25OHD 3 is the product of the first hydroxylation of vitamin D in the liver, which is then transported to the kidney and further hydroxylated by the 1-a hydroxylase to generate the more biologically active 1,25-dihydroxyvitamin D3 [1,25(OH) 2 D 3 ] (Sibilska, Sicinski, Plum, & DeLuca, 2013). In traditional nutrition, it is generally believed that the function of vitamin D is mainly related to the balance of calcium and phosphorus in the body (Kaantee, 1983). In fact, vitamin D not only is involved in the regulation of calcium and phosphorus after being converted into biologically active 25OHD 3 and 1,25(OH) 2 D 3 in the human body but also is involved in the regulation of various physiological processes in the body by binding to VDR. VDR is expressed in various organs, including ovaries and mammary glands (Stumpf & Denny, 1989). Studies have confirmed that the ovary is the target organ of 1,25(OH) 2 D 3 (Dokoh, Donaldson, Marion, Pike, & Haussler, 1983). In animal studies, it was found that the addition of 6 mg/day 25OHD 3 to the feed of cows for 40 days could significantly increase the calcium and phosphorus content in their milk and the calcium and phosphorus content in their serum, and it could significantly improve the quality of the milk, thereby improving the performance of the cow (Guo, Jones, Givens, Lovegrove, & Kliem, 2018). Adding 50 µg/kg 25OHD 3 to the diet for 118 days improved sow reproductive performance, milk quality, bone status and bone quality in newborn piglets (Zhou et al., 2016); in