I. Mechanisms of high temperature affecting milk production The reduction in feed intake in dairy cows during the high season is a direct cause of the decline in milk production. The feed intake of cows often increases or decreases with changes in temperature. This is because the carcass is undergoing digestion and absorption at the same time during the intake process. In a heat stress environment, the hyperplasia heat produced by the intake is An additional burden, so as the temperature rises, the cow's feed intake will decrease. Due to the reduction of feed intake, the lack of nutrients in the body and the increase in energy consumption, especially in the lactating dairy cows, would need to absorb a large amount of nutrients, but at this time the intake is insufficient to meet the nutritional needs of lactation, resulting in production. Milk volume decreased by 17% to 20%. In addition, under the influence of high temperatures, dairy cows like concoctions, and anorexia and coarse foods reduce the proportion of lower fatty acids, such as acetic acid and propionic acid, in the rumen, resulting in a decrease in milk fat percentage. The reason for the decrease in milk production during the high temperature period is not only related to the reduction in feed intake, but also related to the fact that the high temperature slows down the rate of passage of feed through the digestive tract and high temperatures significantly reduce the feed utilization rate of milk production. In general, the following factors affect the intake of dairy cows. 1. The high temperature acts on the feeding center and reduces the feed intake. The bovine hypothalamus has two centers that regulate feed intake, one is a palatable center, and the other is an anorectic center located in the upper part of the hypothalamus and located in the medial nucleus of the hypothalamus. They accept signals from a variety of external receptors, including chemoreceptors, glucose receptors, stomach extension receptors, mechanoreceptors, and temperature sensors. Glucose receptors are sensitive to changes in blood glucose levels. Gastric efferent receptors are sensitive to changes in gastric volume. Temperature sensors reflect body temperature and temperature changes. Many physiological and environmental studies suggest that there are two mechanisms of action that regulate feed intake in cattle: a constant warming mechanism and a chemically constant mechanism. However, the mechanism of how the two mechanisms regulate animal feed intake is not yet clear. Most people tend to think that the constant warming mechanism is the mechanism of action of feed intake regulation in domestic animals. It causes satiety by increasing the intake of calories, and additional heat is produced in certain cells or other parts of the body below the hypothalamus. Change in feed intake. 2. High temperature reduces thyroid secretion. Thyroid hormone is the main hormone regulating the body's metabolism, growth, development and other physiological processes. Westra et al. reported that the thyroid secretes a function that promotes peristalsis of the digestive tract and shortens the transit time of the chyme. When the ambient temperature reaches the maximum temperature of the comfort zone, the secretion of thyroid gland is significantly reduced, and the daily temperature range of the cow is 21.1 to 37.8°C. The activity of the thyroid gland is reduced by 30% compared with the daily variation range of 4.4 to 21.1°C. Less thyroid hormones in the body will undoubtedly affect the motility of the gastrointestinal tract of the livestock and prolong the transit time of the chyme. The filling of the stomach will be transmitted to the thalamus through the gastric extensor receptors on the stomach wall to control the intake center. Feedback will be reduced. Eating. 3. When the temperature rises, the blood vessels of the skin expand, a large amount of blood flows to the skin, and the body metabolizes the heat to the surface to increase the non-evaporative heat dissipation. At this time, the lack of blood flow in the digestive tract affected the rate of absorption of nutrients, filled the digestive tract and increased the tension in the stomach, thus inhibiting intake. 4. At the same time as the body dissipates heat, a large amount of water is evaporated in the body along with the loss of heat, and the thirsty cow's desire to increase. Due to the large amount of drinking water, the body feels full, but it also dilute the gastric juice, thereby reducing the digestive capacity of gastric juice. 