Technical Note: The USDA beef yield grade equation requires modification to reflect the current longissimus muscle area to hot carcass weight relationship

¹ Department of Agricultural Sciences, West Texas A&M University, Canyon 79016-0001

^* To whom correspondence should be addressed. E-mail: tlawrence{at}mail.wtamu.edu.

	Abstract

With adoption of visual instrument grading, the calculated yield grade can be used for payment to cattle producers selling on grid pricing systems. The USDA beef carcass grading standards include a relationship between required longissimus muscle area (LMA) and hot carcass weight (HCW) that is an important component of the final yield grade. As noted on a USDA yield grade LMA grid, a 272 kg (600 lb) carcass requires a 71 cm² (11.0 in²) LMA and a 454 kg (1,000 lb) carcass requires a 102 cm² (15.8 in²) LMA. This is a linear relationship where required LMA = HCW x 0.171 (slope) + 24.526 (intercept). If a beef carcass has a larger LMA than required, the calculated yield grade is lowered, whereas a smaller LMA than required increases the calculated yield grade. The objective of this investigation was to evaluate the LMA x HCW relationship against data on 434,381 beef carcasses in the West Texas A&M University Beef Carcass Research Center database. In contrast to the USDA relationship, our data indicate a quadratic relationship (WTAMU LMA = 33.585 + {0.17729 x HCW} + {-0.0000863 x HCW²}) between LMA and HCW whereby on average, a 272 kg (600 lb) carcass has a 75 cm² (11.6 in²) LMA, and a 454 kg (1,000 lb) carcass has a 96 cm² (14.9 in²) LMA, indicating a different slope and different intercept than that in the USDA grading standards. These data indicated that the USDA calculated yield grade equation grade favors carcasses lighter than 363 kg (800 lb) for having above average muscling and penalizes carcasses heavier than 363 kg (800 lb) for having below average muscling. If carcass weights continue to increase, we are likely to observe greater proportions of yield grade 4 and 5 carcasses due to the measurement bias that currently exists in the USDA yield grade equation.

Key Words: beef, yield grade, longissimus muscle area, hot carcass weight