ANTICIPATING HEAT STRESS: EARLIER SIGNALS & BETTER DECISIONS FOR DAIRY HERDS
- Heather Dann
- 23 hours ago
- 3 min read
Heat stress remains a management challenge for farmers, with lasting impacts on milk production, fertility, and overall herd health. Anticipating heat stress early and responding proactively can prevent costly losses. Over the last month or so, our Miner team has focused on fan and soaker maintenance to ensure our facilities are ready for the summer heat.
The temperature humidity index (THI) is a long-standing indicator of heat load. While still useful, research from Florida using 1280 lactating Holstein cows (doi:10.3168/jds.2008-1370) found that the dry-bulb temperature (Tdb) alone was nearly as effective as various THI formulas for predicting increases in body temperature. One commonly used THI formula, also used at Miner Institute, is: Tdb (°F) − [(0.55 − 0.0055 × relative humidity (%)) × (Tdb (°F) − 58)] (NRC, 1971). The high correlation (r = 0.95) between temperature and THINRC means that, from a practical standpoint, closely monitoring air temperature and cow responses may be just as informative as relying on a calculated index. The onset of hyperthermia occurred at ~83°F, with body temperatures rising predictably above that threshold (rectal temperature (°F) = 0.3622 x Tdb (°F) + 71.208).
While environmental measures are useful, the cow’s physiological response ultimately determines the severity of heat stress, and cows vary considerably in their heat tolerance. Recent research from Australia (doi.org/10.3168/jds.2025-27599) evaluated continuous body temperature patterns in Holstein-Friesian cows and found that daily temperature stability during cool conditions is strongly linked to heat stress resilience. Cows with more stable temperature profiles under thermoneutral conditions maintained lower body temperatures during hot weather. In contrast, those with larger daily fluctuations accumulated more heat under elevated THI conditions. These findings have both management and genetic implications. Heat tolerance may be identifiable before the onset of summer, creating opportunities for grouping strategies based on thermal resilience and for long-term genetic selection. Precision technologies, such as reticular-rumen or vaginal temperature sensors, make this type of monitoring increasingly feasible.
Recent work (doi.org/10.3168/jds.2025-27465) involving more than 900 lactating Holstein cows highlights the value of behavioral indicators, particularly rumination, as early signals of heat stress. Declines in rumination occurred at mean THI values between ~57 to 62, while decreases in milk yield were not observed until THI reached 65 to 68 or higher. By the time milk production drops, cows have already been experiencing heat stress. Rumination is an earlier and more sensitive indicator. Once the threshold is exceeded, rumination can decline by ~2 to 5 minutes per day for each unit increase in THI. Because rumination responds quickly to changes in feed intake and cow comfort, it serves as a more immediate indicator of heat stress than milk yield, which reflects downstream physiological effects. Interestingly, cows did not respond equally to heat stress. High-producing, multiparous cows were more susceptible due to greater metabolic heat production associated with higher feed intake and milk synthesis. In contrast, first-lactation and lower-producing cows were more heat-tolerant.
The duration of heat exposure is also critical. While peak or mean THI are often emphasized, the number of hours per day at elevated THI plays a major role in determining severity. Rumination may decline with relatively short heat exposure, but milk losses typically require prolonged stress, often more than 10 to 15 hours per day. At Miner Institute we see intake and milk yield drop during heat events when our cows have 3 or more days without nighttime cooling.
Taken together, these findings suggest several practical management considerations:
Prepare early: Clean and service fans and soakers well ahead of summer.
Start cooling sooner: Start heat abatement when the THI approaches the high 50s, rather than waiting until it exceeds 68, especially in high-producing herds.
Monitor rumination: Use rumination data as an early warning system and to evaluate cooling effectiveness.
Prioritize vulnerable groups: Focus cooling efforts on high-producing, multiparous cows, as well as cows showing greater temperature variability under cool conditions. Dry cows warrant attention and will be discussed further in a future report.
— Heather Dann