Effect of dietary crude protein level and degradability on performance and carcass characteristics of growing and finishing beef calves
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Two experiments were conducted at two locations to determine the effects of dietary CP level and source on performance and carcass characteristics of beef steers, serum urea nitrogen (SUN), and evaluation of the metabolizable protein (MP) system. British x Continental steers were blocked by BW (357 ± 28 and 305 ± 25 kg initial BW; n = 360 and 225; four and five pens/treatment in Exp. 1 and 2, respectively). Steam-flaked corn-based diets were arranged in a 3 x 3 factorial with three CP levels (11.5, 13, or 14.5% of DM) and three sources of supplemental CP (N basis): 100% urea (U), 50:50 blend of urea and cottonseed meal (B), or 100% cottonseed meal (C). Steers in both experiments were initially implanted with Ralgro and reimplanted with Revalor-S on d 56. Performance and carcass data were pooled across locations and analyzed with mixed model procedures using pen as the experimental unit. Crude protein level quadratically affected ADG (P < 0.05) and carcass-adjusted (to a common dressing percent) ADG (P < 0.10). Increasing the level of supplemental urea linearly increased carcass-adjusted ADG and gain:feed (P < 0.05) and carcass-adjusted gain:feed (P < O.OOI). Dry matter intake was not affected by CP level or source (P > 0.10). Hot carcass weight (HCW), longissimus muscle area (LMA), and dressing percent tended to increase linearly with increasing urea level (P < 0.06), whereas increasing CP level quadratically affected HCW (P < 0.05), with a maximum value at 13% CP. Differences in backfat thickness and yield grade were negligible across treatments. Neither marbling score nor percentage of carcasses grading Choice was affected by CP level or source. Serum urea nitrogen concentrations were affected by CP level and source (P < 0.05) over time. At all times measured, SUN concentrations increased with increasing CP level, but effects of CP source were small and variable across time. Evaluation of the MP system (d 0 to End) revealed no relationship between actual shrunk weight gain (SWG) and MP allowable gain (P = 0.344) and undegraded intake protein intake (UIP; P = 0.209). Linear relationships between SWG and energy allowable gain (P < 0.0001), degraded intake protein intake (DIP; P = 0.0008), CP intake (P = 0.003), and total MP supply (P = 0.003) were determined. For d 0 to 56, there was no relationship between SWG and MP allowable gain (P = 0.143) and between SWG and total MP supply (P = 0.143). However, linear relationships were noted between SWG and energy allowable gain (P < 0.0001), DIP intake (P < 0.0001), UIP intake (P = 0.002), and CP intake (P < 0.0001). Results indicate that increasing CP levels from 11.5 to 13% slightly increased ADG and carcass-adjusted ADG, whereas increasing the proportion of supplemental urea increased carcass-adjusted ADG, gain:feed, carcass-adjusted gain:feed, HCW, LMA, and dressing percent. A CP level above 13%) seemed to be detrimental to ADG and HCW. Serum urea N increased over time with increased dietary CP, but the response to CP source was variable. Evaluation of the MP system revealed no relationship between SWG and MP allowable gain; however, a strong linear relationship between SWG and energy allowable gain was observed.