Effects Of Heat Stress On Milk Performance And Ruminal Methanogen Population In Dairy Cows

As the global climate continues to warn,heat stress has become one of the most adverse problems to dairy production in some areas in summer.In this study,milk protein and milk fatty acid were determined.Based on the correlation of milk fatty acid content and ruminal methane production,we evaluated the effect of heat stress on ruminal methane production in dairy cows.The composition of methanogen community in the rumen was studied by high-throughput sequencing.We further researched the mechanism of the “heatstressed milk protein decrease syndrome” and the mechanism of rumen methane production in dairy cows,to offer new perspective to ameliorate heat stress,improve milk quality and inhibit the methane production in heat-stressed dairy cows.The main contents of this study are as follows:(1)Twenty-three Chinese Holstein mid-lacating dairy cows were randomly selected to study the effect of the heat stress on milk protein and milk fatty acid.By real-time monitoring of temperature and humidity index,we determined the heat stress degrees of each cow.Meanwhile,milk yield and the composition of milk were measured.The results showed that heat stress reduced the milk yield(P<0.01).Milk fat,C18:1n9c,UFA was significantly decreased(P<0.05).Milk urea nitrogen and α-La was significantly increased(P<0.01).SCC,BSA,C18:2n8c,trans-10+11 C18:1 was significantly increased(P<0.05).It is concluded that heat stress may have effects on milk yield,milk composition and milk fatty acid of mid-lacating dairy cows.(2)This study was conducted to investigate whether the population structure of methanogen communities would differ on heat stress in dairy cows.To achieve the objectives,twenty-one mid-lactating Holstein dairy cows were used.The treatments were: heat stress(n=10)and cooling(n=11).DNA of microorganisms in rumen fluid was extracted,followed by high-throughput sequencing of 16 S r RNA genes of methanogens.Then,the structure of methanogenic bacteria communities was analyzed.Results showed that heat stress significantly increased methane production(g/kg DMI)(P<0.05).However,no significant effect of Chao1 index,Simpson index,Shannon index and Observed species were found between heat stress and cooling.Compared with cooling,heat stress reduced the average daily milk yield(P<0.01),and the relative abundance of Methanomassiliicoccaceae Group 8 sp.WGK1,Methanobrevibacter gottschalkii clade,Methanomassiliicoccaceae Group 12 sp.ISO4-H5 were significantly decreased(P<0.05),Methanobrevibacter ruminantium and Methanomassiliicoccaceae Group 10 sp.were significantly increased(P<0.05).It was concluded that the heat stress may have effects on methane production and the population structure of methanogen communities of mid-lacating dairy cows.We can speculate that heat stress may change the ruminal methanogen community structure,increase methane production,and cause cows energy loss,affect milk performance,resulting in changes in milk protein content and fatty acid composition.