The harm of heat stress to dairy cows and the countermeasures

The concept of heat stress: the cow’s own metabolism is relatively stable at a suitable temperature, and its heat production and heat dissipation are balanced. The body maintains a normal body temperature and can carry out normal production. Its efficiency is also the highest at this time. We call this temperature range Suitable temperature zone. When the ambient temperature rises to a certain level, the heat dissipation of the cow is affected. The heat production is greater than the heat dissipation, which will cause the body temperature to rise, and the cow will use the body function to accelerate the heat dissipation rate and maintain the stability of the body temperature. For example: gasping for breath, quicker breathing, drooling, standing unwilling to lie down, unwilling to eat, decrease in milk production, etc. These reactions that the cows are forced to use body functions to regulate their own temperature stably as the ambient temperature rises are called heat stress.
How to determine whether heat stress has occurred? Whether heat stress occurs can be determined by THI (Temperature and Humidity Index). When THI>72, heat stress occurs in dairy cows. The heat stress of high-yielding dairy cows is more obvious and more sensitive than other groups, and heat stress will appear at 67,68.

The hazards of heat stress:
1. Reduce milk production and milk quality and taste:
Under heat stress conditions, dairy cows' feed intake decreases and energy intake is insufficient. Increased breathing rate increases the body's energy requirements to balance body temperature, which increases the body's protein and fat decomposition and decreases its synthesis, leading to stagnant growth of dairy cows, weight loss, and reduced feed conversion rate. Insufficient fat and protein synthesis leads to a decrease in milk volume, changes in milk composition, and a decrease in milk taste.

2. Increased hoof disease:
Under heat stress, cows are unwilling to lie down, standing for a long time, increasing the burden on the limbs and hoofs, and the limbs and hoof joints cannot get adequate rest, which in turn increases wear and tear. In addition, the spray water cannot be drained in time, and the hoofs are soaked in water. Long, softened cuticles, bacteria are easy to invade, and the risk of limb and hoof disease is increased.

3. Break the electrolyte balance:
Dairy cows are affected by heat stress, drinking more water, the loss of sodium ions is accelerated, and the concentration of renal tubules is lost, which leads to an increase in urine volume and accelerated loss of potassium ions, which reduces the blood volume of the kidney tissue and causes electrolyte disorders. In addition, the increased respiratory rate and hyperventilation reduce the plasma carbon dioxide, which in turn leads to an increase in pH. The main buffer system in the body of dairy cows is the chemical balance system of bicarbonate and carbon dioxide. Bicarbonate acts as an alkaline substance, and carbon dioxide acts as an acidic substance. Therefore, the increase of bicarbonate or the decrease of carbon dioxide makes the blood appear alkaline, which in turn causes respiratory alkalosis.

4. Rumen acidosis:
When heat stress occurs, the body temperature of dairy cows increases, and at the same time, the temperature in the rumen increases, which accelerates the production of lactic acid bacteria and decreases the pH of the rumen, leading to the occurrence of rumen acidosis. In addition, in order to reduce the energy consumption of dairy cows to maintain body temperature, some farms adjusted the ratio of fineness to coarseness to reduce ruminant and insufficient saliva secretion. When dairy cows suddenly eat a large amount of feed containing highly fermentable carbohydrates, they will quickly produce a large amount of volatile fatty acids. It is absorbed in time or neutralized by saliva, resulting in a drop in rumen PH. When rumen PH drops to a certain level, saliva secretion and rumen peristalsis are inhibited, rumen microflora changes, a large amount of lactic acid is produced, and rumen acidity drops sharply. Rumen acidosis occurred.

5. Increasing incidence of mastitis and diarrhea:
The high-heat and high-humidity environment in summer is a hotbed of pathogenic microorganisms, which accelerate their growth under suitable conditions. Heat stress will increase the secretion of glucocorticoids, inhibit the production and differentiation of T lymphocytes and B lymphocytes, resulting in a decline in the immune system of dairy cows, and increasing the risk of mastitis. In addition, the feed intake of dairy cows decreases, ruminating decreases, and the increase in drinking water leads to an increase in gastrointestinal permeability, and a large amount of endotoxins are produced to cause diarrhea in dairy cows.

6. Decreased reproduction performance:
For the ranch, this is the most serious loss. Heat stress reduces the secretion of hormones such as follicle stimulating hormone (FSH) and prolactin (LTH), so that cows do not estrus, shorten the estrus time or covert estrus, so that the pasture breeders cannot report estrus in time and miss the best mating time. In addition, the temperature is too high, it also has an inhibitory effect on the gonads of the bull, causing it to decrease libido, poor sperm development, and affect the quality and vitality of semen. The lengthening of fetal space and the increase of lactation days have a great relationship with heat stress.

What measures should be taken to solve the above problems?

1. Improve dietary energy and nutrition. Increase the proportion of concentrate and reduce the heat generated during the digestion process.
2. Change the milking and feeding time, and increase the feeding ratio and milking as much as possible in the early morning and night time when the temperature is lower, and increase the number of feedings per shift.
3. Increase the rumen PH value buffer material, and add 1% sodium bicarbonate as a buffer in the diet, which is beneficial to maintain the acid-base balance of the cow's body.
4. Provide adequate drinking water, especially the cows return to the shed for about an hour after milking, and there will be a drinking peak after eating the forage.
5. Add anti-stress products to the diet.
6. Build sunshade nets or sunshades.
7. Provide spray and fan, especially in the waiting area of the milk parlor, where the density of the herd is high, and the heat dissipation is slow. The number of spray and fan should be increased.