Abstract: The objective of this study was to determine the effects of different starter neutral detergent fiber (NDF) levels on the growth performance, rumen environments and serum biochemical parameters of calves. Sixty newborn Holstein calves (24 females and 36 males) with body weight (42±2.5) kg were randomly assigned to four treatment groups (A, B, C and D), with each group made up of nine males and six females. Each calf was housed individually in a 1.5 m×3.4 m calves island at the experimental farm. Each group was fed one of four starters with different NDF levels of alfalfa hay and oat hay. The NDF levels of the starters were 10%, 15%, 20% and 25%, respectively. The calves were weaned at 70 days of age and the experiment lasted for 112 days. The results were as follows. During 0-42 and 70-112 days of age, the average daily gains were 46.59, 39.79, 117.13 g·d^-1 and 35.33, 153.29, 145.93 g·d^-1 respectively higher in group B than in groups A, C and D (P<0.05). During 70-112 days of age, the starter intake was significantly higher by 255.20, 252.48 and 392.27 g·d^-1 in groups B, C and D than in group A (P<0.05). The A and B starters improved feed conversion ratios (P<0.05). 2) At 35 and 70 days of age, rumen pH was significantly higher in group C than in groups A, B and D (P<0.05). At 112 days of age, rumen pH was greater for groups A and B than C and D (P<0.05), and the ammonia nitrogen concentration was decreased by 16.72%, 44.19% and 52.10% more in groups B, C and D respectively than in group A (P<0.01). 4) The proportion of acetate was significantly higher in groups C and D than in groups A and B (P<0.01). The propionate ratio was significantly higher by 4.24%, 3.18% and 6.43% in group B than in groups A, C and D (P<0.05). The acetate to propionate ratio of group B was lower than the other groups (P<0.05). 5) No significant differences were observed in contents of total protein, glucose and urea nitrogen in serum (P>0.05). In conclusion, starters with approximately 15% NDF could increase Holstein calves’ average daily weight gains and improve their feed conversion ratio and rumen environments, while they do not affect their serum biochemical parameters.

Key words: dairy calf, starter, NDF level, growth performance, ruminant environments, serum biochemical parameters