Breeders of the eight major pure breeds have participated in 10 National Barrow Show (NBS) Sire Progeny Tests since 1991. The purpose of this ongoing effort was to gain information about sires and build a research database to guide breeders as they plan their genetic improvement programs. More than 500 sires and nearly 4,000 pigs have been tested (Table 1).
Entries were submitted by 228 breeders from 22 states and Canada. Results of every trait for each progeny pig are given to each entrant after each test. Several breeders entered more than one sire group each season.
The NBS Sire Progeny Tests have been used by breeders of all sizes - from 20 sows to 5,000 sows - each gaining access to the most modern technology available.
Table 2 lists the traits measured. The records were collected and reported by representatives of Hormel Foods, Iowa State University, the University of Minnesota and Texas A&M University. All pigs in these tests were fed a commercial, five-stage diet regime. This diet did not maximize lean deposition since protein content declines at each stage, starting at about 17% and declining to 13% in the final diet.
Pork Quality Review To better understand the results of the NBS pork quality tests, it is useful to review the limited information that was available on the various traits in the early 1990s. Most of the pork quality research information dealt with intramuscular fat percent (marbling) and ultimate pH of the fresh loin. Table 3 shows a summary of breed differences in loin intramuscular fat percent from research reports going back to 1965.
The U.S. reports of Hiner (et al.) and Jensen (et al.) in the 1960s showed the very high levels of intramuscular fat in pork loins. Duroc pigs had much greater marbling than the other breeds. Foodservice marketers often hear experienced chefs say they wish they could get the well-marbled pork of the past. These results show just how different pork loins were 30 years ago.
Many people debate the importance of intramuscular fat percent to consumers. It is often difficult to show a taste preference between loins of 2% and 3% fat, but almost certainly a taste difference would occur between a 2% and a 6% loin from 1960s genetics. Loins with 6% intramuscular fat are extremely rare today.
By the 1990s, the research studies of Barton-Gade (Denmark) and Cameron (England) showed loin intramuscular fat percentages had been reduced by breeders in those countries as they undertook intensive genetic selection for increased carcass leanness. A genetic correlation of 0.3 exists between loin intramuscular fat percent and carcass backfat thickness. This correlation is large enough to be a problem if ignored. Packers in the U.S. followed suit in the late 1980s and early 1990s, paying higher carcass price premiums for very lean pigs.
Table 4 shows historical research for loin ultimate pH (greater than 12 hours postmortem). This pork quality measure is closely related to water-holding capacity and tenderness of the loin.
Notice the breed loin ultimate pH differences are consistent between the 1960 and 1990 studies. Comparisons of Duroc, Hampshire and Yorkshire breeds show a consistent difference between Hampshires (lower pH) and the other breeds. Ultimate pH is most important to pork processors since it affects cooking and curing yields. Consumers prefer high pH pork because it is more tender and juicy.
Fortunate Discovery Since the 1970s, the stress gene (or halothane gene) was known to cause pork quality problems if pigs had the dimutant genotype (both genes are stress). A standard test for genotype was exposing the young pig to halothane gas. If there was no response to the gas, the pig was considered stress-free or normal. However, if a pig did respond to the halothane gas, there was no way to tell if the pig's genotype was monomutant (carried one stress gene) or dimutant (carried two stress genes). The effect of environment and pig health also can confound a pig's response to the halothane gas. Some breeders believed the monomutant pigs did not have the pork quality problems of dimutant pigs, but there was no way to prove this since the halothane gas test couldn't separate the pigs' genotypes.
A breakthrough by Canadian researchers in 1991 was the deoxyribonucleic acid (DNA) test for the stress gene. This test was able to differentiate between normal, monomutant and dimutant genotypes with high accuracy.
All pigs in the NBS Sire Progeny Tests were evaluated for the stress genotype using the DNA test. The results of these tests show clearly that the monomutant pigs do have many more pork quality problems than normal pigs. The differences, based on a 240-lb. market pig, are shown in Table 5.
Notice that the stress gene does improve loin muscle area, carcass yield and 10th rib backfat thickness. Therefore, producers selling pigs on a Fat-O-Meater carcass grading system will get greater lean premiums if their pigs carry the stress gene. However, most packers have told producers they will not purchase stress-gene carrier pigs because of the pork quality losses associated with those carcasses.
The NBS test results were the first to show pork quality differences from large numbers of pigs with known stress genotypes.
Barrows vs. Gilts The other major gene difference is the sex difference between gilts and barrows. This difference offered some management opportunities, such as managing barrows and gilts separately to maximize feed efficiency. This practice also enhances carcass composition differences.
The NBS pigs were all fed the same diets within a test group, and there were many littermate pairs of barrows and gilts. This design gives very good estimates of sex differences.
Sex differences varied between breeds for some traits, creating a breed by sex interaction. All of the interactions were of size, not direction. For instance, Berkshire barrows are 0.28 in. fatter than Berkshire gilts, while Hampshire barrows are 0.16 in. fatter than Hampshire gilts. In other words, no barrows were leaner than gilts, but the size of the difference between them varied by breed.
