I'll have to put in a word for the Simmental association...they've been pioneers in the development of EPDs going all the way back to EBVs (Estimated Breeding Values) back in the 1970s. Back then we had 5 numbers and they probably weren't very valuable compared to what we have now, but it should tell you how committed the Simmental association and its breeders have been to performance records and calculation.
Visit this link:
http://simmental.org/site/index.php/genetic-evaluation/epds
Hover on the highlighted "Genetic Predictions" tab at the top, then click on any of the tabs in the drop down menu that appears. Since ASA has had an open herdbook since the association was formed, they've collected data on many breeds, and all that information is now a part of our multi-breed EPD calculations. Beginning this fall, Simmental and the Red Angus Association will begin calculating EPDs using a common base, and producers will be able to compare EPDs of any Simmental, Red Angus, or other breed animals that are in the database at that time. It's a progressive move that is sure to receive criticism from some producers who probably don't understand the merit or the motivation behind two separate breed associations partnering, but the basic intent is to position both breeds against their leading competition which is obviously black Angus.
Some breeds have EPDs for the same traits and they express the numbers in the same way. Some will have numbers for completely different measurements, and some associations will measure the same trait, but they'll express the measurement differently. For example, birth weight ratios for Angus and Simmental...a lower ratio for Angus is better (meaning lower birth weight), and a higher index for Simmental is better (meaning more acceptable compared to herd average). You'll need to concentrate on one breed at a time at first and try to understand the language and definitions, then it won't be difficult to relate that understanding to how other breeds do things. If you have a favorite breed, start there. If you haven't chosen a breed yet or you're open to multiple breeds, I would encourage you to study the Simmental database just because the association is very progressive in terms of EPD calculation, because they are the only association that speaks in terms of multi-breed EPD calcuation, and because the language is relatively straight forward.
Another basic element you'll need to understand for this to make sense is "Base" calculation. Every breed determines what their base will be for each trait measured, or their breed average. EPDs are a comparison to that base within that particular breed. In theory, it would make sense if the base for all of these numbers were 0, but that isn't the case. And this is a big reason why you can't compare EPDs between different breeds that are calculated by different associations...they all use a different base for their comparisons. Later this year when Simmental and Red Angus adopt a common base, every animal in those two databases will be on a level playing field in terms of EPDs, and producers will be able to compare the numbers side by side, even for cattle of different breeds. And when I say "for cattle of different breeds" I'm talking about cattle that are in that database...you still won't be able to compare numbers calculated by other breed associations. In case I'm confusing you, the Simmental association has always had an open herd book, meaning you could breed other cattle that were not Simmental to a Simmental bull and register the calf as a %. Most of the American Simmental today are bred up from either other registered breeds or even commercial cattle, so the association's registry database contains a lot of performance calculations for cattle that contain other specified breeds. All that information has helped build our EPD calculations. Now we even have purebred cattle registered as "Foundation" sires and dams to be used in crossbreeding to make registered Simmental offspring, so those Foundation parents, many of which are registered with other breed associations like Angus for example, will help build this calculation as well.
Anther GREAT wealth of information for you would be the MARC website:
http://simmental.org/site/index.php/genetic-evaluation/epds
The Meat Animal Research Center in Clay Center, NB has been a HUGE source of information and learning about EPDs and crossbreeding, just to touch on a couple of important topics. It's probably one of the most important facilities in the United States, if not THE most important, considering the research that's been done there and the information it's provided. If you're interested in livestock production and you haven't learned about MARC research yet, it would be worth your time and I think you would find the information very interesting.
Hope this helps! And good luck to you!