Heat stress is one of the main problems in Iran’s cattle farms, especially in tropical and subtropical regions. Dairy cows are very sensitive to heat stress. Heat stress not only has a negative effect on the producer’s income due to the decrease in production, the decrease in milk quality, and the decrease in fertility, but it also causes problems in the health of dairy cows.
Two heat sources affect the cow, which are the heat of the environment and the heat resulting from the metabolism of food in the animal’s body. When the air temperature rises suddenly or more than the tolerance level of the animal’s body, the body will lose its balance and will cause a decrease in appetite and production and reproduction and even death. To control this problem and minimize it, there are various methods that will be mentioned below. The increase in the cow’s body temperature can be controlled by humans by creating showers in the milking area or by increasing the rate of breathing or by increasing the activity of the animal’s own sweat glands. Since cows sweat relatively little, it is necessary to moisten them with water. Short-term watering is followed by evaporation of water by ventilation equipment. Water spraying or misting and cooling using ventilation have been used in tropical countries.
Definition of heat stress: In a cow, the ideal temperature that does not have any negative effect on livestock production is between 4 and 24 degrees Celsius, which is called the temperature comfort zone. The temperature outside the comfort range causes the animal’s balance to be disrupted, production and fertility decrease, as well as problems in the animal’s health, which are called thermal stress. Heat stress occurs when the thermal load of the dairy cow is greater than its ability to dissipate heat. The severity of heat stress depends on the amount of humidity and temperature. At temperatures above 25°C or even above 20°C, the cow experiences heat stress and its health status and performance are affected. Excessive thermal pressure on the cow due to high temperature and humidity will cause thermal stress. Heat stress causes a decrease in the production and pregnancy rate of dairy cows during the summer. These damages appear in the decrease in the amount of milk produced, the increase in the number of open days and the decrease in the calving rate. The dangerous level of heat stress in dairy cows is 37 degrees Celsius and 20% humidity. And the lethal range for dairy cows is 37°C and 80% humidity.
Effects of heat stress:
1-Increasing the rate of breathing: increasing the rate of breathing to more than 70 times per minute, in this case the cow’s mouth is open and it starts to crush.
2- Reduction of feed consumption
3- Increase in water consumption
4- Increased sweating
5-Reduction of blood flow to the internal components
6-The effect of heat stress on the amount of milk production: reduction of milk production causes economic loss to dairy cattle breeders. Reducing feed consumption by 8 to 12 percent or more, which leads to a decrease in milk production, while part of the consumed food is used to control and maintain the animal’s body temperature, and since high-producing cows consume more feed They are more sensitive to heat stress. In a cattle breeding unit, this stress can reduce the daily production of 4 kg per animal, which can reduce a large amount of production in a period of milk production.
7- The effect of heat stress on reproductive performance in dairy cows: heat stress is an important factor in the infertility of dairy cows and it causes significant damages from an economic point of view.
Various factors affect heat stress: The important result is an increase in temperature and humidity, a decrease in the manifestation of estrus, a decrease in appetite and food consumption. Heat stress affects the reproductive process of most farm species and dairy cows are particularly sensitive to heat stress due to the production of metabolic heat resulting from the milk production process. Among the things that can be affected during heat stress is the activity of the hypothalamus-pituitary-ovary axis. Several studies have investigated the effect of heat stress on the secretion pattern of estradiol and progesterone hormones. Sometimes different results have been published that the type of stress (acute or chronic), animal breed, accuracy, type of feed consumed and uncontrolled factors, etc. can interpret these different results. Ovarian follicles in animals under heat stress are damaged but continue to grow. Theca and granulosa cells in the follicle may also be damaged during heat stress. It has been shown that follicles cannot develop normally and estradiol production is reduced. The decrease in estradiol synthesis has made the appearance of estrus a problem, which is one of the effects of heat stress in the fertility process or low fertility in animals under heat stress. Heat stress causes a decrease in the amount of estradiol, but this decrease may not be significant in the tested treatments.
In addition to the effects of heat stress on ovarian follicles, the corpus luteum is also affected. Considering the need for a significant amount of progesterone for pregnancy, the decrease in progesterone produced by the corpus luteum in animals under heat stress can be one of the reasons for the low pregnancy rate in these animals. Of course, the reports on changes in the amount of progesterone are also different. To reduce progesterone concentration due to heat stress, the shortening of the estrous cycle during the summer and disruption of lipid metabolism during the hot months of the year have been mentioned.
One of the main ways to prevent the reduction of reproduction is the same method of creating shade and drying the environment during artificial insemination, which will help improve this situation to a great extent. Another method that has a great effect in improving the conditions is the timing of estrus and the creation of synchrony in cows, which has been shown that in the summer, cows that are estrused and inseminated at the same time have a higher success rate in comparison to cows that are estrused separately. They have been fertile because it is difficult to find estrus individually in the summer due to the shortening of the estrus period.
The effect of heat stress on the reproduction of female cows: 1-Decreasing the incidence of estrus and further reducing fertility 2-Increasing ovarian cysts 3-Increasing abortion and early fetal death 4-Decreasing the pregnancy rate 5-Increasing uterine temperature-Reducing blood flow to the uterus 6-Increasing the distance Calving due to delayed uterine return after childbirth
The effect of heat stress on bull reproduction: One of the negative effects of heat stress on bull is the decrease in sperm quality. Changes in sperm quality may lead to a decrease in fertility. Experiments have shown that bulls are different in the occurrence of stress symptoms and showing sensitivity to heat stress, and individually there may be bulls that do not show many symptoms in front of heat stress, while others have severe symptoms.