5. Cows develop hot wheezing during heat stress, affecting feeding time and feed intake. II. Coping strategies for raising dairy cows at high temperatures Due to heat stress, the physiological, chemical, immunological and metabolic disorders of dairy cows result in decreased production performance, reduced immune defenses, increased digestive, reproductive and respiratory diseases, and increased mortality. In the production, effective measures should be taken to reduce the adverse effects of high temperature environment on dairy cows and improve the production performance of dairy cows. 1, pay attention to nutrition regulation, improve feeding tube way. Dairy cows have large stomach capacity, large feed intake, and strong digestive system function. The energy conversion and protein conversion rate for feeds are 17% and 25%, respectively, accompanied by more calories generated during the digestion process, and due to particularly strong lactation performance, lactation There are also many calories produced by metabolism, which is the fundamental reason why dairy cows are cold and not heat-resistant. Under hot climate conditions, the adjustment of feed structure, increase of palatability, and maintenance of nutritional balance can compensate for the reduction of nutrient intake caused by heat stress. (1) Feed the green juicy feed. Juicy green feed such as high-quality green grass, carrots, potatoes, etc., rich in carbohydrates and a lot of water, not only good palatability, and can quench their thirst, cool the heatstroke and relieve cow heat stress is very beneficial. Summer should be fed less hay, can be fed properly fermented sugar, bean dregs, brewers, etc., to increase milk production, increase milk fat rate is good. (2) Adjust dietary composition. After the cows eat feed, they produce a certain amount of heat in the body, called heat gain. The amount of heat consumption is not only proportional to the amount of feed but also to the type of feed, and roughage is greater than concentrate feed. The heat consumption of dairy cows for concentrates accounts for about 15% to 20% of the total feed energy, 25% to 40% of the metabolizable energy, about 20% of the total energy, and 40% to 80% of the metabolizable energy. In terms of pure nutrients, protein heat gain is the largest, accounting for about 30% of metabolizable energy; carbohydrates are centered, accounting for about 10% to 15% of metabolizable energy; fat is the least, accounting for about 5% to 10% of metabolism. It can be seen that, in a hot and hot environment, in order to ease the high temperature stress of dairy cows, the amount of roughage should be appropriately reduced, and the content of protein and fat in the diet should be appropriately increased. The content of corn, bean cake and fish meal can be increased in a suitable amount in the compound feed, so that the protein concentration can be increased by about 4% compared with the normal level, and the ratio of crude protein in the rumen protein can be 35% to 38%, and fatty acid calcium, whole-grain cottonseed, and other rumen-trans fat can be added. So that the fat content in the diet accounted for 5% to 7%. (3) Adjust the feeding time. The feeding time should also be adjusted. The temperature is high at noon in the summer and the cows have low appetite. According to reports, during the high-temperature season, the intake of dairy cows accounts for 75% to 80% of the daily intake during the colder nights and early morning hours, while the high-temperature cows can only consume 20% to 25% of the total intake. It is best to add more ingredients in the morning and evening to increase feed intake and meet their nutritional needs. 2, add anti-heat stress drugs. In the high temperature season, the cow's skin evaporation, drinking water and urine output increase, and the loss of potassium is significantly higher than that of sodium. Therefore, potassium chloride can be added to the diet. The dosage is 180 grams per day per cow, divided into three batches. Hey. If you feed more concentrate at high temperatures, you should add sodium bicarbonate at the same time. The recommended dose is 150-200 grams per day per cow. Adding 300 grams of sodium acetate per day to the diet of dairy cows can relieve the inhibitory effect of high temperature on milk production to a certain extent, and the milk production and the total secretion of milk fat increase significantly. In the case of heat stress, adding certain compound enzyme preparations, rumen, yeast cultures, etc. in the diet has a good mitigation effect. In addition, some Chinese herbal medicines that have the effect of clearing away heat and cooling the skin and detoxifying blood, and the dual functions of drugs and nutrients, can effectively relieve heat stress. For example, gypsum, Radix Isatidis, Radix Scutellariae, Atractylodes lanceolata, Radix Paeoniae Alba, Radix Astragali, Codonopsis pilosula, light bamboo, licorice and other Chinese herbal medicines were prepared and crushed at a certain ratio, and then the dairy cows were fed with diets added during the summer for 2 months. The average milk production was daily. The head cows increased by 1.5 kg and the milk composition did not change significantly. 