Effect of nutritionally induced live-weight differences on the ovarian response and fertility in ewes treated with pregnant mare's serum gonadotrophin

Abstract

Abstract Two groups of 120 Romney ewes were differentially fed for 4 months resulting in a mean live-weight difference of 15 kg. Within groups, ewes were treated with 0,600,or 1000 i.u. pregnant mare's serum gonadotrophin (PMSG), on day 13 of the cycle. Ewes were mated with entire rams and were slaughtered 22-24 days later to assesss ovulation rates and the number of normal embryos. Ewes returning to service were slaughtered 1 to 4 days later to determine fertilisation rates. Treatment with PMSG increased the ovulation rate (p < 0.01) and the response was greater in the heavier ewes (p < 0.01).Mean ovulation rates were 1.35,1.97,and 3.88 in low-live-weight ewes and 1.54, 2.95, and 5.34 in high-live-weight ewes ireated with 0, 600, or 1000 i.u. respectively.Conception rates did not differ significantly between the treatment groups and also did not differ between ewes shedding different numbers of ova. In those ewes not returning to service the mean number of normal embryos was 1.23, 1.64, and 2.08; and 1.37,2.07,and 2.37 for low-and high-live-weight ewes treated with 0, 600, or 1000 i.u. PMSG respectively.As the number of ovulations increased the proportion of embryonic mortality also increased.