Dr. Dahl presented a Real Science webinar on heat stress on April 1, 2025. You can find the webinar at balchem.com/realscience. This episode of Real Science Exchange further explores the key elements of Dr. Dahl’s webinar.

Dr. Dahl talks about geographical differences in whether farms provide cooling for dry or lactating cows. Cows get heat stressed long before humans. Some farms are concerned that using misters for cooling will add too much water to their manure handling systems. He notes a study comparing conventional misters and fans, no cooling, and smart soakers that only provide mist if a cow is present. When the total amount of water (drinking + cooling system) was evaluated, the smart soakers cooled as well as the conventional system, but used the same amount of water as the no cooling group.  (7:08)

During the dry period, a main impact of heat stress is a reduction in dry matter intake. However, there are dramatic shifts in immune function and effects on mammary development and redevelopment in cows who experience heat stress in the dry period. This sets the stage for lower productivity in the next lactation. In addition, there are many negative impacts on the in utero calf from heat stress. Calves from heat stressed dams are challenged from a growth standpoint, in addition to organ development challenges in the mammary gland, ovaries, and immune system. These calves are less likely to make it through their first lactation, are less productive, and pass their poor production and survival phenotype on to their offspring. (14:28)

Dr. Tao talks about when during the dry period to provide cooling for cows. Spoiler alert: the entire dry period! He also notes that bred heifers should have cooling provided for the last 60 days of gestation as well. Laura asks about the impact of heat stress on neonatal calves and how it may impact their mammary development. More research is needed in this area, and you also have to wait two years to collect data from the first lactation. Dr. Dahl notes that observations from season of birth data indicate lower longevity for calves who are born to heat stressed dams. (18:26)

Milk production is decreased by 8-10 pounds per day for cows stressed during the dry period, and they also produce a lower volume of colostrum. Calves from heat stressed dams also have a lower rate of passive transfer of antibodies from colostrum. The panel talks about why that might be, whether or not those gut differences persist after calfhood, and how that might be related to growth differences between heat stressed and cooled calves. (27:30)

What about reproduction? It appears that heat stress during the dry period has a negative impact on reproductive function in the subsequent breeding season. Recent research has indicated that calves experiencing heat stress in utero have poor gonadal development and lower follicular reserves. In addition, placental development is also negatively affected. Dr. Tao notes that heat stress negatively impacts mammary gland involution during the dry off period. All of this leads to a decrease in cow longevity. (35:36)

Dr. Dahl describes a retrospective records study using Florida and California herds to evaluate cows in their fifth through eighth lactations. In Florida, about three-quarters of those animals were born in cooler parts of the year rather than in hotter months of the year. The pattern in California was similar, though not quite as extreme. (44:02)

Dr. Tao and Dr. Dahl expand on the economics of cooling cows, including return on investment and the costs of not cooling. The guests also talk about some of their research abstracts at the 2025 ADSA meetings. (48:10)

Panelists share their take-home thoughts. (57:41)

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