Heat stress is created when livestock cannot expel enough heat to maintain their core body temperature for optimal production and health. The temperature-humidity index (THI) is a number that combines humidity and temperature to correlate with the amount of heat stress an animal experiences.

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Larry J. Judge

Research from the University of Arizona suggests cows experience a milk loss starting at a THI of 68ºF.

Costs

A heat stress study found cows suffering from heat stress experience the following:

  • 4,000-pound decrease in milk production per lactation
  • 60-day increase in average days open
  • Nearly 2 percent increase in death loss

For a 1,000-cow dairy, these losses could easily exceed $300,000 per year.

Identification

Professor Jodie Pennington at the University of Arkansas recommends testing the rectal temperatures of 10 cows. If seven cows have a temperature of 103ºF or above, your herd is experiencing heat stress. Watch your cows for the following indicators of heat stress:

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  • Respiratory rate greater than 100 breaths per minute

  • Excessive salivation

  • Open-mouth breathing

  • Decrease in dry matter intake (A 10 percent decrease indicates the animal is under high stress; a 25 percent decrease indicates severe stress.)

Prevention

  • Keep cows comfortable and cool.

  • Supply adequate, fresh, clean water at all times.

  • Provide shade, fans, misters (in low-humidity areas) and coolers.

  • Make sure fans are provided near sprinklers for proper evaporative cooling.

  • Deliver feed more frequently and push up feed more often.

  • Feed during cooler times of the day.

Nutritional recommendations

Heat stress has a significant effect on rumen pH. Consider the following to prevent rumen acidosis:

  • Offer high-energy, palatable diets with high-quality forages.

  • Supplement free-choice bicarbonate.

  • Limit crude protein to 16 percent.

  • Keep ration-degradable protein at less than 61 percent.

  • Provide probiotics like yeast and beneficial bacteria which are shown to improve feed efficiency during heat stress.

Treatment

Take the following steps to treat animals suffering from heat stress:

  • Provide a constant supply of cool water for animals to drink to aid in the recovery process.

  • Add oral electrolytes to the water to ensure adequate hydration.  end mark

PHOTO: Cows at the feedbunk. Staff photo.

Dr. Larry Judge