"Due to the low genetic relationship between marbling and the other carcass traits we can increase marbling, ribeye area and percent retail product; and decrease external fat with the same sires. Our job is to identify the sires."

Carcass evaluation for the Angus breed was conceived by Dr. Richard Willham and co-researchers at Iowa State University (ISU) in 1972 as part of the original Sire Evaluation Program. This very structured program consisted of random-mating schemes throughout several commercial herds using the same set of bulls. These sires were later referred to as the original set of reference sires for the Angus breed and served as foundation benchmarks for future evaluation.
This structured Sire Evaluation Program is still in place. While it has been refined and altered by Dr. Doyle Wilson to fit today’s needs, the basic principles are still intact. Additionally, advances in ultrasound technology provide breeders, both large and small, a nondestructive method of gathering the same information from live yearling bulls and developing heifers.
When we consider the use of carcass expected progeny differences (EPDs), we must remember that the improvement of livestock is somewhat like mapping out a trip. First, we must know where we are. Second, we must determine where we are going. And third, we must plot a route.

Plotting where you are
Before we are able to make improvements in carcass traits, we must establish some knowledge of the genetic merit of the herd for these traits. The time-tested means of retrieving carcass data, of course, is through retained ownership of steers from conception to slaughter with cooperating feedlots and packers in order to obtain carcass data on each individual animal slaughtered.
This is relatively easy for large operations; however, it does provide problems for smaller producers with insufficient progeny numbers to make the system work. In this regard, some groups have pooled resources and livestock in order to efficiently retain ownership, feed steers and recover carcass data.
In measuring carcass merit, we are concerned with the traits that affect carcass value, those being:
1. Age or youthfulness as determined by the color of the bone and the hardness or degree of ossification of the split dorsal processes of the vertebrae. The youngest classification, “A” maturity, is approximately 30 months old or less.

2. USDA yield grade (YG) is an attempt to classify carcasses as to cutability or yield of closely trimmed retail cuts from the round, loin, rib and chuck. Four characteristics considered in determining yield grade are external fat thickness over the 12th rib; percent kidney, pelvic and heart (KPH) fat; area of the ribeye muscle at the 12th rib; and hot carcass weight. YG 1 is very lean, and YG 5 is very fat.

3. USDA quality grade is determined by maturity and marbling. Fig. 1 and Table 1 illustrate various quality grades and corresponding marbling scores.



Since the Angus breed is noted for carcass merit, it is also important to know the adjusted averages for the steers in the carcass database (see Table 2).



For carcass weight (767 lb.) and marbling (Small92, low-Choice, almost average-Choice), the average of the Angus breed is excellent. However, from an idealistic standpoint, we should perhaps strive for an average range in fat thickness from 0.30 to 0.40 inch and slightly increase average ribeye area. The question is, “How can this be accomplished?” The answer is through genetics, over time.
It has been said that most traits expressed by animals are a result of two factors — genetics and environment. Genetic levels controlled by genes regulate portions of traits passed from parents to offspring. Table 3 illustrates the genetic and phenotypic relationship between carcass traits, as well as heritability estimates.



Note that the trait heritabilities on the diagonal are 0.31 for carcass weight, 0.37 for marbling, 0.28 for ribeye area, 0.24 for fat thickness, and 0.24 for percent retail product. These traits are moderately to highly heritable, which means that we can make rapid progress through proper sire selection.
Also note the genetic correlations expressed above the diagonal. The genetic relationship between marbling and ribeye area at -0.08 is slightly negative, but close to zero. The genetic relationship between marbling and fat thickness is even less at 0.07. This means that each of these traits can be improved independently of the other traits. For example, the Angus data suggests that we can increase marbling and ribeye area and simultaneously reduce fat thickness through proper selection procedures.

Understand the EPD
At this juncture, it is important to understand exactly how the four carcass EPDs are defined and what the EPD means.
Carcass weight EPD is the average difference in hot carcass weight from breed average of the sire’s progeny at 480 days of age. It is expressed in pounds. Like other EPDs, the absolute value is not as important as the differences among the sires. As previously pointed out, the average adjusted carcass weight of the steer progeny in the Angus database is 767 lb.; however, due to differences in environment and the genetic values in commercial cows, the majority — or about 70% — of progeny carcass weights for sires that are 0, or breed average, have ranged from approximately 760 lb. to 840 lb.
Ribeye area EPD is the average difference from breed average of ribeye area of the sire’s progeny at 480 days. The measurement is taken at the 12th rib and expressed in square inches (sq. in.). At a constant weight end point, ribeye area has been shown to account for significant variation in percent retail product. There is also a high genetic correlation between ribeye area and total retail product.
Care should be taken when using the ratio of ribeye area to carcass weight as selection on this index will result in changes in mature size. Generally, the higher the ratio, the smaller the mature size. Ranges in ribeye area for the majority of the progeny of sires with 0 EPDs are from approximately 11.0 sq. in. to 14.0 sq. in., with the steer average at 12.4 sq. in.
Marbling EPD is the average difference from breed average of the marbling score of the sire’s progeny at 480 days. It is expressed as a percent of one-third of a marbling score. The average marbling score of the steers in the database is 5.92, or
Small92, which corresponds to low-Choice92; which is very close to average-Choice.
Sires with 0 marbling EPDs sired steers the majority of which exhibited marbling scores ranging from approximately 4.80 or Slight80, which corresponds with high-Select, to approximately 6.80, which is Modest80 and is average-Choice. These averages again depend on nutrition, environment and the genetic strength of the cow herd for marbling.
Fat thickness EPD is the average difference from breed average of the average external fat thickness of the sire’s progeny at 480 days. It is measured over the 12th rib and expressed in inches. Fat thickness has a negative genetic relationship to percent retail product.
Percent retail product EPD expresses the difference from breed average in percent retail product of a given sire’s progeny. This EPD combines the traditional carcass traits used in calculating yield grade and is heavily influenced by external fat thickness.

Putting EPDs to work
Here’s a scenario to illustrate how carcass EPDs can be effectively applied. Let’s say we run a 200-head crossbred cow herd. These cows were bred to a bull named Henry VIII who had the following EPDs: CWT, +0; MS, +0.0; and REA, +0.0.
The steer calves were retained and kept in the same group from birth to slaughter and were all processed the same day. A glance at the data revealed the following:

Avg. carcass weight 700 lb.
Avg. marbling score 4.80 (Slight80)
Avg. quality grade Select
Avg. ribeye area 12.0 sq. in.
The following year, in an effort to increase quality and yield, a bull named Richard II was selected for use. Richard posted the following carcass EPDs: CWT, +20; MS, +0.30; and REA, +0.30.
Assuming that nutrition and environment were the same, the averages of the next calf crop would be expected to be the following:

Avg. carcass weight 720 lb.
Avg. marbling score 5.10 (Small10)
Avg. quality grade low-Choice
Avg. ribeye area 12.3 sq. in.

If yield grades of both groups were the same, assuming a $10/hundredweight (cwt.) spread between Select and Choice, Richard’s steer progeny would be worth on the average $20 more than Henry’s. Multiply this by 100 and we have an increased value of $2,000 for the lot of cattle.

Summary
Profit in the beef industry is affected by many factors. From a genetic standpoint, reproduction is by far the most important trait, followed by growth to harvest per unit of feed and maternal ability. Through proper sire selection, we can keep these traits at optimum levels. These factors combined with acceptable carcass qualities will go a long way toward ensuring the success of the beef industry.