The repeatability of the reproduction rate of commercial Merino ewes.
The repeatability of the reproduction rate of 143 and 423 Merino ewes in two commercial flocks was investigated. Ewes from flocks were grouped as those that failed to lamb and those that lambed (one or more lambs) at first lambing at two years of age. The pooled repeatability of the number of lambs born per ewe of these two groups over the subsequent three years in the first flock, and over the subsequent two years in the second flock, was 0,05 ± 0,02. The repeatability of reproductive performance at second lambing (three years) and the subsequent year could be estimated for the second flock only and was 0,05 ± 0,02. No effect of ewe age on repeatability was found. The data of the second flock made it possible to group ewes as having had no lambs, one lamb or two lambs born per ewe mated. The repeatability of performance at first lambing at two years of age over the subsequent two years was 0,03 ± 0,06 for the initial group which had borne no lambs or only one, but for the initial group which had borne one or two lambs it was 0,24 ± 0,03. The repeatability between performance at the second and third lambings was 0,03 ± 0,01 for ewes with no lamb or one lamb in the second year, but 0,28 ± 0,15 for ewes with one or two lambs in the second year. Culling of dry ewes at first lambing in these flocks would increase the number of lambs born in subsequent lambing seasons of the current flock by approximately 0,01 lamb per ewe per year.