This is intended to help understand the ASA/IGS Stayability EPD and disclaimer, I’m not a geneticist, so the following is explained from my level of understanding.
I recently created a video about the Stayability (STAY) EPD, and I think it’s worth spending a little more time talking about what this EPD is and what it isn’t. I see a number of folks point out that they have 10+ year-old females in their herd, yet some of those females or animals within those cow families have STAY EPDs in the bottom percentiles. In my opinion, some of the confusion comes from discussing STAY as though it is a measure of longevity and that’s not quite accurate or what it’s intended to measure.
Stayability predicts differences in the probability that a sire’s daughters will remain productive through six years of age, given that they first calved at two years of age. This last part is important, especially for seedstock producers and reporting. For a female to enter the Stayability evaluation, she MUST have a calf reported at age two. From there, the evaluation follows whether she continues to produce a calf each year until age 6. This information comes together within the evaluation to help better predict genetic differences in Stayability among sires.
Why six? As a cow/calf operator, we’re over here saying, “heck, we want her to produce until well after 10 if she’s able.” Absolutely! But, there is a reason why most evaluations choose age 6. Research has suggested that it’s at about six years of age at which a commercial female that has produced a calf each year has recovered her development costs. So if you noticed, the question posed isn’t intended to answer, “Which bull will likely give me the most 15-year-old cows?” The question is closer to: Which sire is more likely to produce daughters that remain productive long enough to pay for their development? Think about those conversations we have about the cost of an open coming 3-year-old. We invested in developing her, bred her, got one calf, and then she came up open before ever having the opportunity to pay for herself. Those ones hurt. That’s the economic driver behind STAY and helps us make better selection decisions at the commercial level when choosing replacements.
There is an important limitation worth understanding, too. STAY is evaluating a specifically defined subset of females. Ones that have a calf reported at age two and then every year after within the evaluation. A female without a calf reported at two isn’t automatically a longevity failure. She may never have entered this particular evaluation. Also, if there’s a gap on a female from age 3-6 (no calf reported) that is impacting Stayability calculations. This is why it’s important to report a calf every year and give the female credit, it counts toward her cow card and filters back to the sires STAY numbers.
These are all reasons why I think understanding the definition behind any particular EPD is every bit as important as looking at and analyzing the number. STAY gives us a tool to compare sire’s with regards to daughters remaining until age 6. Anything after that should be gravy. STAY doesn’t tell us everything there is to know about fertility, longevity or lifetime productivity. It’s not meant to. The value comes from understanding exactly what we’re measuring, collecting as much complete and unbiased data as we can, and then using the appropriate tools to make more informed selection decisions that ultimately help increase profitability at the commercial level.
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