3. Greening the environment and improving the microclimate on the site. Greening has the function of alleviating solar radiation and reducing the temperature of the environment, in addition to purifying the air (absorbing oxygen from carbon dioxide, absorbing harmful gases, absorbing smoke, sterilizing, etc.), reducing noise, preventing winds, preventing fire, beautifying the environment, and regulating the microclimate. The cooling effect of greening is mainly reflected in the following points: (1) By means of transpiration and photosynthesis of plants, solar radiant heat is absorbed to reduce the temperature. The area of ​​the leaves of the trees is 75 times that of the trees planted. The area of ​​the grass blades on the grass is about 25 to 35 times the area of ​​the grass. These green leaf areas, which are more than a dozen times larger than the area planted, greatly absorb solar radiant heat through transpiration and photosynthesis, which can significantly reduce the temperature. It was determined that if water, precipitation, and transpiration (which generates 1 gram of tree tissue to evaporate 300-400 grams of water from the root-absorbed soil) accumulated at night on the foliage were added together, the water needed per square meter was added. Ye Lin can evaporate 250 kilograms of water in one summer, evaporating 1 kilogram of water and absorbing 585 kcal of heat of evaporation. In addition, plants also absorb a large amount of solar radiant heat during photosynthesis. Therefore, in the same period of temperature, the forest area can be reduced by 3 to 5°C than the non-forest area. (2) Reduce the radiation by shading. The canopy can reflect 20% to 25% of the solar radiant heat into the atmosphere, 30% to 70% of it is intercepted by the forest canopy, and only 5 to 40% of it is shot through the canopy to the forest surface. In July and August in Beijing, when the temperature of the asphalt pavement is 30-40°C, the lawn temperature is 22-24°C. In the hot summer days, the exposed surface temperature without trees is far higher than the temperature at that time. For example, when the maximum temperature at an altitude of 1.5 meters is 31.2 degrees Celsius in an open field and the highest ground temperature is 43 degrees Celsius, the animal body is exposed to both direct solar radiation and surface radiation heat. Increase the feeling of heat. The ground temperature in the green space is much lower than the open space, which is generally lower by 10 to 17.8°C. (3) Through the moisture held by the plant roots, a large amount of heat energy can be absorbed from the ground to cool down. Because of the above-mentioned cooling effect of greening, the air is cooled and the temperature of the ground is lowered, so that the amount of heat radiated to the roof, the exterior wall and the doors and windows is reduced, and the shade of the trees is blocked by the sunlight and the temperature inside the house is lowered. 4. Strengthen the heat insulation design of the perimeter protection structure in the noon house. In the hot season, there are three causes of overheating in the barn: high atmospheric temperatures, strong solar radiation, and heat generated by cows in the house. The purpose of strengthening the thermal insulation design of the housing protection structure is to prevent and reduce the impact of high temperature and solar radiation on the environment. First, under the premise of comprehensively considering the requirements of other architectures and drawing materials, materials with small thermal conductivity are selected to enhance thermal insulation of roofs and walls. Second, since white and other lighter colors have a stronger ability to reflect light, the use of light-colored construction materials for the external protective structures, especially roofs, can reduce solar radiant heat and have a certain significance for mitigating the effects of strong sunlight on the interior of the house. . 5. Install a fan in a dairy cowhouse or in a place where the cows are resting. Under conditions of high temperatures, the cows will feel comfortable by increasing the airspeed. This method is simple and easy. At high temperatures, as long as the air temperature is lower than skin temperature, increasing the flow rate is conducive to convection heat dissipation. 6, cowshed cooling. Through thermal insulation, ventilation and sunshading, only the influence of solar radiation and temperature on the temperature in the house can be weakened and the heat energy released by the carcass in the house can be dissipated, and the atmospheric temperature cannot be reduced, so when the temperature is close to, equal to or higher than the body temperature of the cow In order to ease the impact of high temperatures on the health and productivity of dairy cows, cooling measures must be taken. Cooling measures commonly used in dairy cow houses include evaporative cooling, the use of chillers, and mechanical refrigeration.
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