8-The risk of acidosis: during heat stress, the risk of acidosis increases. Factors that are related to acidosis problems include: reducing the consumption of dry matter with less fodder ratio, reducing rumination, reducing saliva secretion and finally creating acidosis in livestock, all of these factors are related to reducing food yield, followed by milk production and Often, milk fat is also reduced. In addition, acidosis affects the general state of the animal’s health, fertility and productive life span of cattle.
9- Heat stress in pregnant cows leads to weight loss of the calf, and after giving birth, the calf suffers from metabolic disorders. The cow stands up, which will cause discomfort in the joints.
:Management strategies to control and reduce heat stress in dairy cows
1-Increasing the frequency of feeding, but in small amounts and at cooler times of the day, 4 to 6 times a day – fresh food encourages consumption. – Cows are curious, therefore, if the feeding place is a suitable place, by increasing the frequency of feeding, the cows move more towards the manger.
2- Because the cow has difficulty in removing heat, the blood flow to the digestive system decreases. It is recommended to use higher quality and more palatable foods.
3- Canopy design on top of the manger
4- When using higher ratios of concentrates in the diet, it is recommended to use sodium bicarbonate (sweet soda) to prevent acidification of the stomach environment.
5- Avoid excessive protein in the diet. Diets should have about 18% or less protein. Only cows with the highest production need 18% protein.
6-Niacin (6 grams per day per animal) can be used as an additive in order to improve energy use in animals under heat stress.
7- The increase in temperature and the subsequent increase in sweating and excretion of sodium and potassium increase the need for minerals. It is recommended to maintain a 3:1 ratio of potassium to sodium in the diet during heat stress.
8-Dietary management: When the amount of feed consumption decreases due to heat stress, the concentration of nutrients should be increased to ensure adequate supply of all necessary nutrients. Low-quality forages produce more heat by fermenting in the rumen. High-quality forages are digested faster, resulting in less heat production.
An increase in energy density may lead to the use of larger amounts of concentrates or by-products. Increasing the energy density of the diet, using high-quality fodder and feeding more fat powder and concentrate helps the animal to maintain its required energy even with the consumption of less dry matter. However, in order to prevent digestive disorders such as acidosis and displaced udders, the diet must be properly balanced. High sodium and potassium diets are recommended only for lactating cows.
9-The ration should be completely mixed to minimize the possibility of selection.
10- The water tanks should be close to the manger to increase the frequency of water consumption. Cool and sufficient water should be available to the animals. For every 20 cows, consider a drinking trough with a reserve of 20 liters. Water consumption is significantly reduced due to the reduction of dry matter consumption. Placing clean water near the feeding place of cows encourages them to eat and drink. Cows prefer water between 21 and 26 degrees Celsius to very cold water. The daily water requirement of dairy cows is between 130 and 150 liters. When cows are under heat stress, their water requirement increases 1.2 to 2 times. During heat stress, cows will consume more water after leaving the milking area.
11- Avoid any kind of tension in the herd.
12- Reduce the ratio of fodder to concentrate. In the hot summer months, there should not be any extra fiber, because extra fiber increases the temperature during fermentation in the rumen. Of course, the severe reduction of fodder in the rations consumed in most herds leads to an increase in acid production in the rumen and a decrease in food consumption.
13-Creating canopies on the stables, manger, water trough, leaving around the stables (3.5-4.5 square meters of canopy for each adult cow).
14- Summer cleaning should be observed and the blades of the fans should be cleaned from dust, because it is not possible to move the air properly when the movement of the blades slows down.
15-During the summer, to prevent the accumulation of rotten feed and algae, drinkers should be washed at least once every other day with a diluted disinfectant solution, and more drinkers should be placed near the milking waiting area.
16- Due to the fact that the cows are scattered in the barns in order to cool their bodies, the milking waiting area is the best cooling place, which should reduce the body temperature of the animal immediately after milking by means of ventilators, fans and foggers and using cold water.
17-Reducing waiting time before milking; Reducing the number of cows in each milking group.
18-The place for keeping livestock should be designed and built in such a way that there is good ventilation (building a suitable place in the north-south direction without walls to move air(.
19- The ventilator should be installed at an angle of 30 degrees downwards.
20- Give more attention to cows that are about to give birth and cover their feed troughs to prevent corruption caused by sunlight.
Conclusion: Considering that heat stress is one of the problems that endangers the health of livestock, reduces the production and reproductive performance of livestock, therefore, more attention should be paid to improving breeding conditions and equipment, especially in the hot seasons of the year, rather than the negative effects and Prevent irreparable heat stress on performance and health of cows. Therefore, it is necessary for the breeder to prevent heat stress in his livestock with management solutions and by using equipment in livestock farming, and when heat stress occurs in livestock, he should be familiar with methods of cooling livestock and basic measures to reduce stress in livestock and avoid increasing stress in livestock. Avoid traps. Keeping dairy cows in cool and balanced conditions can bring back as much capital as possible. Also, this work makes the animals more comfortable and, as a result, causes more production. Shelter and cool water should be available to cows and heifers at all times. If possible, cooling devices should be installed in the cowshed and in the feeding area (manger). In the summer, the ration should be balanced and the energy should be increased in general to help compensate for the decrease in the dry matter of the ration.