Significant milk production losses across Michigan during July heat

Record high temperatures in early July resulted in reduced milk production from heat stressed cows.

The Fourth of July brought more than just fireworks to Michigan – it brought record heat as well.  According to weather stations across the state several high temperature records were set in early July. For example, in Lansing, Mich. a record high temperature of 102 degrees Fahrenheit from July 24, 1934, was broken on July 6, 2012 with a high temperature of 103F. In addition to the sweltering heat, many dairy producers in mid-Michigan were not able to run the cooling fans for several days due to power outages from a storm that swept through Michigan on July Fourth resulting in heat stressed cows.

In the south central part of the state, temperatures were even hotter for a longer period of time. According to the Coldwater, Mich. weather station, temperatures peaked over 100F four days in a row between July 4 and July 7 with a high temperature on July 6, 2012 of 103.8F. During this period of time, nighttime lows were in the mid-70’s, not giving cows a chance to recover from the heat.

Lactating dairy cows prefer ambient temperatures of 41-77F, which is considered the thermo-neutral zone (TNZ). When temperatures rise above the TNZ, cows must use energy to dissipate heat from their body. Without heat abatement, temperatures above 77F will result in the cows body temperature rising, reduced milk production and a change in milk composition including a reduction in milk fat (Coppock, 1978). Producers in Michigan have reported production losses in the range of 0-26 lbs. of milk/cow/day during and for a few days following the early July heat, demonstrating the variability in production practices and cow responses across the state.

Humidity can affect the level of heat dissipation in cattle. Temperature-Humidity Index (THI) of 70 or less are considered comfortable for lactating cows, 75-78 are considered stressful, and above 78 is considered extreme distress with cows being unable to maintain normal body temperatures (McDowell et al., 1976). Individual farm response to heat stress will vary based on what cow cooling methods are implemented and milk production level. In 1982, a study by Coppock et al., concluded that high-producing cows are affected more by heat stress than low-producing cows because the thermo-neutral zone shifts to lower temperatures as milk production and feed intake increase.

Thankfully for the cows in Michigan, temperatures have become more normal in the mid to high 80’s with low humidity and many producers are reporting a rebound in milk production as of July 12, 2012. However the periods of high heat are likely to continue throughout the summer so it is not too late to consider management strategies to minimize the impact of high heat and humidity on milk production and cow health.

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