Invited review: Management strategies capable of improving the reproductive performance of heat-stressed dairy cattle

摘要

Impaired fertility during periods of heat stress isthe culmination of numerous physiological responsesto heat stress, ranging from reduced estrus expressionand altered follicular function to early embryonicdeath. Furthermore, heat-stressed dairy cattle exhibita unique metabolic status that likely contributes to theobserved reduction in fertility. An understanding of thisunique physiological response can be used as a basis forimproving cow management strategies, thereby reducingthe negative effects of heat stress on reproduction.Potential opportunities for improving the managementof dairy cattle during heat stress vary greatly and includefeed additives, targeted cooling, genetic selection,embryo transfer and, potentially, crossbreeding. Previousstudies indicate that dietary interventions such asmelatonin and chromium supplementation could alleviatesome of the detrimental effects of heat stress onfertility, and that factors involved in the methioninecycle would likely do the same. These supplements,particularly chromium, may improve reproductiveperformance during heat stress by alleviating insulinmediateddamage to the follicle and its enclosed cumulus-oocyte complex. Beyond feed additives, some of thesimplest, yet most effective strategies involve alteringthe timing of feeding and cooling to take advantageof comparatively low nighttime temperatures. Likewise,expansion of cooling systems to include breeding-ageheifers and dry cows has significant benefits for damsand their offspring. More complicated but promisingstrategies involve the calculation of breeding values forthermotolerance, the identification of genomic markersfor heat tolerance, and the development of beddingbasedconductive cooling systems. Unfortunately, nosingle approach can completely rescue the fertility oflactating dairy cows during heat stress. That said,region-appropriate combinations of strategies can improvereproductive measures to reasonable levels.