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 Knowledge for the CommonWealth

Genetic Relationships

Livestock Update, November 2001

Scott P. Greiner, Ph.D., Extension Animal Scientist, Beef, Virginia Tech

Part 2: Growth/Mature Size and Carcass Traits

In last month's article the concepts regarding genetic correlations and their impact on selection were discussed, along with the favorable and antagonistic relationships that exist between growth and maternal traits. In this article, the relationships between growth and mature size, as well as associations between carcass traits and other performance traits will be discussed.

Growth and Mature Size Relationships
Mature size is an economically relevant trait from several aspects. Mature size is measured in weight and/or height (frame score), and these two measures are highly correlated (genetic correlation = .86) (Bullock et al., 1993). Mature cow size has a strong relationship with nutritional requirements. At the same stage of production (90 days post-calving) and moderate milk production, 1200 pound cows (frame score ~ 5-6) have a 10% higher energy requirement and 7% higher protein requirement than 1000 pound cows (frame score ~ 4). As cow size increases to 1400 pounds (frame score ~ 7), energy and protein requirements increase 19% and 13%, respectively, compared to 1000 pound cows (NRC, 1996). These differences are due in large part to higher maintenance requirements of larger cows, as they simply have more body mass to maintain. Increased nutritional requirements result in higher cow carrying costs throughout the production cycle. Similarly, mature cow size impacts stocking rates and supplemental feed resource needs. Mismatches between cow size and nutritional resources may compromise reproductive efficiency.

Mature cow size also influences the uniformity of the calf crop produced- particularly in single sire herds. Variation in mature cow size frequently results in large differences in feeder cattle grade. This variation in frame size is contributes to differences in finished weight/carcass weight (when fed to a constant body composition) as well as quality and yield grades (with a constant time on feed) in the finishing phase. Long calving seasons that result in large differences in calf age also contribute to lack of uniformity.

Mature size has a strong positive genetic correlation with birth weight (.64), weaning weight (.80), and yearling weight (.76) (Bullock et al., 1993). These relationships would suggest that selection for growth will result in a corresponding increase in mature cow size. Selection for extremes in growth traits can be detrimental. At the same time, small mature size- that may be advantageous in terms of costs of production, is associated with reduced growth. Therefore, optimization of growth and mature size is key. Optimum mature size will vary with production system and feed resources.

Sire selection tools such as Mature Height and Weight EPDs, or frame score, in conjunction with Weaning Weight and Yearling Weight EPDs are necessary to balance growth and mature size. With these tools, significant genetic progress can be made in growth traits without compromising cow size.

Growth and Carcass Trait Relationships
Growth traits have a strong positive genetic relationship with carcass weight. Since carcass weight is a major determinant of total carcass value (carcass weight x price = total carcass value), this relationship is favorable. In a similar fashion, growth also has favorable genetic relationship with ribeye area. Genetically, cattle that are heavier at a given age have more total muscle mass- as reflected in size of the ribeye. However, weaning and post-weaning growth have and antagonistic genetic relationship with fat thickness.

Cutability (yield grade or percent retail cuts) is a composite trait that is influenced by carcass weight, fat thickness, and ribeye area. Since growth and cutability have a positive genetic correlation, selection for increased weaning or yearling weight will result in a favorable combination of carcass weight, fat cover, and ribeye genetics. The relationship between cutability and growth arguably has more significance than the relationships between growth and fat thickness or growth and ribeye. Cutability in the form of a yield grade is an important component of carcass grid pricing schemes, whereas fat thickness and ribeye are individual measurements that influence the composite trait.

The genetic correlations reported here represent a summary of several research studies that have investigated these relationships. Across these studies, the genetic relationship between marbling and various growth measures has been variable. Birth weight and post-weaning gain have been found to have favorable genetic correlations with marbling. However, the growth traits of weaning and yearling weight that are most frequently used in selection programs have negative and antagonistic relationships with marbling. Comparatively, the correlations between growth and marbling are lower in magnitude compared to the correlations between growth and other carcass measures. This suggests that with genetic evaluation tools such as EPDs, cattle can be identified that have a desirable breeding values for these antagonistic traits.

From a selection standpoint, the relationships of growth with carcass weight, cutability, and marbling are most important since these three carcass traits directly influence carcass value. Both carcass weight and cutability are favorably related to growth, while marbling and growth are antagonistic.

Genetic Correlations Between Selected Growth and Carcass Traits
  Carcass Wt. Fat Th. REA Cutability Marbling
Birth Wt. + .60 - .27 + .31 + .05 + .31
Weaning Wt. + .71 + .24 + .49 + .57 - .09
Yearling Wt. + .91 + .32 + .51 + .87 - .33
Post-Weaning Gain + .87 + .19 + .32 + .18 + .11
adapted from Koots et. al., 1994

Relationships Among Carcass Traits
The following table presents genetic relationships among various carcass traits:

Genetic Correlations Among Carcass Traits
  Fat Th. REA Cutability Marbling
Carcass Wt. + .29 + .48 + .00 + .25
Fat Th.   + .01 - .56 + .35
Ribeye Area     + .45 - .21
Cutability       - .25
adapted from Koots et. al., 1994

Examination of these correlations reveals some important favorable and antagonistic relationships between carcass traits of interest. As mentioned previously, the correlations of most importance are those between carcass weight, cutability, and marbling due the economic importance of these three traits.   The genetic relationships of carcass weight with cutability and marbling are generally favorable. Since the correlation between carcass weight and cutability is 0, sires with desirable EPDs for each of these traits are identifiable. Selection for marbling is generally associated with genetics for heavier carcass weights. Provided carcass weights do not exceed industry standards, this relationship is advantageous.   In contrast, marbling and cutability exhibit an antagonistic genetic relationship. The negative genetic correlation between these traits indicates that high marbling genetics are generally associated with unfavorable genetics for carcass leanness and muscularity. This explains to a large extent the difficulty in attaining high quality grades (average Choice or better) in combination with superior yield grades (yield grade 1 and 2), or vice versa.   It is important to recognize that many of these relationships are not perfect, and that exceptions exist. Fortunately, for many of the antagonistic relationships that have been discussed, the genetic correlations are relatively low. Genetic correlations that are small enhance the likelihood that animals exist in the population that have a desirable combination of genes for these traits. As an example, there are a number of bulls in several breeds that have favorable EPDs for both marbling and cutability. Several of these animals are also superior for growth. Knowledge of these various genetic relationships enhance our understanding the importance, challenges, and limitations of multiple trait selection   The final segment of this series will concentrate on relationships between carcass traits and both maternal traits and reproduction, along with considerations important for making genetic progress in multiple traits that may be antagonistic.

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