Effect And Mechanism Of Heat Stress On Synthesis And Quality Of Milk In Lactating Dairy Cows

The purpose of the current study was to explore the effect of heat stress on dairy cows milk synthesis and try to find the possible mechanisms to this effect.The study consists of three parts:1 Effects of high temperature climate on lactating dairy cows metabolism and lactation performanceOn this chapter,we selected 6 dairy cows of similar parturition with regular diet and unlimited drink.The study last for 35 days(from Jun 29 to August 5).We recorded the temperature(at 10:00 and 18:00)and the milk yield.We tested the protein content,fat content and lactose content with the whole day milk sample every week.We also collected the blood samples from jugular vein on both the first and last week to analyze the related blood composition and related hormones contents.Our results showed that with the ambient temperature rise from 26 ? to 38 ?,the milk yield decreased from 15.2 kg/d to 10 kg/d.The milk protein production decreased from 0.47 kg/d to 0.30 kg/d.The milk fat production decreased from 0.46 kg/d to 0.28 kg/d.The lactose production decreased from 0.71 kg/d to 0.46 kg/d.In plasma,the TP(94.18±0.35&115.07±1.04g/L,p=0.049),globulin(69.58±0.20&88.95± 1.10g/L,p=0.045),glucose(2.88±0.04&3.54±0.11 mmol/L,p=0.03)increased with significance.The LDL-C(0.18±0.03&0.29±0.01 mmol/L,p=0.004)and AKP(4.17±0.31&35.50±4.15 U/L,p=0.004)increased with extreme significance.The HDL-C(3.23±0.08&2.20±0.04 mmol/L,p=0.033)decreased with significance.About the plasma hormones,GH(1.19±0.03&1.54±0.06ng/mL,p=0.037)and IGF-I(80.05± 10.62-100.72±21.78ng/mL,p=0.048)increased with significance.Cortisol(288.21±49.73&573.76±11.71 ng/mL,p=0.009)increased with extreme significance.Insulin(22.48±2.53&25.66±1.95IU/L,p=0.057)and glucagon(597.18±58.11&658.02±49.47pg/mL,p=0.06)increased with no significance.In conclusion,under the hot moist weather of summer,the dairy cows are vulnerable to heat stress.The latter alter the endocrine and nutrition metabolism of dairy cows which performs as lactation reduction and milk quality deduction.2 Relationship between the redistribution of milk composition precursors and milk quality under heat stressOn this chapter,we focus on the relationship between milk fat,milk protein and their precursors,i.e.amino acids and NEFA in plasma.Hopefully we can explain the possible mechanisms of milk quality deduction under heat stress.The experimental animal were the same as Chapter 3 with same treatment.We collected the blood sample from jugular vein at the beginning and end of the study.We used a RP-HPLC system and ELISA to test the free amino acids and NEFA in plasma.Our result showed that with the ambient temperature rise(from 26 up to 38 ?),the milk fat and milk protein content decreased.The total NEFA in plasma 178.20± 16.70&272.42±20.53?g/mL,p=0.025)increased with significance,the Glu,Asp,Gly and Val,known as the main amino acids for gluconeogenesis,increased with significance(811.28±259.22&3254.05±666.79?mol/L,p=0.035).And so did the Leu,Ile,known as the main amino acids for ketogenesis(169.26±32.27&502.76± 105.50 ?mol/L,p=0.046).The Branch Chain Amino Acids(Val,Ile,Leu)which took part in the immune response increased with significance(198.36±52.57&1474.85±355.83?mol/L,p=0.0287<0.05).The proportion of essential amino acids increased from 54.75%to 67.89%.In conclusion,under the hot moist summer weather,both milk fat content and milk protein content decrease.However,the NEFA and free amino acids,especially the branched-chain amino acids,increased with significance.It suggested that the high level of NEFA and free amino acids of heat stress dairy cows are not exclusively used for milk fat and milk protein synthesis.They are utilized for other ways like energy supply,gluconeogenesis,immune reaction and etc.These use get higher priority than milk fat and milk protein synthesis.The redistribution of milk composition precursors should be one the mechanism which cause the deduction of milk fat and milk protein under heat stress.3 Preliminary mechanism of milk composition precursors redistributions under heat stressBy the two chapter before,we have discovered that heat stress can cause the distribution of milk protein and milk fat precursors,which lead to the deduction of milk protein and milk fat.In order to explain the mechanism between milk composition distribution and the deduction of milk quality,we collected the liver samples of the experimental dairy cows on this chapter.Using the Real-time PCR system,we tested the gluconeogenesis key enzymes,i.e.PC and PEPCK,in hepatic.We also tested GHR,IGFR,HSP70,Keapl-Nrf2 signaling pathway and its 4 downstream genes,i.e.HOl,GCLM,NQOl and GCLC,in hepatic.Our result showed that during heat stress,the PC mRNA expression level(0.917 ±0.008&1.08 ±0.025,p=0.045)and PECK mRNA expression level(0.87 ±0.009&1.29 ±0.132,p=0.032)increased with significance.GHR mRNA expression level(1.05 ± 0.12&1.39 ± 0.02,p=0.007)increased with extreme significance.IGFR mRNA expression level(1.07± 0.05&1.28 ±0.018,p=0.036)increased with significance.The expression level of HSP70 in hepatic(0.35 + 0.165&1.55 + 0.012,p=0.006)increased with significance.The Keapl(1.07 ±0.24&1.32 ±0.27,p=0.046)and Nrf2(1.59±0.18&1.73 ±0.19,p=0.049)expression levels increased with significance.As to the 4 downstream genes,,HOl(0.98±0.08&1.19±0.02,p=0.037)and NQO(0.92 ±0.08&1.43 ±0.02,p=0.026)expression levels increased with significance,while GCLM(0.69±0.10&0.97±0.05,p=0.059)and GCLC(0.84±0.07&1.29±0.11,p=0.061)increased but without significance.In conclusion:the gluconeogenesis and anti-stress effect are both enhanced during heat stress.Mediated by GH-IGF-I axis and Keapl-Nrf2-ARE signaling pathway,increasing milk protein and milk fat precursors participate in gluconeogenesis and anti-stress injury more than usual.This redistribution costs a lot milk composition precursor and leave less for milk protein and milk fat synthesis.