Induction of estrus with male effect, using vasecomized rams associated or not with ewes in heat in Merino ewes during seasonal anestrus: practical experiences in Uruguay

Abstract

This study reports the practical results of the use of the ram effect to induce heat and ovulation during seasonal anestrus of Merino ewes in an extensive system. For 9 years (2013, 2014, 2015, 2017, 2019, 2020, 2021, 2022 and 2023), cyclicity was induced in a total of 15,300 sheep during seasonal anestrus, through the sudden introduction of vasectomized rams, with ewes with pharmacological heat until 2020, and without them from 2021. In all cases, 4.5% males were used, and until 2020 9% of estrous ewes. After 15 days, the stimulating animals were removed, and other vasectomized rams were introduced. In the last year, 2023, a mixture of vasectomized rams with castrated males treated with testosterone was used instead of only vasectomized rams. Estrus was monitored daily, and the ewes were inseminated with fresh semen. In total, 13,488 sheep showed heat (88.16%), with a variation of between 69.18% and 97.75% depending on the year, although in the years of lower incidence estrus began to be checked later, thus, probably losing the identification of some ewes that came into estrus. Although the ewes that gave birth were not identified, the pregnancy rate was always above 60%. The distribution of heat, although it varied slightly between years, generally corresponded to a bimodal distribution consistent with the response to the male effect. In conclusion, the use of vasectomized rams both with and without ewes in heat allowed for a high incidence of heat in seasonally anoestrous Merino ewes.