Impact of heat stress on health, production, and reproductive performance of dairy cows

Abstract

Heat stress poses major challenges to the dairy industry, disrupting the well-being and productivity of cows. Besides affecting lactating cows, exposure to heat stress during the dry period increases core body temperature (CBT) and alters hormonal profile and mammary gland development, ultimately affecting milk yield in the subsequent lactation. Reproductive performance is severely reduced in dairy cows exposed to heat stress. Even though it is well accepted that estrus expression is reduced during periods of heat stress, it is not clear whether herd-level indicators of estrus-detection efficiency, such as insemination risk, are impacted during periods of heat stress. This dissertation focused on exploring the use of CBT during the dry period as a predictor of postpartum health, production, and reproductive performance during the subsequent lactation. Furthermore, potential implications of heat stress and other seasonal stressors on insemination risk were evaluated. Study 1 investigated the relationships between CBT during the dry period and health, milk production, and reproduction during the subsequent lactation. Dry cows with increased CBT were more susceptible to health disorders and had reduced milk yield early in the subsequent lactation. No association was observed between CBT during the dry period and reproductive performance after parturition. Study 2 explored factors associated with CBT in dry dairy cows and focused on determining the ideal time of the day to assess CBT of heat-stressed dry cows. Core body temperature was increased in cows pregnant with twins and was associated negatively with gestation length. Furthermore, results indicated that 2215 h is the most appropriate time of the day to assess CBT of dry cows exposed to heat stress. Study 3 aimed to compare physiologic and metabolic characteristics of cooled cows classified as having high or low CBT during the dry period. In addition, this study investigated the association between CBT during the dry period and health, milk yield, and reproductive performance after parturition. Cows with high CBT during the dry period had distinct concentrations of pregnancy-associated glycoprotein and indicators of energy balance during the transition period and had reduced milk yield compared with low-CBT cows. Furthermore, CBT during the dry period was a useful predictor of postpartum health disorders. Reproductive performance, however, did not differ between cows that had high or low CBT during the dry period. Study 4 investigated temporal patterns of insemination risk in large dairy herds and explored associations between insemination risk and herd-level traits. Seasonal variation of insemination risk was minimal, with increased insemination risk observed during autumn. Greater values of insemination risk were observed in dry-lot herds, with low mortality of cows, and longer voluntary waiting period for primiparous cows. In summary, assessment of CBT in dry cows may be a useful tool to identify groups of cows more likely to present health disorders and impaired productive performance after parturition. In addition, insemination risk is not reduced during the summer, but it is severely affected by herd-level traits such as housing system, mortality of cows, and voluntary waiting period for primiparous cows.