Dairy cows generally experience "heat stress" on dairy farms in the summer due to rising temperatures and humidity ". Cow comfort is the key to the maximum production performance of cows. Generally speaking, when the temperature exceeds 20 ℃ and the humidity is higher than 85%, cows begin to suffer from heat stress, which is manifested in that more than 70% of cows in the cattle farm have a respiratory frequency of more than 80 times/minute, rectal temperature exceeds 39 ℃, dry matter intake is reduced by more than 10%, and milk yield is reduced by more than 10%.

Lactating cows themselves produce a lot of body heat. If the air circulation in the livestock house is unfavorable, the body temperature of the cows themselves is very close to the temperature of the surrounding air, so the high temperature and humidity will reduce the feed intake of the cows, resulting in a decrease in the milk yield of the cows, thereby reducing the reproductive performance of the cows.

In addition, changes in the supply of new forage in the summer also affect the feed intake of dairy cows, and the difficulty of adapting to the new seasonal feed supply at the same time as heat stress occurs can lead to lower feed intake, which is the reason for the decline in milk production in many cattle farms in the summer months.

The reduction of feed intake of cows subjected to heat stress is to reduce the heat load caused by the increase of feed body, which is the protective response of cows to cope with high temperature. Based on the nutritional balance of dairy cows, to ensure healthy high and stable yield of dairy cows (to maximize the dry matter intake of dairy cows) is a problem to be solved by dairy farmers. This paper focuses on the dynamics of heat stress in dairy cows, nutritional regulation and management measures to prevent heat stress.

Figure 1 depicts the effect on cows when the temperature rises. The temperature suitable for cows is generally 10-20°C. Below this range, cows must consume their own energy to keep warm (increasing feed intake for cows below the appropriate temperature is the way to generate more body heat). Although milk production does not change in the environment of -5-20 ℃, the maintenance needs and dry matter intake of cows do not change only in the range of 10-20 ℃. Above this range the cow will change its behavior to dissipate heat. Cows regulate their body temperature in hot weather mainly by drinking water, panting (shortness of breath), sweating, and finding a cool and comfortable place.

The degree of heat stress was correlated with temperature and humidity. In the case of low humidity, the temperature must reach 27°C before the animal is stressed, but when the humidity is close to 100%, slight stress can occur at 22.2°C. When the temperature increases to close to 37.8°C and the humidity is also high, the cows will be subjected to severe stress. The following temperature and humidity index (THI) formula can help cattle farm managers to test whether cows are subjected to heat stress and the extent to which heat stress affects cows. It was developed by Dr. Frank Wiersma, an agricultural engineer at Arizona State University.

THI = dry bulb temperature +0.36 × wet bulb temperature 41.2(ASAEstandard,1988)

When the THI value is less than 72, there is no heat stress. At 72-78, there is slight heat stress (72 is the critical point of heat stress),78-90 is moderate heat stress, and 90-98 cows suffer from severe heat stress.

A dairy cow with a heavy lactation burden may need to consume more than about 20kg of feed dry matter per day. In addition to providing energy for maintaining high levels of milk production, this feed will also cause the cow to generate body heat, increasing the stress on the cow when the temperature rises. If preventive measures to mitigate heat stress are not taken, feed intake and subsequent milk production will decline. This is a significant reason why milk production often falls in most dairy farms in the summer.

Considering the nutritional factors, the following measures should be taken to deal with the heat stress of dairy cows:

1. The key to rearing heat-stressed cows is to achieve maximum dry matter intake. Provide sufficient clean drinking cold water (13-18 ℃), which should be the primary consideration. Water becomes a very important nutrient for cows under heat stress (drinking water 1/3 more than usual). If the cow does not receive adequate drinking water, the dry matter intake will not reach the maximum. Drinking water should be ensured to be clean and readily available.

2. Increase the nutrient concentration of the diet by increasing the energy density of the feed (referring to the concentration of energy contained in the feed or diet, usually expressed in kilocalories or kilojoules of energy per kilogram of dry matter). For example, increase the energy concentration of the diet by adding fat to the concentrate. Animal fat, whole cottonseed, soybean, fatty acid calcium, etc. are all good sources of fat. Increasing the energy density of the diet will help to compensate for the decline in dry matter intake caused by heat stress.

Reducing the fiber content of the diet, the decline in dry matter intake of dairy cows fed a high-fiber diet is very large. This is because cows produce much more heat when using fiber than when using starch (concentrate), I .e. the crude fiber in the diet produces more heat for cows. Maintain crude fiber levels, including acid detergent fiber (ADF) and neutral detergent fiber (NDF), at the lowest levels provided by the NRC (National Council for Scientific Research). That is, ADF is 19-21% of dry matter and NDF is 25-28% of dry matter. Do not drop below this level or it may cause digestive problems. Note also these general guidelines, including fat in the diet: 1) The total fat in the diet should not exceed 7- 8% of the dry matter of the diet. 2) 1/3 of the fat should come from grains (such as corn),1/3 from oilseeds such as cottonseed or animal fat, and the other 1/3 from rumen-protected fat.

