Hot weather causes heat stress in dairy cattle. Thermal stress causes the body temperature of cows to increase. Lactating dairy cows create a large quantity of metabolic heat and accumulate additional heat from radiant energy. Heat production coupled with compromised cooling capability because of environmental conditions cause heat stress in cattle. As a result, the cows have a lower feed intake.
This is associated with a decline in milk yield, fertility, and growth rate. In hot weather, cattle generally reduce their feed intake. It has been estimated that at 40oC, feed intake (on dry matter basis) is only about half that eaten by cows living in their optimum temperature range. The effect of high temperatures is more marked in high-performance cows with a high milk yield.
Although effects are more severe in hot climates, dairy cattle in areas with relatively moderate climates also are exposed to periods of heat stress. The resultant decrease in milk production and reproductive efficiency can be overcome by implementation of a program consisting of cooling through shades, ventilation, spray, fans etc, and modifying the feeding practices. The economic benefit should be determined before installation of equipment to reduce heat stress.
Heat stress occurs when any combination of environmental conditions cause the effective temperature of the environment to be higher than the animal’s thermo neutral (comfort) zone. Four environmental factors influence effective temperature:
- atmospheric temperature
- relative humidity
- air movement and
- solar radiation.
When the temperature exceeds 27°c, even with low humidity the effective temperature is above the comfort zone for high producing dairy cows. The temperature humidity index (THI) commonly is used to indicate the degree of stress on dairy cattle. When the THI exceeds 72, high producing dairy cows are affected adversely. Estimated milk yield reduction was 0.32 kg per unit increase in THI. Milk yield and TDN intake declined by 1.8 and 1.4kg for each 0.55° C increase in rectal temperature.
The comfort zone of dairy cows is in the range 10-24°C. High temperatures have a marked effect on the performance of dairy cattle. This effect can even be seen at relatively cool temperatures of 25°C, and gets worse as temperatures rise to 30°C or more.. Heat stress can cause losses in production of 20 percent or more and reduce conception rates by 10 to 20 percent. Studies have shown that under heat stress, the hormonal profiles of the cows are altered. Environmental temperatures greater than 86°F may alter the hormonal patterns in cows. It is demonstrated that the levels of estrogen and luteinizing hormones are lower in cows without shade compared to those which can take shelter from the sun in the shade. Heat stress immediately after and at the time of breeding (7-10 days) may result in a lower conception rate.
Strategies to minimize the effect of heat stress
Management strategies to minimize the effect of heat stress are:
- Physical modification of the environment
- Improved feeding practices
- Genetic development of heat tolerant breed
In the long term, dairy cattle can be made more tolerant to hot and humid weather conditions by selective breeding. Colored breeds such as Jerseys and Brown Swiss seem to show greater tolerance to heat stress. Jerseys are also better producers of butterfat and protein, while needing a lesser quantity of high-quality feed. Holsteins are less heat tolerant and they require adequate attention in hot weather.
In the short term, cows must be helped to withstand high temperatures by modifying their environment. The major objective of a cooling system is to reduce the air temperature inside the cow shed, so as to keep the cow’s body temperature as close as possible to the normal (38.5 – 39.3oC). Evaporative cooling can be accomplished in two ways. The first is by direct evaporation from the skin surface of the cows. The second is by indirect evaporation, by cooling the air around the cows with cooling fans and mist in an enclosed cattle shed.
Modifying the Air Temperature
There is no single method of reducing heat stress in cattle and other animals. The only successful approach is an integrated approach.
Cattle shed design
The longer side of the cattle shed should have an east-west orientation. This reduces the amount of direct sunlight shining on the side walls or entering the house .There should be adequate facility for cross ventilation to reduce the thermal stress. An adult cow requires around 800 cubic feet of air space under tropical conditions. In a conventional cattle shed a height of 8 feet at sides and 15 feet at the ridges will be sufficient to give necessary air space.
Thermal stress can be reduced by reducing ground reflection (by coverage of landscape around the shed with grass, shrubs and bushes), attached shade (projection of roof), minimizing the solar radiation (suitable method and material for roofing) etc. There are also various roof systems which give improved natural ventilation by means of roof openings, enhanced solar chimney effect, etc.
Painting the roof white may increase the level of sunlight reflected, thus reducing the amount of absorbed solar energy (All heat at the same wave length must be either reflected or absorbed, so increasing the amount of reflected heat reduces the amount of heat absorbed).
Simply providing some shade around the cattle shed is a good way of protecting cows from the direct rays of the sun during the day. The most effective shade is from trees and other vegetation. They not only protect the cows from sunlight, but also create a cooling effect by the evaporation of moisture from their leaves.
Air movement increases the rate of heat loss from the cow’s body surface. But this is effective only if the air temperature is lower than the skin temperature of the cows.
