Selection Indices for Beefmaster Breeders

By Matt Spangler, Ph.D., Associate Professor and Beef Genetics Extension Specialist University of Nebraska-Lincoln

An economic index is a collection of EPDs weighted by their economic value such that traits with greater impacts on production goals have a larger economic weight associated with them. The basic equation of an economic index is:

I = EPD₁ x a₁ + EPD₂ x a₂ + EPD₃ x a₃ +… + EPD_n x a_n

Where: I is the index value; EPD_n is the EPD for trait n; and a_n is the economic weight associated with trait n.  This basic form was first published in the early 1940s.

Note that EPDs are included in the selection index above.  However, not all traits for which we have EPDs directly impact the profitability of cow/calf operations.

Traits that directly impact a source of revenue or a cost of production are called Economically Relevant Traits (ERT). All traits that are not ERTs are indicator traits, or a trait that is genetically correlated to an ERT but not an ERT itself.

Classic examples of indicator traits include ultrasonic carcass measurements and birth weight. Producers do not receive premiums for IMF levels, rather premiums (and discounts) are applied to quality grades.  Assuming that carcass maturity values are the same, actual carcass marbling is the driver of quality grade.  Although IMF is genetically correlated to carcass marbling it is not the ERT.  Birth weight is another great example of an indicator trait.  Selection to decrease birth weight in an attempt to reduce the prevalence of dystocia is practiced by numerous commercial bull buyers.  However birth weight does not have a direct revenue source or cost associated with it.  The trait that does have a cost associated with it is calving ease (or difficulty).

How are the economic weights derived?  Using historical averages of input costs and the value of sale weight and carcass premiums (and discounts) we use a simulated cowherd to ask the question “if all else is held constant, and we change one of the traits by one unit, how does this change our profitability”?  These values become the foundation for the weightings of the EPDs in the index. The weightings are then modified based on the genetic relationship between the ERT and the indicator trait for which we have an EPD, as well as the genetic relationship between the trait and all other traits in the index (to avoid “double counting”).

Below are some examples of proposed indices for the BBU.

Terminal Index

This index includes EPDs for yearling weight and ultrasound and traits and is designed to select bulls to be used only on mature cows. Producers wishing to enhance growth and simultaneously select for quality and yield grade should use this index. It is a terminal index and caution should be used if replacement females are retained to avoid increasing mature weights of females.

Example:

Bull A +60.0

Bull B +55.0

We would expect that the calves from Bull A would be worth $5.00/hd. more than those from Bull B if retained through a feedlot phase.

Maternal Index

This index assumes that the sire will be used on both cows and heifers and that heifers will be retained as replacements while all other offspring (cull heifers and steers) will be sold at weaning.

Example:

Bull A  +130

Bull B   +120

We would expect that the calves from Bull A would be worth $10/hd. more than those from Bull B.  Over a span of 4 years Bull A could generate $1,200 more revenue than Bull B if mated to 30 females/yr. ($10/hd x 30/hd x 4 yrs.).

Choosing an Index to Use

When making selection decisions based on economic indices, it’s important to consider your particular breeding objective and the traits that will achieve desired production goals. For instance, if your production goals included retaining replacements and selling cull heifers and steer progeny at weaning, then an index that assumes all offspring are retained through the feedlot and no replacements are retained is inappropriate for your operation. It is also important to know the breed average values for particular indexes and to use percentile ranks to determine how far above or below average a particular animal is compared to the rest of a breed.