Heat Stress and Cows: Not A Good Combination

Larry Roth, Ph.D., PAS - Vice President of Nutrition, Agrarian Solutions and Caroline Knoblock, MSc, - Director of Nutrition

While summertime may be relaxing to some people, summer’s heat stress (HS) can be devastating to dairy cow health and production, and by extension to dairy profitability. A clear understanding of a cow’s physiological responses to HS can be helpful for designing economical management programs and protecting animal welfare during summer’s heat.

Ellett et al. (2024)¹ sought to evaluate the direct and indirect responses to HS with 16 multiparous Holstein cows (approximately 100 DIM) allotted to either (1) a thermoneutral environment (THI = 64; PFTN) and pair-fed to a (2) cyclical HS treatment of thermoneutral conditions for 4 d and then 4 d of 76-80 THI.

The HS reduced DMI by 34% and the decreased nutrient intake accounted for 66% of the lost milk yield. In this study, milk production was not different until d 4 of the HS phase and decreased by 20 pounds. Milk urea nitrogen and plasma urea nitrogen was increased in HS cows on d 3 and 4 of the HS period. The HS cows were 7% lower for blood glucose and 71% for insulin than the PFTN cows. There were no differences between HS and PFTN cows for LPS-binding protein or gut permeability markers.

Under the conditions of this experiment, with a low animal number, HS shifted nitrogen metabolism of the cows, but did not increase gut permeability. The reduction in milk production, other than what was caused by reduced feed intake, was likely caused by reduced blood flow to the mammary gland and not by immune-activation. The authors suspect that the June-calved cows had already experienced some degree of HS and were therefore somewhat adapted to heat stress. Other studies that used 4 days of HS saw an increase in gastrointestinal permeability, but they also used more animals and were conducted at a different time of the year. One can suspect that for the cows used in this study, 4 days of heat stress was not enough to increase gastrointestinal permeability. It is likely that lengthier heat stress would have caused increased gut permeability.

This study with mild HS (76-80 THI) inadvertently highlights the variation we see commercially between cows and farms. Would prolonged HS or more elevated HS have resulted in greater gut permeability and immune activation? Each farm will not follow the same pattern of stress, but they are all affected in some ways. We cannot control or predict how the cow will respond to HS, but there are other stressors within our control. If we can support the cow’s response to mycotoxins with DTX, or her response to pathogens with BioCycle, she will be able to better handle the unpredictable nature of HS.

¹Ellett, M. D., R. P. Rhoads, M. D. Hanigan, B. A. Corl, G. Perez-Hernandez, C. L. M. Parsons, L. H. Baumgard, K. M. Daniels. 2024. J. Dairy Sci. 107:5190-5203. Relationship between gastrointestinal permeability, heat stress, and milk production in lactating dairy cows.