A Stayability & $BMI Chat

The most asked questions regarding our British Maternal Index ($BMI) center around the inclusion of the Stayability EPD. With cow costs being the biggest driver of profitability in commercial ranching, longevity of the female is vital to her paying her bills and I think you would be hard pressed to find many people who disagree with that statement. I want to provide some background information in to why the STAY EPD is not currently included in $BMI, as well as some additional information that appears to show that there’s commonality between the two figures, even if one is not included in the other.

When $BMI was created by staff and Dr. Mike MacNeil over a decade ago, there was no genetic predictor for cow longevity or stayability at their disposal. The index was built with the best available tools at the time. Even today, there are traits that would make $BMI even stronger if there were EPDs available for them. Until those are out there for our use, we work with what we have in the toolbox. As has been discussed in this column space before, $BMI is a complex, multi-trait index designed to identify the genetics that are most beneficial in a British cow based crossbreeding system. There are multiple paths to accomplish that goal, whether it is through the cattle that produce elite females, or the lines that can give the commercial man market topping feeder cattle along with good keeper heifers. You can reference back to the August 2022 issue to take a deeper dive into how there’s multiple ways to get to a similar endpoint in terms of selection indexes. One line from that article that I would like to quote here:

“One important takeaway from this exercise is the making selections based solely on a selection index can be just as detrimental as making selections based on a single EPD.”

Circling back to the inclusion of the stayability EPD in the index… if it was as simple as tacking on the Stayability EPD to the end of the current formula, we likely would have done that the day the STAY EPD was released. Unfortunately, it’s not that simple. As you tinker with one piece of an index, you impact the other components as well. At that point, a little tinkering becomes a full rebuild to correct the weightings and values of each component trait. Over the last few months, I was charged with auditing our current index lineup to see if what we have in place is still doing an effective job of identifying cattle to meet the goals of the industry. Even as the ASA Spring Board meeting has passed with plenty of discussion of this topic, the audit project is ongoing. Knowing that stayability and $BMI are an often asked about topic, I investigated if there might be any trend between the EPD and the index in its current form.

One of the methods to analyze was to grab the top 100 $BMI active sires from the database. To qualify as an active sire, a bull must have a calf recorded to him in the last five years. These bulls are all in the top 3% or better for their index ranking. If you study the top 100 active sires for $BMI, 89% of those bulls are above average for their STAY EPD. Thirty-three of those top 100 rank in the top 10% of the breed for ST, and sixty-six of them are in the top quarter for their STAY EPD. The average STAY EPD of these 100 bulls is 18.7, which would rank the group in the top 20% of the breed, knocking on the door of top 15%. I also flipped this script and pulled the top 100 STAY EPD active sires list to look at how they rank on $BMI. Of those top 100 bulls for their STAY EPD, 90% of them are above average $BMI bulls. Thirty-one bulls are in the top 10% for $BMI, and sixty bulls rank in the upper quarter. The average $BMI of this subset is 128.40, ranking them in the top 20% of the Shorthorn active sire population. All these bulls would be in the top 4% or better of active sires for their ST EPDs. There are 15 bulls that happen to rank in the top 100 for both $BMI and STAY.

In a test population we compiled to best mirror the entire Shorthorn population, results look similar when comparing these traits. Looking at the top 100 bulls in this testing group for $BMI, the average is 142.87, good enough to be considered top 5% of Shorthorn sires. Among those 100, 94 of them are also above average for the STAY EPD. Only 3 of these 100 sires rank in the bottom quarter for their STAY EPD. Of all the bulls above average for $BMI in the test group, 73% of them are also above average for their STAY EPD.

While STAY is not a component of the $BMI formula, I would hypothesize that selecting for the kind of cow that works maternally (calves easy, milks, and raises a heavy calf to weaning), it seems that this index has also identified a lot of the bloodlines with longevity and staying power. Again, I do not condone making breeding selections based on any single EPD or index. If you’re going to look at one number, you might as well double your workload and look at two at a time! The index can be a bit of a guide to identify prospects, but I still believe it’s important to look at the component EPDs making sure they are in line with your breeding goals.

Our $BMI index is not a perfect solution, but no index is. And as new tools become available, we will periodically analyze what we have in place and implement those that can help us significantly move the breed forward.