Tools to help reduce the effects of heat stress in dairy cows

Published on Thu, 05/17/2018 - 9:42am

 Tools to help reduce the effects of heat stress in dairy cows

 By Bryan Miller, Ruminant Technical Support Manager, Biomin America

Heat stress is one of the most common management problems that dairy producers must overcome.  Heat stress may be a temporary problem or in some locations may last for many months. Heat stress results in decreased feed intake and reduced milk production.  Additionally it has negative effects on reproduction.

Cattle prefer temperatures of 25 to 65oF (-4 to 18 oC).  Heat stress develops as a result of both actual air temperature and humidity levels.  Facilities, bedding, airflow and crowding will also affect cow comfort and subsequent milk production.
Cows have very limited ability to sweat and their main process of dissipating heat is through heavier breathing or panting.
There certainly are facility changes that can be done to improve conditions.  Shade, airflow (both natural and fans) and applied water, either misting system or cooling ponds (cows must get wet to the skin level and not just the hair) can certainly help.  However facility changes are usually long term ventures and dairy managers are often looking for immediate help or relief.  Which leads to, “What dietary changes can I do to reduce the impact of heat stress?”
In order to make dietary plans based on heat stress, you first need to be aware of how heat affects cows and how they attempt to deal with the problem.
The most critical thing producers can do is to make sure that cows have easy access to clean fresh water.  Water supplies should be in shaded areas that are easily accessed by all cows.  As heat stress increases, cows will spend more time attempting to drink and water access that may have been sufficient in cooler times may not be adequate during heat stress.
Typically during heat stress there is a decrease in dry matter intake (DMI). Many of the feed changes recommended are directed at limiting the negative effects on feed intake.
Dairy producers should re-evaluate the diet’s protein content and components.  Cows having reduced intake may require a greater percent crude protein in order to be sure adequate protein is provided.  However, excess protein requires additional energy for the cow to remove the extra nitrogen and may generate more heat in the process.  Producers should also consider using less rumen degradable and more bypass protein, however, it becomes even more important to be sure adequate limiting amino acid needs are met, such as for lysine and methionine.

As cows pant to blow off heat they also increase the amount of CO2 they exhale.  This in turn affects (decreases) the blood bicarbonate concentration and can eventually result in lower pH.  This is of particular concern if cows are also receiving anionic salts used to prevent milk fever as they also reduce blood pH.  Dietary cation-anion difference, or DCAD, is usually set at negative value in close up cows to prevent milk fever but heat stress calls for a positive (+) balance.
In addition as cows are spending more of their time panting they are also spending less time ruminating.  This reduction in rumination time leads to a decrease in saliva production that provides much of the buffering that is needed within the rumen.  Rumen pH drops resulting in poorer fiber digestion along with reduced general feed intake.  If possible increase the ratio of NDF to ADF in the diet.
If given a choice, cows will reduce their intake of forages and consume more concentrates.  The bacterial fermentation that breaks down fiber also generates heat.  Increased grain, with extra consideration for needed buffering, can improve feed intake.  
Yeast and Aspergillus oryzae product may help in the digestion of fiber, which is normally depressed during heat stress.  These products may help to break down parts of the fiber that can then allow the total fiber to be more completely digested by rumen bacteria.
Rumen bypass fats are often fed during heat stress as they provide energy but do not add to the heat increment.  Added bypass fat may increase energy corrected milk but may not have much impact on total DMI, however the diet will be greater in energy content.
In addition to ruminal changes, there are changes in the digestive tract as well.  Recent work has demonstrated that during heat stress the lower digestive tract develops “leaky gut”.  In this condition, the juncture between intestinal epithelial cells become weakened and there can be passage of material from the digestive tract into the body as well as leakage of material from the body of the cow into the lumen of the digestive tract.  This porous digestive tract is less efficient at absorbing nutrients and has increased energy requirements due to inflammation as well.  This more open gut can allow for the greater absorption of lipopolysaccharides (LPS) which are known to both decrease feed intake and cause inflammation.  This inflammation not only takes energy away from potential milk production, but actually adds the heat produced by the cow herself.  It has been well documented that the decrease in milk production that had been traditionally attributed to the decrease in feed intake is not only the result of decreased energy from the diet, but also increased use of glucose by the cow for purposes other than the production of milk.  Additionally, heat stress directly increases inflammatory conditions within the digestive tract.
Work in other species has demonstrated that drugs such nonsteroidal anti-inflammatory drugs (NSAID) can reduce the inflammation caused by heat stress.  Unfortunately, such drugs are not generally accepted for legal use in dairy cattle.
However, there are new products that may offer some relief to help with the intestinal inflammation.  Phytogenic products, biologically active compounds distilled from plants, may help modulate intestinal inflammation.  The anti-inflammatory and antioxidant activities of these compounds is well documented in other species.  Recent work shows promise to reduce inflammation in the gut of cows as well become a potential tool in reducing the effects of heat stress.
Heat stress affects most cattle at some time during the year.  The severity and length depends upon region of the country and the site specific conditions as they relate to temperature, humidity and airflow.  Regardless of the facilities, nutritional changes can be made and certain feed additives may be used to help modulate and reduce the effects of heat stress.