Variation in the fertility of ewes during the joining period and its modification by prior contact with rams

Abstract

The reproductive performance of ewes mated for the first time in the first, second and third 2 weeks of joining was determined in 19,666 Merino ewes from 46 commercial flocks in the south of Western Australia. The percentage of ewes failing to lamb was higher for ewes mated in the first 2 weeks of joining than in ewes mated for the first time in the second 2 weeks (8.4%, P < 0.001). This difference was not associated with the age of the ewes, but it was related to the stage of the breeding season (19.2%, P < 0.001, in October–November; 4.1%, P < 0-01, in February–March).

The difference resulted from the lower fertility of the ewes mated in the first period and was not associated with either an adverse ram/oestrous-ewe ratio or low fertility of the rams early in the joining period. The data on returns to service suggested that the low fertility of the ewes in the first 2 weeks of joining was associated with a low level of fertilization and/or a high level of early embryonic death. Placing vasectomized rams with the ewes for 2–4 weeks before joining reduced this difference in fertility between ewes mated in the first and the second 2 weeks of joining.

The fertility of maiden ewes improved from the first to the third 2 weeks of joining. Older ewes were more fertile when mated in the second 2-week segment than in the first, but those mating for the first time in the third 2 weeks were least fertile of all.