Effect of age at first joining and genotype on the performance of ewes at 2 to 8 years of age, and that of their lambs, in a grass-based prime-lamb system

Abstract The objective was to evaluate effects of age at first joining (7 or 19 months), ewe genotype and age on ewe performance between 2 and 8 years of age, and that of their progeny to slaughter. Three genotypes were represented: Belclare (Bel), F1 Suffolk x Belclare (Suf×Bel), and ≥0.75 Suffolk ancestry (Suf75). The flock (424 ewes initially) was managed in a grass-based system based on rotational grazing at the Athenry Research Centre; ewes were housed from mid-pregnancy to lambing and exited the flock when culled (health/welfare reasons) or they died. There were no significant interactions between first-joining age and ewe genotype. Ewes joined to first lamb at 1 year of age required significantly less lambing assistance and produced heavier lambs at birth, with lower lamb mortality. Bel and Suf×Bel ewes produced larger litters, and reared significantly more lambs per ewe than Suf75. Barrenness was lower for Suf×Bel than Suf75, and their lambs were younger at slaughter. Ewe longevity was unaffected by age at first joining or genotype. Mortality of lambs from Bel ewes was lower than those from Suf75. Ewe and lamb performance traits increased with ewe age to a plateau starting around 5 to 6 years of age. Number of lambs reared per ewe reached a maximum at age 6 and exhibited a major decline (-0.4) at 8 years, reflecting lower prolificacy and increased barrenness. Lamb growth rate increased and age at slaughter declined as ewes aged.