Making sense of ultrasound and carcass measurements
By Mike Tess, Animal & Range Sciences, Montana State University-Bozeman
U. S. beef production and marketing systems are changing. Production segments are becoming more coordinated; sometimes by formal business alliances, sometimes by improved information transfer and measurement of value. Systems are becoming more product-oriented than commodity–oriented. As a consequence, cattle producers have incentives to make genetic changes in carcass traits. A variety of genetic tools are available to help beef producers tailor cattle to specific markets and environments. EPDs are the most effective for making directed genetic improvement within breeds.
Compared to most growth traits, traditional performance and progeny testing for carcass merit is more difficult to complete. Carcass traits cannot be measured directly on potential parents, i.e., the measurements made on a chilled carcass cannot be duplicated on live animals. Carcass measurements on progeny or other relatives of potential parents are expensive to collect. Ownership of calves can change several times prior to slaughter, and cattle are typically moved to different locations during their lifetime; hence, maintaining animal identification and information feedback to cow–calf breeders are difficult tasks. Nevertheless, carcass traits are moderate to highly heritable.
Problems associated with direct measures of carcass quality have motivated searches for measurable traits on potential parents that could provide carcass quality information. Real–time ultrasound has proved to be an effective technology to meet this goal. Currently, many breeders use ultrasound to identify differences in carcass merit among young cattle. Yet, I sense that confusion exists over how this information relates to the actual carcass measurements made on cattle after slaughter.
Though difficult, such information is desirable because an ultrasound measurement made on potential breeding stock raised under ranch conditions (e.g., yearling bulls or heifers) is not exactly the same as the direct measurement made on carcasses from animals grown in a feedlot, but a genetically correlated trait (see sidebar). Due to this genetic correlation, the ultrasound measurement explains (or accounts for) some but not all of the variation in the carcass trait (see Figure 1 below). For example, an ultrasound ribeye area measured on a yearling bull is useful in predicting the ribeye area EPD for that bull, but not as useful as a direct measurement would be.
Estimates of genetic correlations between carcass measures and ultrasound measures are needed before EPD for carcass traits can be computed from ultrasound data. Accurate estimates of these statistics are not easy to obtain. Therefore, breed associations have taken one of four approaches to computing EPD for carcass traits:
- using carcass data to compute carcass EPD,
- using ultrasound data to compute ultrasound EPD,
- computing separate EPD for carcass and ultrasound and publishing both, or
- using carcass and ultrasound records to compute a single carcass EPD, i.e., via the genetic correlations.
Confusion may arise when bulls have both ultrasound EPD and carcass EPD. It’s possible that these EPD may rank bulls differently. First, we must remember that these are different, although genetically correlated traits. Another likely contributor to this problem may be different amounts of progeny data for ultrasound measurements versus direct carcass measurements, both among the bulls evaluated and among the bulls’ ancestors.
Because ultrasound measurements are made on potential parents, one might predict that future carcass trait EPD will be essentially based on ultrasound measurements. However, electronic animal identification, animal tracking networks, and web–based data transfer might facilitate a different course. If this electronic technology is widely adopted, and if the expense is low, direct carcass measures might eventually be the dominant source of genetic information on carcass traits.
Understanding Genetic Correlations
A genetic correlation is a statistical measure of the degree of association between two traits. Generally genetic correlations are the result of several genes affecting both traits. For example, the genes carried by the calf affecting growth rate before birth also affect growth after birth. Hence, there is a genetic correlation between birth rate and yearling weight.
Like other types of correlations, genetic correlations range in value between +1.0 and -1.0. Correlations near the extremes indicate very strong relationships, while correlations near zero suggest little or no association. Positive genetic correlations suggest that genetic increases in one trait will be accompanied by genetic increases in the second trait, while negative genetic correlations indicate that genetic increases in one trait will be accompanied by genetic decreases in the second trait. For example, birth weight has a positive genetic correlation with yearling weight, but a negative correlation with direct calving ease. Selection for increased yearling weight is expected to increase birth weight but decrease calving ease.
Beef: Questions & Answers is a joint project between MSU Extension and the Montana Beef Council. This column informs producers about current consumer education, promotion and research projects funded through the $1 per head checkoff. For more information, contact the Montana Beef Council at (406) 442-5111 or at beefcncl@mt.net