Abstract: 【Objective】The biological clock system generally exists at all levels of life, which is closely related to the digestive physiology and growth performance of animals. This experiment explored the relationship between circadian rhythm and rumen fermentation parameters and nutrient digestion and metabolism of Hu sheep, in order to explore the relationship between circadian rhythm and physiological digestion and metabolism of Hu sheep, and provide an important theoretical basis for improving the growth performance and nutrient utilization of fattening sheep.【Method】Forty-five healthy Hu sheep with body weight of (21.57 ± 0.77kg weight)  were randomly divided into 3 groups, with 15 sheep in each group. Each treatment group used the same concentrate supplement and coarse feed, and the concentrate coarse ratio of the basic diet was 56:44. The three treatments were respectively set as follows: day treatment group, that was, 70% of the total daily concentrate + 30% of the total daily coarse feed in the morning, which was recorded as DH; In the evening treatment group, 30% of the daily total concentrate + 70% of the daily total coarse feed were fed in the morning, which was recorded as DL; The control group, i.e. 50% of the daily total concentrate + 50% of the daily total crude feed in the morning and evening, was recorded as CON. After feeding for two months, the digestion and metabolism test was carried out.  Total fecal collection and urine were used to determine nutrient apparent digestibility and metabolic rate.  After the feeding experiment, rumen fluid samples were collected from oral cavity 2 hbefore morning feeding and 2 h before evening feeding, respectively, for the determination of related indexes.  【Results】 ① The daily gain of DH was 215.00g, the food intake was lower than that of DL and CON, and the feed conversion ratio was the best, which was 5.35, which was 11.19% and 16.04% better than the other two groups, respectively. ② Under different feeding modes, the digestibility of dry matter (DM), organic matter (OM), crude protein (CP), neutral detergent fiber (NDF) and acid detergent fiber (ADF) of sheep in DH group were higher than those in CON and DL groups. Compared with DL group, DM digestibility increased by 21.42%, CP increased by 22.29%, NDF increased by 9.85% and ADF increased by 28.69%. ③ The biological value of nitrogen in DH group was 20.31% higher than that of CON and 67.41% higher than that of DL (P>0.05). There was no significant difference in the intake of digestible nitrogen among all groups, and the total nitrogen excretion in DL group was significantly higher than that in DH group (P<0.05). The nitrogen deposition, nitrogen utilization and biological value in DH group were significantly higher than those in DL group (P<0.05) and CON group (P>0.05). ④ There were no significant differences in rumen microbial protein (MCP), acetic acid and total volatile fatty acid (TVFAs) among groups with different treatments (P>0.05), rumen pH, the ratio of acetic to propionic acid and butyric acid in DH group were significantly lower than those in DL group (P<0.05). With the alternation of day and night, rumen pH, NH₃-N, acetic acid, propionic acid, total volatile fatty acid and the ratio of acetic to propionic acid were significantly changed (P<0.05), and related parameters were higher in day than in night.【Conclusion】Growth performance and nutrient digestibility of sheep could be improved by changing the ration of concentrate to forage in the early and late days under the same nutrient content in the diet, and feeding high concentrate ratio in the morning and low concentrate ratio in the afternoon. Rumen fermentation was affected by day and night, and feeding high concentrate diet in day promoted the contents of TVFAs, NH₃-N and MCP in rumen. In a word, from the results of this experiment, the effect was better to increase the proportion of concentrate in the morning. It is more consistent with the rhythm of digestion and absorption of the animal body.

Key words: mutton sheep, hu sheep, circadian rhythm, nutrient digestion, rumen fermentation