Effects of prostaglandin F(2alpha) dosage of synchronizing ovine estrus using a modified single injection regimen

Two dosages of prostaglandin F(2alpha) (PGF) were tested to evaluate their efficacy to synchronize estrus in ewes. A selectively administered single injection regimen was used. A total of 329 Targhee, Suffolk x Targhee and Finn x Targhee ewes 3 to 6 yr of age were allotted within breed type to one of three treatment groups: control, 10 mg PGF or 15 mg PGF. Trials were conducted over a 2-yr period and were replicated twice within each year. In each trial, epididymectomized rams were placed together with the ewes (1 ram:40 ewes) for 2 wk prior to a 35-d breeding exposure. Fertile, semen-tested rams were introduced (1 ram:10 ewes) on Day 1 of the breeding period. All ewes that had not mated by Day 5 received one of the three randomly assigned treatments. Treatment with PGF-10 or PGF-15 increased the percentage of ewes mating (and the percentage of those conceiving) 32 to 56 h following treatment compared with the control ewes (P<0.01). Within 56 h following treatment, 11.7, 56.2 and 65.5% of the control and the PGF-10 and PGF-15 treated groups, respectively, had mated. The percentage of ewes that conceived within 56 h was 10.7, 42.3 and 47.8, respectively. Treatment did not affect fertility, prolificacy or fecundity (P>0.05), irrespective of breed. A treatment by breed interaction was found: Finn x Targhee ewes treated with PGF-15 had a lower (P<0.01) lambing rate than those treated with PGF-10 or those in the control group. Lamb birth weight and lamb mortality were not affected by treatment (P>0.05). The cumulative lambing percentage was higher (P<0.01) at 157 d following the introduction of rams for ewes treated with PGF than for the controls. These results indicate that at a dose of 10 or 15 mg i.m. PGF was effective in synchronizing estrus in ewes within 56 h post treatment.