The intensive selection for increased carcass leanness has led to a decline in meat quality and an increase in consumer concerns.
The quality characteristics that play an integral role in consumer acceptance, such as intramuscular fat, have declined as genetic suppliers have focused on intense selection for increased leanness.
Recent developments in ultrasound technology offer producers the opportunity to evaluate and improve meat quality traits in breeding animals, such as intramuscular fat, without extensive and costly progeny-testing programs.
25 Years of Change
The genetic antagonisms that exist between carcass leanness and various indicators of meat quality have been well documented, confirming that long-term selection for components of carcass composition will yield reductions in traits such as tenderness, color, moisture content and marbling. It is not surprising, therefore, that meat quality characteristics, particularly intramuscular fat percentage, were significantly better before intense selection was placed on lean percentage.
In an effort to document the shift in meat quality and eating quality traits, the Duroc breed was selected for a research project aimed at comparing the offspring from boars available in the mid-1980s to those sired by boars currently available.
The magnitude of these changes was evaluated by comparing pigs sired by boars in regional studs in the mid-80s (via frozen semen) to a representative sample of Duroc boars available in 2002.
Results from the study verified that significant progress toward the enhancement of carcass composition has been realized within the Duroc breed since the mid-1980s (Table 1 ).
These long-term changes have also altered the growth patterns and deposition rates of ultrasonically measured 10th rib backfat, loin muscle area and intramuscular fat.
Unfortunately, the enhancement of carcass leanness over time has been at the expense of meat quality traits, namely intramuscular fat percentage, tenderness and color, as well as eating quality traits, such as flavor.
Researchers were also able to identify older genetic lines, which have superior meat quality attributes and maintain adequate growth and carcass composition.
Ultrasound Captures Quality
Ultrasound technology is effective in estimating carcass composition. This information has guided U.S. commercial and seedstock producers toward leaner, more muscular animals.
Likewise, it is widely recognized that greater levels of marbling have a positive impact on consumer acceptance of pork. Improved meat quality, particularly intramuscular fat, has been one of the criteria used in the development of specialty meat products, worldwide.
Conversely, levels of intramuscular fat below 2 to 2.5% have been associated with poorer moisture retention and poorer flavor.
Until recently, the application of ultrasound technology as a tool to accurately predict levels of marbling has not been possible. However, drawing on the methodology used extensively in the beef cattle industry, Iowa State researchers spent approximately 10 years focused on the accurate measurement of intramuscular fat in live hogs.
Over 4,000 pigs have been scanned. The digital images on over 750 pigs have been compared to intramuscular fat values determined by chemical analysis after harvest. This data was used to develop the current model for predicting intramuscular fat in live animals. This ability is especially valuable because it eliminates the cost of progeny testing, and the animals are still available for use as breeding stock.
Estimating intramuscular fat in live pigs was accomplished using an Aloka 500V SSD ultrasound machine fitted with a 3.5 MHz, 12.5-cm transducer for image collection. The transducer is placed longitudinally across the 10th to 13th ribs. Four digital images are captured and saved to a notebook computer for interpretation.
Image capture and interpretation software, developed by Biotronics, Inc., Ames, IA, (www.biometrics-inc.com ) was used to evaluate image quality and generate final predictions of intramuscular fat.
The quality of the captured images is a key component to accurate prediction, and is dependant on several factors, including correct transducer positioning, correct machine settings and consistent image texture.
It is important for producers to gain a solid understanding of the parameters affecting image quality before attempting to apply this technology. For this purpose, training and informational sessions are conducted through a joint effort of the National Swine Improvement Federation, Iowa State University, and Biotronics, Inc. (See www.nsif.com  for upcoming ultrasound training sessions).
Trained technicians are able to accurately measure percent intramuscular (IMF), along with backfat and loin muscle area, using an Aloka 500 ultrasound machine and Biosoft Toolbox for Swine software, which incorporates IMF technology developed by Iowa State University.
Heritability of Quality Traits
The effectiveness of this technology incorporated into current BLUP methodology (best linear unbiased prediction, an estimate of breeding value) has been directly evaluated in a long-term selection project involving intramuscular fat in Durocs.
To evaluate breeding animals in the line selected for enhanced marbling, ultrasound estimates of intramuscular fat were incorporated into a genetic evaluation system similar to those utilized in breeding programs.
After five generations of selection, a 68% improvement has been realized and an average intramuscular fat level of 5.15% has been established (Figure 1 ).
With evolution of value-added markets that focus on meat quality, genetic improvement of intramuscular fat will remain an emphasis in swine breeding programs. At the present state of technology, intramuscular fat percentage can be accurately evaluated in live pigs with the use of real-time ultrasound.
Table 1. Averages for Carcass and Meat Quality Traits in Purebred Duroc Pigs Sired by Boars from Two Different Time Periods
|Item||Current (2002)||Old (mid-80s)|
|Tenth-rib backfat, in.||0.80||1.11|
|Loin muscle area, sq. in.||6.47||5.40|
|Intramuscular fat percentage||3.09||3.48|
|Instron tenderness, kg||5.98||5.31|
|Subjective color scored||3.87||4.09|
|Subjective firmness scoree||2.08||2.14|
|Subjective marbling scoref||3.07||3.54|
|24-hour Minolta reflectance, %g||22.70||23.25|
|24-hour Hunter L valueh||47.67||48.10|
|Water-holding capacity, mg||47.33||47.75|
|Percent cooking loss, %||19.09||18.96|
| aCurrent = pigs sired by boars commercially available in 2000; Old = pigs sired by boars from the mid- to late-1980s. |
bcMeans with different superscripts within each row differ (P 0.05).
dSubjective color score (1 = pale pinkish gray to white, 6 = dark purplish red).
eSubjective firmness score (1 = soft, 3 = very firm).
fSubjective marbling score (1 = 1.0% intramuscular fat, 10 = 10.0% intramuscular fat).
gMinolta reflectance (0 = 0% reflectance, 100 = 100% reflectance).
hHunter L values are objective measures of exposed lean color (0 = black, 100 = white).