The relationship of average backfat thickness of feedlot steers to performance and relative efficiency of fat and protein retention.

Selection for growth and improved carcass merit has resulted in cattle that are variable in composition of gain during the finishing phase. This study assessed the relative performance among cattle with different levels of initial backfat thickness. It also exploited the ability to track carcass composition in the live animal with ultrasound estimates of backfat and marbling. A procedure was developed to partition and estimate relative efficiency of fat and protein gain. The trial periods were the last 43 or 50 d before slaughter and included 10 pens (average of 27 animals per pen) that ranged in average backfat thickness from 6.3 to 13.1 mm. There was no correlation (r2 = 0.0026) between average backfat thickness and G:F (g/kg of DMI). Correlations between average backfat thickness and ADG or DMI were also nearly zero (r2 = 0.0007 and 0.0042, respectively). Fat deposition from NEg was 3.98 times more efficient than protein deposition. Carcass backfat thickness was a poor predictor of carcass marbling score (r2 = 0.083), even though backfat thickness was an important predictor of the percentage of empty body fat (r2 = 0.807). The results indicate that a measure of backfat thickness on the live animal during the finishing phase is not an effective predictor of future feed efficiency. They also confirm that protein accretion is energetically expensive, and that using a single coefficient for predicting gain from NEg is valid regardless of whether gain is predominantly muscle or fat. These data document that there is little relationship between body composition and marbling score, which is contrary to models that assume a USDA quality grade target at a specified percent fat end point.