Plasma metabolic profiling reveals that chromium yeast alleviates the negative effects of heat stress in mid-lactation dairy cows

Chromium yeast(CY)supplementation has the potential to alleviate the negative effects of heat stress in dairy cows,but the mechanism remains elusive.We aimed to identify the metabolic mechanisms whereby CY supplementation alleviates the negative effects of heat stress in mid-lactation dairy cows.Twelve Holstein dairy cows with similar milk yield(24.6±1.5 kg/d),parity(2 or 3)and days in milk(125±8 d)were fed the same basal diet containing 0.09 mg of Cr/kg DM.They were allocated randomly to 2 groups:a control group(CON,without CY supplementation)and a CY group(CY,administered 0.36 mg Cr/kg DM).The experiment was performed over 8 weeks during a hot summer,in which the mean temperature-humidity index was 79.0±3.13(>72),indicating that the dairy cows were exposed to heat stress.Chromium yeast supplementation reduced rectal temperature(P=0.032),and increased the lactation performance by increasing the yield of milk(+2.6 kg/d),protein,lactose and total solid,and protein and lactose percentages in the milk of the heat-stressed dairy cows(P<0.05).Supplementation with CY increased the serum glucose and thyroxine concentrations,but reduced the urea nitrogen,in-sulin,and triiodothyronine concentrations on d 56(P<0.05).Furthermore,plasma metabolomic analysis was performed using liquid chromatography tandem-mass spectrometry,which identified 385 metab-olites in the two groups.Subsequently,16 significantly different metabolites in the plasma,were significantly higher in the CY group(variable importance for the projection>1.0,P<0.05),and found to be involved in 6 Kyoto Encyclopedia of Genes and Genomes pathways,including those involved in nicotinate and nicotinamide metabolism.Specifically,plasma concentration of nicotinamide was higher after CY supplementation,which might also contribute to the reduction of rectal temperature,the regulation of glucose homeostasis,and an improvement in the lactation performance of heat-stressed dairy cows.In conclusion,CY supplementation reduces rectal temperature,influences metabolism by reducing serum insulin concentration and increasing serum glucose and plasma nicotinamide concen-trations,and finally increases lactation performance of heat-stressed dairy cows.