3. Add juicy feed (such as increasing green materials, root tubers, melons) to diversify the forage mix, improve the palatability of the diet and increase water intake. "Effective fiber" is necessary to stimulate rumination and maintain the rumen environment, which means that 1-1.5kg of long hay must be guaranteed in the diet of dairy cows.

4. To compensate for the decrease in feed intake associated with heat stress, it is beneficial to increase the protein content of the diet. Hassan et al. showed that under the condition of temperature fluctuation of 17.8-32.8 ℃, the dietary protein level of dairy cows was increased from 14.3 to 20.8, the feed intake was increased by 11.3, and the milk yield was increased by 6%. Avoid feeding too much rumen soluble or degradable protein, otherwise the excess protein is deaminated for energy, increase the "heat consumption", aggravate the heat stress. Generally, the rumen non-degraded protein (rumen bypass protein) of high-yielding dairy cows can be increased from 28%-30% to 38-40% of the crude protein content.

5. Increase the level of minerals in the diet: in the hot summer months, cows sweat more, sodium, potassium, magnesium loss is larger, should be supplemented. Dairy cows should adopt a diet of 1.3-1.5 potassium, 0.5 sodium and 0.3 magnesium in summer to help relieve heat stress and improve milk production. However, it is not suitable for dry cows. Excessive intake of sodium and potassium can cause or aggravate udder edema.

Rocky Lick BrickPhosphorus, calcium, zinc, magnesium, iron, cobalt, iodine, manganese, selenium, sodium and other mineral trace elements interact with each other, which plays a very important role in improving the health index, reproduction rate and milk yield of dairy cows.

6. Supplemental feed additives such as buffer (sodium bicarbonate), niacin or yeast. Due to the increase in the amount of vitamin A consumed by cows during heat stress, vitamin A is supplied twice as much as usual in summer.

7. Conditional pastures should be fed with total mixed ration (TMR) to improve the dry matter intake of dairy cows, so that dairy cows can not be picky eaters, rumen fermentation is more stable, nutrient utilization efficiency is higher, in order to ensure nutritional balance and full price. Artificial TMR is generally not advocated because it takes into account factors such as the length of roughage, the uniformity of mixing, and labor.

Increasing the energy density in the diet is the most common way to reduce heat stress in dairy cows. The digestion of crude fiber can produce more calories than energy feeds such as fats and carbohydrates. Reducing crude fiber in the diet and increasing energy density is an effective way to ensure the dry matter intake of dairy cows. Care should be taken to maintain an appropriate level of crude fiber (ADF and NDF) in the diet to prevent digestive disorders caused by excessive energy density.

1. Physical cooling measures: trees (such as broad-leaved fast-growing poplar) are planted on both sides of the cattle farm road or in the open space to improve the microclimate of the cattle farm and beautify the environment.

2. In addition to setting up shade sheds in the center of the sports ground, the cattle farm should also set up shade sheds in the feeding trough and drinking trough. The height of the awning is generally about 3.5 meters, and each cow needs an area of 4 square meters. Although shading does not generally reduce the ambient temperature, it can reduce the heat load on cows by blocking solar radiation. The rectal temperature, respiratory rate and the incidence of clinical mastitis were lower in the cows sheltered by the awning.

3. Often clean the feeding tank to avoid secondary fermentation of feed, drinking tank thoroughly disinfect once a week, in the summer when dairy cows are fed with juicy feed, the sanitary management of the feeding tank is particularly important.

4. Provide electric fans and shower systems for cows in cattle barns and milking halls. This cooling treatment means that the cow is first showered with a low-pressure sprinkler (for 30 seconds), and then a high-power fan is used to blow the cow for about 5 minutes to evaporate the moisture attached to the surface of the cow. The cow farm milks the cow 3 times a day and cools the cow 3 times, which can have a good effect on heat release stress. Such a cooling system can also be placed elsewhere outside the to-be-squeezed area, such as the roof of a cattle barn.

5. The use of scattered cattle pens allows cows to feed and drink freely, increasing the contact time with the feeding trough and drinking trough (relative to the tethered feeding model) and increasing dry matter intake.

6. Adjust the work and rest time: Consider the use of night stocking exercise, that is, let the cows exercise freely outside the house at night, which is conducive to the heat dissipation of the cows, and pay attention to feeding in the cool time at night. Increase feeding times, extend feeding time and increase feed intake.

7. Take effective measures to reduce the breeding of mosquitoes and flies and prevent the harassment of harmful insects to dairy cows.

Summary:

The key to raising and managing cows, especially high-producing cows, is to recognize the importance of maintaining their comfort ("cow welfare"). When the temperature rises in the summer, sometimes the cow will even stop eating, so it is very important to keep the cow in the most suitable comfort. The above nutritional control measures and common sense will help to deal with the heat stress caused by the high temperature of dairy cows in summer.