Abstract: 【Objective】The objective of the study was to know the effect of leucine and phenylalanine supplementation on growth performance and serum metabolites of Holstein male calves. The study could provide a reference for the high efficient use of leucine and phenylalanine on dairy cows.【Method】A total of 20 Holstein male calves (birth weight (38±3) kg) were randomly assigned into treatment groups of control, L (1.435 g·L^-1 leucine), P (0.725 g·L^-1 phenylalanine), M (1.435 g·L^-1 leucine and 0.725 g·L^-1 phenylalanine), and each treatment added alanine as isonitrogenous diet. The experiment lasted for eight weeks, one week for adaption and seven weeks for feeding experiment. The calves were fed individually. Within one hour after birth, each calf was fed 4.0 L of colostrum. During the first week, the calves were fed their assigned milk twice daily, 3.0 L each. For adaption, the amino acid supplementation was increased 20% daily until it reached 100% of the target supplementation on the sixth day after birth. From week 2 to 3, the calves were fed their assigned milk twice a day, 3.5 L each, while from week 4 to 8, the volume of the milk fed increased to 4 L each time. From the third week after birth of the calves, a quantitative supply of the starter feed was provided, with free access to the drinking water. Body weight, withers height, body length, heart girth, heart girth/body length, body length/withers height were recorded at the 1, 3, 5, 7, and 8 week old, respectively. At the same time, 10 mL of blood was drawn from the jugular vein and placed at a tilted angle for serum precipitation. The serum samples were used to determine the amino acids, glucose, serum urea nitrogen, insulin, and cholecysto-kinin, respectively. Data were analyzed using the GLM procedure of IBM SPSS Statistics V22.0 software, differences among treatments were compared using LSD’s multiple range tests.【Result】Average daily gain in M group was significantly lower than control, L and P groups (P＜0.05). Compared with control group, other three groups significantly increased the final body length (P＜0.05). L and M treatments tended to increase the final wither height (P＜0.10), while P treatment significantly decreased the final heart girth (P＜0.05). Compared with control group, L and M treatments significantly increased the heart girth/body length (P＜0.05), whereas no difference in initial body weight, body length, wither height, heart girth, heart girth/body length, body length/withers height, final body weight and final body length/withers height was found among treatments (P＞0.05). L treatment significantly increased the concentrations of histidine, leucine, threonine, glycine, and total essential amino acids (P＜0.05), tended to increase the concentrations of aspartate and total amino acids (P＜0.10), while significantly decreased the concentrations of alanine and glycine (P＜0.05). P treatment significantly increased the concentrations of phenylalanine, glutamate and glycine (P＜0.05). Compared with L and M groups, control group significantly increased the concentrations of serum alanine (P＜0.05). For serum concentrations of amino acids, no significant interactions between L and P treatments were detected in our study (P＜0.05). L treatment significantly increased the concentrations of glucose (P＜0.05), while P treatment significantly decreased the concentrations of glucose (P＜0.05). For serum concentrations of glucose, significant interactions between L and P treatments were detected in our study (P＜0.05). Serum urea nitrogen in P group was significantly higher than other three groups (P＜0.05), whereas no difference in concentration of insulin and cholecysto-kinin were found among treatments (P＞0.05).【Conclusion】In conclusion, leucine supplementation had a better effect than that of phenylalanine on the serum concentration of essential amino acids, glucose, heart girth/body length and average daily gain of Holstein male calves, whereas negative interactions existed between the two kinds of supplementation.

Key words: leucine, phenylalanine, calves, growth performance, serum metabolites