Sex differences (Table 6) show barrows grow faster, are fatter, have less lean and have more tender and juicy loins. The many traits that show interactions suggest an opportunity to fine-tune management and marketing programs to match performance differences of a specific breed-sex group of pigs.
Production-Based Traits Pig growth, leg soundness and dressing percent are the traits commonly observed by producers. Profits depend on fast growth, ability to function in modern environments and conversion of feed to carcass weight. Table 7 shows breed differences for these traits.
When the NBS program started, there was much concern about the leg soundness of pigs in confinement. But, breeders have made great strides in improving those structural shortcomings. No pigs have been disqualified for leg unsoundness since 1996. Duroc and Landrace have shown the most improvement.
Carcass-Trait Differences Carcass traits, measured in the packing plant, are shown in Table 8.
Some carcass buying programs use last rib ruler to directly measure backfat, while others use forms of 10th rib backfat measurement such as Fat-O-Meater or AUS ultrasound. Breed differences are apparent.
Loin muscle area and fresh ham composition (one ham) measures are shown in Table 9. Again, breed differences are notable.
Quality-Trait Indicators Reviewing meat quality indicators, the results in Table 10 show the large breed differences in ultimate pH that are consistent across the large, valuable pork muscles. High ultimate pH is very desirable since it is related to low drip losses, low cooking losses, better water-holding capacity and more tenderness of final product. Pork processors continue to develop value-added, enhanced products. Producers can improve the quality of pork for value-added programs and the demand of their product through improved genetic programs.
Measures of NPPC loin firmness score and ham muscle 48-hour drip loss indicate pork processing quality and tenderness. Table 11 shows breed differences for these traits.
The results show the greater firmness and lower drip loss of pork from Berkshire and Chester White entries. These differences are of economic importance and show the range of improvement possible to the industry.
Meat color is measured by Minolta light reflectance and Hunter L color, in which lower readings indicate darker, more desirable color. Table 12 shows the Hunter L color scores for fresh loin, fresh ham muscles and cooked ham muscles.
The color differences (lower scores are darker) are consistent across muscles. Berkshires have the darkest pork, which would be expected from their higher pH levels.
The loin quality traits of intramuscular fat percent, loin cooking loss, cooked loin tenderness and cooked loin juiciness are shown in Table 13. These traits are good predictors of consumer satisfaction with pork loins.
Cooking loss ranges 4% from best to worst. Having a market hog supplier using a breeding program directed to meat quality means pork processors have a better opportunity to provide products that will satisfy consumers.
There is also a large difference in tenderness between the breeds. Value-added products have a better chance of success if the pork is more tender before processing. Overall, better fresh pork quality plus new processing methods can increase consumer demand for pork.
Heritabilities Reviewed Genetic improvement programs depend on the heritability of a trait, which shows the amount of genetic variation in a trait and the genetic correlations among traits.
Genetic correlations between economically important traits that are positive translate to faster genetic improvement. Negative correlations mean genetic improvement will be slower.
Genetic correlations are difficult to estimate and require a large number of records from a well-designed project. They are a very important component of a successful breeding program.
Heritabilities of traits vary somewhat with the populations from which they are estimated. Other research projects may produce somewhat higher or lower estimates for the same trait.
Geneticists group heritabilities into three ranges: high (40-70%), medium (20-40%) and low (5-20%).
Heritabilities estimated from the NBS data (Table 14) tend to be on the high end of the trait ranges. The large numbers of pigs representing many breeds on each slaughter date in the NBS tests gives better estimates, especially for meat quality traits.
These genetic correlations show the path swine breeders must pursue. All of these traits are economically important for increasing pork demand. Fast lean growth is needed to have competitive pork production costs. Tender, tasty pork is needed for consumer acceptance of fresh and processed pork.
The problems are the -0.34 correlation between backfat and Instron tenderness, leaner pigs producing tougher loin chops and the 0.39 correlation between backfat and intramuscular fat percent (Table 15). Ultimate pH does not seem to be related to the production traits but is related positively to Instron tenderness and Minolta color.
Industry Opportunities Seedstock breeders and commercial producers have a great deal of breed variation to use. Some breeds - such as Berkshire, Chester White and Duroc - have superior enough meat quality that they could be used as purebreds for premium pork. Durocs are also popular as a sire breed for crossbreeding programs because of their fast growth. Notice that the largest reproductive breeds, Landrace and Yorkshire, are average to poor in meat quality. However, those breeds are so superior in reproductive traits that meat quality will be improved by sire breeds. Chester Whites had good pork quality and are considered a maternal breed.
The heritabilities of meat quality traits show an opportunity to improve meat quality within breeds. To measure most of these traits requires slaughter of the pigs. Live animal measures of meat quality must be developed for rapid genetic improvement. Research with ultrasound and biopsy techniques is being conducted at several universities. Direct measures on live animals will also be needed to avoid problem genetic correlations that are found.
The genetic tools developed from the NBS tests results are available to all producers to build better pork products.