Genetic Relationships

Livestock Update, December 2001

Scott Greiner, Extension Animal Scientist, Beef, VA Tech

Part 3: Carcass and Maternal Traits

With increased focus on carcass traits in today's beef breeding programs, the logical question arises- how does selection for carcass merit affect maternal traits? Specifically, does selection for improved carcass characteristics result in undesirable correlated responses in important maternal traits? Unfortunately, few selection experiments have been conducted that directly address this very relevant question. However, work conducted at the U.S. Meat Animal Research Station through the breed germ plasm evaluation studies has provided some insight to the carcass trait vs. maternal issue.

The following tables outline important genetic correlations between selected carcass measures and maternal traits. Both studies were conducted at the Meat Animal Research Center in Nebraska, and include a variety of breeds and crosses. The study conducted by MacNeil et al. (1988) evaluated the genetic relationship between fat trim weight and total retail product weight of steers and maternal traits of their female herdmates. All genetic correlation estimates for fat trim weight were antagonistic with female traits. Thus, selection for reduced fat trim (i.e. improved retail product % and yield grade) would be associated with a correlated increase in age at puberty, increased weight at puberty, reduced fertility, larger mature size, and more calving difficulty in females. Similarly, selection for enhance retail product weight (more saleable product) was found to have an undesirable genetic relationship with the female traits of age at puberty, weight at puberty, mature weight, and calving difficulty. Genetic correlations reported by Splan et al. (1998) also indicate that selection for decreased fat (fat thickness, carcass % fat, and retail product %) would negatively affect fertility (calving rate) and maternal calving difficulty. In these same studies, neither ribeye area nor marbling was negatively associated with calving rate or calving difficulty. Thus, it appears the undesirable associations between maternal traits and carcass merit are mediated through fat thickness.

Genetic Correlations for Maternal Traits with Carcass Fat and Retail Product Weight

Female Trait	Fat Trim Wt.	Retail Prod. Wt.
Age at Puberty	- .29	+ .30
Weight at Puberty	- .31	+ .08
Conceptions/service	+ .21	+ .28
Mature Wt.	- .09	+ .25
Calving Difficulty	- .36	- .02

adapted from MacNeil et al. (1988)

Genetic Correlations Between Carcass Traits and Selected Maternal Traits

	Calving Rate	Calving Difficulty
Fat Th.	+ .19	- .14
Fat %	+ .18	- .23
REA	+ .15	- .04
Retail Product %	- .13	+ .18
Marbling	- .05	- .09

adapted from Splan et al. (1998)

These genetic antagonisms present great challenges to beef producers. Traits such as age at puberty, fertility, mature size, and calving ease all contribute significantly to the economic viability of the cowherd. At the same time, we are challenged to produce a high quality, consistent end product with consumer appeal. Due to the unfavorable correlations between these maternal traits and carcass measures, these goals tend to be in contrast to each other. Further troubling selection for a proper balance between maternal and carcass traits is the general lack of genetic predictors (EPDs) for important maternal traits such as fertility. Hence, tools are less readily available to select for simultaneous optimums in the cow herd as well as end product traits. This leads to the question: Can we genetically design a low-cost female that is adaptable and low-cost in our environment, while at the same time produce cattle that have carcass merit attributes desired by our customers?

In this series, genetic relationships that influence selection have been discussed. The major genetic antagonisms that exist include those between:

1. growth rate and calving ease
2. growth rate and mature size
3. carcass cutability and marbling
4. carcass cutability and maternal traits

The difficulty becomes simultaneously making genetic improvement in these important traits. To do so successfully, several tools for genetic improvement need to be considered:

1. Crossbreeding System- Due to the undesirable relationship between maternal traits and carcass characteristics, breeding systems that enable selection to occur somewhat independently for these two areas of importance seems advantageous. A terminal crossbreeding system, which can be used to balance economically important traits in the cow herd as well as end product traits in the calf crop is one such system. Maternal lines that are composed of breeds selected for production characteristics (reproduction, growth, milk, mature size, calving ease) that match the environment, can be tailored to the resources of the operation. To compliment these maternal lines, terminal-cross lines (or breeds) can be selected with primary emphasis on growth performance, efficiency, and carcass specifications. With such a system, compromises between maternal and end product genetics can be largely avoided. Certainly, since the cowherd contributes to the growth efficiency and carcass merit of the calf crop, minimum genetic thresholds for these traits are necessary. However, selection criteria in the maternal lines for carcass merit may focus on achieving acceptable minimums and avoiding extremes, while enhancing uniformity.
2. Heterosis- Our biggest ally in overcoming the genetic antagonisms that affect reproduction in a negative fashion is heterosis. Specifically, maternal heterosis (the crossbred cow) has been documented to have pronounced favorable effects on productivity and profitability of the cowherd. Research has shown an increase of 20-25% in pounds of calf weaned per cow exposed as a result of heterosis in a terminal crossbreeding system. With the lack of genetic predictors (EPDs) available to select directly for reproduction, heterosis is our best tool to genetically improve reproductive efficiency.
3. Breed Complimentarity- Since no one breed simultaneously excels in all traits, utilizing multiple breeds to mix and match strengths and weaknesses allows for balanced performance. For carcass traits, coupling the general superiority of the British breeds for marbling potential with the red meat yield advantages of the Continental breeds offers the opportunity to optimize quality grade and cutability (yield grade). Similarly, optimizing reproduction, milk potential, mature size, growth, and adaptability of the cowherd to the environment will likely require the resources of more than one breed.
4. EPDs- Selection of genetics within breed is equally important as choice of breed, as variability within breed may be as great as between breeds for many traits. EPDs are the most accurate selection tool for identifying desirable genetics within a breed population. At the same time, breed strengths and weaknesses and the genetic merit of a particular breed as a whole for specific trait also warrant consideration when bulls are selected for use in a crossbreeding system. In other words, EPDs need to be considered on both a within and across-breed basis to effectively balance selection in a crossbreeding program. As discussed previously, EPDs allow for the identification of individual animals that have favorable genetics for traits that tend to be antagonistic (i.e. low birth weight and high growth).

In summary, the number of economically important traits involved in beef production coupled with the unfavorable relationships that exist between many of these traits make multiple trait selection in beef cattle challenging. Proper use of existing tools such as crossbreeding and EPDs are necessary to optimize performance in multiple traits. Future development of genetic predictors for traits such as reproduction and efficiency, as well as the application of selection indexes for specific production and marketing systems, will enhance our ability to overcome these challenges.

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