Variations between years in the ram effect when Coopworth or Poll Dorset rams are introduced to seasonally anovular Coopworth ewes

摘要

Abstract Seasonally anovular Coopworth ewes previously isolated from rams were joined with either Coopworth or Poll Dorset entire rams for 28 days from either 6 January or 3 February in each of 3 successive years. Induction of ovulation varied between years, and more ewes ovulated when exposed to rams of either breed in February. When rams were introduced in January, more ewes exposed to Poll Dorset than Coopworth rams ovulated within 28 days of ram introduction. Onset of oestrus also varied between years and treatments. It is concluded that the variation in expression of the ram effect is probably governed by depth of anoestrus of the ewes at the time of joining.