The causes of hoof cracking in sows are many and complex, closely related to biotin nutrition, pig breed, season, pen type, management level and other factors. Among them, biotin has more influence. In recent years, in the pig production, especially in the "lean" pig breed in the closed housing environment, continuous production of large-scale pig farms, sow hoof cracking caused by hoof disease, resulting in the phenomenon of sow elimination is becoming more and more prominent.

1、The function and metabolism of biotin

      Biotin, also known as vitamin H, belongs to the B vitamin group and is a water-soluble sulfur-containing vitamin. In its natural state, it is a colorless needle crystal, soluble in hot water but insoluble in ethanol, and quite stable at room temperature. It is a low molecular organic substance necessary for the animal body to maintain normal physiological functions, and is an indispensable nutrient for the animal body. Biotin is widely distributed in plants and microorganisms, such as wheat, brewer's yeast, alfalfa, yellow corn, barley, wheat bran, whey powder, etc. Among them, the biotin content and utilization rate in yeast is the highest. In addition, pig colon microorganisms can also synthesize biotin.

1.1 Function of biotin

　　Biotin has a role in maintaining the strength and hardness of the cuticle of the hoof, which is related to limb and hoof keratinization and hoof integrity. In pig production, hoof and limb disease is one of the important reasons for sows to be culled, and hoof cuticle crack is the main cause of hoof and limb disease in pigs. The nutrient metabolism of biotin is related to hoof horn cracking, so biotin is beneficial to sow hoof health. Biotin also affects the reproductive performance of sows such as litter size, conception rate and estrus interval. Biotin can shorten estrus interval, increase litter size of sows, promote uterine dilation and placenta formation during gestation, and increase the length of uterine horn and surface area of placenta. The placenta is a filter between the maternal and fetal blood systems, it is able to select the type and level of nutrients that pass through the placental barrier, and the growth of the fetus after implantation will be largely dependent on this filtering effect of the placenta for nutrients. Biotin will better provide nutrients to the fetus and promote healthy fetal development.

　　In recent years, the growth-promoting effect of biotin on piglets and growing pigs has been studied, and it has been reported that biotin can increase pig weight gain and feed conversion ratio. It has also been reported that the addition of biotin to the diet can improve the carcass quality of fattening pigs, and the mechanism of action is to reduce the content of unsaturated fatty acids in the organism, thus reducing soft fat.

1.2 Metabolism of biotin

　　Natural biotin exists in two forms: bound and free. The bound form of biotin needs to be broken down by biotin-degrading enzymes in the intestine before it can be used by animals.

　　The absorption of biotin in the digestive tract of pigs is influenced by the type and composition of the diet. When crystalline biotin is added or the diet contains high levels of available biotin, the proximal part of the small intestine is the main site of absorption in pigs. According to tests, biotin injected into the large intestine of pigs was found not to increase the level of biotin in the blood, thus indicating that although the microorganisms in the large intestine of pigs can synthesize biotin, their absorption capacity is low.

　　Biotin is a cofactor of many enzymes in the organism and has an important role in maintaining the dynamic balance of metabolism. The main function is to participate in the metabolism of carbohydrates, lipids, proteins and nucleic acids of the body, and in the metabolic process of other nutrients such as methyl transfer reaction, etc. It is also closely related to the activation of lysozyme and the function of sebaceous glands, as well as the metabolism of vitamin B6, folic acid and pantothenic acid.

　　Biotin is absorbed by the body and distributed throughout the body tissues, with the highest content in the liver and kidney, and the intracellular distribution is related to the localization of biotinase. When the biotin absorbed by the animal is higher than its storage capacity, the excess is excreted in the urine.