What is heat stress in cows?
Dairy cow heat stress refers to the summation of non-specific responses of dairy cows to high-temperature environments that are not conducive to physiological conditions. Its purpose is to mobilize the defensive function of the body to overcome the adverse effects of high-temperature stimuli and ensure the stability of the body in extreme circumstances.
The normal body temperature of dairy cows is about 38.5℃, and the suitable feeding environment temperature is 10 ~20 ℃. In this range, dairy cows do not need to use extra energy to warm or cool their bodies, which is conducive to the best performance of dairy cows. When the ambient temperature exceeds 25℃, there will be a thermal stress reaction. In areas with high humidity, there will be thermal stress reaction when the temperature exceeds 23 ℃.
How to judge the heat stress of dairy cows?
Ten dairy cows were randomly selected and their rectal temperature was measured. If the rectal temperature of more than 7 cows is higher than 39.4℃, it indicates that the cows are in a state of heat stress. The rectal temperature of dairy cows under severe heat stress may reach 40 ℃.
The respiratory frequencies of 10 dairy cows were measured. If 7 cows breathe more than 80 times per minute, it means that the cows are in a state of severe heat stress. If more than 5 cows breathe more than 100 times per minute, urgent measures should be taken to reduce the fatal damage caused by heat stress to cows.
The dry matter intake or milk yield of dairy cows was measured. If dry matter intake or milk production is reduced by more than 10%, it also means that cows are in a state of heat stress.
Milk quality problems caused by heat stress in dairy cows
1. Milk is low in acidity.
The causes of low acidity are as follows:
(1). The intake of dairy cows decreases when the ambient temperature is between 25 and 27 degrees centigrade; when the ambient temperature rises above 30 degrees centigrade, the intake decreases sharply; when the ambient temperature reaches 40 degrees centigrade, the intake decreases by 60 to 80 percent. During heat stress, the dry matter intake of dairy cows could not meet the needs of lactation, which leads to the decrease of organic acid and inorganic acid content in milk, resulting in the decrease of lactic acid in cows.
(2). Dairy cows alleviate heat stress by themselves, through asthma and heat dissipation, excessive pulmonary ventilation reduces plasma H2CO3 concentration or primary PaCO2, which leads to an increase in pH value.
(3). The high temperature and humidity environment caused an increase in the incidence of recessive mastitis in cows. The milk of cows with mastitis was alkaline and not isolated, which leads to the low acidity and alkalinity of milk.
(1). To alleviate heat stress through physical cooling of the cowshed, using water conditioner and fan.
(2). Rumen fat or cottonseed were added to the diet of dairy cows and feed with high-quality forage to reduce heat consumption and increase DMI of dairy cows.
(3.) Adjusting the feeding ratio of the ranch, morning: noon: evening = 5:2:3, and feeding at night.
(4). Rumen protein was added to ensure amino acid balance in the dairy cow diet and avoid excessive lysine.
(5). Each cow was tested for mastitis and promptly found and treated.
2. Milk is alcohol-positive.
The causes of alcohol positive:
(1) Because heat stress can cause imbalance of ionic balance in body fluid of dairy cows, in order to alleviate the discomfort caused by heat stress, dairy cows regulate themselves through perspiration, asthma, metabolism and other aspects, resulting in a large loss of sodium ions, a small loss of potassium ions, and an increase in the content of free calcium ions. Calcium ion adsorbed on the surface of milk microparticles and combined with it to neutralize the charge of casein, which changed the balance of electrostatic interaction between microparticles, destroyed the balance of colloidal dispersion system composed of microparticles, dehydrated and coagulated casein without charge under the action of alcohol, and formed an alcohol-positive emulsion.
(2) In summer, the amount of milk in the ranch is less, the stirring is not uniform, and the upper layer of refrigeration tank freezes, which results in the weakening of the stability of protein molecular structure in milk. Under the action of alcohol, it is easy to dehydrate and coagulate, forming alcohol-positive milk.
(1). Physical cooling of cowshed through water conditioner and fan.
(2).High-quality forage (alfalfa, leymus chinensis) was preferred, and rumen-passing fat and rumen-passing protein were added to the diet of dairy cows.
(3). vitamin E, C and A should be added appropriately. Increase sodium chloride and potassium chloride 40-70 g/day, baking soda 100-50 g/day, and magnesium oxide 60-80 g/day.
(4). Avoiding unnecessary artificial stress in dairy cows.
3. The decline of the physicochemical index of cow milk decreased
Causes of decline:
(1) Under heat stress, the body temperature of dairy cows is too high, and the enzymes, intermediates, and proteins involved in physiological and biochemical reactions are inactivated, resulting in fewer amino acids and proteins involved in the synthesis of lactoprotein.
(2) Dairy cows are affected by heat stress. Daily dry matter intake is insufficient and energy is relatively deficient.
(1) Physical cooling of the cowshed, through water conditioner and fan.
(2) Use Chinese herbal medicine additives to alleviate heat stress, that is gypsum, licorice, and pulsatilla Chinensis.
(3) To regulate the ratio of concentrate to roughage by 60:40, reduce the heat production of rumen fermentation and avoid rumen acidosis, we should feed high-quality crude material (cut alfalfa) and increase the byproduct of high-fiber fermentable digestive feed, such as beet meal. And supplement buffers (baking soda and magnesium oxide).