Economic Selection Index FAQs

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Economic Selection Index FAQs


By Wade Shafer, Ph.D., EVP, American Simmental Association

Selection Indexes

The American Simmental Association has two economic selection indexes, the All Purpose Index ($API) and the Terminal Index ($TI).

All Purpose Index ($API): Evaluates sires for use on the entire cow herd (bred to both Angus first-calf heifers and mature cows) with the portion of their daughters required to maintain herd size retained, and the remaining heifers and steers put on feed and sold grade and yield.

Terminal Index ($TI): Evaluates sires for use on mature Angus cows with all offspring put on feed and sold grade and yield.

Using $API and $TI: First, determine which index to use; if you’re keeping replacements, use $API. If not, use $TI. Then, just as with EPDs, zero in on the unit difference between bulls. Index units are in dollars per cow exposed. The difference can be used to determine how much a bull is worth compared to another. Or, put another way, how much you can pay for one bull compared to another. For example, when buying an all-purpose-type sire, you can quickly figure a bull scoring +100 for $API is worth an extra $6,000 over a +50 bull if both are exposed to 30 cows over four years ($50 diff. x 30 hd. x 4 yr. = $6,000). A percentile ranking chart is required to determine where a bull’s index value ranks him relative to other bulls in the breed. 

Q.  How should breeders factor in breed composition when using selection indexes?

ASA’s Quick Reference Guide to EPDs and Indexes, which is published in many bull sales catalogs, states the proper approach well: Important disclaimer: DO NOT compare index values of purebreds of different breeds, hybrids of different breed composition or purebreds with hybrids — our system was not developed to make valid comparison among these groups. Therefore, you must first determine the breed and breed composition appropriate for your herd and use index values to compare animals within that population.

Q.  Is growth treated differently between the All Purpose ($API) and Terminal($TI) Indexes?

It is easy to see that there is a strong tendency for high-growth animals to be at the top of the heap for $TI, while it is not unusual to see low-growth animals rank highly for $API. Because of that, it may be logical to conclude that growth is treated differently between $API and $TI. What may be a surprise to many, however, is that growth is treated identically between the indexes. The economic value of growth in beef cattle production is realized through weight at weaning and harvest in cattle not retained for breeding. Since both indexes predict differences in profit in an integrated production system, the economic value of growth in slaughter cattle is factored into both $TI and $API. Further, since the same pricing grids are used, the value for a unit increase in growth in a sire’s slaughter offspring is the same for both indexes.

So, why does it appear that growth is treated differently between the indexes? Why does growth seem so important in $TI, while appearing to not be as important in $API? The reason lies in the fact $TI assumes that all of a sire’s offspring go to slaughter, while $API is predicated on retaining replacement females, i.e., only steers and cull heifers go to slaughter. This means that, though an increase in growth for a single slaughter calf has the identical impact between indexes, since all of a sire’s offspring go to slaughter under the $TI scenario, the collective impact of growth is magnified in $TI. Furthermore, since $API also factors in economic differences in cow herd traits, which tend to have more economic impact on the production system than growth (particularly stayability and cow herd intake), superiority in this area can easily make up for shortcomings in growth.

Q.   Why is selecting on ASA's indexes not the same as singe-trait selection?

The assertion that index selection is akin to single trait selection is only true if the trait being referred to is profit. Unlike single-trait selection for biological traits, selection on $API and $TI simultaneously applies selection pressure to all traits that impact each phase of beef cattle production — cow calf, feedlot, and packing plant. Because ASA’s indexes weight these traits in a mathematically optimal manner, selection on them maximizes genetic progress in the trait we all should be practicing single trait selection on — profit! 

A major distinction that sets ASA’s indexes apart from those published by some organizations is that they simultaneously account for all segments of beef cattle production. Our industry has been inundated with what I call “segmented” indexes — indexes geared toward specific and limited segments of the production cycle, e.g., weaning, feedlot, and grid. Though indexes of this nature may have utility for users who are interested in single phases of beef production, e.g., a feeder sourcing calves or a packer buying fat cattle, they should not be used to provide direction for a population. In fact, if used widely, “segmented” indexes can have unintended and undesirable consequences for a population.   

 Because ASA’s indexes consider all traits a sire will impact across the entire production system, they are “balanced” — this balance prevents selection on the indexes from taking our population down unintended and undesirable paths. It ensures that we will make optimal genetic progress in all traits that have an economic impact on beef cattle production. 

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