Can the "ram effect", or the "ram effect" plus PMSG be used to advance the breeding season of Southdown ewes?

The 'ram effect' induced some seasonally anovular Southdown ewes (23%) to ovulate within 5 days of introducing Southdown rams on 10 December. Priming the ewes with medroxy progesterone acetate from intravaginal sponges for 12 days, immediately before the introduction of the rams, ensured that most of the ewes displayed oestrus at the ram-induced ovulation. The proportion of anovular ewes ovulating increased (P < 0.001) as the date of joining moved from December (4/25) to February (15/16). At all times of joining the proportion of ewes ovulating and their ovulation rate was improved by an intramuscular injection of pregnant mare's serum gonadotrophin at sponge withdrawal. On another farm greater than 92% of Southdown ewes, synchronised with the same regime, lambed to the synchronised oestrus from joinings in late January or early February. An injection of PMSG did not increase the proportion of ewes ovulating but it did increase their ovulation rate (+PMS, 1.75 vs -PMS, 1.17; P(O.001) and proportion of multiple births (+PMS, 42% vs -PMS, 7%; P

Keywords:

• Animal science
• Serum gonadotrophin
• Estrous cycle
• Ovulation
• Intramuscular injection
• Seasonal breeder
• Biology

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