Materials and Methods
Animals
Pigs used in the study were from mating of purebred Hampshire sires to Yorkshire x Landrace (YL) crossbred females classified as negative (rn+, rn+) for the RN gene using the procedures outlined Monin and Sellier (1985). Matings of rn+/rn+ females were conducted through AI using semen from fifteen purebred Hampshire sires of unknown RN gene status. A total of 32 litters, representing from 1 to 3 litters per sire and 3 to 24 pigs per sire were produced.
Pigs were housed in a conventional finishing building allowing 8.0 ft2 of floor space in pens of either 20 or 25 pigs from 70 lb until market weight at 250 lb. A 17.5% CP, 1.15% lysine corn-soy diet was offered for ad libitum intake from 70 to 150 lb , followed by a 16.0% CP, 0.85% lysine corn-soy diet from 68 to 91 kg, and a 15.0% CP, 0.70% lysine corn-soy diet from 91 kg to market weight. Pigs were removed from test 18 to 20 hours prior to harvest at weekly intervals upon reaching market weight and transported to a commercial swine packing facility. Pigs were rested approximately 8 hours prior to stunning.
Carcass Measurements
At 24 hours post-mortem, the right side of the carcass was ribbed between the tenth and eleventh ribs for collection of longissimus muscle area (LMA) and backfat depth (BF10) (NPPC, 1991). A four-rib section (10 to 14th) of longissimus (L) muscle was collected and used for muscle quality assessment and sensory evaluation. Subjective and objective quality scores for color, marbling, and firmness/wetness were taken on the cut surface of the loin (10 to 11th rib chop) following guidelines described in Procedure to Evaluate Market Hogs, (NPPC, 1991). Ultimate pH (pHu) was measured with a portable pH meter (pH Star, SFK, Co., Cedar Rapids, IA. One inch thick loin sections were used to measure intra-muscular fat (IMF) content and GP.
Sensory Procedures
The 12 to 13th and 13 to 14th rib sections were vacuum packaged and stored for 7 to 10 days at temperatures between 0 and 4 C prior to sensory evaluation. Purge loss (PURG), an indicator of water holding capacity and juiciness was evaluated during storage. Loin samples were cooked to an internal temperature of 160oF and served to a trained sensory panel that evaluated samples for juiciness (JUIC) (1 = dry and 10 = juicy), tenderness (TEND) (1 = tough and 10 = tender), chewiness (CHEW) (1 = not chewy and 10 = very chewy), pork flavor (PORK) (1 = none and 10 = intense), off-flavor (OFLAV) (1 = none and 10 = intense). An objective, instrumental measure of Instron (INS) tenderness, which identifies the force, required to compress the cooked loin to 80% of its original height was included as a quantifiable measure of tenderness.
Genotype Classification
Progeny pigs were classified for the RN genotype (RN-/rn+ or rn+/rn+) based upon the bimodal distribution of GP values found in the study using a procedure outlined by Fernandez et al. (1992). The cut-off point for classification of an animal as a carrier (RN-/rn+) of the RN gene was 150 µmole/gram fresh tissue. Based upon this cut-off, RN genotypes were assigned to each pig and subsequent statistical analyses performed.