| In recent years,with the global warming,the duration and intensity of summer high temperature in Ningxia are increasing.Summer heat stress usually results in higher body temperature,lower dry matter intake,lower milk yield,slower growth rate and lower reproductive rate of dairy cows.Therefore,it is of great significance to reduce the effect of heat stress on production performance and reproductive performance of dairy cows.By investigating the reproductive performance of a dairy farm in Ningxia,the effects of heat stress on rectal temperature and respiratory rate,blood biochemical indexes,antioxidant capacity and immune function of dairy cows were studied,which laid a foundation for exploring the mechanism of heat stress on dairy cow production performance.Experiment 1:Effect of Heat Stress on Rectal Temperature and Respiratory Frequency of Lactating CowsThe experiment was conducted in Maoshcng Ranch of Ningxia Agricultural Reclamation Helanshan Dairy Co.,Ltd.in March,August and September of 2018.According to the principle of similar lactation days,parity and milk yield,15 Holstein dairy cows with healthy disease-free,32?47 days postpartum and 40?55 kg/head per day milk yield were selected.The rectal temperature was measured at 8:00,14:30 and the respiratory rate was measured at 8:00,14:30 and 18:00.The results showed that the average temperature in August(26.55±2.51?)was higher than that in September(19.83±2.85?)and April(18.28±2.96?)respectively(P<0.01),and the THI(76.40±3.02)in August was significantly higher than that in April(66.20±4.20)and September(65.89±4.37)The rectal temperature in August was significantly higher than that in April(P<0.05),and the respiratory rate was significantly higher than that in September and April(P<0.01),while the rectal temperature and respiratory rate in April and September were not significantly different(P>0.05),indicating that the increase of temperature had a significant effect on rectal temperature and respiratory rate in dairy cows.Experiment 2 Effect of Heat Stress on Production Performance and Reproductive Performance of Dairy CowsIn spring(April),summer(August)and autumn(September)of 2018,128 dairy cows with similar lactation days and parity were selected to determine milk yield,and milk samples were collected to determine milk composition.The results showed that the average milk yield in August and September was significantly lower than that in April(P<0.01);the milk fat rate,lactose and lactourea nitrogen content in August were significantly lower than that in April and September(P<0.01);the non-milk fat solids were significantly lower than that in April and September(P<0.05);the milk protein rate and somatic cell number were not significantly different among the groups(P>0.05).The number of empty days in August was significantly higher than that in September(P<0.05),and the pregnancy rate in 150 days was significantly lower than that in April and September(P<0.01).Compared with the calves in September and April,the mating times of calves in August had an increasing trend(P>0.05).The conception rates of calves in the first estrus in August were 30.47%,28.85%lower than those in April and September,respectively.Monthly calf production decreased by 3.53%and 6.66%respectively.The results showed that heat stress had an effect on the reproductive performance of dairy cows.High temperature reduced the conception rate of dairy cows during estrus,and led to the prolongation of empty days and the decrease of pregnancy rate in 150 days.Experiment 3 Heat Stress on Blood Biochemical Indicators of Lactating Dairy CowsIn spring(April),summer(August)and autumn(September)of 2018,six Holstein dairy cows with similar lactation days and parity were selected to collect blood samples and analyze blood biochemical,immune and antioxidant indexes.The results showed that scrum creatine kinase concentration in August was significantly higher than that in April and September(P<0.05),serum adrenaline concentration was significantly lower than that in April and September(P<0.01),serum cortisol concentration and heat shock protein 70 concentration were significantly higher than those in September and April(P<0.01).There were no significant differences in serum K+,Ca+,total cholesterol,triglyceride,glucose,total protein and insulin concentrations among different months(P>0.05).Serum immunoglobulin G concentration in August was significantly lower than that in September and April,and that in September was significantly lower than that in April.Serum IL-6 and TNF-alpha concentrations in August were significantly or significantly higher than those in September and April,and there was no significant difference between April and September(P>0.05).The total antioxidant capacity in serum of dairy cows in April,August and September was significantly different(P<0.01),and the lowest in August and the highest in September.The concentration of SOD in serum of dairy cows in August was significantly lower than that in April and September(P<0.05),and the concentration of glutathione peroxidase in serum was significantly lower than that in September and April(P<0.01).The concentration of MDA in serum of dairy cows in August was significantly upper than that in April and September(P<0.05),but there was no significant difference between April and September(P>0.05).The results showed that during the period of heat stress,the serum ions and enzymatic activities of dairy cows were affected,which resulted in the damage of physiological function of dairy cows.Heat stress can also lead to a decline in immune function and antioxidant capacity. |
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