Interactions among seasonality of reproduction, genotype and the ram effect.

SUMMARY In a flock of isolated anoestrous ewes, the introduction of rams can induce oestrous cycles. This technique known as the ‘ram effect’ is a cornerstone of the ‘clean, green and ethical’ management of ovine reproduction. However the response of ewes to the ‘ram effect’ is highly variable among and even within breeds thus undermining its usefulness. The aim of our experiment was to determine the effectiveness of the ‘ram effect’ at different times of anoestrus in seasonal (Vendeen and Charollais breeds) and less seasonal (Ile-de-France and F1, Ile-de-France X Romanov) breeds of sheep. Romanov rams were introduced to anoestrous ewes at the beginning (the date when the proportion of the flock showing spontaneous ovulation fell below 50%), the middle (4-6 weeks after the start of anoestrous) or the end (the date when the proportion of the flock showing spontaneous ovulation rose above 25%) of anoestrus. We followed the ovarian responses to the ‘ram effect’ by measuring plasma concentration of progesterone 11 days after the introduction of rams in different breeds in different locations. In the Vendeen breed, the response to the ram effect varied from 3% to 73% with the poorest response in the middle and the best response at the end, of anoestrus. Unexpectedly the response of the Charollais breed in mid-anoestrus was better than in the Vendeen breed reaching 60 to 63% in terms of fertility. In the Ile-de-France breed and the F1s, the response varied between 65 and 80% at the beginning of anoestrus and between 89 % and 100% in the Ile-de-France breed in mid-anoestrus and the end of anoestrus. The ‘ram effect’ appears to be more effective regardless of breed, at the end of anoestrus and can be used effectively to advance the breeding season by about a month. In less seasonal breeds, the ‘ram effect’ is also effective at the beginning and middle of anoestrus. But in the more seasonal breeds the response appears to be variable especially in mid-anoestrus. The response was very poor in the Vendeen breed and a little better in the Charollais breed. More studies are needed to understand these differences. Contrary to the literature, there was no