Nutritional Management and Feeding Strategies for Lactating Dairy Cows Under Heat Stress

The seasonal effects of heat stress have tremendous impact on dairying in India. Adjustments in nutrition and feeding management can alleviate some of the negative effect on health and performance, but compared with environmental modification, manipulation of the cow’s diet specifically for heat stress, has little effect on productivity. Normal physiological processes require that the body's temperature be maintained within fairly narrow limits. When environmental temperatures are moderate (41^o - 77 ^o F) physiologic demand for body cooling or warming are minimal and optimal performance can occur. However in the face of environmental temperature extremes thermoregulatory activities increase and performance is proportionally reduced. Heat stress has been observed to cause reductions in milk production of anywhere from 10-25%. The primary sources of heat gain from the environment are solar radiation and elevated ambient air temperature. These are complicated by high relative humidity and a lack of air movement. Primary methods for altering the environment include the provision of shade (with feed and drinking water under the shade), evaporative cooling with water in the form of fog, mist or sprinkling with natural or forced air movement, and possibly cooling ponds. The incorporation of these methods into an integrated environmental management system which 1) protects cows from the primary sources of heat gain from the environment, and 2) takes advantage of opportunities to enhance evaporative heat loss have the best potential for successful abatement of heat stress.

Heat stress is a health and economic issue in dairy-producing area inIndia. The country does not fit into any one zone. Situated roughly between 8° N and 37° N Latitude, it occupies a large area ofSouth Asia. December to February is the wintertime in almost all of India. At this time of the year, days are cold with average temperature of 10-15°C, but it can drop down to below0 °C in some higher ranges of northernIndia. Normally winters are dry in northernIndia. In Southern part, the temperature difference is not so marked due to moderating effect of Indian Ocean, Bay of Bengal andArabian Sea.  March, April, May and June are the summer months inIndia. It is a time period when rays of the sun fall vertically on Indian subcontinent. The average temperature is around 32°C but in western region the maximum temperature can be far above the average. Temperatures of 50 °C (122 °F) and higher have been recorded in parts ofIndiaduring this season. Hot wind known as 'Loo' is the marked feature of summers in northernIndia. 

The effects of hot, humid weather are costly to the farmer in many ways. It is reported that production losses up to 50% and reduced conception rates (10-20%) on account of heat stress inIndia. Similar report indicated financial losses in heat stressed cows. These impacts are due to thermoregulatory mechanisms maintain thermal balance in the cow at the expense of productive and reproductive efficiency. In addition to reduced milk yield (15 to 40%), cows have lower milk fat content, impaired reproductive performance and greater susceptibility to health problems during hot weather. Feed intake declines and the energy which is consumed by the cows are used less efficiently for milk production. There are many tools available to help the farmer combat heat stress, but management must be excellent if performance is to be maintained. In the summer months cattle’s are often adversely affected by periods of hot climatic conditions. Summer conditions consisting of above normal ambient temperature, relative humidity, and solar radiation (RAD) coupled with low wind speed (WSPD) can increase animal heat load, resulting in reduced performance, decreased animal comfort, and death. The primary factors that cause heat stress in dairy cows are high environmental temperatures and high relative humidity. In addition, radiant energy from the sun contributes to stress if cows are not properly shaded. As the environmental temperature increases, the difference between the temperature of the cow's surroundings and her body decreases, and her reliance on evaporative cooling (sweating and panting) to dissipate body heat increases. However, high relative humidity reduces the effectiveness of evaporative cooling and during hot, humid summer weather the cow cannot eliminate sufficient body heat and her body temperature rises. The tremendous amount of body heat that the high yielding dairy cow produces is helpful in cold climates but is a severe liability during hot weather. One way to measure the combined effect of temperature and humidity is use the temperature-humidity index (THI). A THI exceeding 72 is sufficient to cause minor heat stress and cause a response such as reduced feed intake in the cows. When the THI is in this range, livestock are at risk and environmental modification will be necessary to avoid sharp declines in production, and in some cases to avoid death losses.

Water is the most important nutrient for the cow. It should always be available, should be fresh, clean and there may be advantages to providing cool water during summer. Waterers should be cleaned regularly and should be conveniently located to encourage drinking. Shading the water trough encourages drinking during the hot portion of the day and may help to keep the water cool. Chilled drinking water helps to cool the cow and improves feed intake. Preventing cool well water from warming by shading troughs or using insulated waterers may be beneficial.

