Heat stress – occurs every year

Heat stress – occurs every year

Cattle farmers cannot really complain of this year’s weather so far, as 2021 had the coldest April of the century but then May was also colder than the average. This was favourable with a view to heat stress, but caused a 2- to 3-week delay in the growth of feed plants harvested in the spring, postponing their harvest time. With the exception of a few days when the maximum temperature rose above 30 °C, there were no hot days at all; of this, this situation may markedly change by the time when this article is published.

Our weather is essentially shifting towards the extremities, with the summers becoming longer and hotter. The number of hours of sunshine is also increasing. These changes more and more aggravate the ‘heat stress’ occurring in such periods, together with its negative economic impact manifesting itself in lower production, reproductive disturbances and the damage done by other animal diseases occurring as a result of immunosuppression.

It is not easy to define heat stress, but we can say that heat stress occurs when the ambient temperature exceeds the range that can still be compensated for by the animal’s body. If the temperature exceeds the thermal comfort zone of cattle, this will clearly have negative consequences as the heat load exceeds the amount of heat that the animal can release.


External temperature zones according to the production and comfort of cows (Bak and Pazsiczki, 2004)

In the case of cattle, the thermoneutral zone (including the optimal temperature zone) covers a rather wide range between –15 °C and +25 °C. Owing to the special characteristics of rumen fermentation, in the thermoneutral zone cows can use the heat generated during fermentation for heating their own body, while they can easily get rid of the produced excess heat.

The degree of heat stress can be expressed numerically with the help of the Temperature-Humidity Index (THI): this index, calculated from the temperature and the relative humidity, can be used for determining the cows’ feeling of comfort and for expressing their heat stress status. According to a theory commonly accepted earlier, as long as the THI does not markedly exceed a value of 70, we cannot speak of heat stress. The degree of heat load is low at THI values between 70 and 80. If the THI is between 80 and 100, we can speak of a moderate or strong heat stress, respectively, which is evident also from the animals’ reaction. THI values over 100 rarely occur; such values can be tolerated by the animals only for a short time and they may even result in death.


Evolution of the THI as a function of relative humidity and the ambient temperature (Takács, 2003)

However, research results from recent years indicate that the production of animals decreases already at THI values of 68 or higher. Under barn conditions this means that at 22 °C air temperature and a relative humidity of 50%, negative consequences such as reduced dry matter intake, lower milk production, and impaired reproductive parameters may occur.

Under heat stress conditions the behaviour of animals changes, their physiological processes and production parameters are modified, and the severity of heat stress is indicated by reduced activity of the animal. It can be observed that the animals spend more time stranding and do not use the lying areas or cubicles. Cows prefer to stay in shady areas or in places where there is greater air movement. Changes in behaviour are related to the animals’ effort to increase their body surface as much as possible, thus getting rid of excess heat, and to avoid places where they could be exposed to direct sunlight. The animal’s specific reaction to heat stress is elevation of the body temperature. The normal body temperature of cows ranges from 38.5 to 39.0 °C during their normal daily activities. In the case of heat stress, this increases to values between 38.5 and 39.0 °C, depending on the heat load.

The respiratory rate increases and sometimes the animals are breathing with their mouth open. When the dry heat loss is no longer sufficient, excessive perspiration will start, and increased respiratory rate will be accompanied by salivation. The feed intake also decreases, at a rate ranging from 10–20% to as much as 30–50% of the usual daily feed consumption. In cows in early lactation and in high-producing animals the clinical signs of heat stress develop more rapidly and are more pronounced than in cows in late lactation. The reduction in feed intake is followed by a drop in milk production. At a temperature of 35 °C this milk production drop exceeds 30%, while at 40 °C temperature it may reach a rate of 50%. If the drinking water supply is restricted, an even bigger milk production drop may occur. As the temperature becomes hotter, cows will use the drinking water for cooling their own bodies rather than for milk production. The cows become excited, restless, the urine volume and the frequency of micturition increase, thus facilitating the rapid removal of electrolytes from the body. The milk component values are also impaired, the milk fat and milk protein percentages decrease.

