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

Abstract

Impaired fertility during periods of heat stress is the culmination of numerous physiological responses to heat stress, ranging from reduced estrus expression and altered follicular function to early embryonic death. Furthermore, heat-stressed dairy cattle exhibit a unique metabolic status that likely contributes to the observed reduction in fertility. An understanding of this unique physiological response can be used as a basis for improving cow management strategies, thereby reducing the negative effects of heat stress on reproduction. Potential opportunities for improving the management of dairy cattle during heat stress vary greatly and include feed additives, targeted cooling, genetic selection, embryo transfer and, potentially, crossbreeding. Previous studies indicate that dietary interventions such as melatonin and chromium supplementation could alleviate some of the detrimental effects of heat stress on fertility, and that factors involved in the methionine cycle would likely do the same. These supplements, particularly chromium, may improve reproductive performance during heat stress by alleviating insulinmediated damage to the follicle and its enclosed cumulus-oocyte complex. Beyond feed additives, some of the simplest, yet most effective strategies involve altering the timing of feeding and cooling to take advantage of comparatively low nighttime temperatures. Likewise, expansion of cooling systems to include breeding-age heifers and dry cows has significant benefits for dams and their offspring. More complicated but promising strategies involve the calculation of breeding values for thermotolerance, the identification of genomic markers for heat tolerance, and the development of beddingbased conductive cooling systems. Unfortunately, no single approach can completely rescue the fertility of lactating dairy cows during heat stress. That said, region-appropriate combinations of strategies can improve reproductive measures to reasonable levels.

Effect of heat stress on udder health of dairy cows

“…However, successful cooling methods for reducing the effects of heat stress all involve changing the farm environment by maximizing heat exchange through convection, conduction, radiation, and evaporation. Details of abatement strategies vary depending on the farm environment, location, management approaches, stage of production and affordability (Negrón-Pérez et al ., 2019). Up till now, providing shade to protect against solar radiation is the most effective, simple way of cooling cows (Collier et al ., 2006; Tucker et al ., 2008).…”

“…Moreover, coating the dorsal midline of cattle with a reflective pigment like titanium dioxide has the potential to mitigate the heat stress in feedlot cattle by reflecting solar radiation (Bartle et al ., 2018). Despite these positive effects, it is important to remember that exposure of dairy cattle to severe heat stress during summer can have profound negative effects not only on lactation but also on reproductive traits (Jousan et al ., 2007; Gernand et al ., 2019; Negrón-Pérez et al ., 2019).…”

Time-lagged and acute impact of heat stress on production and fertility traits in the local dual-purpose cattle breed “Rotes Höhenvieh” under pasture-based conditions

“…Due to carryover effects from summer, HS-induced oocyte damage also impairs fertility in autumn. Hence, reproductive performance remains low for approximately 2 mo of cooler temperatures in autumn, when compared with winter ( Negrón-Pérez et al, 2019 ). Furthermore, the early embryo is very sensitive to HS, because high ambient temperatures on day 1 after insemination decreased subsequent embryonic development ( Ealy et al, 1993 ), possibly contributing to embryonic losses and prolonged calving intervals.…”

Melatonin and CIDR improved the follicular and luteal haemodynamics, uterine and ovarian arteries vascular perfusion, ovarian hormones and nitric oxide in cyclic cows

“…The strategy to advance the reproductive performance activities of animals to minimize the undesirable effects of the global warming environment on animal health and fertility was to supplement animals with antioxidants (Pulina et al., 2017). The use of melatonin to improve the fertility of heat‐stressed lactating dairy cattle was for its antioxidant property to scavenge the increased creation of reactive oxygen species in either oocytes or embryos (Ortega et al., 2016; Payton et al., 2018) and its potent antioxidant activity (Negrón‐Pérez et al., 2019). The ability of melatonin to protect an organism from the overproduction of the free radicals is via direct or indirect pathways (Reiter et al., 1995).…”