Energy is a critical nutrient because of the decline in feed intake which occurs during hot weather. Because energy is usually the nutrient which is most limiting in dairy diets, especially during high production and heat stress, the diet must be made more energy dense to provide sufficient energy to maintain milk yield. Increasing the energy in the diet can be achieved by increasing concentrates (grains) and decreasing forages in the diet. However increasing concentrates to greater than 55 to 60 percent of the diet dry matter is risky and can result in depressed milk fat content, acidosis, cows going off feed, laminitis, and reduced efficiency of nutrient use.

Added dietary fat is an excellent way to increase energy content of the diet, especially during summer when feed intake is depressed. Fat is high in energy (about 2.25 times as much as carbohydrate), does not add starch to the diet (minimizing rumen acidosis), and may reduce heat load in summer. Added dietary fat often boosts milk fat test a point or two. Dietary fat content should not exceed 5 to 6 percent of the total diet dry matter. Oilseeds such as whole cottonseed and whole soybeans are excellent sources of fat and are protein supplements as well. Tallows are also good fat sources. Bypass fats are specialty fats which are inert or inactive in the rumen. These are often used when high milk yield requires energy supplementation above that which can be supplied with oilseeds or tallow’s without causing digestive upsets.  

The amount of crude protein in summer diets must be increased because of lower feed intake. Because of decreased feed intake, careful attention must be given to the amount (pounds) of protein needed by the cow, not the percentage in the diet. Caution must be taken to feed adequate but not excess dietary crude protein. Excess dietary crude protein must be metabolized and excreted by the cow; a process which creates heat and consumes energy that could be used to produce milk. Bypass protein is usually beneficial for cows producing in excess of 60 poundsof milk daily. Bypass protein values of 36 to 40 percent of total dietary crude protein are desirable. The amount and type of bypass protein needed varies by the type of diet being fed, and your nutritionist should be consulted to help make decisions for your herd. Research indicates that heat-stressed cows benefit from bypass protein when simply shaded, but the benefit may be greater for those cows receiving extra cooling. Recent research indicates that diets should not contain an excess of 17 percent crude protein with greater than 62 percent degradable intake protein during hot weather. Again, however, rations should be formulated for adequate protein intake based on the amount of feed the cow is actually consuming.

Fiber is required in the cow’s diet for proper rumen function. However the digestion and metabolism of fiber creates more heat than the digestion of concentrates, and the heat-stressed cow will reduce the amount of forages she consumes relative to concentrates if allowed to select between the two. This upsets the ration balance and can lead to reduced fat in milk, rumen acidosis and digestive upsets. There are several steps which can be taken to prevent cows from sorting and selecting their diets. (a) Chop hays and mix into a total mixed ration. (b)Use silages as the sole forage source and mix into a total mixed ration. (c) Use wet ingredients such as wet brewers grains and silages to make dry hay diets wetter and more palatable. (d) Add water to dry diets during mixing to improve intake and reduce sorting. (e) Feed high quality, more palatable forages. Chopping hays for mixing into total mixed rations also helps to reduce waste and improves utilization of lower quality hays when necessary. Succulent feedstuffs are those feeds which are high in moisture, including silages, green chop forages, and by-products such as wet brewer’s grains. Cows like succulent feeds and their inclusion in the dairy ration may encourage intake during hot weather. However a ration that is too wet may restrict intake so care must be taken to not exceed 50 to 55 percent moisture in hot weather rations. Succulent feeds spoil more rapidly than dry feeds, especially during hot weather, and feed bunks should be cleaned daily to prevent spoiled feed from reducing feed consumption.

The fiber content of the ration should be reduced slightly to encourage greater intake in hot weather. The ADF content should not be less than 18 percent and NDF not less than 28-30 percent of ration dry matter to maintain normal rumen function. As mentioned previously, increasing concentrates to greater than 55 to 60 percent of the ration is risky, because of the reduced amount of effective fiber in the diet. Probably of greater importance than the forage: concentrate ratio is the quality of the forage in the diet. Excellent quality forage helps to maintain feed intake and is especially important during the summer. When excellent quality forage is used in your nutrition program, more forage can be included in the ration. In addition to greater feed intake, feed costs are often lower and fiber levels can be maintained at a higher level.

