Heat stress in dairy animals and the measures to reduce it

India is the world’s largest producer of milk, generating more than 200 million tons every year. The country possesses about 300 million bovines (nearly 200 million cattle and 100 million buffaloes). Nearly half of the milk production comes from buffaloes and a little over a quarter from crossbred cattle. Indian dairy industry, which relies on 80 million farmers across the country, most with small herds, has grown steadily and now accounts for nearly 5 percent of India’s economy.
Environmental heat stress is obviously the problem faced by animals located in hotter (tropical) regions of the world, including India. Heat stress not only reduces milk yield in cows and buffaloes but also affects several other economic parameters viz. animal health and animal reproduction, resulting in a significant economic impact on the dairy owners. The most obvious effect of heat stress is the reduction in feed intake, leading to a decrease in milk yield. But apart from that, there are several other metabolic implications that also contribute to the reduction in milk yield. In addition to that, there is a decrease in milk quality as well, primarily due to an increase in somatic cell count (SCC) and incidence of mastitis, which also results in increased health care costs.
Climatic Stress on animals, especially due to heat stress, is one of the big challenges faced by dairy animals in India, including their owners, which may reduce the milk yield of cows and buffaloes substantially. The reduction in milk yield of cows and buffalos results in an increase in both feed costs as well as milk prices. Thus, apart from affecting dairy farmers, extreme heat is proving crucial to the dairy industry as a whole.
In view of the chronic shortage of green fodder, which gets further compounded during hotter months, farmers have to depend more on straws and stovers to give bulk feed to their animals. This year, due to erratic rains and hot spells, the yield of even wheat crops too dropped, causing a shortage of wheat straw as well, with their prices also shooting up. However, these bulky fibrous straws are only low-quality feeds, being low in energy, protein as well as in minerals. Fortunately, the ruminant animals possess the mechanism in their rumen by which part of these bulky (cellulosic) feeds are partly fermented through a large number of microbiota present in the rumen, and the end product is volatile fatty acids, mainly acetate, which serves as precursors for long chain fatty acids in the liver. Starchy diets produce Propionic acid in the rumen and serve as a precursor for glucose synthesis in the liver. Glucose ultimately serves as a precursor for lactose synthesis in the mammary gland and regulates milk volume. This is because Lactose maintains the osmotic pressure of milk.
But even then, due to the shortage of green fodder and its replacement by straw feeding, the animals have to be fed more of concentrate feed in order to meet their requirement of nutrients fully. So, naturally, the demand for feed also increases, and so is the increase in feed cost.
To overcome the challenge of environmental heat stress in tropical countries on dairy animals, some management, as well as nutritional strategies, have been worked out by several researchers, which may help, but only partially.
Adaptations of animals to hot and hot & humid climate
Camel is bestowed with a big hump, which helps the animal to use the stored energy during the period of its limited nutrient supply and thus fight heat stress. But compared to camels, Bos indicus have a smaller hump, so they do not possess as efficient an adaptive mechanism to fight heat stress as camels possess. However, Bos indicus animals are still in a better position to dissipate heat during the hot season, unlike Bos taurus, like crossbred cows, which are devoid of any hump.
The hot humid conditions are the most stressful for the animals. The best-adapted animal for the humid tropics is the buffalo. Although its sweat glands are far less active than the zebu cattle, however, wallowing in ponds helps it a lot to get rid of the body heat and causes thermo-regulation. Temperature -Humidity Index (THI) is a better predictor of whether or not the cows are “stressed”. THI < 72 is the point at which a dairy cow starts to decrease productivity.
Heat Stress Reduces Feed Intake and the quantity of Milk Produced
Unabated heat stress can decrease feed intake by more than 35%. However, even in well-cooled dairies decrease in feed intake due to heat stress may be between 15-20 %. In noncooled management systems, milk yield can decrease by 40-50 % during severe conditions. Reduction in feed intake due to heat stress reduces consequently nutrient intake, resulting in decreased milk synthesis in the mammary gland. However, the overall reduction in milk production is not just due to a reduction in feed intake only, but also due to heat stress itself. Because, the animal has to spend part of its energy intake on panting, to get rid of its excess heat, just the way an air conditioner requires electric energy to cause a cooling effect.
