Reproductive performance of ewes after 5-day treatment with intravaginal inserts containing progesterone in combination with injection of prostaglandin F₂α

Three experiments were conducted with a total of 1579 ewes to examine reproductive performance in response to synchronization of oestrus during the breeding season, using controlled internal drug releasing (CIDR-G) inserts in regimens designed to provide high concentrations of circulating progesterone. In experiment 1, treatment with two CIDR-G inserts for 12 days produced conception rate (79%) and prolificacy (1.9) to first service equivalent to breeding at natural oestrus (56% and 2.0, respectively). Pregnancy rates to two service periods were 90 and 79%, respectively. In experiments 2 and 3, progesterone was delivered by a single CIDR-G insert for 5 days in combination with prostaglandin F₂α (PGF₂α; 5 mg i.m., twice, 3 h apart) the day before (experiment 2), or at insert removal (experiment 3). The combined treatments improved rates of synchronization of oestrus (p < 0.01) by 23 and 20% points, respectively, and pregnancy rates to the first service period by 19 (p < 0.05) and 13 (p < 0.01) percentage points, respectively, compared to treatment with PGF₂α alone. It is concluded that the combination of treatment for 5 days with a CIDR-G insert and two injections of 5 mg PGF₂α, the day before, or the day of insert removal, were effective treatments to obtain high fertility at synchronized oestrus in ewes during the breeding season.