Help dairy cattle cope as temperatures heat up
By Craig J. Louder, DVM
We are familiar with the billboards that show pets outside in winter conditions with the caption, “If you’re cold, they’re cold. Bring them in!” Perhaps during summer months, we should think about our dairy cows with the caption, “If you’re hot, they’re hot!”
The temperature humidity index (THI) [see page 30] combines the air temperature with the humidity to determine a “real feel.” Cattle begin feeling the effects of heat stress at levels as low as 72 on the THI. This would equate to temperatures that are as low as 77°F with humidity of 50 percent or 81°F with 25 percent humidity.
Cattle living in all parts of the United States can be exposed to the effects of heat stress. The more time that animals experience being in the heat stress zone, the more pronounced are negative effects, possibly even leading to death. In the US dairy industry alone, heat stress causes more than $900 million in annual losses.
Rumen considerations
Since cattle are ruminants, the rumen — the very physiology which allows cattle the ability to generate heat and stay warm during cold temperatures — is what increases the risk of heat stress during hot temperatures.
Microbial fermentation creates several byproducts, including heat. For example, bacteria-fermenting sugars are the source of heat produced in a silage pit or compost pile. This is the same process that takes place inside the rumen. As cattle consume feed, the fermentation inside the body generates heat, increasing the heat stress felt by the environmental temperature. Dry matter intakes of cattle are one of the most visible effects of heat stress seen on dairies. However, there is much more happening than what may meet the eye.
Heat stress effects
Cattle have a limited ability to sweat and, combined with their dense hair coat, this ability challenges body heat dissemination. Increased respiratory rate or panting is one method that cattle use to mitigate the effects of high temperature. But panting leads to challenges of its own.
The increased respiratory rate causes increased oxidative stress within the animal. While cellular metabolism requires oxygen, excess oxygen can cause problems due to its reactive nature as an oxidizer. A common example of oxidative stress is when a cut apple is left exposed to the air and the apple turns brown over time.
This same oxidation can occur inside body cells causing damage to the cell membranes, protein, and even the DNA. When the body’s antioxidant system can’t keep up with the excess oxygen from panting, oxidative stress occurs. Oxidative stress then leads to a pro-inflammatory response by the animal, causing additional issues within the body.
Heat stress can also affect a cow’s hormones. Increased heat decreases prolactin, oxytocin, estrogen, and thyroid hormones. The reduction in these hormones all influence milk production, as well as increase the risk of mastitis due to poor milk letdown and milk harvesting. In addition, there is an increase in the corticoid steroids and progesterone, influencers of the immune and reproductive systems. Cortisol is known to suppress the immune system, so animals are more susceptible to mastitis and other infectious diseases.
Endocrine changes associated with heat stress can negatively affect the reproductive performance of cows. Consider that as little as one hour per day of a THI over 73 can negatively affect conception rates. Or a reduction in placental hormones negatively impacts fetal growth, resulting in lighter birth weight calves.
Heat abatement focuses
Heat abatement should focus on maintaining the cow’s core temperature to under 103°F. Establishing shades in open lot corrals and assuring airflow for indoor barns is critical for cow comfort. The holding pens for parlors become higher risk areas for heat stress because of animal density and the added heat. Fans and water misters are two common methods of alleviating those elevated temperatures. Assuring cows are in lockups for a minimal amount of time allows animals to return to shaded areas and rest.
Calf considerations
Calves are also susceptible to heat stress, and those raised outdoors may experience increased stress due to a greenhouse effect of their hutch. University of Florida and Texas A&M studies show the material of calf hutches can impact inside temperature of the hutch by as much as 7°F. Hutch bedding can impact how much heat the calves feel, as well: straw has more insulative properties versus sand or shavings.1, 2
Changing the direction of the hutch to north-facing during the summer months gives the calves exposure to less direct sunlight and increases their afternoon shade.
Additionally, caution should be exercised to assure adequate drainage around the hutch. Increased moisture coupled with increased heat can greatly increase fly populations for calves, which can lead to more health issues.
Water is critical
Water is critical in managing heat stress in both cows and calves, and clean, fresh water should always be available. Lactating dairy cows will consume between 20–40 gallons of water per day during the summer depending on stage of lactation and size of animal. As a rule of thumb, every 5-degree increase in daily temperature requires an additional one gallon of water consumption.
Water intake depends on the body’s salt levels. While sweating is not a major mechanism of cooling cows, there are enough electrolytes lost through sweat to impact a cow’s salt balance. During summer months, decreased feed intakes also decrease salt intake through the ration. Providing access to free-choice minerals can allow cows the ability to consume more salt, driving water intake.
Lactating dairy cows will consume between 20 - 40 gallons of water per day during the summer depending on stage of lactation and size of animal.
Trace minerals are beneficial
During periods of heat stress, salt is not the only mineral that is important. Trace minerals also play an integral part of the antioxidant system of cattle. A recent study conducted on dairies in Texas and New Mexico showed the benefit of injectable trace mineral supplementation in maintaining cow performance through periods of heat stress.3
Coupled with decreased feed intakes, limited trace mineral intake can lead to lower levels of antioxidants in the body to combat the negative effects of oxidative stress. Implementing the utilization of injectable trace minerals helps ensure that cattle have trace minerals to mitigate heat stress situations.
The bottom line: Prepare for heat before the heat
Preparation for heat stress occurs while the temperatures are still chilly outside. Make sure that fans and misters are in proper working order. Manage stocking density to assure animals spend minimal time in lockups or in holding pens. And realize, that by the time we are suffering from heat stress, cows have been experiencing it for quite some time.
Livestock temperature humidity index (THI)
The THI combines the air temperature with the humidity to determine a “real feel.” Cattle begin feeling heat stress effects at levels as low as 72 on the THI. This equates to temperatures as low as 77°F with 50 percent humidity, for example.
(Noble Research Institute, noble.org)
