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Ask the Vet, July, 2023

By Dayna Locitzer, DVM

At what temperature is heat stress an issue? And how can I deal with it on my farm? Have you heard of the term thermoneutral zone? I think a lot about thermoneutral zones, mainly in the summer when the temperature outside gets above my zone and I start to complain. I consider myself to have the same thermoneutral zone as a cow, about 20-70° depending on humidity and wind.

In this temperature range, the animal does not have to expend any extra energy or resources to maintain health. This means in the winter time, when that wind-chill is predicted to be -5°, you should consider making sure your cows have more food. This means that cows produce less milk on a 95° July day because they are focused on doing things to keep themselves cool rather than on eating. This means a cow is truly content when it is a breezy spring day and she is happily lying down chewing her cud with her nose to the sun and her eyes closed. Those are my favorite days as well. We are coming upon the season when sometimes more than half the day will be above a cow’s thermoneutral zone, so it is important to prepare for that.

Let’s dive in a little deeper on how the upper limit of the thermoneutral zone is determined. Both humidity and temperature are factors. Using them in combination, you can determine the Temperature Humidity Index (THI). Cows begin to feel heat stress when the temperature is 70? with a relative humidity of 65% - this is a THI of 68. When the humidity is 10% the THI drops down to 64. When humidity is 100% and the temperature is still 70° the THI spikes to 70. A cow’s upper limit is determined to be 68 because when THI goes above that, her milk production begins to decrease.

This decrease in milk production is a sign of heat stress, but it is more the consequence of all these other factors. For example, you will see increased respiratory rate (>60 breaths per minute), bunching, cows standing on their feet for longer periods, and decreased feed intake - all of these lead to a decrease in milk production and decreased fertility. On a day when the THI is 90, a cow will produce upwards of 20 lbs. less milk than on a day when the THI is 65. For a herd of 30 cows, that’s about a $200 loss per day.

I’ll go a little more in depth on why there are such significant effects of heat stress. When the temperature is outside of a cow’s thermoneutral zone, she needs extra energy to meet her needs; but on hot days, she will consume even less dry matter than she would on a normal day. Blood circulates to the periphery of their body to help dispel the heat, this means that that same blood flow is not going to the udder or to the uterus. Cows stand more to increase their body’s surface area that is exposed to air, decreasing their lying time. All of these characteristics contribute to immediate decline in milk production and fertility. Then there are also the long-term decreased milk production and fertility effects, like lameness and metabolic stress. Prolonged standing on these hot days contributes to long-term lameness issues. Heat stress also causes systemic inflammation due to changes in body chemistry from altered circulation and panting, causing poor overall metabolic health.

Milk production is a very tangible and visible consequence of heat stress. It is very obvious how heat stress directly affects cows that are in milk. The more insidious consequence of heat stress is its effect on dry cows. A recent University of Florida study showed that fetuses that experienced heat stress during their final 45 days of gestation matured into adult cows that produced less milk and were more likely to get culled than cows that hadn’t experienced the late gestation heat stress. Additionally, the daughters of those cows that experienced heat stress during late gestation also produced less milk. This shows that there are short-term and long-term health and economic consequences of heat stress on both lactating and dry cows.

With all this in mind, it is important to think about strategies to reduce heat stress. When there are more and more of these hot days coupled with the pasture requirement of organic, this can be a challenge. Shade and easy access to water are important to have on these sorts of days. You can designate pasture specifically for days where cows are at risk of heat stress. This can be a sacrifice pasture where you feed hay. These tend to be sacrifice pastures because cows are going to bunch under the shady areas and not necessarily eat what's in the pasture in full sun, causing overgrazing and trampling in specific areas. You can also have pastures in your rotation that you save for heat stress days using the practice of silvopasture. These silvopasture paddocks are cultivated areas of forest that provide forage and shade. Adding these sorts of paddocks to the rotation allows for grazing and mitigating the consequences of heat stress.

If there is not a pasture that fits this bill, then the cows should probably stay in the barn or barn yard so that those amenities can be provided to them. When cows are in the barn with fans directed towards them, easy access to food and available water, they will be able to eat and rest more comfortably than if they were exposed to the heat. You might think you are sacrificing milk production by keeping them in the barn, but as I explained above, heat stress is the real danger to milk production. If you keep them in the barn during the day, you can then put the cows on pasture at night when the temperature is cooler and the sun is no longer menacing.

When you are thinking about heat abatement strategies in the barn, whether it’s for keeping them in from pasture or just having them there for milking, it is important to be strategic. If you have a large free stall, make sure you use shade cloth on sides with sun exposure. If you don’t already have sprinklers in your barn, think about a place where one sprinkler would have a large impact. This could be in the holding area before they enter the parlor. To give this sprinkler even more impact, give them fan access after the sprinkler is off. This provides an evaporative cooling effect to further dissipate heat than just the water alone would do. Strategic fan use is very important. Using them in conjunction with sprinklers is helpful, but it is also important that they are directed appropriately and are of the proper strength. Don’t direct the fans in areas where people walk. Directing your fans where the cows are lying down and eating will go a long way. They will also help with fly control, a nuisance that will cause cows to behave similarly as if they were heat stressed, compounding the issue.

Cows are large animals that have a giant fermentation vat, also known as a rumen, in their abdomen. This generates an enormous amount of heat, so much so that their body temperatures are on average 3 degrees higher than that of humans. While humans are certainly affected by hot and humid temperatures, cows are even more at risk. The consequences of heat stress cause strain on the entire body system of a cow, leading to significant production losses that are felt when looking at your bottom line. Heat stress is stressful, so add a couple of these strategies to your farm protocols this year to help yourself and your cows.