The Effect Of Heat Stress On Endogenous Metabolites In Dairy Cows

The aim of this experiment was to determine the effects of the heat stress on the respiration rates, rectal temperatures, milk yield, milk composition, metabolic profile of lactating dairy cows. According to biomarkers, we can find out those altered pathways and may be able to identify the mechanism of reduction in milk production. Twenty multiparous, middle-lactation Chinese Holstein cows were selected. Based on the value of THI, the experiment was divided into two periods:the spring period (for control, CK) from mid-march to mid-April and the summer period (for heat stress, HS) from mid-July to mid-august in 2014. The each period length was 38d, consisting of 10-d preliminary feeding period followed by 28d of heat stress (HS, THI ranging from 74.80-85.84). Respiration rates, rectal temperatures, milk yield, milk composition were measured weekly. THI inside the barn was calculated everyday. Body weight was weighed at the first and la st day. Blood samples and milk samples collected at the last day were analyzed using 1H NMR. NMR data were analyzed by multivariate statistical methods (PCA, OPLS-DA) to identify the altered metabolites between the CK and HS. Results showed that the respiration rates, rectal temperatures of cows in moderate heat stress were higher than CK cows (P<0.05); respiration rates of cows in mild heat stress were higher than CK cows (P<0.05),but mild heat stress didn’t alter the rectal temperatures. It indicated that respiration rates and rectal temperatures were susceptible to alterations of TH. Heat stress decreased milk yield (P<0.001). Mild and moderate heat stress decreased milk yield by 15.1%(P<0.001)、30.5% (PO.001), respectively. Compared to mild heat stress, moderate heat stress decreased milk yield by 18.1%(P<0.001). Heat stress didn’t alter milk lactose, but milk protein was decreased by 10.01%(P<0.05), milk fat had the trend of rising. The up-regulation of Alanine, Glutamate, Glucose, Urea, Histidine,1-MH, Formate were determined in the serum of heat-stressed cows, on the other hand,3-Hydroxybutyrate, Leu, Lipid, VLDL, LDL were decreased. These altered metabolites demonstrated disturbance of glucocorticoids, carbohydrate metabolism, energy metabolism, gut flora metabolism, gluconeogenesis metabolism and muscle catabolism, which may provide an overview of the ongoing metabolic processes in the heated-stress cows organism and elucidate mechanisms responsible for changes in milk yield. The up-regulation of NAG, Choline, scyllo-inositol, Pyridoxamine were determined in the milk of heat-stressed cows, while OAG, citrate, PC, GPC, MP were decreased. It indicated that heat stress may affect the normal function of mammary gland, promoted the choline synthesis, and inhibited the PtC catabolism.