PIG Animal
Extension Swine Husbandry
ANS 94-803S    

Estimating Carcass Lean in the Live Animal

M. Todd See, Ph.D. and Charles M. Stanislaw, Ph.D.
Extension Swine Specialists
Department of Animal Science
North Carolina State University


Carcass leanness and lean gain per day have become major concerns of both commercial and seedstock swine producers. This has resulted, to a large degree, from the development of the optical fat-lean probe and its addition to the major hog slaughtering plants. The optical fat-lean probe measures the external fat depth and loin eye thickness. These measurements are made by inserting a probe into the hog carcass in the area of the tenth rib. The measurements are then used in an equation that produces an estimate of the amount of lean in the carcass. The amount of lean, plus premiums or discounts, becomes the basis for the price paid for the carcasses. Hence, there is great interest among producers on methods to measure backfat and loin dimension on the live hog, and on the correct procedures to make individual animal-to-animal comparisons.

Ruler and Scalpel

One of the oldest techniques for measuring back fat depth in live hogs is the scalpel and ruler. The scalpel is used to make a small incision on the back of the animal and a narrow metal ruler is inserted. The ruler can be passed through the fat easily but is stopped when it reaches the dense lean tissue of the loin muscle. The ruler is then withdrawn and the depth of skin and fat is read directly off the ruler.

This procedure would appear to be almost foolproof and accurate. However, errors can be made if the ruler encounters strips of lean tissue in the fat layers under the probe site. These lean strips, which are frequently referred to as "false lean", are actually portions of the Trapezius muscle. The ruler must be forced through the false lean in order to reach the loin muscle for an accurate fat depth reading. The dilemma is knowing how much pressure to apply to the ruler. Thin strips of false lean can be traversed easily, but thicker strips can be mistaken for the actual loin muscle.

False lean is most frequently encountered over the shoulder. For this reason, shoulder readings have generally been abandoned as backfat probe sites. The tenth and last rib sections of the loin are less likely to contain false lean, so the fat readings made at these sites with the ruler are more accurate.

Lean Meters

Several A-mode ultrasonic devices are available commercially for measuring backfat. A-mode instruments emit sound waves from a transducer and use the "pulse-echo" technique to quantify tissue depth. The transducer acts as both a sending and receiving unit of these sound waves. These waves reflect or bounce off changes in tissue density, such as the change from fat to lean, and elapsed time is measured from creation to reflection of the signals. This elapsed time is converted to distance measurements and may be indicated by points of light or read directly from a screen.

The A-mode devices also detect false lean, and strong but inaccurate readings can be recorded as the sound waves echo back from the fat-false lean interface. For this reason, shoulder readings should not be used with these instruments.

Some A-mode instruments are available commercially that extend the pulse-echo readings far enough to pick up loin depth. The fat depth and loin depth are automatically entered into an equation that generates an estimated percent carcass lean.

Real-time Ultrasound

In recent years, real-time ultrasound, also referred to as B-mode machines, has been added to the technology for lean content estimation of live animals. These machines have been used for some time to perform diagnostic procedures in human medicine. Real-time ultrasound emits sound waves like an A-mode ultrasound device but, by using a specially designed transducer, it emits many sound waves simultaneously along a linear path. The reflected sound waves are then transcribed and presented on a television monitor as a two-dimensional image or cross section of muscle mass as, for example, the loin-eye muscle. This permits direct measurements of backfat depth and loin eye area, either by making a direct tracing on the screen and using a planimeter (adjusting for any effect of screen size), or by using more advanced computer technology to make the area measurements.

Real-time ultrasound has gained considerable attention recently as a method of measuring leanness in both the live animal and in the carcass. It offers the first opportunity to obtain accurate loin eye area estimates in the living animal. It also permits these measurements to be made in hot carcasses without breaking the loin and reducing its retail value.

Studies comparing loin muscle measurements taken in the live animal with real-time and measurements made in the carcass have given correlations as high as 0.93.1 Perhaps equally as important is the fact that the equipment is compact, relatively light (approximately 22 lbs.) and highly portable.

