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This summer, will you cool cows or cool the barn? According to Dr. Jim Spain, milk production in hot environments is more closely correlated with thermal balance of the cows than the temperature of the environment. What really matters is how hot the cow is, not how hot the barn is. On most farms though, fans are set on a thermostat so they turn off in the evening or early during the night as the barn temperature drops. This can be a problem for a couple of reasons. First, the body temperature of heat stressed cows typically peaks sometime at night, so turning fans off in the evening will mean she may have her highest daily body temperature when the fans are off. Even non-stressed cows typically will have higher body temperatures in the evening than in the morning. In addition, on dairies that milk three times a day, cows' body temperature may increase as they come into the holding pen and milking parlor during the night milking. Second, convective (fan) cooling is much more effective when there is a large temperature gradient between the air temperature and the cow's body temperature. On a hot afternoon the air temperature in the barn may be close to the body temperature of the cow, making convective cooling very inefficient. If hot enough, convective cooling can become convective cooking, as one sees in a convection oven. At night, when the air temperature is lower, convective cooling works much better to cool cows.
Have you ever noticed early on a cool morning, following one or more very hot days that your cows are still breathing rapidly? Or have you ever noticed that on some mornings nearly every cow you check has an elevated rectal temperature? These observations mean your cows are still hot, even though the air temperature may have cooled. Thus, we should design cooling systems to cool the cows until they are no longer heat stressed instead of just cooling until the air temperature drops. There are some challenges in accomplishing this though. Ideally every cow would have those cool rumen boluses that transmit temperature and rumination rate. Then the thermostats could be tied to actual cow temperature. For now though, most farmers will have to use lower-tech solutions. This might mean simply setting the thermostats at a very low temperature on evenings following hot days so that the fans will not shut off. If might mean installing some sort of timer on the thermostats where you can set a shutoff delay so the fans continue to run for a designated time after the barn temperature drops below the shut off temperature. It might mean having someone objectively evaluate cows for heat stress sometime during the night, by evaluating respiratory rates, for example. There are probably many ways to accomplish this, but the bottom line is: If cows are still hot, the fans need to stay on.
A simple test is to check the respiratory rates of at least ten cows in the early morning. Cows with body temperatures below 102.5 will typically breathe 60 times per minute or less. When body temperature approaches 104 degrees, respiratory rates will be near 100 breaths per minute. Count breaths for 20 seconds and multiply times three. If rates are significantly greater than 60, your cows are probably still hot, and you need to adjust fan usage accordingly.
What about dry cows? Do we need to cool them too? It sure looks like we do. A number of studies have shown increases in milk production of three to nine pounds per day during the subsequent lactation when dry cows were cooled. Interestingly, cooled cows seem to eat more during the prefresh period, but this increased consumption does not result in increased body condition; rather, the extra calories go toward increasing calf birth weight. A very important finding is that mammary glands of cooled dry cows have a higher rate of cell proliferation during the dry period. Cooled cows thus calve with more milk producing mammary cells and this likely explains the difference in milk production.
In addition, at least one study has shown that cooled dry cows produce more colostrum than non-cooled cows. In our practice we often hear complaints about cows producing too little colostrum to feed a calf, and these complaints most often come in the late summer or fall.
Can your calves suffer heat stress? Yes, they can. Studies have demonstrated that heat stressed baby calves do not grow as well as when they are cooled. Remember that growth rates before weaning are a very important factor in determining just how much milk the animal will produce once she is in the dairy herd, so reduced growth rates cost you money. You do not get to raise your calves twice, so mistakes count. Even hutch calves can get hot. It is a good idea to lift up a back corner of the hutch to allow more ventilation in the summer. If you have those white, translucent, plastic hutches, your calves are at more risk for heat stress than if they are in wood or fiberglass, opaque hutches. A study done in the fall in Utah found the translucent hutches to be fourteen degrees hotter during the day than opaque hutches. These were not plastic domes, which are likely even hotter. Translucent hutches need to be shaded in the summer. There may be a way to paint the inside with a heat-blocking paint, but I have no personal experience with this. Calves in calf barns get hot too; fans can often be used effectively for cooling.
Cooling cows can reduce or prevent reduced milk production, impaired reproductive performance, lameness, and even mortality. In order to be effective, though, you need to cool animals, not just air.
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