Abstract Angus × SimAngus-crossbred steers (n = 93; body weight BW = 342 ± 29 kg) were used in a randomized complete block design to examine the effect of feeding finishing diets with crude protein (CP) contents similar or below industry averages on growth performance, carcass characteristics, and plasma urea nitrogen (PUN). Steers were blocked by BW and randomly assigned to 1 of 2 treatments (3 pens/treatment; 15 steers/pen). Treatments consisted of a dry rolled corn-based diet offered ad libitum during 206 d with either 11.5% or 13.5% CP (Dry matter (DM) basis). The experimental diets were formulated to be isoenergetic. Interim BW were recorded at 28 d-intervals and blood samples were collected from all steers on d 56, 112, and 197 for the analysis of PUN. All statistical analyses were conducted using the MIXED procedure of SAS 9.4 considering the pen as the experimental unit. No treatment × period interaction was detected for BW, average daily gain (ADG), dry matter intake (DMI), and gain to feed ratio (G: F; P ≥ 0.11). Steers administrated the diet containing 11.5% CP displayed greater DMI (P = 0.04) and tended to be heavier (P = 0.08) than their counterpart fed the diet with 13.5% CP. No treatment effect was evidenced for ADG and G: F (P ≥ 0.15). A treatment × period interaction was detected for relative DMI (% BW), where steers fed the diet with 11.5% CP showed a greater DMI during the early and mid-stages of the feeding period (P = 0.01). Steers fed the 11.5% CP diet had greater carcass-adjusted final BW and ADG than steers fed the 13.5% CP diet (P ≤ 0.01). A tendency for a treatment × d interaction was observed (P = 0.07) for PUN, with steers fed the 11.5% CP diet displaying lower PUN concentration in d 56 compared to those fed the 13.5% CP diet. In addition, the concentrations of PUN in d 197 were lower compared to those observed in d 56 and 112 (P 0.01). Steers offered the diet with 11.5% CP had a heavier hot carcass weight and increased dressing percentage relative to steers fed the diet with 13.5% CP (P ≤ 0.02). The remaining carcass characteristics were unaffected by treatments (P ≥ 0.15). Taken together, results from this investigation demonstrate that finishing diets with 11.5% CP provides steers with sufficient protein, and although the feed intake relative to BW substantially declines during late-stage finishing, this decline does not impose protein-induced constraints on productivity.
Pittaluga et al. (Tue,) studied this question.