As the summer heat blazes a trail across the U.S. this year, humans and animals are suffering alike. Steve Nickerson, PhD, University of Georgia, says the stresses associated with hot and humid environments have huge adverse effects on a dairy cow’s metabolism by greatly increasing her body maintenance requirements.

“It must be kept in mind that dairy cattle are of northern European origin, and are generally intolerant of high environmental temperature above 77°F, especially when the relative humidity is greater than 80%,” Nickerson says. “Moreover, older, heavier, high-producing cows are more susceptible than smaller and younger animals.”

Nickerson says we manage our dairy herds to counter heat stress in the summer season, and because we expect more production from our cows, controlling heat stress becomes extremely important.

The negative effects of heat stress on dairy cattle include:

  • Depressed feed intake
  • Decreased milk yield, fat, and protein
  • Increased weight loss
  • Elevated SCC
  • Increased clinical mastitis
  • Elevated rectal temperatures
  • Increased respiration rates
  • Impaired reproduction

“But, under conditions of excessive heat and humidity, we need to help her keep more comfortable,” he says. “Strategies to control heat stress are aimed at maintaining feed consumption, preventing milk production losses, and minimizing mastitis and other disorders such as acidosis and dystocia.”

Methods to minimize stress include:

  • Providing adequate shade to reduce solar radiation
  • Cooling cows with sprinklers and fans, commercial coolers, tunnel ventilation, or cooling ponds
  • Providing fresh, cool (50°F), drinking water (cows drink 50% more water at temperatures of 80°F and above compared to 40°F)
  • Feeding a high energy ration with high quality forages, and doing so in the early morning or late afternoon to encourage consumption
  • Increasing dietary potassium, sodium, and magnesium, and providing free-choice salt
  • Supplementing the diet with vitamin A, ß-carotene, vitamin E, and selenium
  • Feeding a fat supplement to increase the energy density of the diet
  • Avoiding the handling of cows (ex; pregnancy checks) during the hottest part of the day

Helping the cow to cool herself is probably most important, and shade is probably the easiest and least expensive way to do this, Nickerson notes. “However,  it is imperative to keep shade structures away from feed bunks because cows tend to defecate and urinate where they eat, and if there is shade over the bunk, they will lay down in feces and urine, which is a prime environment for mastitis-causing bacteria, which grow even better when it’s hot and humid.”

Under these conditions, factors such as rain, mud, manure, and bedding become important as they influence the numbers and types of microorganisms present on udders and teats. “Environmental mastitis pathogens such as E. coli and Strep. uberis love to grow where it is warm and moist, so it is imperative to keep bedding materials and calving areas as clean and dry as possible.”

In addition to decreasing milk yield and quality, and increasing mastitis and SCC, heat stress impairs animal reproduction. Factors include:

  • Poor estrus expression
  • Increased embryo mortality
  • Retained placenta
  • Compromised uterine environment
  • Early calving
  • Increased dystocia
  • Reduced calf weight
  • Increased calf mortality

Read Part II from Nickerson on using cow cooling methods effectively here.