ISSUE FOCUS FEED & ADDITIVE MAGAZINE July 2022 41 during the final phases of gestation during the dry period versus those animals that are cooled. It’s not just the one study that shows this, but a number of studies show the same thing. Cooled cows make more milk in the next lactation than cows that are heat stressed when they are dry. Heat stress has a profound and consistent effect on production. One reason for this is that we do see differences in various aspects of mammary development. We know that the dry period is important not only to have a reduction in older, less productive cells in the mammary gland, but also to regenerate capacity for milk yield in that next lactation. When we look at mammary cells, we see that the structure of mammary gland cells is different in heat stressed dry cows versus cows that are cooled during the dry period. You’ve conducted a wealth of research examining the implications of heat stress. What have you seen recently in terms of the effects of heat stress on dry cows? We conducted a study looking at aspects of animals’ seasonally being heat stressed when dry. We studied more than 1,500 dairy cows in Florida on one farm that had consistent, high-level management but were housed on pasture when dry during June, July and August — the hottest months of the year. We compared this group with another 1,000 cows that were housed on pasture when dry during the cooler months of the year — December, January and February. We saw about 1,100 pounds less milk in the next lactation in those cows that were dry during the hotter months of the year. Drying the cows in cool months improved performance. There is also definitely an effect on disease incidence in animals if they experience heat stress when they’re dry. Cows that were dry in hotter months ended up having increased rates of mastitis, respiratory disease and retained fetal membranes. Reproduction was also negatively affected in the cows that were dry in the hotter months. Do I have to cool cows throughout the entire dry period? We set up a “switch-back study” to determine when we need to start cooling and whether we can just wait to cool cows during the close-up phase. Cows were either cooled from the time they dried off until calving or heat stressed, and then halfway through the period, we switched half of those groups — so we ended up with cows that were either cooled for the entire dry period or cows that were heat stressed for the entire dry period, as well as cows that were cooled and then switched to heat stress and cows that were heat stressed and then cooled. Our results showed that no, you cannot just cool them during the close-up phase. It didn’t matter when the cows were stressed in the dry period; whether it was early
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