GUIDELINES FOR UNIFORM SWINE IMPROVEMENT PROGRAMS

IV. EVALUATING MARKET HOGS USING A
LIVE-PRODUCTION TO CARCASS-MERIT APPROACH

(Adapted from Procedures to Evaluate Market Hog Performance, 1991,
and updated from NPPC committee reports, 1997)

General Considerations

The most complete and comprehensive evaluation of market hogs includes both live-hog production and carcass merit, and incorporates all steps listed in the next section, Steps to Determine Pounds of Quality Lean Pork Gain per Day on Test.

Because both composition (how much) and quality (how good) of lean are essential for assessing carcass merit, use of the most complete procedure will ensure maximum accuracy. This entails cutting the chilled, post-rigor loin muscle (longissimus thoracis) between the 10th and 11th ribs so that actual muscle area, fat depth and quality characteristics can be assessed. If ribbing the carcass is impractical, substitute either last-rib fat thickness and muscling score, or % lean as predicted by electronic devices. If only live data are available, electronic methods such as ultrasound can be used to predict composition, and equations have been included for this. Unfortunately, live weight is subject to variations of fill, and muscle quality is ignored.

Steps to Determine Pounds of Quality Lean Pork Gain per Day on Test

1.Develop production testing

A.On-farm and Central Test. Visually screen pigs for health and abnormalities; properly identify with ear tags and tattoos; pen pigs according to breed, sex and weight; provide a preliminary acclimation period of at least seven days; and record initial on-test unshrunk weight in pounds.

B.Production Competition Test (sometimes called a "derby" contest). Bring pigs from participating herds to one or more central locations at a specified time. Screen pigs for health and abnormalities. Weigh pigs individually to the nearest pound, ensure that all are ear notched, place a unique tattoo or tag in the ear of each pig, and record. Photographs would provide for additional identification. After verification of each producer=s pigs, return them to the farm for testing. Carefully clean and disinfect equipment before proceeding to the next producer's pigs. See the next section, Production Information, for details.

2.Record dates of test.

3.Visually evaluate hogs off-test.

A. Eliminate a hog exhibiting undesirable traits such as serious injury, evidence of stress susceptibility or other heritable defects.

B.Eliminate a hog possessing a structural score of 1 to 3 (see Appendix D).

For steps 4-13, details are found in the sections Transforming the Hog to Pork and Carcass Information.

4.Tattoo hog (preferred) or use ear tag to identify prior to slaughter.

5.Visually evaluate each warm carcass and eliminate from consideration if it is condemned or excessively trimmed (greater than 5%). Carcasses that exhibit cryptorchidism or objectionable traits that appear to be the result of improper management practices should also be eliminated from consideration.

6.Obtain carcass weight, adjust for minor trim losses (if necessary) and adjust to a warm, skin-on basis. Eliminate from consideration if adjusted carcass weight is less than 150 pounds.

7.Measure carcass length (inches).

8.Cut carcass between the 10th and 11th ribs and measure fat depth at the 3/4 location.

9.Determine loin muscle area using a grid or tracing.

10.Assess color score of loin muscle and eliminate scores of either 1 (pale pinkish gray) or 6 (dark purplish red).

11.Assess firmness/wetness score of loin muscle and eliminate score of 1 (very soft and very watery), where 3 is very firm.

12.Assess marbling score of loin muscle and eliminate scores of 1 (devoid to practically devoid) or 5 (moderately abundant or greater).

13.Assess quality of fat throughout the carcass and eliminate if it is soft and oily.

14.Determine pounds of quality lean pork gain per day on test containing 5% fat, using the following formula:

Pounds of acceptable quality lean pork gain per day on test

a This is the preferred equation. Substitute others if working with live ultrasonics, carcass electronic probe or unribbed carcasses. Equations are from Orcutt et al. (1990).
b Equation is from Brannaman et al. (1984).
c Days include both first and last days on test

For details and alternative calculations, see page 25.

Production Information

If records are to be verified, identify young pigs by using ear tattoos or tags and have this information recorded by a disinterested party such as, a representative of the county pork producer organization, extension service, agriculture education program, industry livestock specialist or similar service.