If they persist for a longer time, the adverse processes described above have a negative effect on the animals’ body condition and an intensive loss of body mass commences. The prevalence of milk fever increases and the time needed for postpartum uterine involution is prolonged. Under the less favourable hygienic conditions and because of the weakened immune system of cows the prevalence of mastitis and metritis increases. In the summer period, the farm average of milk somatic cell count is usually elevated as well. Fertility is impaired and the occurrence of early embryonic mortality increases. Reduced appetite and lower dry matter intake, coupled with a more sluggish rumination, will increase the chance for the occurrence of acidosis. The general energy deficiency status becomes more and more severe, which facilitates an increased incidence of ketosis.

In order to ensure the normal functioning of physiological processes the animal has to lose heat when the ambient temperature is higher than normal and if the relative humidity in the barn is higher than the value comfortable for the animal.

There are several possibilities for diminishing the adverse effects of heat stress: the primary focus should be on the modification of environmental effects, changing the feeding technology and reformulation of the ration.

The following procedures are suitable for providing protection against the harmful environmental effects:

  • When building new barns, the impact of the climate change must be considered and, accordingly, barns with large air space, open ridges and fold-up curtain side walls must be constructed.
  • Shading of farm structures used by the cows (feed alleys, pens, alleyways, pre- and post-milking holding areas, etc.)
  • Direct cooling of the animals: cow bathing, cow showering.
  • The use of fans and humidifiers in the pre-milking holding areas and in the appropriate parts of the milking parlours.
  • Indirect cooling: facilitating the heat loss of animals by the operation of fans.
  • If evaporative cooling is used, cooling of the ambient air of the barn by fine-droplet mist generation.
  • Particular attention should be paid to the elements of feed bund management:
    • Distribute fresh TMR several times a day; avoid the premature feed-out of fermented feeds from the depots.
    • When using once-a-day feeding, a stabiliser containing propionic acid may be added to the TMR in order to avoid the heating of the total mixed ration.
    • Cover but at least shade the feed bunks on the outside.
    • Move the feeding times to the cooler parts of the day.
    • Clean the feed bunks daily and remove the remnants of feed at all times.
    • Utilise the entire feed bunk space, but distribute more feed to the parts of the barn which are more intensively used by the animals.
    • Avoid overcrowding and provide sufficient resting and feeding place for the animals.


In connection with what has been said about the THI, the procedures of cow bathing, cow showering and humidification should preferably be used only in combination with fans. According to recent research results, the most efficient cooling procedure is when we cool the cows. We wet the animals with the help of spray nozzles. Subsequently, spraying is cyclically suspended, when the previously sprayed water starts to evaporate. The water sprayed out cools the cow and removes heat from her body surface during its evaporation. This evaporation must always be supported by the operation of fans, and the generated vapour must be removed from the air-space of the barn before the subsequent spraying. The elements of the above system should preferably be operated by automatic control, based on sensors mounted in the barns, in harmony with one another.



When setting up the feeding technology for the summer period, it is important to use TMR (Total Mixed Ration) based nutrition in all cases, as in this way the sorting of specific components by the animals can be avoided. It is preferable to time feed distribution to the dawn-early morning or late-afternoon-evening hours of the day. Particular attention should be paid to the condition of the feed bunks and feeding alleys and it is important to remove any feed leftovers. When dividing the year’s feed stocks, it is advisable to reserve the best batches for the summer months, with special attention to the volatile fatty acid content of fermented feeds and the phenological state of the plants at the time of harvest. The right phenological state at harvest is important for ensuring a high nutrient density and good digestibility of the ration. In the summer period the animals’ drinking water requirement may increase by 20–50%, and thus it is important to provide additional drinkers, primarily types that have a large water space and high capacity. By positioning a certain proportion of the drinkers in the post-milking waiting area or along the alleyways, we can reduce overcrowding within the barn in the post-milking period.

The summer heat stress period also requires the modification of certain parameters in the ration. However, it must be mentioned that these changes can be successful only if the appetite and the dry matter intake is stabilised. The previously calculated dry matter quantities must be reduced, while the nutrient density of the ration must be increased. In practice this means that the concentrate to forage ratio must be lowered and the quantity of certain rumen-protected components must be increased. As digestion generates heat, the it is advisable to lower the fibre levels, in the case of acid-detergent fibre (ADF) to 17–18%, in the case of neutral detergent fibre (NDF) to 17–18%, while in the case of the NDF of forage to 21–22% on dry matter basis. In addition to high-quality forages it may be beneficial to feed concentrated fibre sources of outstanding digestibility and NDF content (beet pulp, soybean hulls, cottonseed). It is not recommended to feed a high quantity of grains, as cows are predisposed to acidosis in that period. To support the energy supply, it is more favourable to raise the fat concentration up to 5–7%, as well as to feed gluconeogenetic ingredients. In early lactation it is advisable to adjust the crude protein to a level close to 17% and to increase the ratio of rumen-protected protein up to 40% of the total feed protein level.