Mineral content of the ration should be boosted before the onset of hot weather so that the cow is prepared and a sharp drop in production can be avoided. Potassium (K), sodium (Na), magnesium (Mg) mineral requirement increase in heat stress animal. As per NRC the K, Na, Mg (% of diet DM) should be increased from normal 0.9, 0.18 and 0.2 to 1.5, 0.6, 0.35 respectively in heat stress animal. Buffers such as sodium bicarbonate should be used during hot weather, especially in low fiber, high concentrate diets. Diets should contain at least 0.75 percent sodium bicarbonate on a dry matter basis. This amounts to a minimum of 0.34 poundof buffer per day for a cow consuming 45 poundsof dry matter. Magnesium oxide at 0.35 to 0.4 percent of diet dry matter also helps to maintain milk fat.

Feeding buffers can be beneficial during heat stress periods for two reasons. First, if fiber content of the diet is minimized and/or cows are selecting against eating forages, buffers can help prevent a low rumen pH and rumen acidosis problems. Secondly, the most common macro minerals in a buffer is usually Na, exception of K in KHCO₃, which when increased in diets fed during heat stress has increased DMI and milk production.Feeding Aspergillus oryzae reduces heat stress in cows through lowering rectal temperatures (average .86° F) and milk production was increased which is due to improved fiber digestion in the rumen. Feeding niacin during the summer was found to increased milk production across all cows by an average of about 2 pounds per day, but cows producing over 75 pounds per day increased over5 pounds per day. No change in milk components occurred.

The concentration of all nutrients will need to be increased in diets as DMI decreases during heat stress. Guidelines for nutrients that have been specifically shown to have an influence on DMI and milk production during heat stress are:

Management during hot weather should be aimed at whatever measures are necessary to encourage feed intake because it is difficult to provide sufficient nutrients if feed intake is severely depressed. If grazing is a part of feeding program, cows will consume more if grazed very early in the morning or at night. During the day cows will seek shade. If cows are moved a long distance from pasture, heat stress will be increased, especially if cows are moved during the hot afternoon hours. Hurrying cattle to the barn further stresses the cow. Since long distances often separate pastures from the dairy barn, perhaps green chopping forage is an option. This allows the use of fresh green forage, allows better control of the ration and keeps cows near the barn, increasing the opportunity to cool the cow.

Frequent feeding provides fresh feed, stimulates the cow’s curiosity and encourages more frequent eating, all desirable during hot weather. The heat associated with digestion of feed peaks about 3 to 4 hours after feeding. Time of feeding also is important. During hot weather, cows will eat mostly during the nighttime and after milkings. Having fresh feed in the mangers after milking, especially when cows have been cooled in holding areas, is a good way to encourage DMI. The majority of fresh feed should be fed at night when heat stress loads. Feeding at sunset and then again about an hour before sunrise are good times. Feeding a total mixed ration (TMR) is preferable to component or separate ingredient feeding during heat stress periods. A TMR with forages mixed in will help reduce the cow's tendency to selectively consume concentrates rather than forages. A well balanced TMR will allow diets to be formulated at minimum fiber levels encouraging DMI and minimizing rumen fermentation fluctuations and pH declines. Adding water to diets may help DMI during summer months. Water will soften fiber feeds, and reduce dustiness and dryness of the diet increasing palatability and DMI. A three to five percent addition of water is recommended. Also be sure managers or bunks are kept clean. Remove refused feed every day. Check and clean any moldy and/or heating feed from the corners and edges of feeding areas at least three times a week or oftener if animal protein and fats are fed. Feeding areas with a decaying feed smell reduce DMI even when fresh feed is offered.

The management of heat stress and its effects through environmental modification involves reducing heat gain via solar and thermal radiation and high ambient air temperatures. This may be reasonably accomplished with shade and evaporative air cooling. Prepare for hot weather before it arrives, and remember that the weather is comfortable for you does not mean that it is not costing production from your dairy cows. A reduction in DMI is the primary reason milk production declines during heat stress periods. At the same time DMI decreases, maintenance cost of the cow increases in an attempt to maintain body temperature and thus, the overall availability of nutrients and energy for milk production is decreased. The most effective feeding management strategy to minimize production losses during heat stress periods is to provide a cool, comfortable environment by shading, sprinkling and/or forced air flow. The concentration of all nutrients will need to be increased in diets as DMI decreases during heat stress. The ration should be properly balanced, and generally the energy density should be increased in the summer to help compensate for decreased dry matter intake of the cow.

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