title: Effects of dietary protein level on growth performance and nitrogen excretion of dairy heifers.
abstract: OBJECTIVE
  Protein supplementation is costly and can result in excess nitrogen (N) excretion. The objective of this study was to evaluate the effects of feeding different levels of dietary protein on average daily gain, body size, rumen fermentation, and nitrogen excretion of 8 to 10 month-old Holstein heifers.
  
  
  METHODS
  Thirty-six Holstein heifers were divided into 12 blocks according to age (273±6.2 d) and were randomly assigned to diets containing a low (10.2% dry matter [DM]), medium (11.9% DM), or high (13.5% DM) level of dietary crude protein (CP). All diets contained approximately 70% roughage and 30% concentrate with similar dietary metabolizable energy (ME) content (2.47 Mcal/kg).
  
  
  RESULTS
  Dry matter intake did not differ among the treatments, and average daily gain increased with the increasing dietary protein, 0.79, 0.95, 0.97 kg/d for low, medium, and high group, respectively. Body height increased linearly with increasing dietary CP but no other significant differences in body dimensions were found among the treatments. The increased ratios of dietary CP improved the rate of rear teat length growth remarkably (p<0.05). There was no difference in rumen pH or ruminal major volatile fatty acid (acetate, propionate, and butyrate) concentration among the 3 diets, but rumen ammonia-N concentration increased with the higher dietary CP (p<0.05). Increasing N intake led to increased total N excretion; urinary N excretion was significantly increased (p<0.05) but fecal N excretion was similar among the treatments.
  
  
  CONCLUSIONS
  These data suggest that the diet containing 11.9% CP (ME 2.47 Mcal/kg) could meet the maintenance and growth requirements of 9 to 11 month-old Holstein heifers gaining approximately 0.9 kg/d.
authors.affiliation: Institute of Dairy Science, College of Animal Science, Zhejiang University, Hangzhou 310058, China.
authors.forename: Bin
authors.initials: B
authors.lastname: Zhang
authors.affiliation: College of Animal Science and Technology, Zhejiang A&F University, Hangzhou 311300, China.
authors.forename: Chong
authors.initials: C
authors.lastname: Wang
authors.affiliation: Institute of Dairy Science, College of Animal Science, Zhejiang University, Hangzhou 310058, China.
authors.forename: He
authors.initials: H
authors.lastname: Liu
authors.affiliation: Institute of Dairy Science, College of Animal Science, Zhejiang University, Hangzhou 310058, China.
authors.forename: Jianxin
authors.initials: J
authors.lastname: Liu
authors.affiliation: Institute of Dairy Science, College of Animal Science, Zhejiang University, Hangzhou 310058, China.
authors.forename: Hongyun
authors.initials: H
authors.lastname: Liu
keywords: Growth Performance; Heifers; Nitrogen Excretion; Protein