In the cold winter in the north, especially in the case of extensive feeding, the low temperature and poor nutritional conditions outside, the insufficient energy intake of cows, negative balance of energy metabolism, easy to cause body condition decline or even fat loss, so that when the back fat of these cows is measured by B-ultrasound of dairy cows, it is found that the back fat thickness is decreasing. The reproductive function is affected, showing a long period of lack of estrus.

When the ovaries of dairy cows are examined by B-ultrasound of dairy cows, it is found that many ovaries are in a dormant state. Among the environmental factors, the environmental temperature has the greatest impact on the maintenance needs. It is directly related to body temperature and body heat production. Too high or too low environmental temperature increases the maintenance needs. The maintenance state of animals refers to the adult animals or non-production animals keeping the same weight, the nutrients in the body are relatively stable, and the decomposition and anabolism processes are in zero balance. This need only maintains the basic metabolic processes in life activities.

In the cold winter, the use of plastic film to build a plastic film greenhouse increases the house temperature by about 8.2℃. By building a plastic film greenhouse and supplementing feed, it has a significant effect on maintaining the body condition of cows and promoting early estrus of cows. The construction of plastic film greenhouses still cannot alleviate the accelerated metabolism of cows caused by low temperature, so it has little effect on maintaining the body condition of cows and promoting early estrus. When using dairy Cow B-ultrasound to measure the back fat of these cows, it is found that the back fat thickness is well maintained. It can be seen that insufficient energy intake, negative balance of metabolism, and decreased body condition are the main reasons for the lack of estrus in cows.

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