Effects Of Chromium Yeast On Production Performance,Blood Indexes,Rumen Microbial Structure And Metabolites In Heat-Stressed Dairy Cows

This study aimed to evaluate the effects of appropriate amount of chromium yeast(CY)on lactation performance,blood index,and trace element contents in the plasma and milk,plasma metabolites,and rumen microbial diversity and metabolites of mid-lactation dairy cows under heat stress,which might provide theoretical basis for the application of CY in dairy production during heat stress.The specific contents are as follows:Experiment 1: The objective of the experiment was to evaluate the effects of different doses of CY on production performance,antioxidant status and immune function,and trace element contents in the plasma and milk of dairy cows under heat stress.Twenty-four healthy Chinese Holstein dairy cows were randomly allocated to four groups: a negative control group(without CY supplementation,CON),and experimental groups that received CY at 0.18,0.36,and 0.54 mg Cr/kg DM,respectively.The experiment lasted 10 wk,including a pre-feeding period of 2 wk.The results showed that:(1)Supplementation with CY reduced the respiratory rate and rectal temperature of dairy cows under heat stress in a linear manner(P<0.05).(2)CY supplementation linearly increased dry matter intake and milk lactose content(P<0.05).(3)Furthermore,increasing amounts of CY increased glucagon,prolactin content and activity of glutathione peroxidase,superoxide dismutase and total antioxidant capacity in serum,but decreased urea nitrogen,albumin and malondialdehyde content(P<0.05).(4)Supplementation with CY decreased the concentrations of IL-2,IL-4,and IL-1β,but increased that of IgG in the serum(P<0.05).(5)The plasma and milk Cr concentrations increased as the dose of supplemented Cr increased(P<0.05).(6)Under the conditions of this experiment,the appropriate dose of CY at 0.36 mg Cr/kg DM is recommended for lactating dairy cows experiencing heat stress during hot summers.Experiment 2: The objective of the experiment was to evaluate the effects of CY on lactation performance,blood index and plasma metabolites of dairy cows under heat stress.Twelve healthy Chinese Holstein dairy cows were randomly allocated to two treatments: a control group(without CY supplementation,CON),and an experimental group that received CY at 0.36 mg Cr/kg DM(CY).The experiment lasted 10 wk,including a pre-feeding period of 2 wk.The results showed that:(1)The respiratory rate of heat-stressed cows in the CY group tended to decrease(P=0.08).(2)Dry matter intake of the CY group was significantly higher than that of the CON group(P<0.05),and the milk protein content in the CY group had a tendency to increase(P=0.07).(3)Sixteen different metabolites were significantly increased in the plasma of CY group,which promoted vitamin absorption,glucose metabolism and amino acid metabolism.Experiment 3: The objective of the experiment was to evaluate the effects of CY on rumen microbial diversity and metabolites of dairy cows under heat stress.Compared with CON group,the addition of CY had no significant effect on the rumen microbial diversity,but thirty-six different metabolites were significantly decreased in rumen fluid.Supplementation with CY has significant effects on amino acid metabolism,mineral absorption,protein digestion and absorption,aminoacyl-tRNA biosynthesis,biosynthesis of amino acids and glutathione metabolism.In conclusion,supplementation with CY to the diet for heat-stressed dairy cows reduced respiratory rate and rectal temperature,increased dry matter intake and serum antioxidant capacity and immune function,and elevated the concentration of chromium in plasma and milk.The appropriate dose of CY at 0.36 mg Cr/kg DM is recommended for lactating dairy cows experiencing heat stress during hot summers.CY promotes plasma vitamin absorption,glucose metabolism,and amino acid metabolism in heat-stressed dairy cows.CY did not affect the rumen microbial diversity of heat-stressed dairy cows,but had a positive effect on rumen metabolism.