  1. On-test weight. Acclimate feeder pigs to the test environment for at least seven days. This is an adjustment period for pigs that have lost weight due to handling stress before and during delivery to the test station or derby weigh-in site. Beginning unshrunk weights should range between 40 and 90 pounds.

  2. Off-test weight. To ensure uniform ingesta and excreta content, weigh each hog on an unshrunk basis. Live weights are only necessary if (a) growth rate is to be expressed on a live-weight basis, (b) there is concern about the carcass meeting the minimum requirement of 150 pounds, or (c) an assessment of dressing percent is desired. Because warm carcass weight is less subject to errors due to fill variation, and because it is used in the equation for predicting pounds of lean pork gain per day on test, off-test weight has limited value if fill is not standardized.

  3. Days on test. Record in whole days, including first day on test (day0)(after the acclimation period) and last day on test.

  4. Chronological age. Record in whole days, including day of birth as first day (day0) and slaughter day as last day. If using age in programs to predict lean growth per day of age, or in competitions to establish eligibility, the actual age should be used.

Transforming the Hog to Pork

  1. Identification. Tattoo each hog with approved edible ink at two locations on each side. If hogs are skinned rather than dehaired, clip tags to both ears. After bleeding and before removing the head, the tags should be removed, placed in a plastic bag and pinned to the foreshank.

  2. Inspection for unusual conditions. Even if a carcass passes inspection, some unusual conditions may exist (e.g., jowl abscesses, arthritis, cryptorchidism, etc.). Inform the producer of these conditions.

  3. Warm carcass weight and trimming losses. The warm carcass weight should be recorded sequentially on a weigh sheet, written on the carcass with an edible ink marker, or recorded by automatic processing. Writing on the carcass helps to minimize errors in matching weights with identification numbers.

    Most carcasses shrink about 1.5% during chilling, so if chilled weights are recorded, convert them to a warm-weight basis by dividing by .985 (or a value provided by the plant management). Adjust skinned carcasses to a skin-on basis by dividing warm weight by 0.94 (skin is approximately 6% of the carcass), or by a value provided by the plant management.

    If jowls are removed due to abscesses or if muscle, fat or bone have been removed from locations where measurements are taken, or if excessive (greater than 5%) muscle, fat and bone have been removed because of bruises or localized infections, the carcass should be eliminated from consideration. If the trim loss is less than 5%, estimate the amount missing and adjust the warm carcass weight.

    The recommended minimum warm carcass weight is 150 pounds. The acceptable weight range for carcasses, however, should match the weight preference of the cooperating packing plant. This ensures that carcasses with high evaluations do not receive large discounts for being outside the packer=s acceptable weight range.

  4. Ribbing the carcass. To measure quality and composition characteristics, first saw the vertebra of the untrimmed carcass perpendicular to the long axis of the loin between the 10th and 11th ribs. Start adjacent to the 11th rib and just cranial to the 10th - 11th thoracic vertebrae junction to permit a perpendicular cut to the loin muscle and avoid cutting the 10th rib. After sawing the vertebra, use a knife to extend the cut no more than one inch beyond the lateral (outer) edge of the loin muscle. Extending the cut further will damage the belly for bacon processing.

Carcasses should be sufficiently chilled prior to ribbing (at least 12 hours is recommended for conventional procedures or 6 hours for surface freezing procedures). To allow full expression of the quality characteristics finish ribbing at least 10 minutes before examination.

Carcass Information

1.Quality Characteristics. Desirable fresh pork quality is defined as a combination of traits that provides an edible product that is nutritious and wholesome after processing and storage, suitable for processing, attractive in appearance, and appetizing and palatable after cooking. In addition, quality must be consistent. The following traits should be useful predictors of pork quality:

A.Muscle color. Fresh pork should be reddish pink. Individual muscles are usually uniform in color, but muscle groups often vary considerably in color (such as in the ham). Consumers object to muscles that are too pale or too dark. Abnormally pale muscles quickly turn gray in the retail display case and often shrink considerably, resulting in economic losses during processing, and dry-tasting products after cooking. Dark muscles will have a shorter shelf life because they are less acidic and therefore support bacterial growth.

Charts illustrating the color scores are available from NPPC. Carcasses scoring 1 or 6 are considered undesirable.

