August 2020 Articles

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Heat Stress Impacts All Aspects of Cattle Reproduction

Heat Stress Impacts All Aspects of Cattle Reproduction A. Edwards – LSU AgCenter When summer begins to approach, farmers and ranchers begin to prepare for extreme heat and the humidity that often accompanies it. For cattle producers, this often includes checking shade structures and water sources, as well as making a grazing plan for the warm-season pastures. When high temperatures set in for days (or weeks) at a time, the internal body temperature of livestock can also increase, potentially resulting in decreased performance. Stressors, including heat, stimulate a release of corticotropin-releasing hormone (CRH) then adrenocorticotropic hormone (ACTH) from the brain. In turn, ACTH triggers production of catecholamines and glucocorticoids to ease the effects of stress. However, as these work to alleviate stress, the brain also decreases the release of reproductive hormones (gonadotropin releasing hormone, follicle stimulating hormone and luteinizing hormone). Consequently, this compounds to reduce reproductive function in both males and females. When heat stress occurs during the breeding season, pregnancy rates can decline as much as 25-50%. Similarly, pregnancy losses may increase 25% or more through the first trimester. This is largely due to decreased production of reproductive hormones from the brain and reproductive tract. Comparably, increased environmental temperatures impact quality of sperm, despite multiple thermoregulatory mechanisms working to cool testes for sperm production. Ultimately, compromised embryo quality results in poor survival rates when fertilization does take place. Moreover, decreased libido is also common during times of heat stress. Cows subjected to heat stress in mid- to late gestation may experience decreased blood flow to the uterus and placenta. Decreased blood flow leads to a reduction in nutrients being carried to the developing calf. While loss of pregnancy may not occur, lower birth weights are possible if stress is prolonged. If heat stress is experienced post-calving, an extended post-partum interval may occur. Decreased milk yields are also common in times of excessive heat stress. For more information, please feel free to contact me at akedwards@agcenter.lsu.edu or 512-818-5476.