Adjusting the Data

If measurements gathered on live hogs are to be used for making comparisons within a contemporary group, these numbers should be adjusted to a constant weight basis. Contemporary groups are animals of the same sex and breed type that are reared under identical management conditions at one time. The fat and muscle measurements should be taken by one technician using the same equipment on all animals. The following adjustment equations are used to convert backfat and loin muscle measurements to a constant live weight basis for more accurate comparisons.

Live backfat measurement to a constant weight.2

Where a is equal to -20 for boars, 30 for barrows and 5 for gilts. Live loin-muscle area (LMA) to a constant weight.2

Assume that a gilt came off-test weighing 240 pounds, probing .9 inches of backfat and a 6.2 sq. in. loin muscle area (LMA). The backfat measure adjusted to 230 pounds is:

And the LMA adjusted to an off-test weight of 230 pounds is:

Estimating Percentage Lean in Live Hogs

The ultimate objective of the pork industry is the production of maximum quantity of high quality lean per unit of energy input. An estimate of percentage lean for a live hog can be predicted if fat depth and loin muscle area are collected using ultrasonics. The following equation can be used to predict percentage carcass lean (containing 5% intramuscular fat) adjusted to a 230 pound live weight basis.3

An estimate of percentage carcass lean for the gilt in the previous example would be:

Estimating Percentage Lean With Carcass Measurements

An estimate of percentage lean can also be predicted with electronically measured fat and muscle depths on carcasses. The following equation can be used to estimate percentage carcass lean (containing 5% intramuscular fat) adjusted to a 170 pound carcass weight basis.3

Estimating Average Lean Gain Per Day

To determine the genetic potential for lean gain in a given production operation an on-farm evaluation can be performed as follows:

  1. Groups of hogs representative of the operation should be raised in the fall or spring, removing the effects of weather.
  2. Record average pig weight at the start of the trial.
  3. To get the best estimate of lean gain per day a high density amino acid diet should be fed so animals may perform to their maximum genetic potential.
  4. At the end of the trial, record the number of days on feed, weigh the hogs and sell them as a group on grade and yield.
  5. The kill sheet for the load of hogs provides the average carcass weight, average backfat and average muscle depth. The measurements should be converted to inches.
  6. The National Pork Producers Council provides a formula to use when estimating average lean gain per day:2

High lean gain pigs are generally defined as having a lean gain per day of .76 lbs or greater from 40 to 240 lbs of body weight. Medium lean gain pigs have an average lean gain per day of .6 to .75 lbs and low lean gain pigs gain less than .6 lbs of lean per day.

Assume that a trial was started with 100 feeder pigs with an average weight of 42 pounds. The group was fed for 120 days and sold at an average market weight of 246 pounds. The kill sheet provided by the packer indicated that the load averaged .95 inches of backfat, had an average loin depth of 2.20 inches, and a total carcass weight of 18,204 pounds. The average carcass weight is 18,204 / 100 = 182 pounds. Using the above formula the pounds of lean gain per day on this group is estimated to be .65 or medium lean gain pigs.


Animal evaluation, genetics and understanding the "numbers" is important today; all affect your bottom line. To make sound selection decisions you must understand how the measuring is done, what equipment is being used, and how the data are being reported.

As a producer you should be able to identify weakness in your market hogs; ie, weight sort, fat depth and/or loin depth. If fat and/or loin depth needs improvement, first determine if the limiting factor is genetics or environment by testing the genetic potential for lean gain in your operation. If carcass problems are due to genetics, implement a genetic program. If they are due to environmental effects, change your management or implement a new nutritional program.


  1. Turlington, L.M. 1990. Live animal evaluation of swine and sheep using ultrasonics. M.S. Thesis. Kansas State University, Manhattan, KS.
  2. NSIF. 1994. Guidelines for uniform swine improvement programs. (in press).
  3. NPPC. 1991. Procedures to evaluate market hogs (3rd. ed.).

Reviewed by:
Darwin G. Braund, Dale C. Miller, Department of Animal Science, and
Dwain H. Pilkington, Department of Food Science, North Carolina State University.

Since June 1, 2000