By this time of year, keeping cows comfortable in the heat challenges most dairy farmers. Dr. Pete Hansen, professor of animal science at the University of Florida, says heat stress is a major problem for dairy cows and can impact milk yield and fertility.
“Cows that produce more milk are more sensitive to the effects of heat stress,” said Hansen. “As we continue to improve milk yield per cow genetically, nutritionally or through other management strategies, we’re also making cows that are more susceptible to heat stress. The cow of tomorrow will be more sensitive to heat stress than the cow of today.”
Hansen compares the dairy cow to a furnace. “Just like a furnace, she takes fuel sources (feed) and burns that fuel to produce heat,” he explained. “The dairy cow uses that heat to power all the actions in body to remain alive and produce milk. If cows produce more milk, they need to burn more fuel and more energy to power the synthesis of milk, and as a result, the heat production of the cow increases.”
To clarify cow cooling, Hansen explains four modes of heat exchange: radiation, conduction, convection and evaporation. Radiation is the transmittal of heat by infrared radiation; mostly from the sun. Cows also acquire heat from hot concrete and hot roofs.
Conduction is the transfer of heat from one substance at a high temperature to another at a lower temperature. “Typical types of conductive heat exchange are heat loss to air heat that isn’t moving around the cow, heat exchange between the cow and the ground, and heat exchange with surface water,” said Hansen. “When a cow is wet and the air surrounding the cow is replaced by water, the cow loses heat 22 times faster than if surrounded by air.”
Heat loss through convection involves two substances are moving past each other, increasing the rate of heat loss. For cows, convection is facilitated by wind. In contrast, evaporative heat doesn’t depend on air temperature. The rate of heat loss depends on the humidity of the air – the more humid the air, the lower the rate of heat loss via evaporation.
Cows can effectively lose heat by sweating and respiration. When cows take air into their lungs, the air is humidified. The heat required to humidify the air comes from the body temperature of the cow. For a cow in severe heat conditions (94 degrees F and higher), conduction and convection ability is greatly reduced. If humidity is also high, the cow can’t lose heat through sweating or panting. Cows in the sun gain heat through solar radiation. The result is that the cow can’t lose body heat resulting from milk production, and her body temperature rises.
Heat stress is usually measured by a temperature/humidity index, but Hansen says such charts don’t provide sufficient information. “What’s more important is whether cows can keep their body temperature low enough to prevent effects on milk yield or conception rate,” said Hansen. “The critical number is a body temperature of 102.2 degrees F. Milk yield and conception rates decline in cows with rectal temperatures above this.” Hansen noted that high-producing cows can become hyperthermic when barn or outdoor temperatures range from 75 to 80 degrees F.
The best way to cool the hyperthermic cow is a combination of shade and evaporative cooling via sprinklers or foggers. Evaporative cooling increases evaporative heat loss as water evaporates from the cow.
“A sprinkler or soaker system is designed to wet the cow,” said Hansen. “Put enough water on the back of the cow to penetrate into the hair and allow heat loss by conduction, convection and evaporative heat loss. In contrast, a fogger system is not designed to wet the surface of the cow — it increases the evaporation of water into the air surrounding the cow.” Hansen added that fogger systems are like primitive air conditioning where evaporation cools the air, and are ineffective when humidity is high.
Not all shade is equal — the key is providing sufficient shade to effectively block solar radiation. Allow about 50 square feet per cow to prevent animals from crowding. Crowded cows cannot effectively lose heat to the air because they’re in contact with each other.
Fans used along with water help increase the rate of heat loss by convection and prevent the build up of humidity that occurs when animals are sprinkled or fogged. The cow might still be hyperthermic due to a very high air temperature, but to a lesser degree.
Tunnel ventilation to facilitate evaporative cooling can lose effectiveness depending on where cows are located. The main consideration is airflow and whether cows or equipment are blocking that flow. Cross-ventilated barns offer the advantages of tunnel ventilation and reduce the problem of airflow. Fans are located on the side of the barn and push air across the barn rather than through it. The downside of cross-ventilation is higher cost.
Although lactating dairy cows are the most sensitive to the effects of heat stress, other animals including dry cows, calves and growing heifers are affected by heat stress.
It’s important to keep dry cows cool because the mammary system is in the final stages of preparing for lactation. Hansen’s research on reproductive performance showed that heat stress during this period has a long-term effect on the upcoming lactation. He says pre-fresh cows housed in barns with sprinklers and fans had significantly higher milk yields.
University of Wisconsin research on lactating cows showed that as air temperature increases, body temperature increases. “We think of infertility occurring at body temperature of 102.2 degrees F,” said Hansen. “These cows were getting that hot in an air temperature less than 75 degrees F.”
Heifers are more resistant to heat stress, but heat abatement is still important because elevated body temperature can lead to fertility issues. Young calves in hutches or transition groups also require cooling. In studies, body temperatures of calves in hutches in summer measured as high as 106 degrees, which influences long-term growth.
Hansen encourages dairy farmers to monitor cows’ body temperatures. He uses iButton®, a thermal-sensitive computer chip that records body temperatures at intervals throughout the day. “The iButton® can be used over and over again, and you can download the data to a laptop,” said Hansen. “Put it in an empty CIDR, pop it in the vagina and measure body temperature over a 24-hour period to see when cows are becoming heat stressed.”
Information from the chip will help identify times of day or physical locations when cows are not sufficiently cooled. Hansen also suggests tracking differences in lactation and reproduction performance between summer/winter to determine the effectiveness of cooling systems.