The duration of treatment with intravaginal sponges impregnated with medroxyprogesterone in the reproductive season influences the response to oestrus synchronization in Corriedale and Merino ewes

Abstract

The aim of this work was to compare the effect of 12 or 14 days of treatment with intravaginal sponges (IVS) containing medroxyprogesterone on the reproductive performance of Corriedale and Australian Merino (AM) ewes during the breeding season. Corriedale (weight: 42.4 ± 4.2 kg; mean ± SD) and AM (weight: 38.6 ± 4.0 kg) multiparous ewes were used under randomized block experimental design. The IVS were removed at 12 days (Group IVS12d; n = 224; Corriedale: 110; AM: 114), or 14 days (Group IVS14d; n = 187; Corriedale: 100; AM: 87). The estrus was detected AM and PM during 5 days after IVS withdrawal, using 10% of vasectomized rams. The ewes were submitted to cervical insemination (AI) with fresh semen obtained from rams of their same breed, 12 and 24 h after estrus was detected. Pregnancy and number of fetuses were diagnosed with ultrasound 45 days after AI. The longer permanence of the IVS increased the percentage of estrus (IVS14d: 94.7% vs. IVS12d: 82.6%; P = 0.0003), decreased the interval IVS – estrus (IVS14d: 48.8 ± 1.3 h vs. IVS12d: 58.8 ± 1.3 h; P < 0.0001), and increased the fertility (IVS14d: 72.7% vs. IVS12d: 60.7%, pregnant ewes/treated ewes; P=0.01), without affecting the conception rate (pregnant ewes/inseminated ewes) or the prolificacy (fetus/pregnant ewes). The fecundity was affected by the breed (Corriedale: 0.8 vs. AM: 0.6, fetus/treated ewes; P = 0.04). We concluded that 14 days of treatment with IVS during the breeding season increased and concentrated the estrus presentation without affecting conception rate. As a result, as we used AI associated with estrus detection, the fertility improved Corriedale ewes had greater fecundity than AM ewes, independently of the length of hormonal treatment.