Part One: The Hidden Costs of Heat Stress

By Dr. Tricia Wood, Technical Services-Ruminant, Lallemand Animal Nutrition

This article is the first in a series of three about heat stress in dairy cattle.

As temperatures climb, dairy producers can expect more than a drop in milk yield. Rising temperatures, combined with humidity, trigger subtle but significant changes in cow behavior, immune function, reproduction and calf development — and the long-term financial consequences can stretch for years. The temperature-humidity index (THI), which gauges the severity of heat stress, is now recognized to impact cows at a lower threshold than once thought: 68.¹

Heat stress increases respiration rates, which elevates carbon dioxide loss and disrupts blood buffering systems. To compensate, cows divert bicarbonate away from the rumen — where it would typically help stabilize pH — to protect blood chemistry. Rumination slows, drooling increases, and saliva production drops, compounding the problem. It’s no mystery then to understand that a heat-stressed cow is an acidotic cow.²

The Dry Period Dilemma: Invisible Impacts with Lasting Consequences
The dry period, when cows are most vulnerable, coincides with the final trimester of gestation, making heat stress especially damaging. According to industry estimates, U.S. dairy farms incur over $1.5 billion in losses annually tied to heat-stressed dry cows through:^3,4,5,6

Calves Are Behind Before Birth
Research continues to uncover how maternal heat stress challenges the next generation.⁴ Calves born to heat-stressed cows face a trifecta of challenges:^7,8
Weakened immune function. Calves absorb colostrum antibodies less effectively, compromising early-life immunity.

Stunted growth and organ development. Redirected maternal energy prioritizes survival over fetal growth, leading to lower birth weights and poorly developed organs.
Poor thermoregulation. These calves struggle to cope with heat after birth, increasing their risk of future heat stress and increasing the probability of culling.

The Takeaway
Heat stress isn’t just a summertime inconvenience. It’s a production, health and herd development issue with long-term financial implications. While immediate milk yield losses focus attention on the lactating herd, there are the silent, cascading effects on dry cows and developing calves that quietly erode profitability year after year.

Proper investment in heat abatement strategies, especially as they impact the success of your future herd, is the key to the long game. It is critical for producers to take steps to mitigate heat challenges before they occur. Learn more about nutritional and management strategies that are proven to ease heat stress in dairy cattle and keep your operation efficient and profitable.

Read Part 2 — Feeding Through the Heat: Practical Nutrition and Management Adjustments That Work.

References :
¹ Collier R. J. et al 2012. Quantifying heat stress and its impact on metabolism and performance. Proceedings of 23^rd Annual Florida Ruminant Nutrition Symposium.
 
² Mishra M. et al 1970. Effect of diet, and ambient temp-humidity on ruminal pH, oxidation-reduction potential, ammonia, and lactic acid in lactating dairy cows. J. Anim. Sci 30: 1023-1028.
 
³Baumgard L.H., Rhoads RP. 2012. Ruminant Nutrition Symposium: ruminant production and metabolic responses to heat stress. J AnimSci. 90(6):1855-65
 
⁴ Marsola R.S., et. al.2010. Effect of feeding live yeast on performance of Holstein dairy cows during summer. J. Dairy Sci.(88)E-Suppl.2:432.
 
⁵ Macko A. R. et al 2017. How heat stress during the dry period reduces milk yield in the next lactation. Proceedings of South West Nutrition Conference.
 
⁶ Baumgard L. H. et al 2017. Low summer milk fat. Is it the same as diet-induced milk fat depression? Proceedings of South West Nutrition Conference.
 
⁷ A. P. A. Monteiro, S. Tao, I. M. T. Thompson, and G. E. Dahl. In utero heat stress decreases calf survival and performance through the first lactation. J. Dairy Sci. 99:8443–8450.

⁸ J. Laporta, F. C. Ferreira, V. Ouellet, B. Dado-Senn, A. K. Almeida, A. De Vries, and G. E. Dahl1Late-gestation heat stress impairs daughter and granddaughter lifetime performance. J. Dairy Sci. 103:7555–7568.