The Effect of Protein Concentrations in Finishing Diets on Growth Performance and Carcass Characteristics of Steers

The objective of this study was to evaluate the effect of four crude protein concentrations (7, 11, 16 and 21% CP) in finishing cattle diets. One-hundred thirty-two steers (initial BW: 314 kg) were stratified by initial BW across five pens and randomly assigned to one of the four dietary treatments (n = 33 per treatment) containing 7, 11, 16 and 21% CP. Corn silage and wheat straw were offered at 10 and 5% DM in all diets as the roughage source. Metabolizable protein supply was formulated to differ by a similar amount between adjacent treatments and provide 627, 912, 1195, and 1475 g/d, respectively. Urea was used as the supplemental protein source for the 11% CP diet to meet the DIP requirement, and urea and dried corn distillers grains with solubles were used as the supplemental protein sources for diets containing 16 and 21% CP. Corn oil was provided in the first three diets to match the amount of oil in the diet containing 21% CP. Cattle were marketed in 5 groups balanced across treatment at an average BW 594 kg. Steers fed the 7% CP had reduced (P ≤ 0.01) ADG and DMI when compared with diets with 11, 16 and 21% CP. Steers fed the 7% CP diet had greater (P < 0.01) gain:feed ratio in comparison with the steers fed 11 and 21% CP. Similarly, gain:feed ratio tended to be greater (P < 0.10) in steers fed the 16% CP, compared with 7% CP. In addition, steers fed the 7% CP diet had decreased (P < 0.01) hot carcass weight, marbling score, back fat, and kidney, pelvic and heart fat (KPH) in comparison with steers fed the other three diets. However, rib-eye area did not differ among the treatments. These data indicate that feeding diets containing 11% or greater CP are necessary to optimize growth performance and carcass characteristics of steers.

Table 1. Effects of dietary protein levels on carcass measurements of finishing cattle

Item	Treatment
	7% CP	11% CP	16% CP	21% CP	SEM	P-value
Hot carcass weight, kg	344a	365b	369b	373b	3.7	< 0.01
Marbling score	431a	481b	500b	479b	17.3	0.01
Rib-eye area, cm2	83.8	84.5	85.1	85.8	1.43	0.59
Back fat, cm	0.91a	1.22b	1.35b	1.32b	0.066	<0.01
Kidney, pelvic and heart fat, %	1.81a	1.92b	1.98b	1.92b	0.032	<0.01

^abMeans ± SEM within a row with different superscripts differ; P ≤ 0.05