The Difference Between The Performance And Fat Metabolism Of High-and Low-yielding Dairy Cows In Summer

The limited environment temperature of the production of dairy cow is 5-25 ℃.They can not cooling themselves sufficiently when the ambient temperature is above on 25℃,which lead to the heat accumulation in vivo and the body tempreature will rise.If the body tempreature is above 39℃,it instruct that the dairy cows are at the state of heat stress.The decreased dry matter intake at the stage of heat stress will exacerbate netagive energy balance of high milk yield dairy cows.At this very moment,dairy cows will mobilize bodyfat to compensate for the energy deficitas a result of the insufficient dry matter intake.The body fat mobilization level of high milk yield dairy cows may be higher because of its high milk production.The experiment detected the changes of milk yield and milk composition during the heat stress and the level of hormone and biochemical index related to fat metabolism in plasma of high-producing dairy cows and low-producing dairy cows to understand the performance and fat metabolism characteristics of dairy cows in summer and provide the basis for the production of dairy cow.Part 1:The experiment was conducted to explore the effect of heat stress on milk yield and milk composition of high-producing dairy cows and low-producing dairy cows.Forty Holstein healthy cows were selected and allocated into high-producing group(mean>35 kg/d)and low-producing group(mean=20-30 kg/d)with 20 cows in each group.The experiment was carried out in summer and respiration rate and rectal temperature were measured three times daily at 8:00,14:00 and20:00.Meantime,milk yield was record with three days every week and milk sample was collected one time to determine the milk components.The results showed that the average temperature-humidity index(THI)was 81.89 units and the milk yield of high-producing and low-producing dairy cows decreased by 29.3%(P<0.01)and 7.6%(P<0.05),respectively.The milk fat percentage of high and low milk yield dairy cows increased 10.21%and 8%(P<0.05),while there is no significant differences of milk protein percentage in a hot environment(P<0.05).The milk fat percentage of high milk yield dairy cows was higher than that of low milk yield dairy cows(P<0.05).Conclusions:compared to the low lactating cows,the milk yield of the high lactating cows is negatively affected severely in high temperature seasonand the high lactating cows have a high level of milk fat percentage.Part 2:The experiment was conducted to analyse the condition of body fat deposit of dairy cows with a different milk yield.We measured the body condition score,heart girth,abdominal girth and back-fat thickness of all mature-cows in a farm of Taicang City and gather data of production record.The results showed that no sinificant differences were observed about the level of the thickness of backfat and body condition score in high-producing group and low-producing group(P>0.05).The thickness of backfat showed reducing at first thenturn to increase as the milking time passing.Compared to low-producing dairy cows,the back-fat thickness of high-producing dairy cows took on a small range drop in the early lactation and slowly rise in mid-lactating,but climbed quickly in late lactation.Part 3:The xperiment was conducted to evaluate the differenceof factors related to catabolism and transportation of lipid and lactation between high and low milk yield dairy cows in a hot environment.Forty Holstein healthy cows with a similar parity were selected and allocated into high-producing group(mean>3 5kg/d)and low-producing group(mean=20-3 0kg/d)with 20 cows in each group according to the matching principle ofexperimental design.The blood samples were collected from caudal vein to determine and compare the difference of factors related to lipid metabolism and lactation.The results showed that no sinificant differences were observed about the level ofnonestesterified fatty acid(NEFA),β-OHB,total cholesterol(TCHO)and Insulin(INS)in high-producing group and low-producing group(P>0.05).The level of very low density lipoprotein(VLDL),apolipoprotein B100(apoB-100)and high-density lipoprotein(HDL-C)of high-producing group weresignificantly higher than that of low-producing group(P<0.05).There was no significant difference in the level of prolactin(PRL)and progesterone(PROG)between high-producing group and low-producing group(P>0.05);the level of insulin-like growth factor-I(IGF-I)of high-producing group werehigher than that of low-producing group(P<0.05).Conclusions:The level of lipid catabolism in high-producing and low-producing dairy cows is similar,but hepatic lipid transportation rate of high-producing dairy cows is higher than that of low-producing dairy cows.