The reproductive performance of Border Leicester × Merino ewes immunized against testosterone and cortisol

Abstract A flock of 200 Border Leicester cross Merino ewes was used to determine if prolificacy, lambing percentage and weaning percentage could be increased, over 3 successive years, in ewes immune to testosterone. In the first 2 years half of the flock was immunized against a testosterone-3-human serum albumin (HSA) conjugate. The remainder were untreated controls. The effect of treatment was assessed on antibody concentration, oestrus, ovulation rate, prolificacy, fertility, lambing percentage, weaning percentage, birth weight and weaning weight. In the third year half of each group from the previous 2 years was also immunized against cortisol-21-HSA to observe the effect of cortisol immuno-neutralization on prolificacy and fertility. Immunity to testosterone in Year 1 was associated with increased ovulation rate, prolificacy, lambing percentage and weaning percentage. It was associated with increased ovulation rate and prolificacy, but reduced fertility, lambing percentage and weaning percentage in Year 2. The difference in lambing percentage and weaning percentage between Years 1 and 2 was associated with higher concentrations of antibodies to testosterone present at the time of conception, and during the period of development of the blastocyst in Year 2. In Year 3, immunity to testosterone was associated with increased ovulation rate, prolificacy, lambing percentage and weaning percentage. Immunity to cortisol was associated with increased ovulation rate and prolificacy but reduced fertility and weaning percentage. These effects were attributed to the ability of serum from ewes immunized against cortisol-21-HSA to bind significant amounts of circulating progesterone. There was no consistent effect of immunization per se on either birth weight or weaning weight. However, testosterone-immune ewes produced a higher proportion of twin and triplet litters and this resulted in a significant effect of immunization on birth weight and weaning weight. These results suggest that immunity to testosterone is a practical method of increasing prolificacy, lambing percentage and weaning percentage, without adversely affecting fertility, provided the concentration of antibodies to testosterone can be kept within as yet undefined limits around the time of conception and during the period of development of the blastocyst.