what is an outcross sire in simple terms relating to thoroughbreds?

Answer 1

An outcross is simply when you use a stallion who's pedigree does not include the same bloodlines as your mare's.

For example, Rainbow Quest is an outcross for mares by Sadler's Wells. Rainbow Quest has no "Northern Dancer" blood in his pedigree as he is by Blushing Groom (by Red God) out of I Will Follow (by Herbager). The Northern Dancer bloodline is the most potent in the world at the moment, and probably since TB breeding began. It can be hard to find stallions that are "Northern Dancer-free".

It is important to try and find outcrosses for your mare to make sure that you do not inbreed her to certain bloodlines.

Inevitably, as ALL Thoroughbreds are related, there will be some inbreeding, but we need to be very careful about how closely we allow them to breed.

Hope that hasn't baffled you with science and pedigrees!

Answer 2

One dictionary definition of an outcross follows. A cross between relatively unrelated individuals.
It's up to you to decide what "relatively unrelated" means.

The use of Rainbow Quest and Sadler's Wells is a case in point. Many people would agree that is an outcross; some would disagree as Sadler's Wells is inbred to Nearco 3 x 6 x 6 and Rainbow Quest is 4 x 5. The resulting foal of breeding a Sadler's Wells daughter, with no Nearco influence from the dam side, to Rainbow Quest would be inbred to Nearco 5 x 5 x 6 x 8 x 8. There are other horses in common as well as Nearco but he was the most obvious choice for me to illustrate my point.
I have no problem with calling that an outcross; but if you want to have an outcross make a hypothetical pedigree and check to see if it works for you.