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OSU Extension

College of Food, Agricultural, and Environmental Sciences

May 28, 2019 - 8:21am --

For most of us, the term heat stress conjures up images of discomfort.  For the dairy cow, however, heat stress goes beyond discomfort and produces both direct and indirect negative production and health effects.  One very visible and direct impact of heat stress is reduced milk production. Correlated with this, is reduced dry matter intake.  Another direct effect is reduced or impaired fertility in which cows may fail to conceive or, in more severe heat stress conditions, experience early embryonic death of the fetus.  Indirectly, heat stress may cause increased incidence of lameness.  The reasoning is that heat stressed cows stand for longer periods as a way to increase airflow around their bodies to dissipate heat.  Heat stressed cows get less rest, which can depress their immune system and higher somatic cell counts are associated with heat stressed cows.

It is crucial for the dairy producer/manager to recognize that cattle experience heat stress long before a person experiences heat stress.  When measuring heat stress in dairy cattle, a temperature humidity index (THI) provides some indication of the potential severity of heat stress.  For dairy cows, heat stress can begin at a temperature/humidity index somewhere around 68.  A temperature of 72 F with humidity around 44% provides that index.  Although the THI is one measure of potential heat stress, an important concept is that the severity of heat stress is a result of the accumulated heat load in the animal over time.  The animal’s body temperature is a key indicator.  The normal body temperature of a cow is in the 101 to 101.5 F range.   Heat stress begins when body temperature reaches the 102 to 102.5 degree range.  How severe those effects are depends upon how long the cow maintains elevated body temperature.  For example, if body temperature becomes elevated during the day but is able to cool back down during the night hours, this heat stress will not produce as severe effects as a cow that maintains an elevated body temperature for more hours without recovery.

Heat stress and heat abatement discussions most frequently center on lactating cows, but do not ignore heat stress in dry cows.  Research demonstrates that heat stress in dry cows has economic consequences in terms of lost production and increased health costs.  Cows that are heat stressed during the dry period produce less milk in the subsequent lactation compared to non-heat stressed cows.  They may also have a depressed immune system, leading to greater risk of infections and sickness. Studies have shown that dry cows under heat stress as compared to cows not under heat stress calve 2 to 8 days earlier, resulting in a lighter birthweight calf.  These calves typically continue to grow slower and have lighter weights through the first 12 months of age, compared to calves born to non-heat stressed dams.  Heifers born to heat stressed dams also produce less milk in their first lactation compared to peers born to non-heat stressed dams. Therefore, dairy farms should have a plan to implement heat abatement strategies to minimize heat stress in both lactating and dry dairy cows.

Good ventilation is a pre-requisite to any heat abatement program.  There must be adequate air exchanges.  Then, add in circulating fans to create air movement.  We want both good volume of air movement, determined by size of fan and good air speed.  Fans move air effectively at maximum in feet of 10x the fan diameter.  Therefore, place 40-inch fans no more than 40 feet apart.  Provide an air speed of four to six mph, as measured at the height of the cow lying down.  Fans need to be angled somewhere in that 15-20 degree range to provide air movement over the backs of cows lying down.  Do not add water to the system unless air movement and speed are adequate.   Added water not removed by air exchange will create an even worse heat stress condition.  Most water systems use either misters to provide indirect evaporative cooling or sprinklers that wet the skin of the cow and provide direct evaporative cooling.

Don’t overlook shade and drinking water as components of heat abatement.  For example, bedding in freestalls exposed to direct sun can get hot and the result is that those stalls don’t get used.  It creates a temporary increase in stocking density because those stalls are not used.  Providing shading of those stalls can put them back into use.  Cow’s consumption of clean, cool water can increase up to 50% under conditions of heat stress.   The recommendation is to make sure that cows do not have to walk more than about 50 feet to water and not through areas of sunlight.  Provide at least two linear inches of water trough space per lactating cow and up to 3 inches per dry cow.

In closing, I will put in a plug for the 2019 Dairy Twilight Tour hosted by Harmony Farms in Kidron on the evening of July 9.  One of the features of this farm is a good heat abatement program.


Rory Lewandowski is an OSU Extension Agriculture & Natural Resources Educator and may be reached at 330-264-8722.

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