An observation of tenderness, yield, and carcass composition in serially slaughtered steer carcasses implanted with Revalor-XS

Tenderness, carcass composition estimates using the 9-10-11th rib sections, and the current USDA yield grading equation were observed and evaluated in serially slaughtered beef steers. These comparisons were made in an effort to better understand evaluations of steers in current marketing processes in the United States. Charolais x Angus steers (n=80) were randomized to implant treatments (REV: Revalor-XS on d-0 and d-190 or CON: no implant) and harvest date in a 2 x 10 factorial design. Four pairs of steers were randomly allocated to one of 10 harvest dates and harvested in 42d intervals (0, 42, 84, 126, 168, 210, 252, 294, 336, and 378 days on feed). Growth promotants are commonly administered to increase rate of weight gain and improve feed efficiency, but in some instances may increase the incidence of tough beef. One objective of this study was to investigate objective mechanical tenderness of serially harvested steers with and without exogenous growth promotant administration. Samples of the M. longissimus dorsi from the 13th rib section were obtained from the left side of each carcass, aged for 14d, and frozen at -29⁰C. Frozen samples were cut into 2.54-cm-thick steaks for Warner-Bratzler shear force (WBSF) and sliced shear force (SSF) measurements, and vacuum packaged. Samples were thawed for 24h at 2⁰C, initial weight was obtained prior to cooking, and steaks were cooked to an internal temperature of 71°C. After a 5 min cooling period, cooked weights were recorded; WBSF samples were cooled 24h at 2⁰C prior to coring and shearing. Slice shear force samples were taken immediately following attainment of cooked weight. All samples were sheared according to AMSA protocol with a texture analyzer and peak shear force data were analyzed via Pearson correlation and mixed models. There were no TRT x DOF interactions (P ≥ 0.51) observed for objective tenderness determined via WBSF or SSF. Peak force values did not differ between REV and CON for WBSF (P = 0.10) or SSF (P = 0.83). Peak force did not differ (P ≥ 0.17) across DOF. Moderate correlation was observed between WBSF and SSF (r = 0.41). Results from this study indicate that growth promotion had little effect on objective tenderness. Estimates of beef carcass cutability are determined via the USDA yield grade (YG) equation, which has remained unchanged for over 55 years. Previous studies have shown that variables used for predicting YG do not adequately reflect changes in genetics and composition of the current fed beef population. Carcass yields were determined by fabricating carcasses and separating lean, fat, and bone components and weighing to the nearest 0.05 ± 0.005 kg. Linear models were developed to compare predictive red meat yield of steers to the original USDA boneless closely trimmed rib-loin-chuck-round (BCTRLCR) equation and actual BCTRLCR yield. Additionally, ribeye area (REA) to hot carcass weight (HCW) relationship was compared to the standard expected when beef carcasses are graded according to USDA methods. Carcasses less than 317.5 kg in this study had larger REA than required from the USDA REA standard, whereas carcasses greater than 317.5 kg had smaller REA than required. In comparison to HCW, REA trended in a quadratic manner, differing from the original YG equation which is represented in a linear fashion. Yields of BCTRLCR were analyzed without trim fat using a forced intercept of 51.34 from the original USDA YG equation, in order to generate new coefficients for the carcass parameters used to estimate carcass cutability. Additionally, correlations were calculated between four individual parameters (HCW; REA; fat thickness in inches, (FAT); and percentage kidney pelvic and heart fat, (KPH)) previously determined by the USDA equation, and BCTRLCR outcomes. Calculations of BCTRLCR were strongly correlated (r = -0.86, -0.72, -0.64, -0.82 to HCW, REA, FAT, and KPH respectively) to all individual parameters, indicative of a shift from the original USDA BCTRLCR equation. The new equation would reflect the four original parameters with adjusted coefficients; BCTRLCR = 51.34 – 2.71 (FAT) – 0.848 (KPH) – 0.678 (REA) – 0.0043 (HCW). An updated equation would improve the accuracy in estimating beef carcass cutability. The third objective was to compare separable lean, fat and bone of samples taken from the ninth-tenth-eleventh rib section to those of the carcass, as well as proximate analysis from each component. Carcass yields were determined by fabricating the right side of each carcass and separating lean, fat, and bone components and weighing to the nearest +/- .05 kg. Ninth-tenth-eleventh rib sections were collected from the left side of each carcass, dissected into lean, fat, and bone, and weighed to the nearest +/- .005 g. Regression models were created to determine linear associations between rib sections and carcass parameters. Carcass fat and lean were strongly correlated (r = 0.87 and 0.80 respectively) to 9-10-11 fat and lean, however carcass bone was only moderately correlated (r = 0.59) to 9-10-11 bone. Correlations between whole carcass parameters and rib sections were more closely correlated when weights of each respective method were compared. Proximate analysis of carcass and rib section components produced varying levels of correlation. A REV × DOF interaction was detected for carcass ash, moisture, and bone (P = 0.04; and 0.05; and < 0.01, respectively) and rib section ether extract (P = 0.05). Both carcass components and rib section differed across DOF (P < 0.01) for ether extract, crude protein, moisture, lean, fat, and bone. No treatment effects (P ≥ 0.13) were observed in rib section components, however ash and ether extract differed (P < 0.01) between treatments for carcass components. Results from proximate analysis indicated differences between the 9-10-11th rib and whole carcass parameters.