Since the late 1980s, hog procurement has moved steadily toward a carcass weight and lean premium system. This emphasis on lean composition has forced pork producers to utilize good estimators of lean composition in breeding stock selection programs.

Loin muscle area and 10th rib backfat are good predictors of carcass lean content. Therefore, researchers investigated the heritabilities and genetic correlations of these key carcass traits.

Ultrasound technology has made it possible to measure these highly heritable traits in live boars and gilts. With the best individuals identified and used in commercial breeding programs, backfat thickness has decreased and loin muscle size of market hogs has increased dramatically.

Still, packers and processors are not paid based on weight of total lean. Rather, they are paid on weight of component primal or boneless sub-primal cuts. Naturally, the price of each component varies by weight. Pork use trends have increased the value of bellies relative to loins and hams. Further, the bellies from some very lean, heavily muscled pigs do not have the quality that bacon processors require.

New Carcass Component Data

Carcass component pricing is feasible. It's only a matter of time before packers and processors begin procuring hogs based on total weight and value of individual carcass components.

Table 1. Heritabilities (on diagonal) and Genetic Correlations (below diagonal) for Primal Cut Weights and Estimates of Carcass Composition

Traita	HAM	LOIN	BB	PIC	BLY	BF10	LMA
HAM	0.60
LOIN	0.57	0.61
BB	0.26	0.70	0.13
PIC	0.64	0.81	0.59	0.23
BLY	-0.53	-0.54	-0.66	-0.67	0.66
BF10	-0.66	-0.63	-0.19	-0.57	0.52	0.59
LMA	0.53	0.75	0.56	0.59	-0.39	-0.57	0.67
aHAM=Ham 401, LOIN=Loin 410, BB=Boston Butt 406, PIC=Picnic Shoulder 405, BLY=Belly 409, BF10=Carcass Backfat, LMA=Carcass Loin Muscle Area.

Data from two national checkoff-funded programs were used in this study. The National Pork Board's Genetics of Lean Efficiency project included 285 purebred Yorkshire and Duroc barrows and gilts. And, 171 barrows and gilts from the National Barrow Show (NBS) Sire Progeny Test represented the Yorkshire, Duroc, Berkshire, Chester White, Hampshire, Landrace, Poland China and Spotted breeds. All pigs were classified HAL 1843 nonmutant.

All pigs were slaughtered at the Quality Pork Processors plant in Austin, MN. Loin muscle area (LMA) and 10th rib, off-midline backfat thickness (BF10) were measured on each carcass. One side of each carcass was dissected into primals and boneless sub-primals at Geneva Meats, Geneva, MN.

Primal weights included the Ham 401 (HAM), Loin 410 (LOIN), Picnic Shoulder 405 (PIC), Boston Butt 406 (BB) and Belly 409 (BLY) [See Table 1].

Heritability estimates for primal cut weights — ham, loin and belly — were as high as backfat and loin muscle area — 0.60, 0.61 and 0.66, respectively. Heritability was 0.59 for backfat and 0.67 for loin muscle area. Shoulder components — Boston butt (BB) and picnic (PIC) — were less highly heritable at 0.13 and 0.23, respectively.

Genetic correlations between components with a high lean-to-fat ratio (ham, loin) were positively correlated with loin muscle area but negatively correlated with backfat thickness. The belly showed the opposite relationship, being positively correlated with backfat thickness (0.52) and negatively correlated with loin muscle area (-0.54). These are large genetic correlations.

Once again, nature shows you can't get everything for free. Selection for decreased backfat thickness and increased loin muscle area has indirectly increased weight of the ham and loin while decreasing the weight of the belly. The value of the entire carcass can be reduced if belly weight discounts exceed loin and ham premiums. A few packers have reduced lean premiums for very lean pigs due to lost belly value.

Given economic values, heritability estimates and genetic correlations for carcass components, selection indices can be developed. With these high heritabilities and large genetic correlations, an index incorporating ham, loin and belly weights should protect a breeder from reducing the value of any of these three primals.

As long as packers and processors are procuring hogs based on a carcass weight, backfat and loin lean premium basis, producers will continue to use breeding stock that maximize economic returns from those buying grids.