Double artificial insemination in sheep: a comprehensive review.

Artificial insemination (AI) in sheep presents variable results, especially when combined with estrus induction treatments during the anestrous period. Alternatives for obtaining better results without significantly altering the costs of hormonal protocols are essential because of the importance of this biotechnology in production systems. One alternative that potentially meets these requirements is double-AI. Therefore, this article aims to review the literature on double-cervical AI in sheep and identify gaps in existing knowledge. Double cervical superficial (CS) AI with frozen-thawed (F.T.) semen after estrus detection significantly increased pregnancy rates (PR) in most (6/8) evaluated studies, with an increase of 7 to 34.2 percentual points (p.p.), compared to single AI. Regarding fixed-time AI (FTAI), all studies used fresh (F) or chilled (C) semen, and no positive effects were observed for double FTAI in most cases (8/9). Most studies have not applied current estrous synchronization protocols and insemination doses. Therefore, further studies are needed to evaluate the potential benefits of double FTAI, especially using F.T. semen in combination with hormonal protocols and insemination doses aligned with current practices.