Heat stress is one of the biggest challenges for dairy cow health and productivity, especially during hot and humid seasons. When environmental temperature and humidity exceed the animal’s capacity to dissipate heat, the cow experiences physiological stress. This leads to increased respiration rate, heart rate, and hormonal changes, ultimately reducing feed intake and digestive efficiency (Giannone et al., 2023; Rhoads & Baumgard, 2013). In the U.S. alone, the annual economic loss from heat stress in the dairy and beef industries is significant (Hein, 2023).
The rumen is a vital part of the cow's digestive system. Under heat stress, reduced feed intake leads to less chewing and rumination, which reduces saliva production. Saliva contains bicarbonate that buffers rumen pH. A decrease in saliva lowers the buffering capacity, causing a drop in rumen pH (Kim et al., 2022).
This pH drop leads to a condition called rumen acidosis, where the ratio of acetate to propionate decreases, and lactic acid-producing bacteria increase. This disrupts microbial balance, reducing fiber-digesting organisms and energy yield (Baumgard & Rhoads, 2013; Kim et al., 2022).
Heat stress causes microbial imbalance, with an increase in lactic acid bacteria like Streptococcus and a decrease in beneficial cellulolytic bacteria. This condition is referred to as "rumen dysbiosis" (Kim et al., 2022), and it impairs fermentation, digestion, and energy production.
Additionally, heat stress damages the epithelial lining of the rumen. Microscopic studies show cellular lesions, reduced barrier function, and increased permeability of the rumen wall, allowing harmful bacteria like E. coli to enter the bloodstream (Guo et al., 2022).
One of the first physiological responses to heat stress is decreased appetite and dry matter intake (Baumgard & Rhoads, 2013). For every unit increase in the Temperature-Humidity Index (THI), dry matter intake drops by approximately 0.45 kg (Fabris et al., 2021).
Even if feed quantity remains unchanged, heat stress disrupts nutrient absorption. Blood is redirected from the digestive tract to the skin for cooling, reducing enzyme activity and blood flow to the intestines. This leads to damage in intestinal villi, lowering nutrient absorption (Baumgard & Rhoads, 2013).
Heat stress also weakens the intestinal barrier, allowing endotoxins like lipopolysaccharides (LPS) to leak into the bloodstream. This causes systemic inflammation and increases the risk of diseases like mastitis, diarrhea, and respiratory infections (Hein, 2023).
Chronic or repeated heat stress leads to long-term issues:
• Reduced milk yield and milk fat/protein percentages (Giannone et al., 2023)
• Reduced growth rate and meat quality in beef cattle (Hein, 2023)
• Impaired reproductive performance, including lower fertilization rates, increased embryo loss, and more abortions in pregnant cows during hot seasons (Dahl et al., 2020)
• Calves born during hot seasons may have lower birth weights and weaker immune systems
Over time, heat-stressed cows are culled earlier and have shorter lifespans, decreasing overall herd productivity and economic return (Hein, 2023).
Heat stress affects not just the external thermal regulation of cattle, but also directly disrupts the gastrointestinal system. It reduces feed intake, alters rumen microbial populations, causes inflammation in the rumen and intestines, impairs nutrient absorption, and decreases reproduction and productivity (Giannone et al., 2023; Kim et al., 2022; Guo et al., 2022; Hein, 2023).
Management solutions include improving ventilation, using live yeast or antioxidants in feed, and breeding for heat-resistant genetics (Hein, 2023; Kim et al., 2022).