PS10-6. Does Dietary Guanidinoacetic Acid and Rumen-protected Methionine Supplementation Improve Performance and Carcass Traits of Beef Finishing Steers?.

Abstract This study evaluated the effects of guanidinoacetic acid (GAA) and rumen-protected methionine (RP-Met) supplementation on performance, skeletal muscle energy metabolism, and meat quality of finishing beef steers. Eighty Angus steers (444 ± 31 kg BW) were fed a basal diet containing 60% high-moisture corn, 20% cracked corn, 12% haylage, and 8% mineral-vitamin supplement on a dry matter basis. Animals were randomly assigned to one of four dietary treatments (n = 20 per treatment): no supplementation (CON), RP-Met (12 g/head/day), GAA (60 g/head/day), or GAA + RP-Met (60 and 12 g/head/day, respectively). Steers were housed in pens of 10 at the Ontario Beef Research Centre (Elora, ON) for 143 ± 16 days until slaughter. Individual feed intake was recorded daily, and blood samples, body weight, and carcass ultrasound measurements were collected every 28 days to assess metabolic status and growth. At the end of the finishing phase, steers were slaughtered, and liver, kidney, and longissimus dorsi muscle samples were collected for tissue analyses. Post-mortem, muscle pH was measured, and rib sections of the longissimus muscle were collected for surface colour and Warner-Bratzler shear force analysis. Data were analyzed in SAS, with dietary treatment and day of measurement as fixed effects and sire, pen, and animal as random effects. No effect of dietary treatment was detected for dry matter intake (DMI; P = 0.70), final body weight (P = 0.69), average daily gain (ADG; P = 0.12), or residual feed intake (RFI; P = 0.29). Though the RP-Met-fed steers tended to have a greater (P = 0.06) Gain:Feed than the steers fed the CON diet. In comparison, ultrasound measures of intramuscular fat (P = 0.77), rump fat (P = 0.88), and ribeye area (P = 0.93) were similar across dietary treatments. GAA+RP-Met-supplemented steers had a subcutaneous rib fat deposition 1.6 mm and 0.8 mm greater (P = 0.02) than the RP-Met and GAA-supplemented steers, respectively. No differences were observed for hot carcass weight (P = 0.48) and dressing percentage (P = 0.30), regardless of dietary treatment. However, the GAA-supplemented steers tended to have a greater (P = 0.08) percent carcass yield than the GAA+RP-Met-supplemented steers. Among treatments, no differences were observed for carcass pH postmortem (P = 0.63), Warner-Bratzler Shear Force analysis (P = 0.80) or meat lightness, redness or yellowness (P > 0.05). Tissue-specific responses at slaughter, including arginine:glycine amidinotransferase (AGAT) abundance, guanidinoacetate N-methyltransferase (GAMT) abundance, glucose concentrations, and creatine concentrations, also did not differ (P > 0.05). Overall, supplementation of GAA and RP-Met during the finishing phase did not enhance growth performance or carcass characteristics in finishing steers.