$API and $TI A Bull Buyer's Best Friends

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$API and $TI A Bull Buyer's Best Friends

by Lilly Platts


An index is not an EPD, but rather a measure of the difference in profitability for the designed scenario. Understanding each index can help you move the profitability of your cow herd in a positive direction. Lane Giess, ASA Geneticist, answers some common questions about index selection.

Bull selection impacts a cow herd for many years, often long after the bull has left an operation. You might spend months leading up to sale season combing through catalogs, studying numbers, making phone calls, and putting in the time to make sure your selection will live up to your operation’s needs. Or, you may have a simple approach and assess bulls the day-of with your eye on a few select criteria. Either way, the bull you want is going to walk through the sale ring or across your screen, and you will have a small window of time to balance the cost of the bull and the criteria he meets (or doesn’t). Even if you have a longstanding relationship with a seedstock breeder or a buyer who picks out bulls for you, simplifying the process is beneficial. 

Indices take into account many EPD and combine them into one value, expressed in dollars of profitability prediction among bulls. ASA publishes two indices:  $API (All Purpose Index), and $TI (Terminal Index). $API estimates profitability when a percentage of daughters remain in the herd as replacements. $TI estimates terminal profitability.

All-Purpose Index ($API): Dollars per cow exposed under an all-purpose-sire scenario.
Terminal Index ($TI): Evaluates sires for use on mature Angus cows with all offspring put on feed and sold grade and yield.
Interpreting $API and $TI: Just as with EPD, zero in on the unit difference between bulls. (As described above, index units are in dollars per cow exposed.) The difference can be used to determine how much a bull is worth compared to another. 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 4 years ($50 diff. x 30 hd. x 4 yr.= $6,000). Percentile is required to determine where a bull’s index value ranks him relative to other bulls in the breed.


Why is it important to have $API and $TI?
Why is there value in simplifying things?   

Giess: An economic index such as $API or $TI greatly improves a breeder’s ability to select for genetics associated with commercial profitability. Without the use of an index that accounts for all traits, selecting for balanced genetic improvement is too burdensome. Commercial cattlemen can use a single value to simplify their selection criteria and use $API or $TI as a starting point in their decision-making process.

 How should each index be used?

Giess: Seedstock breeders have the responsibility of improving the genetics that influence the entire commercial beef industry. Because of this, the goal of commercially minded breeders should be to develop genetics that are commercially viable and profitable. An economic index helps with this goal.

Instead of breeders selecting animals based on individual traits related to a single aspect of an operation’s profitability (i.e., Weaning Weight, Calving Ease, etc.), an economic index combines all economically relevant traits and weights their importance in a single value — one expressed in dollars.

If your goal as a breeder is to develop genetics with the intention of retaining females and being more maternally focused, then the All Purpose Index ($API) is the one you should use. This index focuses on the whole-life-cycle and accounts for traits such as maternal calving ease, docility, fertility, and, perhaps most importantly, female longevity. As a whole-life-cycle index, the $API still needs to account for carcass performance since many of the daughters will contribute to the terminal genetics of their progeny.

On the flip side, if your goal as a breeder is to develop genetics with the sole purpose of maximizing terminal profitability, then the Terminal Index ($TI) is what you should use. The $TI was designed for breeding systems with the assumption that all females were purchased or developed separately. A terminal mating system places greater emphasis on carcass performance, growth, and direct calving ease.

It’s important to note that all seedstock breeding programs are different and have commercial customers with varying goals, so the best suggestion would be to use the $API and $TI as a starting point and then focus on other traits to develop a breeding program.


Dr. Ken Odde runs commercial cattle in South Dakota and is the past department head for the Animal Sciences and Industry Department at Kansas State University. He shares his insight on utilizing $API and $TI: “The purpose of a selection index is to help the commercial producer, or the potential buyer, in appropriately balancing traits. We now have EPD on such a wide variety of traits that it is difficult to decide what is actually the most important. Indices are really a breed association effort to help producers with appropriately balancing traits.”

Since each producer’s operation is unique, how can we trust that an index will do what it’s supposed to? Giess: Indices are designed to be robust in their function. And while every operation is unique, the beef industry’s profit centers are fairly constant across all regions. The commercial industry needs females that stay in the herd until profitable, don’t exceed reasonable nutrition requirements, produce and wean a heavy calf that is born healthy, grows well, and kills with excellent terminal merit. This is obviously a simplification of the nuances of the beef industry, but over all this is what an $API index is designed to accomplish. And while breeders can tailor their breeding programs to fill niches, an index will guide a breeding program in the direction of commercial profitability.

Do the inputs for $API and $TI change over time, or does the formula stay the same?

Giess: The economic inputs for $API and $TI largely stay the same across years. They are designed to be robust in market fluctuations and not inappropriately discount an animal’s genetic worth if corn prices rose suddenly and then dropped back to normal a few months later. Since breeding decisions happen at a fixed point in time, you want an index that can withstand those market fluctuations.


How long can it take for a producer to see a measurable change after committing to using indices in selection decisions?

Giess: Unlike single trait selection, realizing the genetic progress from economic-index-based selection will inevitably be slower. Since $API uses a balanced approach to weighting every economically relevant trait, you will not be using the extreme outliers for a single trait. For traits with low heritability, this noticeable difference will be even slower. This process can take years with long generation intervals, and the need to keep females around until profitable.

Do you have any tips for producers wanting to utilize $API or $TI? 

Giess: One thing that is important to note about indices is the reliability of the prediction. We do not calculate an “accuracy” for indices since they are aggregated from many EPD, each with their own accuracy. If comparing a highly proven sire’s index to that of a yearling bull, the reliability of the younger bull’s index is inevitably lower. 

Also, it’s extremely important to remember that an index allows superiority in one trait to outweigh poor genetic merit in a range of other traits. This means that if a sire is exceptional for only one of the key traits in an index and poor for the others, he may appear more favorable than bulls with average performance across all traits.


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