Phase Feeding Strategies To Meet Metabolizable Amino Acid Requirements of Calf-Fed Holstein Steers

One hundred eight Holstein steer calves (114 ± 8 kg) were used to evaluate effects of phase feeding metabolizable amino acids (MAA) on growth performance and carcass characteristics. Three feeding strategies were evaluated: 1) control, single-phase feeding (steers were fed a single urea-based growing-finishing diet that meets the average MAA requirements for the overall feeding period); 2) 2-phase feeding (steers were fed a diet formulated to meet the average MAA requirements for the first 112 d on feed, and thereafter finished on the control urea-based diet); and 3) 3-phase feeding (2 diets were formulated to meet average MAA requirements during the first and second 56-d feedlot feeding periods, and thereafter cattle were finished on the control urea-based diet). Two-phase feeding and 3-phase feeding management strategies produced similar (P > 0.20) growth performance. Multiple-phase feeding increased ADG (18%, P < 0.01), DMI (4%, P < 0.05), and observed dietary NE to expected dietary NE ratio (16%, P < 0.01) during the first 112 d of the study. From d 112 to harvest, all steers received the same diet. There were no treatment effects (P > 0.20) on growth performance; however, multiple-phase feeding increased overall (351-d) ADG (6.3%, P < 0.01), DMI (3.7%, P < 0.10), gain efficiency (2.8%, P < 0.01), and observed dietary NE to expected dietary NE ratio (3.4%, P < 0.01). There were no effects (P > 0.20) of 3-phase vs. 2-phase feeding strategies on carcass characteristics. Multiple-phase feeding increased hot carcass weight (5.2%, P < 0.01), dressing percent (1.0%, P < 0.10), fat thickness (25%, P < 0.05), and LM area (8.8%, P < 0.05). There were no treatment effects (P = 0.81) on marbling score. Based on the NRC (2000) Level 1 approach, the single-phase feeding program was deficient in metabolizable Met, Lys, and His during the initial 112 d of the feeding period, providing 71.4, 73.0, and 67.6% of respective requirements during the first 56-d period, and 85.8, 87.9, and 81.1% of respective requirements during the second 56-d period. Thus, observed dietary NEg was similar (96%) to that expected for the 2- and 3-phase programs during the first 112 d, but was only 83% of expected for the single-phase program. It was concluded that a 2-phase feeding program was needed to improve overall ADG, G:F, and energetic efficiency of calf-fed (placed in the feedlot as lightweight calf) Holstein steers compared with a conventional single-phase system. At a minimum, a 2-phase system would need to separately consider MAA requirements from feedlot placement to 280 kg BW (phase 1) and from 280 kg BW to harvest (phase 2). Methionine, Lys, and His appear to be the first limiting amino acids during the first feeding phase. During the second phase (280 kg to harvest), supply of all MAA acids will exceed requirements, even where urea is the sole source of supplemental N. Maximizing growth rate during the first feeding phase may enhance carcass dressing percent and LM area.