Epidemiology of Pregnancy Losses and Practical Strategies for Prevention

Pregnancy losses from fertilization to term are extensive in lactating dairy cattle. In many cases, they are underestimated because of the inability to determine conception early after insemination. For most producers, pregnancy losses are only quantified after the initial di‐ agnosis is performed at approximately 30 to 45 d of gestation. Recent studies that provid‐ ed reliable assessments of fertilization have led to the conclusion that more than 60% of all pregnancies are lost before term in lactat‐ ing dairy cows. The critical period is from fer‐ tilization until the end of the differentiation phase (embryonic period) in which approxi‐ mately 85% of those losses occur. Therefore, strategies to prevent embryonic losses should result in the greatest improvement in repro‐ ductive performance. Nevertheless, although fetal losses are minor (approximately 15%) compared with embryonic losses, those are more costly than embryonic losses; therefore they cannot be overlooked. Known risk fac‐ tors for pregnancy loss include prolonged pe‐ riods of follicular dominance, low progester‐ one concentration during ovulatory follicle development, heat stress, postpartum diseas‐ es and disorders such as dystocia, metritis, endometritis, mastitis, fever, ketosis, lame‐ ness, and digestive problems, negative energy balance, excessive body weight loss, toxins found in feedstuffs such as gossypol, myco‐ toxins, and ergot alkaloids, and infectious agents such as Neospora caninum, Leptospira spp., Campilobacter spp., BVD virus, and IBR virus among others. Strategies that prevent or control known risk factors should help de‐ crease pregnancy loss and improve reproduc‐ tive performance.