B.Muscle firmness/wetness

The following scoring format is used to assess this characteristic:

1 = Very Soft and Very Watery. Fluid accumulations appear on a soft, open-textured muscle surface subsequent to ribbing, the quantity of which can be easily assessed by the filter paper test. This condition is often related to a pale pinkish gray color, but is also common to the reddish pink color. The product will shrink excessively during processing and lack juiciness after cooking. A carcass that scores a 1 is unacceptable.
2 = Slightly Firm and Moist.
3 = Very Firm and Dry. This rigid, closed structure exhibiting no visible surface fluids is often associated with a purplish red or darker color.

C.Marbling (intramuscular fat). Marbling is the visible fat within the boundaries of the muscle. NPPC has color charts illustrating the five degrees of marbling. The scores are based on the amount of chemical fat and described as:

1 = devoid to practically devoid,
2 = traces to slight,
3 = small to modest,
4 = moderate to slightly abundant, and
5 = moderately abundant or greater.

Some marbling (scores 2 to 4) is considered desirable for providing a juicy and flavorful cooked product. Pork without marbling may be less flavorful and less juicy and is more susceptible to dryness when abused with high broiling temperatures. Carcasses devoid to practically devoid of marbling (score 1) are unacceptable. At the other extreme, large quantities of marbling do not make pork proportionately more palatable but do supply excess calories from fat. A carcass possessing moderately abundant or greater amounts of marbling (score 5) is undesirable and should be eliminated.

D.Other tissue conditions. The following conditions should eliminate the carcass from consideration:

1)Steatosis (fatty infiltration in muscle due to problems such as muscle atrophy).
2)Soft, oily fat (fat should be firm and white).

2.Composition Characteristics. Composition refers to the proportionate quantity of lean pork or muscle in a carcass. Degree of fatness and extent of muscling (reflecting variations in muscle-bone ratio) are the primary factors associated with composition. Carcasses should have as much muscle and as little fat, bone and skin as biologically possible without jeopardizing quality and live production factors.

When comparing carcasses or measuring the production efficiency of a hog, it would be ideal to determine the proportion of muscle by dissection and chemical analysis. To date, however, this is impractical, so simpler and less accurate methods are used to estimate composition. Each of the following indicators is recommended.

A.Fat depth (including skin) over the loin at the 10th rib. Cut the untrimmed carcass perpendicular to the backbone between the 10th and 11th ribs to expose the loin surface. Divide the longest axis of the loin muscle=s cross-sectional surface into quarters. Measure the fat depth at the 3/4 distance along the loin muscle (closest to belly side) in .05-inch units from the edge of the loin muscle to the outer edge of and perpendicular to the skin. If carcass has been skinned, add 0.1 inch (or whatever is recommended by the plant management) to the depth measurement. For more accuracy, rib and measure both sides and average the values.

B.Loin muscle area (LMA). At the same place backfat was measured, determine the area in .05-square-inch units either by using a plastic grid directly on the cross-sectional surface or by tracing on acetate paper and using either a compensating polar planimeter or the grid. (Grids are available from Cooperative Extension Service, Iowa State University, Ames, IA, and are identified as grid AS-235e.) For more accuracy, rib and measure both sides and average the values.

C.Visual carcass muscling score. Make every effort to minimize the effects of fatness when assessing muscling scores.

1 =Thin. Carcass is angular in shape. The hams and shoulders are tapered and narrow in appearance and lack bulge and thickness. The muscle-to-bone ratio would be low. A carcass with thin muscling is unacceptable.
2 =Intermediate. Carcass would be intermediate between Scores 1 and 3. Most carcasses are within this range.
3 =Thick. Carcass is bulging in appearance. The hams and shoulders are significantly thicker (as a result of more muscle, not fat) than the loin region. They are convex in shape and the muscle-to-bone ratio would be high.

D.Last rib fat thickness. Last rib fat is used to calculate USDA grades. Measure to the nearest .05 inch. Measure perpendicular to and including the skin and all layers of fat exposed on the split surface of the backfat adjacent to the last rib. Do not include the connective tissue layer next to the vertebra. Measurement made on one side of the carcass is usually satisfactory for accuracy, but measure both sides and average the values if the carcass is split off-center. Make subjective adjustments only when there are mechanical disfigurations and/or very unusual fat distributions. If the skin has been removed, add 0.1 inch (or whatever is recommended by the plant management) to adjust to a skin-on basis.

