VARIATIONS BETWEEN YEARS IN THE RAM EFFECT WHEN COOPWORTH OR POLL DORSET RAMS ARE INTRODUCED TO SEASONALLY ANOVULAR COOPWORTH EWES

Seasonally anovular Coopworth ewes previously isolated from rams were joined with either Coopworth or Poll Dorset entire rams for 28 days fr om either 6 January or 3 February in each of 3 successive years. Induc tion of ovulation varied between years, and more ewes ovulated when ex posed to rams of either breed in February. When rams were introduced i n January, more ewes exposed to Poll Dorset than Coopworth rams ovulat ed within 28 days of ram introduction. Onset of oestrus also varied be tween years and treatments. It is concluded that the variation in expr ession of the ram effect is probably governed by depth of anoestrus of the ewes at the time of joining.