The impetus for authoring this article was when a producer asked me about choosing his next bull. Picking sires is where producers can make the greatest impact on their herds. Bulls are responsible for many calves a year, while cows usually contribute one calf a year.
This producer wanted to focus on Weaning Weight (WW) and Calving Ease (CE). It surprised me when I discovered that the numbers he passed me were the bull’s own performance numbers. While better than basing bull selection solely on sight and picking the best-looking bull, the bull’s performance numbers provide a limited basis for comparison. A bull’s performance is a combination of his genes and the environment he experienced. By relying on individual performance, a producer may choose a bull with an exceptional environment rather than a bull with exceptional genes. For example, during a good year – or when the mother cows had plenty of feed – it is possible that all the calves had large WWs. Conversely, during a dry summer – or when the cows had limited feed – the calves may all be light.
Looking at the differences between individuals and herdmates provides a reference to performance. For example, if two bulls have 205-day adjusted WWs of 750 pounds, we have no basis for comparison. If we add in information about the bull’s birth herd, we learn about the environment where he was raised. For example, Bull A’s herd averaged a 700-pound 205-day adjusted WW, and Bull B’s herd averaged 725 pounds. While both bulls had the same adjusted WW, we now know that Bull A was 50 pounds heavier than herdmates, while Bull B was only 25 pounds heavier. The added information about the birth herds allows us to differentiate the two bulls.
The industry standard is to adjust WWs to 205 days of age. Standardizing the WWs to 205 days eliminates differences due to age at weaning. If we want to compare bulls from different herds, we need to adjust their performance to account for differences between herds as well. By comparing the differences in performance relative to herdmates, we adjust for differences in the environments, providing us with a better understanding of the bull’s genetic makeup.
We can improve the accuracy of the estimation of the bull’s genetic potential by including information about the bull’s parents. We see numbers like this in most sale flyers and A.I. stud books. They are called expected progeny differences (EPDs). As the name suggests, EPDs identify the differences we expect to see in the bull’s progeny as compared to the progeny of other bulls.
The accuracy of EPDs are indicated by an associated accuracy number. This number is between 0 and 1 and represents the probability that a particular EPD represents the true genetics of the animal. They include accuracy numbers with EPDs in A.I. stud books. It varies for two main reasons: (1) how much the given trait is influenced by genetics, and (2) the amount of information included in the calculation. A highly heritable trait – such as Frame score – will start out with a higher accuracy than a lower heritable trait – such as CE.
The more information included in the calculation, the more accurate the EPDs. Some breeds include genetic testing results. In breeds that genomic test large numbers of bulls, the weight of the genomic testing has a greater impact on the accuracy of EPDs. Lastly, once a bull has offspring, the offspring’s records can further improve the accuracy. Therefore, A.I. stud books contain higher accuracy numbers for bulls with offspring.
Why are EPDs not 100% accurate? Nature, specifically the mix of genes the individual offspring receives, impacts the measured trait. Each sperm and egg carries half the parents’ genes, but the mix of genes varies for every reproductive cell. We can accurately measure how big a calf is at birth, weaning and as a yearling, but science has not reached the point where we can predict the exact mix of genes that will be passed to an individual calf. We can improve the accuracy of our estimations, but we still cannot measure the exact values. Accuracy ultimately is based on the heritability of the trait and how much information was available to calculate the EPD. That is why we see older bulls with accuracies above 85% and young bulls that are only 25% accurate for a given trait.
Most commercial producers buy young bulls. We should still use EPDs to compare the bulls to determine purchases. While the accuracy of a young bull may be low, it is still the most complete package of information available.
For example, let's compare two bulls from a recent sale flyer. Both bulls are the same age and have similar accuracy numbers. Bull A’s EPDs for CE and WW are, respectively, 8.6 and 57. Bull B’s EPDs for the same traits are 6.5 and 76.
CE numbers are expressed as the difference in the percentage of unassisted births when a sire is bred to first-calf heifers. In this case, when using Bull A on first-calf heifers, we would expect 8.6% more unassisted births when calving first-calf heifers than a bull with a 0 CE EPD and Bull B to have 6.5%. When comparing just these two bulls, we would say that Bull A has a 2.1 advantage (8.6 - 6.5) over Bull B. This means, on average, 2.1% fewer of Bull A’s calves will require assistance than Bull B’s calves.
WW EPDs tell us the difference between the bull’s average offspring compared to the breed average in pounds. In this example, Bull B has a 19-pound (76 - 57) advantage over Bull A. For this example, each bull sires 25 calves a year. Bull A’s calves would wean with 475 (19 pounds times 25 calves) more total pounds of calf to sell than Bull B’s on average. The producer then needs to determine if the extra calf pounds are worth the extra work of having to assist with approximately two more births out of 100 of Bull B’s calves compared to Bull A’s. Keep in mind that this example only looks at two traits, not all of them. Different producers have different goals and will therefore look at different traits.
Remember to look at all the EPDs to avoid problems. I had a conversation with a producer once who focused on the CE numbers without looking at any others. What he missed was the very low birthweight EPD. He did not pull any calves, but the small calves did not do well during the winter calving season. Similarly, for cows grazing on semiarid rangelands with limited feed resources, one would do well to avoid using bulls with high yearling weight and milk EPDs for sires of replacement heifers.
EPD calculations are done by breed associations. It is important to only use the EPDs to compare sires within a breed. If you want to compare sires across breeds, you need to adjust the EPDs using breed correction factors, but that is a topic for another day.
Back to my producer, who was only looking at the bull’s own performance numbers. After showing him where to find the EPDs, he chose a sire that met his herd goals.
EPDs may not be a perfect representation of a bull’s genetics, but they are the best indicator we have at the time they are calculated. As you look over bull sale flyers or A.I. stud books, focus on EPDs to find the most accurate estimation of the bull’s genetics available. If you have further questions, contact your local extension educator.
