Active immunisation of Romney ewes with an androstenedione-protein conjugate

Romney ewes were immunised on one occasion with androstenedione-6-hemisuccinate-bovine serum albumin during mid-anoestrus (December). The onset of seasonal oestrous activity, the number of animals displaying oestrous behaviour, and the number of ovulations per ewe were recorded during the following breeding season. The results were compared with those from untreated (control) animals and from animals treated with pregnant mare serum gonadotrophin (PMSG). The immunised animals showed a small but highly significant increase (P< 0.001) in the mean ovulation rate (1.43 ovulations per ewe) relative to control (1.08 ovulations per ewe). The increased ovulation rate in the immunised ewes was comparable with that achieved in the animals treated with PMSG (1.37 ovulations per ewe). None of the animals that were immunised with the androstenedione-protein conjugate ovulated more than two follicles, and all but one showed oestrous activity. Many of the animals with low antibody titres against androstenedione had an increase in ovulation rate (11 ewes with titres less than 1:5000 had 1.55 ovulations per ewe), whereas those with high antibody titres more commonly had low ovulation rates (11 ewes with titres greater than 1:5000 had 1.27 ovulations per ewe). A possible interference in behavioural oestrous activity appeared to be associated with high antibody titres. If further studies can show an increased number of live births per ewe mated, a single-injection procedure with an androstenedione-protein conjugate may be of practical importance in enhancing the fecundity of New Zealand Romney ewes.