Synchronized breeding and reproductive resilience in dairy cattle: mitigating metritis, heat stress, and transgenerational fertility impairment.

Abstract

Heat stress profoundly compromises reproductive efficiency in dairy cattle by disrupting follicular and oocyte development, altering steroid hormone profiles, impairing estrous expression and ovulation, reducing fertilization success, hindering embryonic development, and increasing early embryonic loss. Although the molecular mechanisms underlying these disruptions remain incompletely understood, significant advances in reproductive technologies have been developed to mitigate the negative impact of heat stress on reproductive performance. Estrous cycle synchronization programs, enabling timed artificial insemination (AI), emerged in the mid-1990s and reduced heat stress-related reproductive inefficiencies by eliminating the need for estrus detection, improving submission rates, and optimizing the ovulation-to-insemination interval, thereby enhancing pregnancy rates per AI. Embryo transfer (ET) has been adopted as a complementary strategy to bypass fertilization failure during hot periods, whereas genetic selection, most notably the introgression of the SLICK-1 allele of the prolactin receptor gene (PRLR), confers thermotolerance via a sleek hair coat and improved heat dissipation. Nevertheless, heat stress also exacerbates common postpartum disorders such as metritis, which negatively affect fertility regardless of AI, ET, or thermotolerant genotypes, underscoring the importance of preventive herd-health management. Furthermore, exposure to heat stress during fetal development imposes transgenerational consequences, diminishing the health, productivity, longevity, and fertility of subsequent generations. This review evaluates current and emerging technologies, including timed AI, ET, genetic selection for thermotolerance and reproductive resilience, and preventive approaches to postpartum disease, while also addressing strategies to mitigate the transgenerational impairments induced by heat stress in dairy cattle.