Heat Stress Mitigation is Important for Dry Cows

Heat Stress Mitigation is Important for Dry Cows

As the summer heats up, heat stress alleviation is often one of the top things on every dairy producer’s mind. Historically, heat stress mitigation strategies have been heavily researched and promoted for lactating cows. Indeed, the economic consequence of heat stress in lactating cows can be substantial because of lost production and fertility. However, recent attention has also been given to the studying the impact of heat stress in dry cows. While the dry cow isn’t actively working to contribute to the milk check per se, she is working to prepare herself to do so in the near future, so it is important not to forget about her until she’s about to calve in again.

Implications of Heat Stress in Dry Cows

University of Florida researchers estimate that heat stress during the dry period costs the United States dairy industry over $800 million each year. Heat stress occurs when the cow's body is unable to dissipate excess heat through natural physiological or behavioral mechanisms, such as panting, sweating, seeking shade, and reducing feed intake. These behavioral changes, combined with an excessive heat load, alter the cow’s physiology and metabolism. The dry period is a time marked by hormonal and metabolic changes to support rapid fetal growth and mammary involution (i.e., remodeling) to support subsequent milk production. Adding heat stress into the mix can have lasting negative impacts on both the cow and the calf.

“calves born to cows exposed to heat stress during the dry period…produced 5 pounds less milk per day during both their first and second lactations.”

Researchers at the University of Florida have extensively researched the physiological effects of heat stress in dairy cows, and have focused specifically on heat stress in dry cows. In a recent study, they found that cows who experienced heat stress during the dry period produced significantly (9 pounds) less milk per day during the following lactation than cows who were provided with cooling measures, which included shade, fans, and sprinklers. This response was similar regardless of whether the cows experienced heat stress during the early or late dry period. These researchers also observed that heat stress reduced the length of gestation, or, in other words, caused cows to calve earlier than expected.

More recently, University of Florida researchers have also examined the long-term effects that heat stress during the dry period has on the unborn calf. They showed that calves born to cows who were exposed to heat stress during the dry period have lower birth weights, slower growth rates, and lower weaning weights. They also showed that these calves have reduced immune function, which was likely related to their reduced ability to absorb the antibodies in colostrum. A separate analysis of the performance of calves born to cows exposed to heat stress during the dry period showed that these calves produced 5 pounds less milk per day during both their first and second lactations. Furthermore, these calves were more likely to be culled from the herd sooner than their counterparts and had a shorter productive life by 5 months. Altogether, the annual economic impact of reduced performance and health of heifers born to cows exposed to heat stress during the dry period was estimated to be $4 million in Maryland, $8 million in Virginia, and $33 million in Pennsylvania.

Management Strategies

Dairy producers often utilize a variety of heat stress mitigation approaches for lactating cows. However, implementing the same strategies for dry cows can be complex because many producers house dry cows on pasture or in older facilities for practical reasons, which may not provide ideal ventilation or access to electricity. Despite this, there are still measures that can be taken to reduce heat stress in dry cows in these more limited situations.

1. 

   Provide Water. At a minimum, dry cows should be provided with plenty of clean, fresh water. During periods of heat stress, Holstein dry cows will consume between 15-20 gallons of water per day, so it is important that troughs are large enough or refilled often enough to accommodate this demand.
2. Provide Shade. Dry cows should also be provided with access to shade, at least during the heat of the day. Studies show that cows will naturally seek shade and if it’s not readily available, they will spend their time trying to find it rather than resting or eating. They may also display other behaviors, such as bunching, which make heat stress worse. They may also congregate and lay in any muddy or moist area available, which includes areas where there is manure build-up. Utilizing trees as a natural source of shade can be effective, as long as the cows don’t create a muddy mess. Permanent structures, such as barns, lean-tos, or sheds can also be useful for providing shade, but these need to be maintained and monitored for excessive manure build-up. Portable shade structures may also be an option. At the University dairy, we utilize a shade cloth mounted on a running gear for pregnant heifers managed in a rotational grazing system, which allows them to have access to shade in areas of the pasture where there is no permanent or natural shade available (Figure 1).
3. Ventilation. If dry cows are housed in barns or sheds, it is important that these facilities have adequate ventilation. This still applies even if the cows only use these facilities for shade during the day. If they spend several hours per day in these locations, ventilation is important. Opening up sides or windows of older barns can provide much needed air flow and help reduce the temperature in the facility. If electricity is available, fans can also help improve ventilation and increase the air speed at the cow-level to improve convective cooling.
4. Evaporative Cooling. This strategy requires access to electricity and water, so it is often difficult to implement unless dry cows are housed in a barn. In order for this strategy to work, there must be enough air movement, usually provided by fans, to facilitate the evaporative process. There also needs to enough water to thoroughly wet the cows. Simply misting the cows will only create a more humid environment, which may make heat stress worse.

In conclusion, heat stress can have substantial, long-lasting impacts on a dry cow and her unborn calf that negatively impact future productivity and profitability. Adopting management strategies to alleviate heat stress during the dry period is essential to mitigate the economic implications of heat stress on the dairy.

References:

• Ahmed, B.M.S., U. Younas, T.O. Asar, A.P.A. Monteiro, M.J. Hayen, S. Tao, and G.E. Dahl. 2021. Maternal Heat Stress Reduces Body and Organ Growth in Calves: Relationship to Immune Status. JDS Communications. 2:295–299. https://doi.org/10.3168/jdsc.2021-0098.
• Fabris, T.F., J. Laporta, A.L. Skibiel, F.N. Corra, B.D. Senn, S.E. Wohlgemuth, and G.E. Dahl. 2019. Effect of Heat Stress during Early, Late, and Entire Dry Period on Dairy Cattle. J. Dairy Sci. 102: 5647-5656. https://doi.org/10.3168/jds.2018-15721.
• Ferreira. F.C., R.S. Gennari, G.E. Dahl, and A. DeVries. 2016. Economic Feasibility of Cooling Dry Cows across the United States. J. Dairy Sci. 99: 9931–9941. https://doi.org/10.3168/jds.2016-11566.
• Laporta, J. F.C. Ferreira, G.E. Dahl, and V. Ouellet. 2020. Late-Gestation Heat Stress Impairs Daughter and Granddaughter Lifetime Performance. J. Dairy Sci. 103:7555–7568. https://doi.org/10.3168/jds.2020-18154.

This article appears on July 28, 2023, in Volume 4, Issue 2 of the Maryland Milk Moos newsletter.