Strategies to reduce the adverse effect of heat stress in Dairy animals
It has been proved through research that only the 40 % of the decrease in milk production is due to reduced DM intake, the rest of the 60 % reduction in milk production can be explained by heat-stressed induced other biochemical changes. However, the adverse effect of heat stress on the overall health and productivity of dairy animals can be overcome to some extent by improving the microenvironment and feed management. Following the approaches may help.
Providing Shelter/ Cooling System: The decline in old heat mitigation mechanisms such as the shared village ponds and rivers helped these animals, especially buffaloes to cool off. These village water bodies have vanished and the rivers are polluted. Under Indian conditions, the heat stress can now be reduced somewhat by providing proper shelter, controlling air moments, and causing a cooling effect inside with the combination of mist cooling and fan in sheds. This is being adopted by many big dairy farmers in India. Of course, the effect of heat stress on milk production can only be partially reduced.
At NDRI Karnal, some experiment is being performed on music therapy, as a way of reducing heat stress on dairy animals. They have reported that 40 to 60 decibels of sounds may be the best to reduce heat stress to some extent.
Reducing the quantity of fibrous diets and increasing concentrate: It is a known fact that fibrous diets generate more heat during rumen fermentation, called heat of fermentation. By reducing the quantity of highly fibrous feeds in the diet, the animal’s heat load can be slightly reduced. Instead, the animals may be offered succulent green fodders to meet their fiber requirement. In the absence of sufficient green fodder, the quantity of concentrate must be increased to meet the animal’s nutrient requirements.
Bypass Nutrient feeding: Bypass nutrients, instead of being degraded in the rumen, are protected against ruminal enzymes. Thus, proteins are not attacked by proteolytic enzymes and fats are not attacked by lipolytic and hydrolyzing enzymes of microbes. These nutrients are passed on to the lower tract where they are digested and subsequently absorbed from the intestines.
1. Feeding of bypass protein: Since rumen-protected proteins (bypass proteins) are not degraded in the rumen, these are digested in the abomasum (true stomach, or 4th compartment) and then absorbed from the intestines. After absorption, amino acids are then converted to glucose in the liver, to serve partly as an energy source and partly used as a precursor for lactose synthesis in the mammary gland. Apart from the generation of less heat in the rumen, feeding of bypass protein increase the efficiency of nutrient utilization, and thus, try to maintain milk production to some extent.
2. Feeding of bypass fat: The inclusion of Bypass fat in the diet of ruminants, increases the energy density of the ration. Accordingly, it compensates for lower feed intake, by providing energy for maintenance and productive purpose in a more concentrated form. In fact, in high-yielding animals, the inclusion of bypass fat as a dietary supplement is very essential, especially during hotter months, in order to maintain cow’s milk production.
Use of Feed Additives like Antioxidants: Heat stress generally increases the production of free radicals in the body, which then leads to oxidative stress as well. Oxidative stress has a negative impact on immune and reproductive functions. This may lead to increased frequency of mastitis, higher somatic cell counts in milk, decreased fertility, increased embryo mortality, post-partum retained placenta, and early calving. Glutathione peroxidases family of anti-oxidative enzymes, containing selenium incorporated within an amino acid (organic/chelated form), plays a major role in maintaining the anti-oxidative balance, protecting the cells from damage against heat stress. If given along with Vit E, which too is an antioxidant, the result is twofold.
Supplementing the diet with other feed additives like buffers (Sod. Bicarbonate), Niacin and Yeast also has a beneficial effect on the rumen environment and has an overall positive effect on milk production and stress reduction.

By Dr TK Walli, Former Head, Dairy Nutrition division, NDRI, Karnal