Mist and Fan System
Mist particles are sprayed onto the body of the cow, to wet the hair. A fan is then used to evaporate the moisture, as a way of cooling the cow. Cooling in the form of fans and sprinklers is
usually beneficial and is easily installed in these cattle shed. Research shows an 11% increase in milk yield when cows were cooled with fans and sprinklers compared with shading alone. Fans and water sprays are much more effective when they are used together. Either kind of cooling method used on its own is not very effective.
Mist and spray systems are expensive to install. They will only give a profit if the extra milk yield covers the installation cost. Economic analysis has shown that the break even point for this system is an increase in milk production of 0.81 kg/day.
Cattle sheds may remain hot even after the sun goes down. Letting the dairy cows graze out in the night is a good way of helping them lose heat.
Adjusting the Feeding practices
The energy needs of cattle rise in hot weather, which means they need more feed for energy and maintenance. However, at the same time, feed intake tends to fall. Effect of heat stress on thyroid gland activity reduces gut motility and rate of passage. More over under heat stress the blood flow to the internal organs especially to the intestine is reduced to compensate the increased flow to the peripheral organs for effective heat loss through evaporation and conduction. This causes decreased absorption of nutrients from intestine. While heat stress causes a decline in dry matter intake, the cow’s energy and protein requirements for maintenance and production increase. It is important to increase the energy and protein content of diets, if dairy cows are to maintain their performance in hot environments.
During times of heat stress, feed intake is reduced by 8-12% or more causing a reduction of volatile fatty acid production in the rumen and decreased milk production. In order to address this problem, more nutrients must be packed into smaller volumes of feed. (Requirements for lactation do not change, but her energy needs increase). Increasing the energy in the diet can be achieved by increasing concentrates (grains) and decreasing forages in the diet. Added dietary fat is an excellent way to increase energy content of the diet, especially during summer when feed intake is depressed. Often the amount of crude protein in summer diets must be increased because of lower feed intake. Bypass protein values of 36 to 40% of total dietary crude protein are desirable. The fiber content of the ration should be reduced slightly to encourage greater intake in hot weather. Feeding good quality roughage into lactating cows in summer is recommended since it reduces heat buildup supplies adequate long fiber in the diet. The ADF content should not be less than 18% and NDF not less than 28-30% of ration dry matter to maintain normal rumen function. For heat stressed cows alkaline diets are preferable. Buffers such as sodium bicarbonate and magnesium oxide should be used during hot weather, especially in low fiber, high concentrate diets. Addition of sodium bicarbonate to the diet helps to maintain the ph of rumen and also increase DMI and milk yield. Provision for ad-lib water supply is an essential element of a good thermal management system.
Minerals are also readily depleted during the hot months through excessive water loss. The proper use of buffers in the diet can help to address this problem. Water requirements increase dramatically as environmental temperatures increase. Profuse sweating by heat-stressed cows results in a considerable loss of potassium. The level of potassium, and also sodium and magnesium in the diet should be increased. Increasing the concentration of dietary potassium to 1.2% or more, results in a 3-9% increase in milk yield and also on DMI. Heat stress causes up to 30% decline in stored vitamin A in liver which necessitates its supplementation during summer. Heat stress generally increases the production of free radicals, leading to oxidative stress. The use of anti-oxidants such as vitamin E reduces the impact of heat stress on the oxidative balance, resulting in improved milk quality and cow’s health.
A combination of fans, wetting, shade and well-designed housing can help to alleviate the negative effect of high temperatures on dairy cows. Careful management, feeding strategies are important in achieving the optimum milk production. While there are many methods of reducing heat stress, selection of the most appropriate technique and its proper application is essential. If one method proves successful in one place, this does not guarantee success elsewhere. There are also limitations related to the local climate, the educational level of farmers, and the amount of money farmers can afford to invest.
R.E. Patil. Effect of heat stress on dairy cattle and procedures to reduce it. Paper presented during training course “Dairy cattle Production” Shefayim , Isreal. 9/200
Armstrong, D.V. 1994. Heat stress interaction with shade and cooling. Jour. Dairy Science 77: 2044-2050.
Armstrong, D. V., S. K. DeNise, F. J. Delfino, E. J. Hayes, P. H. Grundy, S. Montgomery, and A. Coma.1993. Comparing thne different dairy cattle cooling system during high environmental temperatures. J. Dairy Sci. Suppl. 1:24,
D.K. Beede and R. J. Collier Potential Nutritional Strategies for Intensively Managed Cattle during Thermal StressJ. Anim Sci. 1986. 62:543-554
The author can be reached at: Dr.R.Venugopal. M.V.Sc.,Veterinary surgeon, Veterinary Dispensary, Maranalloor, Thiruvananthapuram,Tele: +919400489150,E-mail: [email protected]