Effect of Different Protein Levels on Nutrient Digestion Metabolism and Serum Biochemical Parameters in Calves

<正>The current study has been performed to examine the effects of different dietary protein levels on the growth,nutrient digestion and absorption,as well as biochemical parameters in calves.Nine healthy newborn calves were selected, randomly divided into 3 groups and fed 3 milk replacers with different protein levels（18,22,and 26%）,respectively.Five period-digestion-metabolism trials were taken between 12-20,22-30,32-40,42-50,and 52-60 days after birth.All 3 groups showed a similar growth curve during 11 to 61 experimental days,however,the growth rate of the 22% crude protein（CP） group was 8.89% higher than that of the 18% CP group and 19.48% higher than that of the 26% CP group,respectively. The apparent digestibility of dry material（DM）declined gradually with age,whereas,the apparent digestibility of N, extract ether（EE）rose slightly.Compared to the 22% CP and 26% CP group,calves fed with 18% CP apparently had a lower digestibility than DM,EE,and nitrogen（N）.The average apparent digestibilities of N were 69.39,75.36,and 74.55%, respectively.Both the apparent digestibility and retention of calcium（Ca）and phosphorus（P）were steady throughout the experiment,but the average apparent digestibility of P in the 26% CP group was only 63.83%,which was markedly lower than that of the 18% CP group（70.40%）and 22% CP group（69.73%）.In addition,the serum concentrations of total protein（TP）,albumin（ALB）,and globulin（GLOB）of the 22% CP group were higher than those in the 18% CP and 26% CP groups.The urea N（BUN）content in the 18% CP group,on the other hand,was significantly lower than that of the other two groups.The highest glucose（GLU）concentration was found in the 22% CP group(5.38 mmol L-1),at the end of the trials.The protein levels in the milk replacer affected the digestion metabolism of nutrition and the serum biochemical parameters of calves at different physiological phases.Calves fed with 22% CP milk replacer had a better growth performance than the other groups.