Abstract
The effects of two diets containing different levels of crude protein (CP), rumen-degradable protein (CPd) and rumen-undegradable protein (CPu) on growth performance and nitrogen balance were examined in 60 Holstein heifers. The experimental period was from 3 months of age until a body weight (BW) of 350kg was reached. Dietary energy treatments were designed to achieve average daily weight gains of 1.0kg/day. Two groups of heifers were assigned based on different CP levels : a low-CP (LP) group, n = 30 ; and a high-CP (HP) group, n = 30. The dietary CPu contents were 4.8% and 6.5% in heifers weighing ≤ 200kg in the LP and HP groups, respectively ; the dietary CPu contents were 3.7 %and 4.8% in heifers weighing > 200kg in the LP and HP groups, respectively. In the HP group, the time taken to reach a BW of 350 kg was 9.7days shorter than in the LP group, although this difference was not statistically significant. During the experimental period, daily BW gain and the gains in specific body parts did not differ between the two groups. Plasma urea nitrogen concentration was higher (P < 0.05) in the HP group than in the LP group. The ruminal ammonia-N level in the HP group was significantly higher than in the LP group (P < 0.01) when the BW was 300kg. A nitrogen balance test was conducted when the BW was 200kg and when it was 300kg. Nitrogen retention did not differ significantly between the groups, but nitrogen urinary excretion was greater (P < 0.05) in the HP group than in the LP group. MCP synthesis in the HP group was significantly higher than that in the LP group (P < 0.05) when the BW was 200kg. The metabolizable protein supply of the HP group was significantly higher than that of the LP group (P < 0.05) when the BW was 200kg and when it was 300kg. Therefore, when administering a high-energy diet to heifers, increased dietary CP and CPu did not affect the growth performance or nitrogen retention. The results showed that because most of the increased CPu is excreted in urine, CPu was not utilized efficiently for growth performance.
