You already do the thing an EPD does. You just do it with two animals at a time.
Stand two bulls side by side and you can guess which one throws the heavier calves. You have watched enough cattle to make that call. An EPD is that same guess, made bigger and fairer, put into a number so you can compare bulls you will never stand next to.
An EPD is a prediction about calves
EPD stands for Expected Progeny Difference. Take the three words one at a time.
Expected means predicted. It is a forecast, not a measurement of the animal in front of you.
Progeny means calves. The number is about the calves the animal will sire or raise, not about the animal itself.
Difference means it is measured against other animals, not on its own.
So an EPD does not tell you what your calves will weigh. It tells you how much heavier or lighter one bull’s calves should run compared to another bull’s calves, bred to the same kind of cows. The American Angus Association, which calculates these numbers, puts it the same way: an EPD predicts how an animal’s future calves will perform compared to the calves of other animals (Association definitions).
A worked example you can repeat
Here is the whole idea in one comparison.
Say you are looking at two bulls. Bull A has a yearling weight EPD of +25. Bull B has a yearling weight EPD of +15. Yearling weight is what a calf weighs at about a year old, and the number is written in pounds.
Take the difference: 25 minus 15 is 10.
You expect Bull A’s calves to weigh about 10 pounds more at a year than Bull B’s calves, bred to the same kind of cows. That is it. The number in front of the trait is in pounds, and the gap between two bulls is the pounds you expect to see in their calves (Bullock guide, Example 1).
Notice what the number does not say. It does not say Bull A’s calves will weigh 25 pounds. The plus sign and the number only mean something next to another animal’s number. Alone, a +25 is half a sentence.
Only against its own breed
There is one fence around this comparison. An EPD only means something next to another EPD from the same breed.
An Angus number and a Hereford number are measured from different starting points, so you cannot lay one next to the other and call it fair. An Angus +25 only tells you something when you set it beside another Angus number. Keep the comparison inside the breed and the number stays honest.
How sure the number is: accuracy
Every EPD comes with a second number that tells you how much to trust it. That is accuracy. It runs from 0 to 1. The higher it is, the more real information stands behind the guess.
Think of two used trucks with the same price on the windshield. One comes with a thick folder of service records, the whole history written down. The other comes with the seller’s word and nothing else. Same price, very different buy. The folder is accuracy. It does not change the price on the glass. It changes how sure you are that the price is right.
A young bull with no calves on the ground yet has little real record behind him, so his accuracy sits low. A DNA test can lift it some before he ever sires a calf, and the deeper essay covers how. As he sires calves and those calves get weighed, more data comes in and his accuracy climbs. A proven bull with hundreds of calves behind him carries high accuracy. So a +25 at low accuracy and a +25 at high accuracy read the same on the page but are not the same buy. The low one can still move a good ways as data comes in. The high one is close to settled.
The “I” flag on young animals
Sometimes you will see a small “I” printed in front of an EPD. That “I” stands for interim, and it is worth knowing what it means before you spend money.
An interim EPD is a placeholder for a young animal whose own records are not in the national evaluation yet. Until that animal’s data comes in, the “I” number is little more than the average of its two parents, carried at very low accuracy (Association definitions). It is a starting guess, not a proven figure. Seeing that “I” is your cue to read for accuracy, not just the size of the number, before you treat it like a sure thing.
Who does the math
One last point, because it is where the trust comes from. You do not compute an EPD, and neither does any breeder, not us and not anyone. The numbers come out of the American Angus Association’s national evaluation, run across the whole breed every week on more records than any single ranch could hold (national evaluation). A breeder’s job is to read them well and pair them with what a number cannot see. That is the work here at Deluisio Angus.
So the whole thing fits in one line you could tell a neighbor at the counter: an EPD is a prediction of how much better or worse one bull’s calves will be than another’s, in the trait’s own units, plus or minus, within the same breed.
This is piece 1 of EPDs, Plain and Simple. For the fuller picture of how these numbers work and why a calf is never exactly what the paper promised, see what EPDs really are.
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