ISSUE FOCUS 42 FEED & ADDITIVE MAGAZINE July 2022 or late, they all responded the same way as the cows that were heat stressed for the entire dry period. All heat-stressed cows made less milk than cows that were cooled for the entire time. We need to get cows cooled from the initiation of the dry period. What are the effects of heat stress on the developing fetus? We see significant effects of heat stress on the fetus that’s developing within that animal. While we tend to look at the effect on the cow as sort of programming her for less milk yield in that next lactation, the effects on the fetus program it for lower productivity and lower survivability in the herd. So, it’s a lifetime effect. In fact, the effects on the fetus may be even greater, when we look at them in aggregate, than the effects on the cow herself in the next lactation. What has your research shown about the effects of feeding OmniGen® to cows that are heat stressed? We looked at cows that were heat stressed from the time they were dried off until calving versus cows that were cooled for the entire dry period, and we fed OmniGen (or did not feed OmniGen to our control group) late in lactation and continued those treatments during the dry period. We continued those OmniGen treatments into lactation. Cows 1. Seasonal influence is profound, and it might not occur when you’d think. Milk production tends to peak in March and April and bottom out in September or October. Plot this trend against the highest average daily temperatures, and it’s interesting to see that the highest temperatures do not overlap with absolute production lows. This indicates that not all effects of heat stress are seen immediately. In addition to the acute effects that producers usually see, heat stress has lingering effects that vary according to the duration of the heat stress period and the stage of lactation. 2. Cooling dry cows increases milk in the next lactation. You’re looking at a reduction in milk yield of 8 to 10 pounds per day if cows are heat stressed during the final phases of gestation during the dry period. Numerous studies show that cooled dry cows make more milk in the next lactation, demonstrating that heat stress has a significant and consistent effect on production. This is due in part to various aspects of mammary development. The dry period is important not only to reduce the number of older, less productive cells in the mammary glands, but also to regenerate capacity for milk yield during the next lactation. 3. There’s a lot happening early in the dry period that can be affected negatively by heat stress. It doesn’t matter whether cows are heat stressed early or late in the dry period; they respond the same way as cows that are heat stressed for the entire dry period. In numerous studies, all heat-stressed cows produced less milk than cows that were cooled. This finding shows the importance of cooling cows at the beginning of the dry period. 4. Heat stress has a lifetime effect on calves before birth. While we tend to look at the effects of heat stress on the dry cow as sort of programming her for the next lactation, the effects on the fetus are likewise programming it for lower productivity and lower survivability in the herd. This carryover effect on the fetus, when aggregated, can be even greater than the effect on the cow herself. 5. Most cows are potentially heat stressed for at least a quarter of the year. Almost every cow endures heat stress (defined as days with the temperature humidity index exceeding 68) for three months of the year - and not just in Southern states like Florida and Arizona. There is a significant potential for dry period heat stress in places like Indiana, Pennsylvania, Michigan and Wisconsin. In fact, unless your dairy is in Alaska, your cows are subject to heat stress. And given that most cows are dry during the hottest months of the year, that equates to a lot of unrecognized heat stress. FIVE THINGS YOU MIGHT NOT HAVE KNOWN ABOUT THE EFFECTS OF HEAT STRESS ON DAIRY COWS
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