Effect of ractopamine hydrochloride on beef carcass composition and estimates of intermuscular fat

A total of 160 predominantly Angus, medium-framed steers (389 kg) were fed a finishing diet for 42 to 49 d. Steers (452 kg) then were estimated to be 4 6 d from slaughter condition, allotted to five blocks [weight and fat thickness (FTK) at 12th rib] and ractopamine hydrochloride (RH) was introduced into their diet (0, 10, 20 or 30 ppm). Carcasses were evaluated 24 h postmortem by three experienced evaluators for USDA yield grade (YG) and quality grade factors, nine subcutaneous (SC) fat indicators and four intermuscular (IM) fat indicators. Right sides (n = 40) varying in composition (YG 1.1 to 3.8) then were randomly selected (2 per block per treatment) for evaluation/determination of 10 carcass traits, four chemical traits (fat, moisture, protein and ash), two measures of cutability, and total IM fat from the round, loin, rib and chuck. Marbling score, adjusted FTK, ribeye area (REA) and USDA grade (yield and quality) were not influenced by RH treatment. Percentage IM fat, SC fat, boneless defatted primal cuts (round, loin, rib and chuck) were not influenced by RH treatment. No differences were found across RH treatments among four chemical traits (fat, moisture, protein and ash). Increased feeding levels of RH may be required to realize improvement in beef carcass composition. Simultaneous consideration of adjusted FTK, REA, kidney, pelvic and heart fat percent (KPH) and hot carcass weight (HCW) accounted for 60.4% of the observed variation in percentage of chemical carcass fat, whereas, an equation containing an IM fat estimate (IM fat estimate at the 12th rib, REA, KPH and HCW accounted for 59.4% of the observed variation. Simultaneous consideration of adjusted FTK, REA, KPH and HCW accounted for 58.9% of the observed variation in cutability (boneless, closely trimmed retail cuts from the round, loin, rib and chuck containing 6 mm SC fat and zero IM fat), while an equation containing an IM fat estimate (IM fat estimate at 12th rib, REA, KPH and HCW) accounted for 64% of the observed variation. These data suggest SC fat can be removed via hot fat trimming and cutability can be reliably predicted using an IM fat estimate at the 12th rib instead of adjusted FTK.