Abstract: 【Objective】 This experiment was aimed at studying the effects of licorice,Picrasma quassioides extract (hereinafter referred to as licorice extract) and montmorillonite on the digestion and metabolism,rumen fermentation parameters and microbial composition of lactating calves.【Method】 45 newborn Chinese Holstein male calves were selected and subjected to a single factor randomized experimental design.The calves were divided into three treatment groups (5 replicates per treatment,3 calves per replicate):The control group (CON group),the licorice extract group (CPE group),and the compound preparation group (MMT group).The calves in CON group were fed with milk replacer powder and basic feed,the calves in CPE and MMT groups were fed with milk replacers supplemented with 1 g/kg licorice extract and 1 g/kg licorice extract+5 g/kg montmorillonite,respectively.The basic feed and other feeding management were consistent with CON group.The experimental period was 67 days,including a pre-trial period of 7 days and a main trial period of 60 days.Ruminal fluid was collected at 67 days of age to measure rumen fermentation indicators and microbial composition.At 68 days of age,a 7-day digestion and metabolism test was conducted to determine the apparent digestibility of nutrients and nitrogen metabolism indicators.【Result】 ①Compared with CON group,the apparent digestibility of organic matter,crude fat,and neutral detergent fiber in MMT and CPE groups of calves were significantly increased (P＜0.05),and there was no significant difference between the two additive groups (P＞0.05).② Compared with CON group,there were no significant differences in total energy digestibility,total energy and digestible energy metabolism,nitrogen apparent digestibility and deposition rate,pH of rumen fluid,rumen total volatile fatty acid content,and various fatty acid contents between MMT and CPE groups of calves (P＞0.05).The levels of NH₃-N and microbial protein in MMT group of lactating calves were significantly higher than those in CON and CPE groups (P＜0.05),while there was no significant difference between CPE and CON groups (P＞0.05).③ The Simpson index of rumen microbiota in lactating calves in MMT group showed an increasing trend compared to the CON and CPE groups (0.05＜P＜0.10).PCoA analysis revealed that the community composition differences between samples in MMT group were relatively small,and the sample similarity was high.The relative abundance of Firmicutes,Trichomonas,Bacteroides,and Vibrio butyricum in the rumen of lactating calves in MMT group were significantly higher than those in CPE group (P＜0.05),while there was no significant difference compared to CON group (P＞0.05).【Conclusion】 Adding 1 g of licorice extract and a composite preparation (1 g of licorice extract and 5 g of montmorillonite) to every kilogram of milk powder during lactation had an improving effect on the digestion and metabolism of nutrients in calves.The apparent digestibility of organic matter,crude fat,and neutral detergent fiber reached a significant level,and the addition of a composite preparation could increase the synthesis efficiency of rumen microbial proteins and the relative abundance of Helicobacter,Pseudomonas,and Vibrio butyricum genera.

Key words: plant extract; licorice; Picrasma quassioides; montmorillonite; calves