Currently, the majority of U.S., Canadian and European pork processors utilize either carcass ultrasound or optical probe measurements to predict carcass lean composition and carcass value. Fat-free lean percentage (FFL%) is an estimate of the amount of muscle tissue in a pork carcass. It is used to estimate daily lysine and essential amino acid requirements in the National Research Council (NRC) state and national swine nutrition guides.

In this research initiative, equations were developed using fat-free lean data from 203 pigs of seven genetic lines to predict FFL% from backfat depth and loin muscle depth taken by three different optical probes — Hennessey, Destron and PG-200. The equations included linear and curvilinear variables of backfat and loin depth (Figure 1).

The accuracy of equations is best evaluated by the residual standard deviation (RSD) for FFL%, with smaller RSDs indicating a greater accuracy. The RSD is approximately the average difference between the actual and predicted value.

Overall, the equations had RSDs of 2.36 to 2.68% and accounted for 71 to 74% of the total variation in FFL%. The addition of the curvilinear terms had little impact on the accuracy of the equations. Plots of the relationships of predicted percent fat-free lean to backfat showed that the equations were nearly identical for each optical probe and each equation was almost totally linear in nature.

In contrast, plots were compared for the new prediction equations from trials funded by the United Soybean Board and National Pork Board, published by Rodger Johnson, University of Nebraska, in the Journal of Animal Science in 2004. These equations were designed to replace the current, less accurate, linear-only equations in the Pork Composition and Quality Assessment Procedures published in 2001.

These new equations had RSD’s for FFL% from 3.74 to 4.23% for either ultrasonic or optical probe measurements of backfat and loin depth. Since the total standard deviation for FFL% was approximately 5%, this indicates that these new equations only account for a very small percentage of the total variance in FFL%.

The predicted percent FFL% at a 220-lb. carcass weight from equations predicting FFL mass (FFLM) or FFL% are shown (Figure 2) for the animal ultrasound (AUS), fat-o-meter (FOM) and ultrasonic FOM (UFOM), which measure loin muscle depth and fat depth. A 0.01 in. reduction in loin depth was predicted for each 0.04 in. increase in fat depth. At 220-lb. carcass weight, the prediction equations for FFLM and FFL% should produce identical results.

The AUS equation predicted that FFLM decreased as backfat increased to a minimum of 99.4 lb. (or 45.2%) at 1.61 in. backfat depth. Above 1.61 in. backfat depth, FFLM increased as backfat depth increased. Predicted FFLM was almost constant at 102.3 lb. FFLM from 1.18 to 2.05 in. backfat. The AUS equation for FFL% predicted that FFL decreased from 47.7% at 1.06 in. backfat depth to 45.95% at 1.58 in. backfat and then increased back to 47.7% at 2.05 in. backfat depth.

The UFOM equation for FFL mass predicted that pigs achieved a minimal FFLM (and percentage) of 96.2 lb. at 1.34 in. backfat. Pigs with 1 in. and 1.73 in. backfat depth are predicted to have 99 lb. FFLM. Pigs with 1.06 and 1.58 in. backfat depth are predicted to have 97.2 lb. FFLM. Pigs with 0.79 and 1.79 in. backfat were predicted to have 103.4 lb. predicted FFLM. The FOM equations for FFL% and FFLM are different from each other and different from the other equations.

Using these equations, pigs with drastically different backfat depth are predicted to have the same FFL% and daily lysine requirements. Yet, depending on the instrument and equation used, pigs with the same backfat depth have drastically different predicted FFL% and lysine requirements.

Currently, the U.S. pork industry has not established any requirements relative to the level of physical carcass dissection and the accuracy of the prediction of any measurement of carcass composition. The European Community has established a requirement that lean yield percentage should be predicted with a RSD of 2.5% or less. At present, published and widely used prediction equations have substantially greater RSDs and account for a relatively small percentage of the true variation in FFL% between pigs or genetic lines.

Researchers: Allan P. Schinckel, M. E. Einstein, John Forrest and J. R. Wagner, Purdue University. E-mail Schinckel at