Replacing the macroelements lost with the large volume of urine and via perspiration and increasing their intake levels are of outstanding importance: the target intake levels are 1.4–1.9% for potassium and 0.4–0.6% for sodium, whereas the level of magnesium should be increased together with potassium, to a level of 0.3–0.4%. At the same time, the chloride intake should be reduced.

Among the feed additives suitable for mitigating the adverse effects of summer heat stress, live yeasts, the use of increased amounts of buffers, preferably those of mixed composition, and the beneficial effects of using bypass niacin should be mentioned. The beneficial effects of live yeasts have been demonstrated by numerous studies: they stabilise rumen function, work as a biological buffer and improves the digestibility of forages. Its use increases the quantity of milk produced and the levels of milk solids.

Bonafarm-Bábolna Feed Ltd. has developed numerous feed additives against heat stress with the purpose of mitigating the harmful effects of hot summer weather in dairy farms. The mineral, vitamin and live yeast supplement designated Termo-Min has been developed specifically for ambient temperatures markedly exceeding the thermal comfort zone of cattle. They contain elevated levels of buffer substances and macroelements playing an important role in water balance (Na, K, Mg, Cl), which ensure a correct cation-anion balance. Under heat stress conditions, the saliva production of cows decreases, and the concentrate ratio may also increase due to the more concentrated feed. This is why it is important to use buffer substances in amounts higher than usual. The additive also contains live yeast, which also helps stabilise the rumen pH, improves fibre digestion and protein utilisation, and strengthens the rumen microflora. Its bypass niacin content improves peripheral blood circulation and increases perspiration (thus supporting the heat loss). At times of heat stress, the body produces so-called heat shock proteins, which protect cells and especially their proteins from the harmful effects of heat stress. Rumen-protected niacin increases the production of heat shock proteins, which may protect cows against heat also on cellular level.


Our Company’s Termo-Fit additive combines phytogenic substances from the plant kingdom with the beneficial effects of live yeast. The phytogenic substances contained in the plant extracts work synergistically with one another. The ingredients of the additive stimulate rumen function and saliva production. Tannin and saponin present in the additive slow down protein digestion, thus increasing the quantity of bypass protein. Besides reducing the feeding costs, this has beneficial effects during the hot summer period, as protein degradation in the rumen is associated with substantial heat generation. This means that by increasing the bypass protein ratio the animals’ heat load can be decreased considerably. Due to its further selected phytogenic active substances the additive exerts a cooling effect through special stimulation of the taste buds (induction of a sensation of cold) and reduces sensitivity to high ambient temperature. The product also contains natural antioxidants of plant origin, which provide protection from oxidative stress occurring as a result of higher body temperature. A further beneficial effect of the phytogenic active substances is that, acting as aromas and flavours, they increase the cows’ dry matter intake, which also has critical importance in combatting heat stress.

Since last year, we have been dealing intensively with the use of aromas in bovine nutrition from multiple aspects, in order to improve the fattening performance of beef cattle as well as to increase the dry matter intake of calves and dairy cows. Among the various available options, we have gained very favourable experience with combining vanilla, fenugreek and different fruits, primarily citrus fruits. Jointly with Agroprodukt Co. Ltd., last year we started the large-scale testing of a feed supplement containing an aroma combination and a sweetener. Continuing that large-scale trial, from March to late July this year we will study the feed intake and milk production of 300 experimental and 300 control cows.

Due to its numerous negative and adverse effects discussed above, summer heat stress has become one of the factors causing the biggest economic loss in dairy farms. The losses caused by heat stress are aggravated further by management deficiencies, the outdated, obsolete and low barn buildings and the deep bedding technology. Any and all measures and technological modifications capable of mitigating the damage done by heat stress are of outstanding importance.

Dávid Éliás
Senior Cattle Consultant
Bonafarm-Bábolna Feed Ltd.

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