Heat stress is bad news for dairy cows. We often hear about abnormal heat waves that roll through regions leaving behind millions of dollars of damage, but what about the losses incurred during a typical summer? Across the U.S., the economic losses attributed to heat stress are approximately $100 per cow per year. With summer knocking at our door, have you considered how hot weather impacts the cows on your dairy?
Interesting facts about heat stress in dairy cows:
- Temperature-humidity Index (THI). THI is commonly used to gauge the severity of heat stress dairy cows experience under specific environmental conditions (ambient temperature and relative humidity). THI ≥ 72 is categorized as heat stress; however, recent research has indicated that the heat stress threshold should be lowered to a THI of 68. THI = ambient temperature – [0.55 – (0.55 * relative humidity/100)] * (ambient temperature – 58.8) Ambient temperature is recorded in Fahrenheit and relative humidity is recorded as a percentage.
- Core Body Temperature. As THI increases and, subsequently, core body temperatures increase, cows spend more time standing rather than lying down. Cows with rectal body temperatures ≥ 102.2 °F (measured in the afternoon) are at risk for decreases in milk production and fertility.
- Water Intake. Cows will consume 35 – 50% more water during heat stress conditions. Ensure cows have access to an adequate supply of clean, fresh water.
- Feed Intake. Heat stress causes decreases in feed intake during the daytime, decreases in feed efficiency, and decreases in nutrient absorption. Nighttime “slug feeding” may lead to higher incidences of acidosis.
- Milk Composition. Cow exposure to environmental heat stress has been linked to higher milk somatic cell counts and bacterial counts, with lower milk fat and protein percentages.
- Cooling Strategies. Natural ventilation, fans, sprinklers/misters, shade access (esp. open lot dairies), and cooling pads all offer heat stress abatement for dairy cows; however, each dairy needs to assess which strategy will work best for each facility. Adjustments to feeding schedules, ration formulations, and stocking densities may also assist with heat stress abatement.
- Milk Production. Depending on the severity of heat stress conditions, cows will decrease their milk production by 10 – 50%.
- Reproduction. Conception rates during heat stress can plummet by almost 20%. For example, a dairy with a typical conception rate of 31% fell to a conception rate of only 12% during hot weather.
- Health. Higher incidences of mastitis, respiratory problems, retained placentas, and higher respiration rates have been diagnosed in cows under heat stress conditions.
- Gestation. Calves exposed to heat stress during the last 45 days of gestation have lower birth weights, lower weaning weights, and depressed immune systems.
Source: June 2015 WSU Dairy Newsletter
