean litter size of 1 .77. There was no difference in fertility between AI (thawed frozen semen a nd laparoscopic insemination) and natural (single sire) mating, although th e M were higher than the BLM ewes (P < 0.01). Gestation length was 2.3 days shorter (P < 0.01) for BLM than for ewes and there was no difference betwe en D and T sire breeds. There was little effect of birth type on gestation length but if increased with birth weight within birth type categories. Bet ween year repeatability of gestation length for ewes was 0.33 (s.e. 0.05).