With the popularization and popularization of cow frozen semen technology, the quality requirements for the breeding bulls are getting higher and higher, the selection of breeding bulls is becoming increasingly stringent, and the importance of breeding bulls for improving and improving the entire herd appears more prominent.

Bulls are the main factor affecting the genetic quality of herds and are the main driving force for herd genetic improvement. An excellent cow naturally breeds more than 10 calves for a lifetime. However, it is possible to breed more than 20,000 cows by artificial insemination of a bull for 1 year. In developed countries such as Europe and the United States, the bulls have started to breed from yak. ​​Only about 5% of the bulls can finally be measured and put into use through their descendants. It can be said that they are the best in the world. These excellent bulls have greatly promoted the improvement of dairy cows and are an important reason for the high production performance of dairy cows in developed countries. In China, bull determination should also be carried out to increase the bull's selection intensity and promote the use of excellent semen frozen semen. This is an economical, rapid, and effective method for improving China's herd.

In the breeding of dairy cows, artificially inseminated bulls can bear the mating of more than 10,000 cows per year. The genetic advantage of excellent breeds of bulls can be maximized. The level of production of dairy cows depends to a large extent on the genetic level of the bulls. The contribution of bulls to the genetic improvement of dairy herds can reach 75% to 95% of the total genetic progress. Therefore, the selection of excellent bulls plays a very important role in the breeding of cows.

The choice of dairy bulls can be divided into three stages:

(1) Prior to sexual maturity, the yak stage, selection is generally made using the pedigree index combined with the growth and development of male calves. According to the data of the ancestors, predicting the performance of milk production by male calves is the most common method for selecting bulls.

According to estimates, of the four factors affecting genetic progress, the father of the bulls accounts for about 39% and the mothers 32%; the fathers of cows account for 26% and the mothers only 3%. It shows that the parents of male calves have a great impact on genetic progress, which can reach 71%. Therefore, the choice of male yak is the first choice of parents of male yaks.

When choosing a male resident, you must select the offspring of the best male cow. The requirements for the mother of the bull were as follows: a child had a milk output of 7,500 kg in 305 days, an output of more than 8,000 kg in a second child, a milk fat percentage of 3.3% or more, and an appearance score of 80 points or more; breast structure, limbs, and chest and abdomen volume were not Obvious defects, relative to compound breeding values ​​above 102%. Requirements for the bull's father: Bull's daughter's first child has a milk output of more than 8000 kg in 305 days, a milk fat percentage of 3.5%, and a milk protein rate of 3.0% or more; main functional appearance traits such as breast structure and texture, limbs and hooves, and breeding values Can not be negative, and there must be a clear improvement, the overall appearance score of 80 points or more, the Bull's lifetime benefit index ranked within 50.

(2) When the calf grows to be over 1 year old, it can select (individual grade selection) with reference to its own performance and directly measure some of its economic traits. The self-selection is based on the phenotypic value of the individual species and one or more traits of the breeding cattle to determine their species value, so as to determine whether the individual chooses to stay. This method is also called performance measurement and performance test. The specific approach is to compare it with other individuals in all herds, or to compare with the average level of the herd, in a condition where the environment is consistent and with accurate records. It can also be compared with the identification criteria.

The body appearance of the bull mainly depends on the size of the body, whether the body structure is uniform, whether the appearance and the coat color meet the requirements of the breed, whether the male characteristics are obvious, and whether the appearance of the bull is obvious. Such as the bull mother phase, the limbs are not strong and strong, limbs are not correct, the back line is not flat, neckline thin, chest narrow abdomen, shoulder oblique people. Reproductive organs are not well developed, testicular size is not normal and there is no elasticity. Anybody who has obvious defects in body appearance or genital malformations will not be planted. The total score of the bulls must not be lower than one level.

In addition to the appearance, the body scale and body weight of the breeding bull were also measured, and grades were graded according to the breed criteria. In addition, you need to check the quality of semen.

(3) After sexual maturity, that is, the stage of young bulls, descendants are determined according to their performance and appearance. Since the characteristics of milk production traits are restricted by gender, it is necessary to identify the descendants by measuring the milk production performance of the selected bull's daughter. Therefore, the selection of bulls takes a long time, often 5 to 6 years. In order to save the cost of selection, many countries have collected semen at the age of 12 months of bulls. After saving 10,000 to 40,000 heads of bulls, the bulls will be eliminated. If the descendants are determined to be excellent bulls, then use the semen of these bulls, otherwise all semen will be discarded. The excellent young bulls cultivated in China are mainly cultivated from abroad with excellent seeds (frozen sperm and embryos). Young bulls are generally used to breed populations and production groups before being verified, and some bulls are measured for offspring.

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