334. Effects of Guanidinoacetic Acid and Rumen-Protected Methionine Supplementation on Growth Performance, Carcass Merit, and Meat Quality of Finishing Beef Steers.

Abstract A randomized complete block design study with a 2 × 2 factorial arrangement of treatments was conducted to evaluate the effects of guanidinoacetic acid (GAA; 0 or 60 g/steer daily) and rumen-protected methionine (L-Met; 0 or 6 g/steer daily) supplementation on growth performance, carcass traits and meat quality characteristics of crossbred beef steers during the finishing phase. Forty steers (initial body weight [BW]: 452 ± 18 kg) were stratified by BW and randomly assigned to treatments (n = 10 per treatment), with five animals per pen (2 pens per treatment). Treatment consisted of 1) no GAA or no L-Met (CON); 2) L-Met only (L-MET); 3) GAA only (GAA); and 4) GAA + L-Met (GAA+L-MET) and were fed for 102 d within each group. Body weights were recorded every 28 days, and daily feed intake was monitored using the GrowSafe system (Vytelle, LLC, Lenexa, KS, USA) until slaughter. Steers were assigned to two blocks and processed one week apart according to the slaughter schedule. Data was analyzed using mixed models with treatments and groups as fixed effects and pens as a random effect. There were no interactions or differences in G: F among the treatments (P > 0.1). Additionally, no GAA × L-Met interactions were observed for any of the growth performance variables evaluated (P > 0.1). However, a tendency was observed for dry matter intake (DMI) with lower DMI in steers supplemented with L-MET (P = 0.08). Also, steers receiving GAA supplementation tended to exhibit greater average daily gain (ADG) from 0 to 28 d (P = 0.07) and higher BW at 28 d (P = 0.08). In longissimus thoracis muscle, a GAA × L-Met interaction was observed for shear force values at 6 d pm, with lower shear force in steers receiving GAA supplementation compared with those receiving GAA+L-MET, while L-MET and CON were intermediate (P < 0.05). In addition, steers supplemented with L-MET tended to have lower backfat thickness (P = 0.08) and retail cut yield (P = 0.06). Carcass characteristics including hot carcass weight, cold carcass weight, dressing percentage, rib eye area, meat pH (at 45 min, 3 h, and 96 h postmortem), color (lightness, redness, yellowness, chroma) at 4 d pm, marbling, and proportions of metmyoglobin %, myoglobin %, and oxymyoglobin %, were not altered by treatments (P > 0.1). In conclusion, L-MET supplementation tended to reduce the DMI without affecting the efficiency, while GAA supplementation tended to increase the BW and ADG the first 28 d of feeding. Meat from steers supplemented with GAA had improved tenderness, while that from steers supplemented with L-MET had lower backfat thickness and increased retail cut yield. Preliminary findings from the first year of a two-year project.