The meat quality of pork is quickly becoming a hot topic in the swine industry. During the 1980's and 1990's producers strove to produce a high lean product. According to some, this trend went too far, and consumers were left with low fat, but less tasty pork. Now, some producers are focusing in on niche markets to supply consumers with a product that will provide a more satisfactory eating experience.

Intramuscular fat (IMF), or marbling, is an important factor in determining meat quality. In market hogs, IMF is typically measured at the tenth rib in fresh pork loins 24 hours post-mortem. This method is effective, but only provides sibling and progeny data when determining Expected Progeny Differences (EPDs) of breeding stock. Advances in real-time ultrasound allow for assessment of marbling in the live pig. Utilizing this data along with actual carcass data of siblings and progeny allows for more accurate genetic assessment of breeding stock and more rapid genetic progress.

A study at Iowa State University was conducted to determine the effect of phenotypic selection for IMF. The study also analyzed the changes in other economically relevant traits incurred by IMF selection pressure. The project started in 1998 with the purchase of 40 registered Duroc gilts from ten Midwest breeders. The females were mated to random Duroc boars at regional boar studs over two generations to create a base generation of 56 litters for the experiment.

Two boars from each litter (when available) were randomly selected to remain intact, and the remainder was castrated. Pairs of litter mate gilts were randomly selected and one was placed in the control line (CL) and the other in the select line (SL). These littermate gilts were then mated to the same Duroc boar to establish genetic ties between the two lines. A total of 24 sires from 14 sire families were used to produce 50 CL and 45 SL litters in generation one. In the following generations, four boars (when possible) remained intact to increase selection intensity.

The study continued over six generations. Selection of breeding stock for the SL was based on Estimated Breeding Value (EBV) for Carcass IMF (CIMF) by using a combination of data from individual Ultrasound IMF (UIMF) and sibling CIMF. For each generation, the ten boars and 75 gilts with the highest EBV for CIMF were selected. The CL included a similar number of randomly selected boars and gilts.

The offspring were raised in confinement buildings and were fed a three phase corn-soy diet. Pigs went on test at a mean body weight (BW) of 42 kg (92 lbs). Pigs were weighed and went off test at a mean BW of 110 kg (242 lbs). Ultrasonic measurements of 10th rib loin eye muscle area (ULMA), off-midline backfat (UBL), and UIMF were collected. Lean gain was calculated using formulas from the National Pork Producers Council (NPPC).

Upon completion of the test period, all available barrows and randomly selected gilts were harvested, and corresponding carcass measurements were taken. Loin samples from the carcasses were also evaluated for other meat quality indicators including pH, color, firmness, tenderness, and water holding capacity. Additionally, the loin samples were cooked and evaluated by a trained sensory panel for juiciness, tenderness, chewiness, flavor, and off-flavor.

At the end of the study, UIMF data of all animals progeny tested was compiled with a random sampling of CIMF data of harvested barrows and gilts. An 88% improvement in IMF was realized over the six generations of the study (4.53% in SL vs. 2.41% in CL). Some studies suggest that a minimum of 2-3% IMF is required for desirable eating quality. The study proves IMF can be significantly increased through EBV selection for CIMF to meet the demands of consumers in meat-quality focused niche markets.

As stated before, the study also analyzed the effects on other economically relevant traits produced by selection for IMF. No significant differences in growth rate between the SL and CL were observed. However, there was a difference realized in carcass composition. The average carcass from the SL measured with a smaller loin eye muscle area (approximately -0.6 square inches) and increased backfat thickness (approximately 0.25 inches) compared to an average carcass from the CL. Measurements of other meat quality factors found an increase in tenderness in the SL, while changes in other factors were statistically insignificant. The sensory panel found a desirable increase in pork flavor and a decrease in the incidence of off flavor from the SL. Previous studies show a positive correlation between IMF and juiciness, flavor, tenderness, and overall palatability.

This study provides valuable insight for producers looking to earn an extra dollar as value-added markets for consumer preferred pork products continue to increase. Farmers can select breeding stock genetically superior for IMF to produce offspring with significantly increased marbling, but with slightly lowered cutability. The improvements in meat quality will have to be weighed with the associated cost of decreased lean carcass composition to optimize profitability.

Results from six generations of selection for intramuscular fat in Duroc swine using real-time ultrasound. I. Direct and correlated phenotypic responses to selection. 2009. J. Anim. Sci. 87: 2774-2780.