Effect And Mechanism Of Thermal Environment On Physiological And Biochemical Indexes Of Mid-lactating Dairy Cows In Different Seasons

Changes of thermal environment lead to stress will have a great effect on midlactating dairy cattles'behavior,growth and development,production performance,immune regulation,metabolism,reproductive performance.Meanwhile,this effect will cause huge economic losses to the dairy industry development,we should pay full attention to it.this experiment aims to investigate the dynamic changes of the thermalenvironment in four seasons of the year and its effect on physiological function,production performance,blood biochemical index and blood routine index of lactating Holstein cows.The main results are as follows:1.During the four test periods of spring,summer,autumn and winter,the temperature and humidity of the test cowshed were measured every three days,and the respiration rate,heart rate and rectal temperature of 15-20 experimental cows in the middle period of lactation were measured every seven days.Results show that the temperature humidity index?THI?in cowshed between spring and summer,autumn and winter around the year was extremely significant different?P<0.01?.In summer,the THI of cow barn between 75 and 89,most of the time greater than 80,the average value of it is 80.80±3.41,extremely significant higher than other seasons.Thus it can be seen,lactating cows in summer were in sustained heat stress.Respiratory rate of dairy cows in summer was extremely significant higher than that of other seasons?P<0.01?,which were 1.90,2.50 and 3.04 times of spring,autumn and winter respectively.The rectal temperature of dairy cows in summer was significantly higher than that of other seasons?P<0.05?.Compared with the spring,autumn and winter the rectal temperature in summer increased by 1.82%,1.33%,1.84%respectively.The heart rate of cows in summer was significantly higher than that in spring and winter?P<0.05?.Compared with spring and winter,heart rate of lactating cows in summer increased by 11.29%and4.69%respectively.The correlation coefficient of THI with respiratory frequency,rectal temperature and heart rate respectively were 0.82?P<0.01?,0.71?P<0.01?and0.38?P<0.01?.2.The thermal environment has great influence on the production performance and milk quality of lactating cows.During the four test periods of spring,summer,autumn and winter,the dry matter intake and milk yield of experimental cattle were recorded every 3 days,and the composition index of milk was measured once a month.The results showed that THI was significantly negatively correlated with DMI and milk yield,and the correlation coefficients were-0.80?P<0.01?and-0.88?P<0.01?,respectively.Compared with the spring,autumn and winter,as THI value increased 1unit,DMI in summer reduced 0.35kg/d,0.11 kg/d,0.14 kg/d,respectively,and milk yield decreased by 0.74 kg/d,0.19 kg/d,0.19 kg/d.The results of milk composition analysis showed that the difference of milk fat percentage in each season was not obvious?P>0.05?,and the level of milk protein content in summer was significantly lower than that in winter?P<0.05?.In summer,the levels of somatic cell counts and urea nitrogen content were significantly increased.3.During the four test periods of spring,summer,autumn and winter,enzymes and metabolites related indicators in the plasma of experimental cows were measured once a month.The activity of ALP and CK in summer was significantly lower than that in other seasons?P<0.05?.The level of GLU decreased in summer,and the level of BUN in plasma was significantly higher than that in other seasons?P<0.05?.The NEFA concentration in spring was higher than that in summer?P>0.05?,which was significantly higher than that in autumn and winter?P<0.05?.The content of CO2 in blood in summer was significantly lower than that in spring?P<0.05?,and the concentrations of K⁺,Na⁺and Cl⁺in plasma were the highest in winter,there was no significant difference among different seasons?P>0.05?,and Ca²⁺concentration in plasma was lowest in summer and significantly lower than that in other seasons?P<0.05?.4.During the four test periods of spring,summer,autumn and winter,Immune and antioxidant related indicators in the plasma of experimental cows were measured once a month.The plasma T-AOC in autumn was extremely significant lower than that in other seasons,T-AOC in summer was extremely significant lower than that in spring and winter?P<0.01?,plasma MDA in summer was extremely significant higher than that of other seasons?P<0.01?,IL-6,TNF-alpha in plasma in summer was significantly higher than that of other seasons?P<0.05?.T4 and Cor in plasma were the highest in summer,and extremely significant higher than those in other seasons?P<0.01?.PROG in plasma was extremely significant higher in summer than in autumn and winter?P<0.01?,and plasma PRL in summer was significantly lower than that in spring and winter?P<0.05?.5.During the four test periods of spring,summer,autumn and winter,whole blood of erperimental cows were collected once a month for routine blood test.The number of white blood cells?WBC?and the number of neutrophils?GC?,neutrophil percentage?Gran%?in in spring and autumn was significantly higher than that in winter and summer?P<0.05?,eosinophil percentage?Baso%?,lymphocyte percentage?Lymph%?,platelet count?PLT?in winter season were significantly higher than that in other seasons?P<0.05?.The percentage of mononuclear cells?Mono%?,the number of lymphocytes?LC?and the number of monocytes?MC?in spring was significantly higher than that in other season?P<0.05?,the percentage of eosinophils?Eosin%?was significantly higher than that in other seasons in summer,the number of eosinophils?EC?in winter was significantly lower than that in other seasons?P<0.05?.The number of basophils?BC?was significantly lower in summer than in other seasons?P<0.05?.The number of red blood cells?RBC?in winter was significantly higher than that in summer?P<0.05?.The concentration of hemoglobin?HGB?in autumn and winter was significantly higher than in spring and winter?P<0.05?.