High temperatures and the impossibility of giving away heat to the environment could lead to serious changes in the body and even death of the animal. Therefore, heat stress is a problem that needs to be considered, understood and prevented in advance. Disregarding the symptoms of overheating can expose us to serious financial losses.
What is heat stress?
Cows producing milk intensively can maintain a constant body temperature when the ambient temperature does not exceed 25°C. The more a cow produces milk, the metabolism is higher and the more milk is produced.Therefore, high-performance cows must be able to return excess heat to the environment. When high temperatures prevent heat from being released into the environment, this can lead to heat stress.
Heat stress is a big loss
In the USA, losses caused by heat stress in the dairy industry reach up to 1.5 billion dollars. Overheating causes many cows to loses in their bodies. Each of the problems described below leads to large economic losses that cattle farmers have to face with them.
Heat stress causes physiological changes in a cow’s digestive and endocrine system. It causes, among other things, an increase in peripheral blood flow and breathing, sweating, looking for a place to cool down, which does not affect productivity well.Then the metabolism of nutrients changes, which results in a decrease in productivity. There is a visible decrease in milk production in the range of 10 to 25%. It should also be added that cows in lactation are more exposed to high air temperatures than dry cows. It has been shown that, in the case of dry cows, the occurrence of heat stress during the 60 days prior to calving has negatively affected the productivity of the cow.
Then the metabolism of nutrients changes, which results in a decrease in productivity. There is a visible decrease in milk production in the range of 10 to 25%. It should also be added that cows in lactation are more exposed to high air temperatures than dry cows. It has been shown that, in the case of dried cows, the occurrence of heat stress during the 60 days prior to delivery has an adverse effect on the start and half of the lactation, yields and fat content of milk are reduced.
During heat stress, the insemination rate falls from 61% to 45% when the temperature measured in the rectum rises by 1°C. Cows calved in spring or summer have difficulties in insemination, the inter-calving period is extended. High temperatures in late pregnancy can lead to a reduction in the placenta, lower production of hormones and, consequently, a reduction in the mammary gland. The summer season has a negative impact on embryo evolution, especially from 8th to 16th day or 27th to 40th day of pregnancy. Heat stress causes a reduction in the flow of cord blood. This results in changes in the formation of the embryo, and its smaller size. Cows exposed to heat stress in the last three weeks before birth and 36 hours after birth show lower concentration of immunoglobulins in colostrum.
At higher ambient temperatures, the levels of somatotropin, triiodothyronine and thyroxine also decrease in order to reduce metabolic heat. There are also biochemical changes in blood, and an increased number of breaths intensifies carbon dioxide losses. This causes a decrease in the concentration of carbonic acid in the blood. In such a situation, the balance between carbonic acid and bicarbonate, necessary to achieve pH, is disturbed. Sweating determines a significant loss of potassium from the body. High temperatures reduce the success of the first insemination between 60 and 66 days after birth.
How to recognize heat stress in advance?
In order to prevent such great losses, the behavior of the animals should be observed. If cows consume less fodder, have trouble with reproduction and thus reduce milk production by up to 68-2072 kg/cow, this is probably due to too high an ambient temperature. Studies have shown that during heat stress, the average time cows spend lying down decreases from 10.9 to 7.9 hours per day. Healthy animals spend about 12-13 hours per day lying down. There is a linear relationship between the temperature outside and the number of cows lying down and an inverse relationship between the number of cows lying down and temperature. During hot weather, the time that cows spend on water intake also increases from 0.3 to 0.5 hours per day.
How to prevent the disease ?
Cooling cows during the last three weeks of during dry period the hot season reduces stress, so that the drop in production is not as drastic as for animals exposed to high temperatures. Decreasing the ambient temperature improves liver metabolism, followed by lactation metabolism. There is a higher fat level in milk and an average yield of 7,5 kg for cooled cows. Cooled animals consume more dry matter 1,6 kg/day. The temperature measured in the rectum, in the afternoon of cows exposed to heat stress is increased to 39.5°C, while in cows undergoing the cooling procedure to 38.2°C. Their milk yield during 305 days of lactation is about 13.6% higher compared to the control group.
