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Heat Stress? Sharpen Focus On Summer Feeding

October 29, 2020

Heat stress in dairy cattle causes increased water intake, rapid respiration, elevated body temperature, altered hormone profiles and nutrition partitioning, reduced dry matter intake, decreased milk production, depressed milk fat and protein, impaired reproduction (decreased conception and increased early embryonic death), increased incidence of metabolic disease, increased metritis, mastitis and milk somatic cell counts, ruminal acidosis, lameness and death. In other words, it’s bad, and it’s often devastating effects extend beyond the period of heat stress itself.

Heat stress occurs in dairy cattle when heat produced in the body from rumen fermentation, digestion and metabolism exceeds the ability of the dairy cow to dissipate it and core body temperature begins to rise. When this occurs it is primarily a function of environmental temperature and humidity (measured as Temperature Humidity Index: THI) and level of milk production, and results in a cascade of events to increase the rate of heat dissipation, as well as decrease the amount of heat produced to survive this potentially fatal situation.

As THI increases initially, water intake, blood flow to peripheral tissues and sweating increase to maintain the rate of heat dissipation required. However, as the THI increases above 68, heat stress begins to set in and the cow responds by increasing its respiration rate to increase evaporative heat loss from the lungs. Respiration rate has been shown to be a good indicator of both the degree of heat stress and rise in core body temperature as shown in the table below.

Temperature–Humidity Index 68–71 72–79 80–89 90+
Heat stress level None – Mild Mild – Moderate Moderate – Severe Severe
Respiration rate (breath/min) > 60 > 75 > 85 120 – 140
Rectal temperature (oC (oF)) > 38.5 (101.3) > 39.0 (102.2) > 40.0 (104.0) > 41.0 (105.8)


*Based on R.B. Zimbleman and R.J. Collier, 2011: see Hoards Dairyman, April 25, 2011, page 281 for THI chart.

When the measures to increase heat dissipation are not sufficient to prevent an increase in core body temperature, the body responds to decrease heat production by actively 1) decreasing milk production, through decreased blood flow to the mammary gland and altering hormone profiles and nutrient partitioning, and 2) reducing feed intake, by decreasing nutrient demand for milk production, but also by decreasing rumen motility and blood flow to, and nutrient absorption from, the rumen.


Recent research has shown that the reduction in feed intake associated with heat stress only accounted for 35% of the observed drop in milk production. The remaining milk loss appeared to be due to increased energy use for active dissipation of heat. This suggests that the potential to recover lost milk through diet manipulation during periods of heat stress may be limited and that investment in cooling technology should be the primary defense against heat stress.

Research on commercial dairies in Israel (a very hot place in summer) has shown that aggressive cooling (10 periods of 45 minutes cooling comprised of cycles of 0.5 minutes wetting every 5 minutes with air movement maintained at 10 feet per second (6.8 miles/hr) for 7 cumulative hours per day) essentially eliminated the effects of heat stress as compared with moderate cooling (45 minutes in the holding pen at each of 3 milkings).

Without aggressive cooling, the increase in respiration rate with heat stress decreases the amount of bicarbonate in the blood and saliva and further decreases bicarbonate entry into the rumen as drooling increases. This decrease in rumen buffering, combined with the other changes in rumen function, is associated with reduced ruminal pH and an increased incidence of acidosis during heat stress. In turn, the incidence of laminitis and lameness increases. This is evidenced by a rise in hoof problems and need for treatment seen in the fall. In addition, the negative energy balance resulting from reduced intake and increased energy requirements during heat stress reduces fertility well beyond the period of heat stress itself.

Consequently, it is important to take steps to fine tune TMR rations for periods of heat stress to minimize heat production (improve efficiency) and decrease the risk and consequences of acidosis, and thereby help maintain dry matter intake and milk production, as well as reproduction, during this challenging time.

The TMR formulation tips below are intended to compensate for the physiological and metabolic changes and effects that are imposed on the dairy cow by heat stress. These are generally supported by scientific research and are best viewed as helping animals cope with the effects of heat stress, rather than methods of “over–coming” them, for which aggressive cooling is likely the only effective method. For the most part, the following tips are best viewed as what is required to prevent an already challenging situation from becoming needlessly worse.

Increase ration density: Increasing ration density, within parameters of safe rumen function, will increase the amount of nutrients delivered per unit of dry matter consumed, increase feed efficiency, and decrease heat of fermentation and digestion, thus decreasing the heat produced per unit of metabolizable energy consumed.

Feed highest quality forages: Save your highest quality forage, especially alfalfa, to be fed during periods of heat stress. Heat increment of feeding (fermentation and digestion heat) decreases as forage quality increases. Alfalfa also has one of the highest buffering capacities of all the forages. For the same reason limit the feeding of poorer quality forages and straw.

Replace starch with digestible fiber: As starch levels increase, the risk of acidosis increases. Substituting with high energy fiber sources such as soybean hulls, distillers grains and citrus pulp allows energy levels to be maintained while reducing the rate of VFA production and risk of acidosis.

Lower starch with bypass fat: Fat contains over twice the metabolizable energy as grain allowing starch to be reduced while at the same time increasing the density of the ration. Research has not yet demonstrated a benefit to adding higher levels of by-pass fat to normal diets during heat stress.

Decrease protein solubility and degradability: Rumen ammonia levels and extent of protein degradation both increase during heat stress, likely due to decreased ruminal motility and increased particle retention time in the rumen. Feeding of soluble protein and increasing protein content with bypass protein have both been shown to have negative effects during heat stress.

Add yeast: Adding either live yeast or yeast culture to the TMR ration improves rumen fermentation and reduces risk of acidosis. Adding yeast culture also been shown to lower core body temperature and maintain milk fat content during heat stress in one study.

