Effects of active immunization against androstenedione on ovulation rate and fecundity in Galway and Finnish landrace sheep

Abstract

Galway and Finnish Landrace ewes were immunized with an androstenedione protein conjugate (Fecundin, trademark of Glaxo Group Ltd.) on two occasions 30 days apart at the beginning of the breeding season. All sheep were treated with intravaginal progestagen sponges to synchronize cycles and exposed to rams for 6 weeks. The antibody titre to androstenedione, plasma androstenedione concentrations, display of oestrus, ovulation rate and litter size were determined. The results were compared with those for non-immunized animals and, in the case of the Galway breed, with ewes given 750 i.u. PMSG at the time of sponge removal. Over 98% of all ewes were mated within 30 days of sponge removal, but immunized ewes were delayed in their expression of oestrous cycles after withdrawal of progestagen treatment. The proportion of ewes mated that ultimately lambed was similar for all treatments, although during the first 5 days after sponge removal the conception rate was lower in immunized ewes. The mean ovulation rates for control, immunized and PMSG-treated Galway ewes were 1.75, 2.52 and 2.05, respectively; the corresponding values for litter size at birth were 1.48, 1.89 and 1.53, respectively. Comparison of the effects of immunization in the Galway and Finnish Landrace breeds indicated that ovulation rate increased in response to treatment in both breeds. Although litter size also increased in the Galway there was no evidence for a similar effect in the Finnish Landrace breed. Plasma concentrations of androstenedione were significantly higher following immunization in Galway than in Finnish Landrace ewes. There was an association between high antibody titres to androstenedione and the reduction in behavioural oestrus and conception rate immediately after sponge removal. The implication of this effect in sheep management is to initiate the immunization earlier in the season so that the introduction of the ram can take place 3–4 weeks after the booster injection when as these data have shown, the fertility of the ewes is similar to untreated ewes but the fecundity is higher.