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The JULY Issue

July's Monthly Mastitis Minute is all about REDUCING HEAT STRESS IN THE DAIRY HERD. This issue is a quick guide to monitor production, udder health, and cow management challenges during the summer heat.

Health Risks, Production Loss and Reproductive Stress

Heat stress can have a major impact on the health, production and reproductive success of your herd. Based on a milk price of $13/cwt, researchers calculated annual losses of $897 million for the dairy industry in the United States even if producers used fans, sprinklers, high-pressure evaporative cooling, and other heat abatement strategies to alleviate summer stress. This loss is almost $100 per dairy cow per year. If no heat abatement systems are used, the total annual loss would be $1.5 billion, or about $167 per dairy cow per year.

Most dairy cows start to experience heat stress at temperatures over 70 degrees or a heat index (a combination of heat and humidity) of about 68 degrees. This heat stress can cause a loss in appetite, which can lead to a drop in milk production by 10 to 20 percent. Milk fat and protein yield also can decrease.
Heat stress also suppresses the immune system, leading to increased susceptibility and severity of common diseases, such as mastitis, retained placenta and metritis, along with higher probability for metabolic diseases like ketosis and displaced abomasum.

When cows are hot, they will attempt cooling maneuvers by continually moving around or standing, thus impacting the needed resting time for health sustainability, well-being and overall productivity.
Studies have shown a 10-20% increase in milk production for cows offered shade in pasture verses those without access to shade. For high-producing, lactating cows this is essential, but don’t forget the heifers and dry cows out on pasture need shade too!

The table below shows the estimated annual production and economic losses for dairy cows and duration of heat stress periods using minimal heat abatement strategies in Wisconsin (St-Pierre et al. 2003).

Heat can also have long-term effects on reproductive suppression and lowered fertilization rates. In fact, reproduction suffers more from heat stress than milk production.

According to Sartori et al. (2002), inseminated heat-stressed cows experienced a 55 percent fertilization rate, 33 percent lower than inseminated cows that were not heat stressed.

BEAT THE HEAT: Summer Heat Abatement Strategies

Shade: Shading from direct sunlight is very important. Although shade from trees is the most natural environment, cows will often compact the area around the trees. Pay attention to these areas to prolong the life to the trees and to avoid mudholes. Mudholds can result in greater mastitis as animals will often lie in the mud after milking and before the teat canals close following milking. Portable or temporary shades can be used and rotated so that cows use shade in different pastures while the muddy ones dry.

Water: Water is the primary nutrient needed to make milk. Cows drink up to 50 percent more water when the heat index is above 80 percent. A good rule of thumb is there must be at least 3 inches per cow in the pen of space along the water trough; this will decrease competition and ensure that all animals have access to clean water.

Be extra-diligent about monitoring waterers to make sure they are clean.

Providing water access immediately after milking will also help keep them cool throughout the day as cows consume most of their daily water intake right after being milked.

Ventilation: One of the hottest places on the dairy farm is the holding pen, due to the high density of the cows. Cows need a minimum of 36 to 48 square feet to prevent heat transfer between cows. When moving cows up to the holding pen, bring up smaller groups instead of a whole pen.

Fans will help remove radiant heat. Fans should be spaced across the barn to create good airflow in all areas. Increasing air-exchange in freestall resting areas, over feed bunks, and in the holding area and parlor is important. Open sidewalls and end walls, and provide a shady resting area for cows that are outdoors.

Sprinklers: Sprinklers over the feed alley or exit lanes out of the milking parlor (combined with fans) provide the best heat removal in most commercial barns. The most effective sprinkler systems soak the cows to the skin. The cows will be cooled as the water evaporates. Such cooling devices need to be used with care when mastitis or somatic cell count problems occur. The object is to wet the cow's back without having water run onto the udder.

Soakers over the beds should be avoided as it causes increased moisture, which can contribute to environmental mastitis.

Remember, air velocity is needed for quick evaporation. If there is crowding in areas, adding more water to the environment will only increase the humidity and cause even more stress. Some barns may require additional circulation fans on still days to maintain enough air velocity to evaporate water.

Alternative Breeding Techniques: Reproduction is always a top concern for dairy farmers but especially becomes a concern in times of heat stress. The large decrease in reproduction due to heat stress has motivated much research into ways to increase pregnancy rates in the summer (Hansen and Aréchiga, 1999; Jordan, 2003).

Two popular management strategies used to offset heat stress on reproduction are embryo transfer and delayed insemination (seasonal herds). Embryo transfer can significantly improve pregnancy rates during the summer months by bypassing the period in which the embryo is more susceptible to heat stress, while herds that are more seasonal focus optimal calving and milk production during the cooler season.

For more information on improving fertility in the summer, check out: Economics of Heat Stress: Implications for Management.

The Takeaway: Remember, the effects dairy cows feel are much greater than that of what we feel. Don't use your own comfort levels to indicate the need to help cool the herd in the summer heat.



Hansen, P.J., and C.F. Aréchiga. 1999. Strategies for managing reproduction in the heat-stressed dairy cow. J. Animal Sci. 77:36-50.

Jordan, E.R. 2003. Effects of heat stress on reproduction. J. Dairy Sci. 86:(E. Suppl.):E104-114.

Sartori R, Sartor-Bergfelt R, Mertens SA, Guenther JN, Parrish JJ, Wiltbank MC. Fertilization and early embryonic development in heifers and lactating cows in summer and lactating and dry cows in winter. J Dairy Sci 2002; 85:2803-2812.  

St-Pierre, N.R., B. Cobanov, and G. Schnitkey. 2003. Economic losses from heat stress by U.S. livestock industries. J. Dairy Sci. 86:(E. Suppl.):E52-77.


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