E.Carcass length. Length itself is not necessarily related to composition, but it is associated with live production characteristics. Measure length in as straight a line as possible from the cranial (anterior) tip of the aitch bone to the cranial edge of the first rib and next to the thoracic vertebra and report to the nearest 0.1 inch.

Equations to Predict Composition

The most appropriate equations are those listed for ribbed carcasses, because both composition and quality can be assessed. Where it is not possible to rib the carcass, the following equations (Orcutt et al., 1990) use either fat thickness, muscling, weight and sex, or an electronic method to determine percent lean. To calculate fat-free lean, multiply each equation by 0.95.

1.For Live Hogs Using Ultrasonics.

A.Pounds of acceptable quality lean pork (containing 5% fat) adjusted to a 230-pound live weight basis (Corresponds to a 170-lb. warm carcass. To convert to % lean on a live weight basis, divide by 230 and multiply by 100, or to convert to % lean on a carcass weight basis, divide by 170 and multiply by 100.)

85.000- .042 x live wt., lb.
-16.326 x 10th rib fat depth, in.
+4.582 x 10th rib LMA, in.2

B.Pounds of acceptable quality lean pork (containing 5% fat) when live weight is not adjusted. (To convert to % lean on a live weight basis, divide by actual live weight and multiply by 100, or to convert to % lean on a carcass weight basis, divide by actual live weight, then divide by .74, and multiply by 100.)

3.950+.308 x live wt., lb.
-16.440 x 10th rib fat depth, in.
+4.693 x 10th rib LMA, in.2

To use predicted percent lean (PPL) in multiple-trait indexes (see Chapter 3), use the enpoint weight (e.g., 250 lb) in place of live weight in equation B. Adjust backfat and LMA to the endpoint weight and use the adjusted values in the equation. NSIF indexes use PPL on a carcass basis.

2.For Ribbed Carcasses.

A.Pounds of acceptable quality lean pork (containing 5% fat) adjusted to a 170-lb. carcass weight basis. (To convert to % lean, divide by 170 and multiply by 100.)

88.307 -.036 x adj warm carc. wt., lb.
-18.574 x 10th rib fat depth, in.
+3.734 x 10th rib LMA, in.2

B.Pounds of acceptable quality lean pork (containing 5% fat) when carcass weight is not held constant. (To convert to % lean, divide by actual warm carcass weight and multiply by 100.)

7.231+ .437 x adj. warm carc. wt., lb.
-18.746 x 10th rib fat depth, in.
+ 3.877 x 10th rib LMA, in.2

4. For Unribbed Carcasses Using Grading Measurements.

A.Pounds of acceptable quality lean pork (containing 5% fat) adjusted to a 170-lb. carcass weight basis. (To convert to % lean, divide by 170 and multiply by 100.)

88.506 -.045 x adj. warm carc. wt., lb.
- 15.077 x last rib fat thick., in.
+ 6.062 x muscling score
+ 3.957 x sex code
(barrow = 0, gilt = 1)

B.Pounds of acceptable quality lean pork (containing 5% fat) when carcass weight is not held constant. (To convert to % lean, divide by actual warm carcass weight and multiply by 100.)

8.179 + .427 x adj. warm carc. wt., lb.
- 15.596 x last rib fat thick., in.
+ 6.290 x muscling score
+ 3.858 x sex code
(barrow = 0, gilt = 1)

4. For Unribbed Carcasses Using Electronically Measured Fat and Muscle Depths

A.Pounds of acceptable quality lean pork (containing 5% fat) adjusted to a 170-lb. carcass weight basis. (To convert to % lean, divide by 170 and multiply by 100.)

83.883 - .007 x adj. warm carc. wt., lb.
- 18.210 x fat depth, in.
+ 9.664 x muscle depth, in.

B.Pounds of acceptable quality lean pork (containing 5% fat) when carcass weight is not held constant. (To convert to % lean, divide by actual warm carcass weight and multiply by 100.)

2.827 + .469 x adj. warm carc. wt., lb.
- 18.470 x fat depth, in.
+ 9.824 x muscle depth, in.


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