Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 16, 1998
Publication Date: N/A
Interpretive Summary: Three major U.S. beef processors have "close trimmed" lines of box beef, accounting for about 45% of total boxed beef production. The objectives of this study was to determine the effects of yield grade on retail product of beef carcass trimmed to .76 or .00 cm of fat thickness and to determine the relative effects of subcutaneous fat and intermuscular fat on retail product yields. Approximately 3.5% difference in total retail product can be expressed between carcasses of different USDA yield grades at either .76 or .00 cm fat trim level. Trimming to .00 cm should lower the percentage of retail product approximately 5.3% compared with trimming to .76 cm. Most of the marbling increase in cattle would be expected to occur between .80 to 1.3 cm 12th rib fat thickness. Percentage of intermuscular fat would be expected to be greater than the percentage of s.c. fat at all yield grades and to be more important in accounting for differences in retail product. On the other hand, the rate of s.c. fat deposition would be expected to be greater during the last 60 days of feed. Percentage of retail product trimmed to .00 cm can be predicted accurately using carcass traits in an equation.
Carcass data from one side of 610 steers born from 1988 to 1990 in Cycle IV of the Germ Plasm Evaluation research program were analyzed to develop means for carcass traits and retail product percentages at two fat trim levels (.76 and .00 cm) by yield grade categories. Weights of s.c. fat and intermuscular fat were recorded separately at each trim level. Quadratic regression curves were plotted for percentages of roast and steak meat (R&S), retail product (RP), and fat trim components relative to incremental changes in USDA yield grade. Prediction equations were developed on a ran- dom half of the 610 carcass to predict weights and percentages of R&S, RP, and fat trim using carcass traits obtained at the time of USDA grading and then tested on the remaining half of the carcasses. In addition, prediction equations were developed using subcutaneous and intermuscular fat plus car- cass traits to evaluate the contribution of each to carcass fabrication yields. Percentage of RP, trimmed to either .76 cm or .00 cm of fat, de- creased an average of 3.5% for each full yield grade increase. Trimming to .00 cm of fat resulted in about a 5.3% lower percentage of RP compared to trimming to .76 cm. A prediction equation for percentage of RP trimmed to .00 cm using adjusted fat thickness, carcass weight, ribeye area, and per- centage of kidney knob had a R squared 2 value of .54. The variations in percentage of R&S and percentage of RP at both trim levels were reduced by removing SC fat trimmed to .76 cm; however, considerable variation still existed. Subcutaneous fat expressed as a percentage of the sum of inter- muscular fat and s.c. fat increased as yield grade increased but the percentage of intermuscular fat was higher for all yield grades than the percentage of s.c. fat.