The aim of this experiment was to investigate the effects of melatonin on rumen fermentation param-eters and microflora of dairy cows in mid-lactation period.Forty-eight Holstein cows with similar lactation age[(143±43)d],parity[(2.75±1.23)parities]and milk yield[(36.80±4.18)kg]were selected and ran-domly divided into four groups of 12 cows each,On the basis of feeding a basal diet,different doses[80(tri-al group I),120(trial group Ⅱ)and 160 mg/d(trial group Ⅲ)]of melatonin were administered at the time of daily morning feeding in three groups,and the melatonin were pre-dispensed in edible glutinous rice cap-sules,while the control group was administered empty glutinous rice capsules.The experimental period was 37 d,including 7 d of pre-test period and 30 d of test period.The results showed as follows:1)compared with the control group,the concentration of ammoniacal nitrogen in the rumen of dairy cows was significantly increased in the trial groups Ⅱ and Ⅲ(P＜0.05),and the concentration of propionic acid in the rumen fluid of dairy cows was significantly increased in the trial groups Ⅰ,Ⅱ and Ⅲ(P＜0.05).The concentration of total volatile fatty acids(TVFA)in the rumen fluid of dairy cows increased with melatonin feeding amount increas-ing,but there was no significant difference between the control group and the trial groups(P＞0.05).2)At phylum level,the relative abundance of Firmicutes in rumen bacteria in the trial group Ⅱ was significantly higher than that in the control group(P＜0.05),and the relative abundance of Spirochaetes in rumen bacteria in the trial groups Ⅰ,Ⅱ and Ⅲ was significantly lower than that in the control group(P＜0.05).3)At genus level,relative abundances of Prevotella and Ruminococcus in rumen bacteria in the trial groups Ⅱ and Ⅲ were significantly higher than those in the control group(P＜0.05).In summary,feeding melatonin can increase the concentrations of ammoniacal nitrogen and propionic acid in the rumen fluid of dairy cows in mid-lactation peri-od,increase the relative abundances of Firmicutes,Prevotella and Ruminococcus in the rumen bacteria,and reduce the relative abundance of Spirochaetes.The recommended feeding amount is 120 mg/d.[Chinese Jour-nal of Animal Nutrition,2023,35(12):7879-7891]
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