Abstract: This study evaluated the effects of different levels of dietary digestible energy and crude protein on nutrient digestion and metabolism, and plasma physiological and biochemical parameters and hormones during early lactation of Yili mares, in order to provide reference information on the nutritional requirements, particularly digestible energy and crude protein, of the mares. A total of 25 Yili mares within 7 days after parturition were selected and divided into 5 groups (Groups Ⅰ-Ⅴ) using a stratified random procedure according to their body weight, with 5 animals (replicates) in each group. In the first month of lactation, dietary digestible energy and crude protein levels of the five groups were: 101.41 MJ·d^-1 and 1.38 kg·d^-1 (Group Ⅰ), 112.05 MJ·d^-1 and 1.50 kg·d^-1 (Group Ⅱ), 122.40 MJ·d^-1 and 1.63 kg·d^-1 (Group Ⅲ), 133.27 MJ·d^-1 and 1.75 kg·d^-1 (Group Ⅳ), and 143.84 MJ·d^-1 and 1.87 kg·d^-1 (Group Ⅴ). In the second month of lactation, dietary digestible energy and crude protein levels were 129.73 MJ·d^-1 and 1.82 kg·d^-1 (Group Ⅰ), 140.37 MJ·d^-1 and 1.94 kg·d^-1 (Group Ⅱ), 150.72 MJ·d^-1 and 2.07 kg·d^-1 (Group Ⅲ), 161.60 MJ·d^-1 and 2.19 kg·d^-1 (Group Ⅳ), and 172.17 MJ·d^-1 and 2.31 kg·d^-1 (Group Ⅴ). The trial lasted for 60 days, with two test cycles of 30 days. At the end of each test cycle, feces and urine were collected to determine feed digestion and metabolism of nutrients and blood was collected to determine nitrogen and lipid metabolism parameters, and concentrations of various hormones. It was found that the body weight, average daily gain, apparent digestibility values of dry matter, organic matter, gross energy, neutral detergent fiber, crude protein and phosphorus, as well as the nutrient utilization rates of digestible energy, digestible nitrogen and phosphorus, were not affected by dietary digestible energy and crude protein levels (P>0.05). The apparent digestibility of calcium in Group Ⅴ mares was 14.14%, 27.08% and 24.13% higher, respectively, than that of Group Ⅰ, Ⅲ and Ⅳ mares (P<0.05), while the apparent digestibility of calcium of Group Ⅱ mares was 21.91% and 19.09% higher than that of mares in Groups Ⅲ and Ⅳ (P<0.05). The retention rate of digestible calcium of Group Ⅴ mares was 33.56%, 54.79% and 26.46% higher than that of mares in Groups Ⅰ, Ⅲ and Ⅳ (P<0.05), and the retention rate of digestible calcium of Group Ⅱ mares was 36.02% higher than that of Group Ⅲ mares (P<0.05). The availability of digestible calcium in mares of Groups Ⅱ, Ⅳ and Ⅴ was, respectively, 23.24%, 27.67% and 34.16% higher than in mares of Group Ⅲ (P<0.05). Dietary digestive energy and crude protein levels had no significant effects on plasma GLT, AST, FFA, I-FABP, VLDL-C, TG, TP, BUN, estrone, estradiol, pituitary prolactin, progesterone, T3, GH and INS levels. The concentration of T4 in Group Ⅱ mares was, respectively, 34.83%, 26.57%, 30.36%, 34.39% higher than that of mares in Groups Ⅰ, Ⅲ, Ⅳ and Ⅴ (P<0.05). In conclusion, in the first and second month of lactation, dietary digestible energy levels of 112.05 and 140.37 MJ·d^-1, respectively, and crude protein levels of 1.50 and 1.94 kg·d^-1, respectively were able to supply the early lactation nutritional requirements of Yili mares, and promoted calcium digestion and metabolism, and levels of plasma thyroxine.

Key words: Yili mare, early lactation, digestive energy, crude protein, digestion and metabolism, blood physiological-biochemicals, hormones