Relationship Between Genome-wide Methylation Rates And Lactation Performance With The Effected By Heat Stress In Dairy Cows

In recent years,epigenetics has been regarded as one of the important molecular mechanisms regulating lactation performance in dairy cows.DNA methylation,histone modification,and non-coding RNA are the major types of epigenetic modification.Under different physiological and pathological conditions,milk protein expression may be regulated by DNA methylation.Epigenetics is affected by two major factors,heredity and environment.The management and feeding conditions of dairy cows change the epigenetic patterns of the body through some mechanisms,and can transfer this attribute to the offspring.Heat stress will make dairy cows under uncomfortably high temperature,and is one of the main factors restricting the development of dairy farming in China.This study mainly related the performance of dairy cows to the frequency of blood DNA methylation to explore the relationship of dairy production performance with DNA methylation(Expt.1);and then investigated the performance and physiological variables of dairy cows under heat stress to analyze the effects of heat stress on DNA methylation and physiological status of dairy cows(Expt.2).1.Relationship of DNA methylation rate and production performance in dairy cows(Expt.1)A total of 196 mid-lactating dairy cows were fed the same diet under the same management situation in Expt.1.Milk yield was recorded and the blood samples were collected from the jugular vein before morning feeding.The blood global DNA methylation rates were quantified using the Methylated DNA Quantification Kit.Overall,the average blood global DNA methylation rate of all the cows was 12.4%,with a range from 3.0 to 22.5%.When DNA methylation rates were further compared between high(n=40,37.0 to 42.0 kg/d)and low milk yield(n=33,24.0 to 30.0 kg/d)cows,the DNA methylation rates in the lower milk yield cows(14.1±0.7%)were significantly higher than those in the higher milk yield animals(11.6±0.7%).Our result indicated an association of milk and protein yield with global DNA methylation rates in dairy cows.2.Lactation performance,blood physiological variables and DNA methylation in dairy cows under heat stress(Expt.2)In order to investigate the effects of heat stress on dairy cow production,physiology and biochemistry,and its relationship with blood DNA methylation frequency,18 high-yielding Holstein cows were selected in Expt.2 to record their feed intake and milk production under high heat-humidity environment.The ambient temperature and humidity,and the rectal temperature of the cows were also recorded during the test.Blood samples were collected before noon feeding under two conditions of mild heat stress(72<THI<78)and moderate heat stress(78<THI<88)to analyze blood indicators and levels of heat stress proteins.Blood DNA was extracted to detect DNA methylation frequency.During moderate heat stress,the rectal temperature was significantly higher(P<0.01),and dry matter intake decreased by 3.5 kg/day,and milk production was significantly reduced(P<0.01)compared with mild heat stress.Under moderate heat stress,blood DNA methylation frequency was significantly reduced(P<0.01),level of peroxidase dismutase,total antioxidant capacity,and blood y-globulin concentrations were also significantly increased(P<0.01).No significant differences were found in heat stress proteins between the two periods.It is indicated that moderate heat stress can seriously affect the lactation performance of dairy cows,reduce the frequency of methylation,and enhance the antioxidant capacity of the body.The underlying mechanism needs to be further explored.In summary,there exists the DNA methylation in lactation dairy cows and it may exerts some influences on milk yield and milk protein yield.Lactation performance and blood biochemical variables is adversely influenced by heat stress,which may have some association with DNA methylation.