Effect of PMSG dosage on the reproductive performance of adult ewes and ewe lambs bred at a progestagen-PMSG synchronized estrus

Abstract

The effect of dose of pregnant mares' serum gonadotropin (PMSG) on the reproductive performance of adult ewes and ewe lambs and lamb survival at birth after treatment with fluorogestone acetate (FGA)-impregnated intravaginal sponges and PMSG (250 IU or 500 IU) to synchronize estrus was evaluated. Ewes were exposed to rams for breeding at the synchronized and subsequent estrous cycles. The flock, comprised of three synthetic strains and two control breeds, was maintained in a controlled environment and exposed to an artificial light regimen which alternated at 4-mo intervals from 16h of light daily to 9h of light daily. Trials were conducted during January, May and September at the end of a 9-h daylength cycle. Adult ewes were bred in May and 8 mo later in January. Ewe lambs were bred in September at 6.5 to 7.5 mo of age. The overall reproductive performance of the adult ewes was similar at the two breedings: fertility ∼90%, prolificacy ∼2.7, fecundity ∼240% and lambs born alive ∼2.4. Dosage of PMSG had no effect. Reproductive performance of ewe lambs was lower and there was a strain × treatment interaction, suggesting greater variability in response. The results indicate there is no advantage to using a higher dose of PMSG in ewes with a natural relatively high fecundity. Moreover, the use of the artificial photoperiod appears to overcome the natural seasonal variation in reproductive performance.