Back to the Basics: Economic Selection Indexes

by Dr. Elizabeth Dressler, ASA Lead Geneticist

Cattle producers make breeding decisions with a common goal in mind: raising cattle that support the profitability of their operation. Many factors influence profitability; for example, growth, maternal performance, feed costs, and carcass merit. But improving one trait doesn’t necessarily mean you’ve improved the overall picture. 

How do you weigh the importance of all those traits against one another? How do you account for the fact that selecting to improve one trait can diminish performance of another trait? Economic selection indexes help address these challenges.

What is an economic selection index? 
It’s a comparison tool. An economic selection index combines genetic predictions for multiple traits into one value to help breeders compare animals based on expected progeny profitability. Rather than evaluating each trait individually, an index considers overall genetic merit for profitability within a defined production system. Animals with higher index values are expected to produce more profitable progeny, on average, than animals with a lower index value under the same production scenario. 

It’s a prediction, not a profit guarantee. An index doesn’t predict how much money an individual animal will make you or guarantee a certain level of profit. Instead, indexes compare animals based on their genetic potential to contribute to progeny profitability, accounting for a more complete picture than any single trait can. 

Myth: Selecting for the right individual EPDs leads to the same progress. 
EPDs are valuable tools that estimate genetic merit for individual traits. But selecting for traits one at a time doesn’t account for how those traits influence each other. An economic selection index brings multiple traits together into one value, helping you compare animals based on their overall genetic merit for profitability. 

How is an economic selection index developed?
In a profitable cattle operation, generating revenue matters, but so does controlling costs. Cattle that return greater value at harvest may generate more revenue, but if that comes at a greater cost in feed and cow herd maintenance, it cuts into overall profit. An economic selection index accounts for these factors together rather than focusing on maximizing revenue. 

Developing an index starts with defining the production system it is intended to represent. $API evaluates sires for use on an Angus cow herd that retains some daughters as replacements and the remaining offspring marketed on a grid. $TI evaluates sires for use on an Angus cow herd where all offspring are marketed on a grid, with no replacements retained. 

From there, genetic predictions for relevant traits are combined with economic information to account for how those traits influence revenue and costs. The economic and phenotypic assumptions behind ASA’s updated indexes are informed by industry data from CattleFax and the US Meat Animal Research Center (USMARC). Genetic relationships among traits and the effects of heterosis are also considered. The result is a single value that accounts for multiple traits, their economic importance, and the relationships among them. 

The process can be summarized into these four steps: 
Define the production system 
Identify the relevant traits 
Account for genetic relationships, economics, and heterosis 
Combine the information into an economic selection index

Myth: An index is just a formula that adds up EPDs. 
An index is more than a sum or average of individual EPDs. Traits differ in economic importance, and genetic relationships among traits affect how selection for one trait can influence others. Simply adding EPDs together or emphasizing traits that seem most valuable wouldn’t account for how those traits work together to influence profitability. 

How do I use an index in selection decisions?
Choose the index that best fits your production scenario. Start by choosing the index that best fits your operation. Consider whether you retain replacement females or market all offspring. $API is designed for operations where replacement females are retained. $TI is designed for operations where all offspring are marketed. 

Use the index to compare animals. Use the appropriate index to rank and compare selection candidates based on their expected contribution to progeny profitability. 

Use individual EPDs to refine decisions. EPDs can help refine your selections among animals with similar index values, if you need to consider specific traits due to requirements related to your environment, management, or market. 

Myth: A higher index means an animal is better for every trait. 
Because traits contribute differently to profitability, an animal can rank highly on an index without excelling in every trait included. The goal is to balance traits to improve overall profitability, rather than maximize every trait individually.