Effects Of Heat Stress On Milk Fatty Acid Composition Of Dairy Cows

Heat stress is a great challenge for future animal husbandry,especially dairy farming,and fully understanding the effect of heat stress on dairy cows’production and health is the basis and premise of formulating strategies for controlling dairy farming environment and implementing measures to deal with heat stress.However,the current research on the effects of heat stress on milk composition of dairy cows is not comprehensive enough,especially on the study of the effects of heat stress on milk fatty acid composition.Milk fatty acid composition is not only closely related to the nutrition,sensory and physical and chemical characteristics of milk,but its changes can also be used as an important basis for dairy cattle breeding,methane emission reduction and productivity prediction.The lack of research of it is not only unfavourable to a comprehensive understanding of the effect of heat stress on milk quality,but also restricts the progress of research on cows related to heat stress.Therefore,in order to further understand the milk composition characteristics of heat-stressed cows and clarify the processes and mechanisms by which heat stress affects the milk fatty acid composition,this study firstly used the before and after comparison test of 30 lactating cows in the actual production environment of the dairy farm to study the impact of summer heat waves on dairy cow performance and milk fatty acid composition(Experiment 1),and then on this basis,a 2×2 crossover test in the climate chambers and 4cows with fistula were used to study the effects of duration of heat stress on milk fatty acids and the possible mechanism(Experiment 2).The results were shown as follows:1. Judging from the measured environmental parameters of the farm and the physiological indexes of the cows,the cows experienced a 9-day heat wave in the summer,which caused the cows to suffer mild heat stress and moderate heat stress successively.Compared with the non-heat stress period before the heat wave,the mild heat stress caused by the heat wave significantly reduced the energy correction milk,4%milk fat correction milk and milk fat production(P<0.05),while the moderate heat stress caused by the heat wave not only reduced the above indicators,but also reduced milk yield,milk protein rate,casein rate,milk protein production and lactose production(P<0.05),and significantly increased the urea nitrogen content in milk(P<0.05),this heat wave has no effect on the milk fat rate,lactose rate,and non-fat solids and total solids’rate(P>0.10).2. Compared with the non-heat stress period,the mild heat stress caused by the summer heat wave has little effect on the milk fatty acid composition,while the moderate heat stress significantly reduces the content of short and medium chain fatty acids(SMCFA,C6-C16)and polyunsaturated fatty acids(PUFA,C18:2,CLAc9,t11,C18:3n-3)in milk(P<0.05),it also significantly increased the content of single LCFA such as C18:0 in milk,but it had no effect on the content of total LCFA(P>0.10).3. Short-term heat stress(SH)compared with long-term heat stress(LH),the proportion of 18 single fatty acids in milk of cows differed significantly(P<0.05)or showed a trend of difference(0.05≤P<0.10),exceeding 1/3 of the total number of single fatty acids measured.The proportion of the main single fatty acids and the total SMCFA and LCFA in milk affected by heat stress gradually decreased or increased with the extension of the heat stress time.Compared with the control period,moderate heat stress exceeding 4 days is likely to cause significant changes in the composition of milk fatty acids(P<0.05),and the effects of moderate heat stress lasting 7 days on the milk fatty acid composition of dairy cows need 2 to 4 days to eliminate.4. Compared with the control period,SH significantly reduced the proportion of C18:2n-6 and total C18:2 in the rumen content of dairy cows,LH significantly reduced the proportion of C14:0iso,C15:0anteiso and total SMCFA,increased the proportion of C24:0,C26:0 and total LCFA in the rumen content of dairy cows(P<0.05).LH also caused the molar ratio of acetic acid,propionic acid,butyric acid,isovaleric acid and total volatile fatty acid(VFA)concentration in rumen of dairy cows to decrease significantly(P<0.05),and the rumen p H was significantly rise because of LH(P<0.05).In addition,heat stress had no effect on the concentration of non-esterified fatty acids(NEFA)in the plasma of dairy cows(P>0.10),but LH significantly reduced the concentration ofβ-hydroxybutyric acid(BHBA)in the plasma(P<0.05).5. Continuous heat stress led to a significant decrease in the proportion of CLAc9,t11in milk and mammary△~9-desaturase index(P<0.05),and the relationship between them is significantly linearly positive(P<0.01,R~2=0.500).The above results showed that:(1)Under actual production conditions,mild heat stress had almost no effect on milk fatty acids composition of dairy cows,while moderate heat stress has a significant effect on it,mainly manifested as the decrease of content of SMCFA and PUFA(C18:2,C18:3n-3,CLAc9,t11)and the increase of content of single LCFA such as C18:0.(2)The duration of heat stress is one of the factors that affect the milk fatty acid composition of dairy cows,and the effect of heat stress on the milk fatty acid composition of dairy cows needs some time to eliminate.(3)The decreased content of SMCFA and CLAc9,t11 in dairy milk caused by heat stress may be related to the decreased production of acetic acid and BHBA in rumen and the decreased activity of mammary gland △~9-desaturated enzyme,respectively.