Nutrition and heat stress of dairy cows

In 1986, Flamenbaum suggested a practical and innovative solution to the negative effects of high temperature on dairy cattle health and performance in his paper “Cooling dairy cattle by a combination of sprinkling and forced ventilation and its implementation in the shelter system” (J. Dairy Sci., 69:3140-3147). A 30 kg milk-producing cow must dissipate the same heat amount as 16 100-watt incandescent lamps. Cows mostly dissipate temperature by increasing respiratory frequency: the dissipation mechanisms can fail with high environmental temperature and temperature humidity index (THI). Different mechanisms to reduce metabolic heat production and to increase dissipation are activated at 22°C with 40% humidity in order to maintain the body temperature constantly around 38.5°C. The first reaction is the dry matter intake reduction, to reduce rumen fermentation and the consequent heat production. Then, the animal moves less to contain the muscles’ heat. At the same time, the water intake increases as well as respiratory frequency. All these mechanisms have as a negative consequence the reduction of productive and reproductive performance.

Heat stress depends on the efficiency of the ventilation and air conditioning system and the increase of the THI. This pathologic status is defined on the basis of the body temperature and the respiratory frequency: a 0.5°C improvement of the body temperature with more than 80 breaths per minute means that the animal is under heat stress and is unable to manage the THI increase. If more than 15% of the animals are under heat stress there is a risk for the entire herd, otherwise, only the single animal needs to be helped.

Studies indicated that low production, feed intake, and fertility lasts, without apparent reason even after the reduction of environmental temperature and until winter. This “autumn low milk production syndrome” depends on a number of different aspects. Figure 1 reports milk production over the years of Italian Holstein cows included in the genetic selection national system. Results indicate that the production begins to fall before summer and this condition lasts until the end of the autumn, even if the THI is already under the risk cut-off value and the days-in-milk are comparable to the spring ones. This is a global problem: European Countries, Russia, the USA, and China have the same results, while Countries in the southern hemisphere experience the opposite because of the different photoperiod.

Figure 1: Average milk production of Italian Holstein cows included in the national system for genetic selection.

Over the years a lot of farms have implemented cooling systems reducing the economic loss due to heat stress. The next step will be to select animals genetically more resistant to high temperature: in the meantime, clinical and functional nutrition can mitigate the reduction in performance. The lower feed intake requires the revision of the ruminant diet and close management during the hot period. Cows eat mostly during the night: lights should be kept on and the ration should be given at the same time as the afternoon milking. The gold standard is to feed the animals with total mixed ration (TMR) twice a day, at the morning and afternoon milking times. The summer diets must be composed of palatable feed additives and highly digestible forages.

Summer water consumption increases by 50% (about 120 L/cow/day), and the animals drink for the most part after milking (60% of the total intake). Drinking areas must be placed preferentially in the rear part of the feeding lane and at the milking room/robot exit. If there are external paddocks, their water access should be in the shade. The water supply issue is generally rather complex: a liter counter upstream of the water pipeline can be useful to know exactly how much cows drink and if it is enough to fulfill their requirements. Increased water consumption and reduced feed intake cause a higher risk of acidosis. Another consequence, also connected with the reduction of time that animals spend lying down, is the increase in signs of clinical laminitis at the end of the heat stress period.

During times of heat, diets normally have lower starch and higher sugar and digestible fiber (from concentrates such as bran, soy hulls, or beet pulp) than in other periods, with lower metabolizable protein derived from rumen bacteria. At the same time, the reduced feed intake causes lower metabolizable protein from low rumen degradable feeds. The consequence is the reduction in milk protein. It is useful to increase the protein intake with more digestible sources in the diet and rumen-protected amino acids (methionine and/or lysine, according to the suggestion of CNCPS models). The dose-response test is a useful tool to decide the possible dietary supplementation with rumen-protected amino acids. After ten to fourteen days of supplementation, milk may be analyzed: the increase of protein and casein concentration means a positive response. The corrected amount of amino acid supplementation is determined through consecutive tests. This procedure is facilitated by feeding robots and the division of the animals per group.

Rumen fermentation reduction causes the reduction of vitamin synthesis by rumen microflora: their rumen-protected forms can be useful to fulfill animal requirements, especially for the B group vitamins. Niacin (vitamin PP or nicotinic acid) has multiple positive effects in summer. Negative energy and protein balance during this period, especially during the last weeks before calving and the 90 days after, lead to high fat mobilization. Vitamin PP (at least 6 g/day) can reduce NEFA production and the risk of hepatic steatosis and metabolic ketosis. This last one is the most important pathology that causes low productive performance in Holstein cows (-7% production for 40 kg/day producing cows with sub-clinical ketosis). Moreover, niacin helps to dissipate endogenous heat because of its peripheral vasodilatory activity.

When the THI is higher than 68 the use of dietary macro-minerals changes. The most important molecule is sodium bicarbonate: it is a buffer for the rumen and is necessary to reduce the accumulation of hydrogen ions into rumen epithelial cells, reducing epithelial damages. The increased respiratory frequency under heat stress increases the risk of metabolic acidosis because of the high elimination of CO2. Farms with a high risk of acidosis increase the chlorine and sulfur concentration in the diet for the dry period: the aim is to reduce the risk of blood alkalization by potassium and phosphorus ingested with hays extremely rich in these elements. In Table 1 are reported the suggested concentrations of macro-minerals for dairy lactating cows during hot periods.

Table 1: Suggested values during heat stress periods. From Fantini Professional Advice srl 2015.
Molecule Suggested values (DM %)
Sodium ≤ 0.6
Potassium No limits
Chlorine < 0.25
Magnesium 0.33
Calcium > 0.9
Phosphorus > 0.4
Dietary Cation-Anion Difference (DCAD) meq

/100 g


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