Heat stress is a concern for dairy producers across the U.S. In fact, the Journal of Dairy Science estimates heat stress costs U.S. dairy producers over $1 billion each year.
ADM Animal Nutrition dairy experts Don Jaquette, dairy nutritionist; and Dwain Bunting, ruminant technical manager; share their insights on the causes and effects of heat stress.
Q. What dairy productive parameters are most often impacted by heat stress?
A. Jaquette: Feed intake and subsequent milk production are the measurable parameters most often affected by heat stress. Milkfat usually decreases when cows are subject to heat stress. Reproductive efficiency is negatively impacted as well.
Q. What physiological systems are most impacted by heat stress in the lactating dairy cow?
A. Bunting: The cow has a notoriously limited lung capacity. During heat stress, the cow pants for cooling, which causes imbalances in blood acid-base balance. In addition, the cow’s immune system is often suppressed by both long- and short-term heat stress.
Q. How does heat stress lead to rumen acidosis?
A. Jaquette: There are several key factors:
- Consumption of grain may be increased deliberately or accidentally. Sometimes there are attempts to increase the energy density of the ration by adding more grain. In addition, sorting and preferential consumption of grain at the feedbunk may be increased by the heat-stressed cow.
- During heat stress, the cow’s acid-base balance in the blood is disturbed, which reduces production of bicarbonate and secretion of this buffer into saliva.
- Water consumption may be inadequate, which reduces dilution of acid in the rumen.
All of these factors contribute to a decline in rumen pH, leading to rumen acidosis.
Q. How does vasodilation (blood vessel expansion) relate to heat stress?
A. Bunting: Vasodilation of blood vessels is a natural response of mammals to dissipate excess heat. It is under tight physiological control but quickly reaches a maximum capacity to dissipate heat as ambient temperature rises.
Q. How is gut integrity impacted by heat stress?
A. Jaquette: This is still an active area of research. One theory is: Heat stress may reduce blood flow to the gut, thereby limiting cellular activity at the gut. This may somewhat reduce nutrient absorption and may reduce the gut’s ability to maintain barrier functions, such as mucus secretion and intracellular “tight junctions.”
These barrier functions are part of the cow’s defense system and are designed to prevent pathogens and other gut toxins from reaching the blood.
Q. Is it possible to prevent some of the compromises in gut integrity associated with heat stress, and if so, how?
A. Bunting: This is also still an active area of research, but there is evidence that certain feed additives (live organisms, yeasts, etc.) may stimulate gut immunity and help strengthen impaired gut barrier functions.
Q. How does heat stress impact a lactating cow’s immune system?
A. Jaquette: Reduction in immunity is multi-factorial and results from the effects of higher stress hormones, impaired gut immune function and reduced glucose supply to the immune system.
Q. How does heat stress contribute to energetic deficiencies?
A. Bunting: The cow’s metabolic “drive to eat” always declines faster than the cow’s hormonal “drive to produce milk” as heat stress unfolds. This results in an energy lag (negative energy balance) that consistently reduces the cow’s body energy reserves: fat.
Q. Does the reduction in dry matter intake (DMI) associated with heat stress account for all of the decrease in milk production?
A. Jaquette: This is often described as the “push versus pull” theory of milk production. That is: Does DMI pull milk production, or does the hormonal drive to produce milk push DMI? Most scientists argue that reduced hormonal drive to produce milk is the primary cause of reduced DMI.
DMI and milk production are correlated but not necessarily cause and effect during heat stress. Recent research suggests that only about 50 percent of the drop in milk that occurs during heat stress is attributed to the decrease in DMI.
Q. What is THI, and what is its relevance to heat stress and dairy cows?
A. Bunting: The THI (or temperature-humidity index) is a simple measurement that relates the compounding effects of humidity on increased ambient temperature. Because the cow can vaporize less moisture (natural or artificial) when humidity is high, the cow’s ability to dissipate heat is reduced.
Thus, at a given temperature, THI values increase as humidity is increased.
Q. At what temperature or THI does milk yield start to become negatively impacted?
A. Jaquette: Around a THI of 67.
Q. What are the primary management strategies for improving milk production during heat stress?
A. Bunting: Any strategy will be beneficial if it increases cow comfort (e.g., reduced crowding or shade), increases air flow, increases water accessibility or increases the potential for effective water vaporization from the cow.
Feeding strategies that maintain feed freshness (frequent feeding) or increase feed availability during cooler hours will be beneficial. Cows should be encouraged to rest when the ambient temperature is the warmest.
Q. Are there feed additives that have potential for helping the lower gut counteract the effects of heat stress?
A. Bunting: This is still an area of active research. Several bioactive materials based on yeast and other organisms seem to have effects primarily in the lower gut. It should be noted that in ruminants, it is never completely clear where exactly these types of feed additives are having their primary effects during heat stress.
In addition, some of the better-researched feed additives in the market for heat stress are blends of several materials, including yeasts, clays, botanical compounds, etc. As such, it is difficult to ascertain which specific ingredients may have the greatest impact on the health of the lower gut.
Q. What type of ration modifications will help improve production and components during heat stress?
A. Jaquette: Using the highest-quality forage available and optimizing the usage of buffers (both sodium and potassium) as electrolytes will help improve production during hot weather. It is also recommended that you feed starch-containing feeds carefully.
Fat can be used to increase energy density of the ration; however, it should be used with caution with rumen-active fats. Rumen-inert fats are usually a safer option. A strong focus should be put on ration structure, such as moisture content and particle size, to minimize the potential for sorting at the bunk. 
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Lynn Jaynes
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