Effects on the fecundity of sheep of immunisation against androstenedione

Fecundin, which actively immunises sheep against androstenedione, has now been commercially available to British farmers for three seasons. By its use an average increase of 20 to 25 per cent in the number of lambs reared can be expected, although the results on individual farms have been variable. During 1984-85 and 1985-86 ADAS carried out trials involving over 2000 ewes on 13 commercial farms. The responses to the treatment were statistically significant in both years (+23 per cent in 1984-85 and +27 per cent in 1985-86). However, the ranges in response were from -2 per cent to +49 per cent and +9 per cent to +42 per cent in 1984-85 and 1985-86, respectively. Similar results have been reported by other workers and commercial farmers using the technique. From these data it is clear that there are interactions between the treatment and animal factors including breed, body condition, nutrition, season and management. These factors are discussed with regard to the implications for the use of the product in commercial flocks.