Dairy cows eating
Updated: May 7, 2021
By Sarah Potts

There is no doubt that dairy cattle become heat stressed at point during the summer months in the Mid-Atlantic region.  The temperature humidity index (THI) is a tool used to assess the risk for heat stress in livestock.  The THI threshold for high producing dairy cows is 68; when values are greater than 68, lactating cows are at risk for heat stress.  With relative humidity levels ranging between 50 and 80% and average monthly highs of 87°F, 85°F, and 78°F during the months of July, August, and September, respectively, most cows in Maryland easily experience several days where the THI is above 68 during these months.

Signs of heat stress in dairy cattle include panting, excessive drooling/foaming from the mouth, and open-mouth breathing.  Heat stress also induces behavioral changes such as increased standing time, bunching, and reduced feed intake, which have direct effects on production. 

Temperature Humidity Index Chart for Cattle

Short-term Effects of Heat Stress

The most obvious effect of heat stress is reduced milk production.  Heat stress is also detrimental to fertility because it reduces display of estrus (heat) behavior and negatively affects oocyte quality and embryo viability.  In Maryland, if minimal heat stress abatement strategies for lactating cows were implemented, the predicted result would be a reduction of annual milk production by 950 lb per cow and an increase in the average number of days open by 18 days.  

Most producers will attest that bulk tank somatic cell count and the number of mastitis cases usually increase during the summer months.  Not only are pathogens more prevalent during the summer months due to conditions more conducive to their growth and proliferation, but high temperatures also depress the immune system by reducing immune cell function, making cows more susceptible to disease.  Lameness also often increases during the summer months, which is likely related to increased standing behavior under periods of heat stress.  Because feed intake is reduced and rumination activity is depressed when body temperature rises, risk for sub-acute ruminal acidosis also increases, which further exacerbates immune dysfunctions brought about by heat stress.  

Often overlooked when it comes to providing heat stress abatement, replacement heifers and calves are also affected by heat stress.  There is evidence suggesting that heat stress negatively affects the growth and immune status of replacement dairy heifers. Furthermore, a recent study from the University of Florida showed that cooling calves under heat stress conditions from birth to weaning reduced total number health events requiring treatment and increased milk and starter grain intake. 

Long-term Effects of Heat Stress

In addition to the more obvious, short-term effects, heat stress also has long-term effects that may not be fully realized until months (or even years!) later.  While the historical focus of heat stress has been on its effects on lactating cows, recent attention has been given to its effects on dry cows.  Heat stress negatively affects mammary cell proliferation and development during the dry period and prolonged heat stress during this time can reduce milk production during the subsequent lactation by as much as 8 to 11 lb/d.

There is also mounting evidence that heat stress during late gestation transcends the dry period and can have effects on the growth and milk production of the calf in utero and her offspring (2 generations).  A recent study analyzed production records obtained from 10 years of heat stress research in Florida and showed that calves whose dams were cooled under heat stress conditions during late pregnancy produced an average of 8 lb/d more milk during their first three lactations than calves whose dams were not cooled.  Effects on production were also observed in the second generation, where calves whose grand dams were cooled during the dry period produced 2.8 lb/d more milk during their first lactation.  Thus, the long-term effects of heat stress during the dry period on milk production may not be fully evident for 3.5 to 4 years! 

Combating Heat Stress

As highlighted above, management of heat stress is important for current and future animal productivity, health, and reproduction.  When looking at your heat stress mitigation strategies, there are a few key areas you should focus on.

  1. Ensure ample access to fresh, clean water.  Water is required for all animals to maintain body temperature and, under normal conditions, a high producing cow will drink up to 50 gallons of water per day.  Hot weather will increase water consumption by 50 to 100%.  To accommodate times of increased water demand, there should be at least 3 inches of accessible water-trough space per cow and flow-rate should be sufficient so that troughs do not run dry during periods of high demand.  If possible, provide waterer access near the milking parlor exit.

  2. Provide shade.  This is most basic component of heat abatement and should be provided for all animals during high temperatures.  This is often in the form of a barn or shed for confined animals or in the form of natural shade or shade cloth for animals on pasture.  Don’t forget about your dry cows, heifers, and calves when it comes to this basic component of heat stress abatement.  Calves in hutches can also benefit from supplemental shade by installing a shade cloth or other covered structure over the hutch area.

  3. Ensure adequate ventilation.  Poor ventilation is often an issue inside barns or other manmade structures.  These facilities should be opened up as much as possible to promote natural airflow by raising side curtains, opening/removing glass windows, etc.  Fans should also be installed in key areas, such as the feed bunk, over the free-stalls or bedded pack, and holding pen to promote airflow.  Additional ventilation can be achieved with calf hutches by propping up the backside of the hutch using 4x4 blocks of wood.

  4. Consider cooling with water only after there is shade and adequate ventilation.  To be effective, this heat stress abatement strategy must be paired with sufficient airflow or fans to promote evaporative cooling.  Simply soaking animals without adequate airflow will only succeed in creating a more humid environment around them.  Sprinklers/misters can be strategically placed at the feed bunk and the holding pen for optimal cooling.

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