Results and Discussion
RN- allele Frequency
The Glycolytic Potential distribution deviated from normal in the population and the cut-off for classification of a RN- carrier was 150 µmole/gram, with animals above 150 classified as RN-,rn. Sire genotype (Table 1) were established based upon progeny classification and statistical probability. Three sires were classified as rn+/rn+ with a probability of failure to detect the RN- allele of < 0.02 (Sires 2, 3, and 12) and one classified as rn+/rn+ (Sire 9) with a probability of failure to detect the RN- allele = 0.125. Two sires were classified as RN-/RN- (Sire 6 and 14) with the probability of failure to detect the rn+ allele of < 0.04. The remaining nine sires were classified as RN-/rn+. The dominant RN- allelic frequency was estimated to be 0.43 based upon sire genotype classification.
The estimated RN- allele frequency was much lower than reported by Enfalt et al. (1997a) (0.61) in a Swedish purebred Hampshire population and LeRoy et al. (1990) (0.60) in a French Hampshire population. A primary difference between this study and other studies listed above was that RN status was based upon the allele frequency derived from the genotype classification of the sire and not the allele frequency calculated from progeny GP values. The estimated allele frequency from the sire genotype should closely match the progeny expectation for these sires when mated to known rn+/rn+ females.
The RN- allele frequency estimate from the population evaluated results in an estimated genotype distribution within the Hampshire population of 19% homozygous dominant (RN-/RN-), 49% Heterozygous (RN-/rn+) and 32% homozygous negative (rn+/rn+) under assumptions of Hardy-Weinberg equilibrium. The expected proportion of rn+/rn+ and RN-/rn+ sires in the Hampshire population provides a large genetic base for selection against the dominant RN- allele through the use of progeny testing procedures utilizing GP analyses.
RN- effects on Performance, Carcass, Muscle Quality, and Sensory Attributes
Differences among RN genotypes for all traits measured in the study are presented in Table 2. No differences were observed between the RN-/rn+ and rn+/rn+ progeny for average daily gain, carcass backfat at any anatomical location or loin muscle area measurement.
RN-/rn+ progeny had lower ultimate pH values, greater GP and poorer firmness scores than rn+/rn+ pigs, but no differences were observed for drip loss percentage or purge loss during storage. The differences between RN-/rn+ and rn+/rn+ genotypes for ultimate pH and GP values agree with previous research reports (Fernandez et al., 1992; Monin and Sellier, 1985; Lundstrom, et al., 1996; Enfalt et al., 1997a,b) but disagree with previous reports (Sutton, 1997; Miller et al., 1998; Bidner et al., 1999b) where RN-/rn+ pigs were reported to have higher drip loss.
Objective measurements of muscle color (L*) and reflectance (Y) showed the RN-/rn+ pigs were paler in color (P < 0.07), but no differences were observed for subjective visual color scores. These findings are supported in research described by Enfalt et al. (1997a) and Lebret et al. (1999) and greater Hunter L* values reported by Lundstrom et al. (1996). Along with the paler color, firmness/wetness scores were lower for the RN-/rn+ genotype indicating the inability of the longissimus to maintain its normal shape and increased visible surface moisture. Marbling score and intra-muscular fat content were not different between genotype as was expected.
No differences were found between genotypes for sensory attributes or objective measures of tenderness, which is in contrast to previous research reports. Interestingly, off-flavor scores were quite high in this data set, with sensory panelist comments (no statistical relationship evaluated) indicating a significant metallic taste to the pork served. Pork flavor scores, in contrast, were very low for the group. Cooking loss and cooked moisture, both indicators of juiciness, were not different in this study.