Effect of active or passive immunization against steroids on reproductive performance of ewes in different levels of body condition at mating

Abstract

ABSTRACTBorder Leicester × Scottish Blackface (Greyface) ewes of three groups, each comprising 118 animals in a wide range of body condition scores, were mated at a synchronized oestrus in mid October. The ewes were passively immunized against testosterone (group P), actively immunized against androstenedione (group F), or not treated (group C). All ewes were slaughtered at return to service or at 35 to 45 days of pregnancy and ovulation rates and numbers of embryos present were determined. Mean ovulation rates of ewes in group P were higher than in those in group C (P < 0·05) and this difference was evident at most levels of body condition. The absolute increase in ovulation rate, compared with the control group, was similar at all condition scores. Mean ovulation rates of ewes in group F were higher than those in group C (P < 0·001) and the magnitude of the increase was greater in ewes in higher condition scores. The incidence of ova wastage was variable but differences between treatments in mean ovulation rate were generally reflected in mean litter size. The conception rates of immunized ewes were depressed compared with those of control animals, particularly in ewes with a body condition score less than 3·0 at mating. Consequently, there was no improvement in the potential lambing rate of immunized ewes following only one cycle of mating. Circulating antibody titres were not related to conception rate or body condition at mating and were related to ovulation rate only in group F ewes. It is concluded that immunization against steroids, using either passive or active techniques, can improve the reproductive performance of individual ewes but improvement in the performance of the flock as a whole may only be achieved under optimal conditions of nutrition and season.