Temperate dairy cows are particularly susceptible to heat stress, and often show marked decreases in productivity in hot climates or when introduced into tropical or sub-tropical regions for genetic improvement of local breeds. Focus on feeding is then more important than ever.
Under heat stress conditions, the amount of energy expended by the cow to maintain homeothermy increases (e.g. 20% more at 35° compared to 20°C). Panting also increases the maintenance requirement by 7 to 25% under heat stress. Therefore, dry matter (DM) intake must increase to cover this additional energy cost. However, during hot weather, DM intake decreases meaning that the energy status of the cow gets a double hit – greater energy costs to try to maintain homeothermy and lower energy intake. In this case, it is not surprising that milk production goes down (Table 1).
This situation can be improved by feeding the cows early in the morning and at night. In these cooler periods of the day, cows can consume up to 80% of their total daily DM intake. However, in the cases where the night- and morning times still exceed the upper critical Temperature Humidity Index (THI) of 72, the amount of feed consumed will not compensate for the greatly depressed intake during the day. Physically cooling the cows can lessen the intensity of the heat load and improve the physiological status and performance of the cow. In one study, sprinkling has decreased respiration rates from 102 to 80 breaths per minute. DM intake was increased from 16.2 to 19.6 kg/d, and average milk production was increased from 17.6 to 20.2 kg/day. Similar responses were noted in beef steers where the daily intake was 6.19 kg and 7.27 kg and the daily gain was 1.09 kg and 1.39 kg for the non-sprinkled and sprinkled steers, respectively.
Feeding protein deficient diets results in an increased heat load on animals due to increased heat production for turnover of tissue proteins. Therefore, supplemental protein is needed to alleviate heat stress. A study was conducted to examine the effect of protein supplementation for Holstein cows during natural heat stress. Dry matter intake was 11% greater with the high-protein diet, and the FCM was 4.3% higher. Excess protein should, however, be avoided as it reduces energy utilisation due to the extra amount of energy needed for disposal of surplus protein via the urea-cycle.
It is generally thought that the fibre content of diets should be reduced during times of heat stress in order to reduce the metabolic heat load on the cow because fibre has a greater heat increment than concentrate. Recent studies, however, have shown that cows fed a diet with the greatest energy density (low ADF diet) needed to cut back the most on DM intake in order to reduce her metabolic heat load to tolerable levels during times of heat stress. This means that total energy intake and diet digestibility may be a more important factor influencing metabolic heat production and resulting DM intake reduction than is dietary fibre concentration alone. An adequate amount of fibre should, therefore, be fed in order to improve rumen functions and animal performance under heat stress (Figure 1). This holds true only with high quality forage since lower heat production occurs with the fermentation of high-quality forages compared with lower quality forage.
Added fat is important in combatting heat stress in hot climates and should be at the maximum amount that can reasonably be fed. This reduces the heat increment associated with alimentary tract fermentation and tissue metabolism which in turn reduces total body heat load. Whole cottonseed is a highly valuable feed ingredient used for such a purpose. Because of its high energy and fibre contents, it stimulates both milk yield and milk fat percentage. The inclusion of soybean oil or hydrogenated vegetable fat may also offer similar advantages. It should be noted that only 3-8% fat is tolerated by rumen microorganisms. Recent advances in nutrition have brought new forms of fat, coated with either calcium soaps or with formaldehyde-treated proteins, which can be added to the rations at higher levels without adverse effects on rumen microorganisms or on animal productivity.
Vitamin E is among the micronutrients playing an important role on mammary gland immunity and health. Also on the alleviation of infertility induced by heat stress in dairy cows. Due to its antioxidant properties, vitamin E also has an effect on the prevention of chronic diseases associated with oxidative stress. Because heat stress could cause an increase of oxygen free radicals and could induce oxidative stress, the supplementation with vitamin C could be relevant considering that ascorbic acid is one of the most important antioxidants in a biological system. Niacin also helps to alleviate heat stress both by increasing evaporative heat loss from the body and also by reducing the effects of heat at the cell level. Cows fed diets supplemented with 6 grams niacin per day during summer had about 1 kg greater milk yield.
Supplementation of minerals during heat stress is a recommended practice, particularly with high producing dairy cows. Of greatest importance is potassium (K) since more K is required to counteract losses through milk and sweat. Increased use of concentrates for heat-stressed cows also contributes to higher K requirements. Most concentrated feeds are potassium deficient, making the use of supplements necessary. Potassium supply should be considered along with the supply of sodium (Na) since both are closely related to the regulation of many body functions. Urinary losses of Na increase during heat stress, due to decreased levels of blood aldosterone. Such losses should then be balanced out through dietary supplementation. In one study, the effects of varying proportion of K and Na on milk production of heat-stressed dairy cows have been examined. Milk production was markedly reduced when only 1.0% K was combined with 0.38% Na in the supplement. There was some improvement in milk yield with 1.5% K and 0.38% Na but when the 1.5% K was combined with 0.67% Na milk yields were superior.
Sodium bicarbonate is a frequently used feed additive that is especially useful during hot weather. Because high concentrate, low forage rations are often fed to encourage DMI during hot weather, or because cows selectively reduce fibre intake in response to high temperatures, the potential for acidosis due to inadequate dietary fibre content is real, especially when feeding low-quality forages. Buffers minimise pH fluctuations, usually enhance fibre digestion, and often encourage greater DMI. A trial conducted under severe heat stress conditions (THI>80) indicated that live yeast fed to dairy cows improved feed efficiency by up to 7%, equivalent to an extra 120g of milk/kg DMI. The same trial also indicated a positive effect on rumen pH, resulting in lower acidosis risk,
As a further strategy, increasing feeding frequency should reduce heat production because this would promote a uniform rate of absorption of nutrients and spread the total heat increment due to feeding over a longer time period. This should not overload the animal’s heat dissipating mechanism at any one period. Frequent feeding of heat stressed cows also helps prevent the milk fat problem by maintaining uniform rumen fermentation and permitting a higher intake of concentrates without a decrease in the rumen pH or the acetate-to-propionate ratio.
Water is a basic molecule in the animal’s body and is essential for the maintenance of vital functions such as tonicity of tissue, osmotic regulation, thermoregulation, and nutrient transport and excretion. Under heat stress conditions, the cow loses water via the skin and respiration as she works to minimise her rise in body temperature. In one study, the loss of water increased by 59% via the skin and 50% via respiration when ambient temperature increased from 18°C to 30°C. It is, therefore, important to provide cool, clean water in ad libitum amounts and eliminate any management factor that may inhibit cows from drinking. If water intake is restricted in hot weather, the drop in milk production will be precipitous.