Impaired reproduction in heat-stressed cattle: basic and applied aspects

Area of Science:

• Reproductive Physiology
• Animal Science
• Environmental Stress

Background:

• Summer heat stress is a primary cause of reduced fertility in lactating dairy cows in hot climates.
• Existing cooling systems on dairy farms have not fully resolved fertility issues associated with heat stress.

Purpose of the Study:

• To review the mechanisms by which heat stress impairs reproductive system function in dairy cows.
• To identify the effects of heat stress on follicular dynamics, steroidogenesis, hormone concentrations, and embryo development.

Main Methods:

• Literature review summarizing existing research on heat stress and dairy cow reproduction.
• Analysis of studies detailing physiological and hormonal changes in cows exposed to heat stress.

Main Results:

• Heat stress suppresses dominant follicle development, compromises the steroidogenic capacity of theca and granulosa cells, and lowers progesterone secretion.
• Elevated FSH and reduced LH and inhibin levels are observed, alongside impaired oocyte and embryo quality and increased embryo mortality.
• Chronic heat exposure leads to decreased plasma progesterone, compromised endometrial function, and pregnancy loss, with delayed effects impacting fertility even in cooler months.

Conclusions:

• Heat stress profoundly disrupts multiple facets of the dairy cow reproductive system, leading to significant fertility losses.
• While hormonal treatments offer partial improvement, the inability of high-yielding cows to maintain normothermia remains a critical challenge.
• Targeted hormonal protocols can enhance pregnancy rates and reduce open days during summer, but comprehensive management strategies are needed.