Effects of postruminal amino acid supply on growth performance and feed cost of beef steers

Abstract Text: Seventy-two steers (initial BW 243 ± 4.55 kg) were used to evaluate supplying excessive AA flow during growth phase on performance, feed cost per gain, blood metabolites and subsequent carcass characteristics of beef cattle. Steers were randomly assigned to three alfalfa haylage-based diets (30% diet DM) with increasing ruminally undegraded AA; A diet was formulated to meet the requirement (BAL) and two diets formulated to exceed (EX1 or EX2) 100% of the most limiting AA during 83d growing phase. Steers were fed a corn-based diet during 83d finishing phase and slaughtered when gain cost exceeded gain value. Initial and final BW did not differ among treatments (P>0.8), however BW at the end of growing phase tended to decrease (P=0.09) as postruminal AA supply increased (BAL 398; EX1 391; EX2 385 kg ± 4.05). Calves fed EX1 tended greater ADG (BAL 1.43; EX1 1.76; EX2 1.62 kg ± 0.13; P<0.1) during the first 21 days on feed, however this compensatory growth, was not maintained by the end of 83d growing phase (BAL 1.82; EX1 1.74; EX2 1.69 kg ± 0.05; P=0.14). ADG during finishing phase and overall, feed intake during growing and finishing phase, and feed:gain during growing phase and overall did not differ among treatments (P>0.6), however, EX1 tended greater feed:gain than BAL and EX2 during finishing phase (BAL 7.59; EX1 7.95; EX2 7.24 ± 0.25 P=0.14). Feed cost per unit of gain ($/Kg of gain) increased (P<0.001) as postruminal AA was included in the diets (BAL 1.89; EX1 2.07; EX2 2.25 ± 0.05). Quality grade, marbling, HCW, ribeye area, 12th-rib fat thickness, dressing percent, KPH and yield grade, did not differ among treatments (P>0.6). Blood glucose did not differ (P=0.99), whereas EX2 had greater blood urea nitrogen compare to BAL and EX1 (BAL 17.29; EX1 16.5; EX2 20.45 ± 0.48; P<0.001). Our laboratory has demonstrated diets formulated to meet absorbable AA requirement, but have not determined the effect of exceeding absorbable AA requirement during growth phase on overall performance. AA supply exceeding requirement during growth phase may impact finishing phase efficiency. Calves consuming diets with postruminal AA supplied above requirements may respond with greater compensatory growth as occurred in this study, however improved growth was not sustained over the feeding period and feed cost per gain were increased.

Keywords: beef, RUP