Heat Stress: Effects on Milk Production and Composition

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Abstract

Lactating dairy cows begin to seriously experience the negative effects of heat stress when rectal temperatures exceed 39.2 °C. A respiration rate of >60 per minute is an indication that stress due to heat and humidity is becoming excessive. Pregnant cows provided relief from thermal stress during the nonlactating period will have heavier calves at parturition, show improved colostrum quality, and will produce more milk postpartum. Higher producing cows as well as all cows in their first 50 days of lactation are most sensitive to heat stress. Both intake of feed and milk production were reduced by about 15% as rectal temperatures increased from 38.8 to 39.9 °C. Management of feed and water intake is important in helping cows cope with conditions of thermal stress. Cows exposed to significant heat stress are more susceptible to ruminal acidosis. Ruminal pH may be lowered because of reduced buffering of the rumen and a reduction in the number and intensity of ruminal contractions. Percentages of milk fat and protein are often lowered (∼0.5 and 0.2 percentage units, respectively) during the times of heat stress and during the summer season. There is an increase in somatic cell count, growth hormone, and prolactin and a decrease in potassium content in the milk of cows exposed to greater heat stress.

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Change History: September 2016. Pasquale Ferranti (Section Editor) updated Further Reading.

Update of C.R. Staples and W.W. Thatcher. Heat Stress: Effects on Milk Production and Composition. Encyclopedia of Dairy Sciences, 2nd Edition 2011, Pages 561–566.

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