Increase sodium bicarbonate and add potassium and magnesium: Sodium bicarbonate compensates for reduction in saliva contribution and neutralizes accumulation of VFAs and lactate in rumen to maintain rumen pH. Increased sodium and potassium intake replaces increased rate of loss in sweat and urine. Magnesium is required to balance the increased potassium. These changes should be made with the advice of a professional nutritionist.

Add Zinc Methionine: Dairy cows have been shown to double their amount of time standing during heat stress, apparently to aid body cooling, thus contributing to the increase in feet and leg problems. Zinc methionine helps improve hoof health and decreases incidence of lameness, as well as helps decrease somatic cell counts and risk of mastitis.

Consider adding bypass niacin and/or biotin: Preliminary research has shown that rumen protected niacin decreased body temperature in lactating cows that were heat stressed. Biotin has been shown to improve hoof hardness and decrease the incidence of lameness, as well as improve milk production of dairy cattle on high grain diets.

TMR management tips

The tips below are intended to compensate for the physiological and metabolic changes and effects that are imposed on the dairy cow by heat stress. These are generally supported by scientific research and are best viewed as helping animals cope with the effects of heat stress, rather than methods of “over–coming” them, for which aggressive cooling is likely the only effective method. For the most part, the following tips are best viewed as what is required to prevent an already challenging situation from becoming needlessly worse.

Watch bunker silo management and feed ingredient spoilage in general: The heat of summer increases the rate of spoilage of all ingredients, and especially moist ingredients such as silage and wet byproducts. Spoiled ingredients dramatically increase the rate of heating and spoilage of the whole ration in the feedbunk, which further depresses intake. Silo face management needs to be impeccable during heat stress to ensure as little material as possible is disturbed, and that loose material does not remain in piles on the floor between feedings. In general, wet byproducts should be cycled at twice the rate compared to other times of year, or even omitted if they cannot be kept from spoiling appreciably. Any feed with mold or risk of mycotoxins should be avoided as these can be particularly problematic during times of heat stress.

Batch feed at least twice per day: Feed spoilage is an exponential function with the number of spoilage organisms doubling with each successive generation, often in only a matter of minutes. Consequently, the longer feed stays before dairy cattle, the more it will spoil and negatively affect intake, especially for the second and third milkings if only fed once per day. Furthermore, feeding the full day’s allotment of feed at one time increases the potential for sorting, and increases the risk of acidosis. If feed can only reasonably be batched once per day, then retain a proportion in the mixer to be fed at the subsequent milking. Though less than ideal, the extent of spoilage in the mixer should be less than what would occur in the feedbunk, and the potential for sorting should be reduced as well.

Monitor particle size and uniformity of mix: Many people advocate ensuring the ration contains adequate physically effective fiber to stimulate rumination during heat stress. To what degree this is possible is unclear since rumen motility is reduced during heat stress, and addition of less digestible forages increases heat production. Nevertheless, caution is advised to ensure that any addition of long fiber to the ration does not lead to sorting, which would increase the risk of acidosis and otherwise be counter-productive. Regular use of the Penn State Particle Separator is advised to ensure not only that there is adequate effective fiber, but also that there is a uniform distribution of particle sizes within the ration, and a uniform mix across the feedbunk that is resistant to sorting.

Add water if sorting is a problem … but not too much(?): If sorting appears to be a problem, first consider rotating or replacing the knives in the mixer to achieve a more uniform, as well as perhaps shorter, cut length. If sorting continues to be a problem consider adding water to bind the ration together. Start with 5–10 lbs per cow. Watch the results for 2–3 days. If dry matter intake increases, try another increase of 5–10 lbs per cow. As long as dry matter intake increases, and evidence of sorting decreases, keep increasing the water. One should be cautious about increasing the moisture content of the ration beyond 55%, however, as one study showed that dry matter intake began to decrease, and sorting began to increase at this point, during heat stress.

Feed largest proportion of feed after late afternoon/evening milking: Research has shown that dry matter intake is maximized during heat stress when the majority of the feed is fed in the late afternoon or early evening when air temperatures start to decline. It is believed that the extent of spoilage is decreased as the majority of feed is consumed before the onset of the higher temperatures during the day. Therefore, consider calling the day’s feed, and making feed adjustments just before the afternoon or early evening milking, and start the 24 hour feeding cycle with the late afternoon or early evening feeding. Batch sufficient feed to last through the morning milking (perhaps 60% of daily call), and then top-up the balance at the morning milking, ensuring the cows never go without feed.

Ensure cows do not go without feed at any time: Cows in heat stress eat their daily intake in less than half the number of meals as under normal conditions. This is thought to result in larger meal sizes that predispose the cattle to higher rumen acid loads and lower rumen pH (i.e. acidosis). During heat stress it is therefore imperative that the cows always have sufficient feed in front of them and never go without feed. Doing do risks building up hunger that could result in binge eating that would exacerbate the risk of acidosis.

Push up feed multiple times during the day: The risk of going without feed, and the effects of sorting increase as the time between feed push –ups increases. Ideally feed should be pushed up 2–3 times between each milking, and within 2 hours of feeding. This will increase dry matter intake by ensuring an adequate amount of relatively unsorted ration will be available to all animals within the first 6 hours following milking, when most feed is consumed.

Clean water … and lots of it!: Cows under heat stress can more than double their water intake, mostly to replace moisture lost as sweat, but also to increase water content of the body to absorb heat and subsequently excrete it in the urine. Thus it is essential to have an unlimited supply of clean water available in multiple locations for the cattle. Ideally, separate waterers should be available upon exit from the milking parlor such that all animals can drink their fill at the same time before entering the free-stall area. And, all waterers should be cleaned daily to a level of cleanliness that one would drink from them him/herself.

Feel free to share these tips with friends and fellow producers.