Cindy, the "LB's don't root" is a myth. They will root as much as any other hog given the right circumstances. The reason people think they don't root is that we all raise LBs on pasture. As long as there is adequate growth (short, tender) pigs will not tend to root. It is when the pasture gets very short, or you put them in a pen or barn, that pigs root. They get hungry, stressed and bored.
Hogs that are not used to grazing will root even when the pasture is in good shape simply because they haven't yet learned that the food is on top of the ground and rooting was a behavior they learned while in "captivity". Once they get used to grazing the rooting generally stops. But I've also learned that rooting can be taught. One good rooter can spoil a whole herd of nice grass eaters
If the pasture gets wet then all hogs will root as the soil becomes so easy to turn. If it gets soggy, well, plant your garden there next year
Some people will ring their hogs to stop them from rooting. This is usually wholly unnecessary and is a really bad idea as it interferes with their ability to forage and can interfere with boar mating behavior. Ringed hogs are generally less productive than those left in their natural state.
Rooting also is beneficial to the soil and pasture. It mixes the top layer so that nutrients and seeds are brought to the surface. It also encourages the growth of native plants which are often just as nutritious as transplanted grasses and will endure the environment better.
The key to controlling rooting is to always have the hogs on good pasture. When they have eaten the grass down to the point at which leaves start growing (read this
) it's time to move them to the next paddock. As long as you keep them moving there should always be lots of leaves to eat and they should not be going after the roots. They will root here and there to get at salt and the critters that live just under the surface but this is a natural and desirable behavior.
Now, Large Blacks do seem to be able to benefit more from eating pasture than modern breeds. They seem to be able to fulfill more of their nutritional needs from forage. This is one reason they make such good pasture hogs and may contribute to the perception that they root less.
Cross breeds inherit the traits of their parents (and whatever ancestors came before them). If you cross a Large Black with a modern breed the offspring may or may not inherit the LBs ability to do better on pasture. That's why I and some other heritage hog breeders only cross a heritage that is known to do well on pasture with another heritage with similar traits. I just sold the last of my latest Hamp X GOS litters and never had to feed them anything. They lived from birth to nine to twelve weeks completely off mama and pasture and had nice growth rates and health. They were a big hit locally.
I can't say whether your York X Poland will create good pasture hogs or not. I stopped raising those breeds long ago as the more rare heritage breeds are just so much better in so many ways. Yorks are considered "heritage" but it seems that their wild ways have been bred out of them in too many barns and pens.
But, having said all that, you just might do well with any cross if they can adapt well to pasture grazing and if you manage the pasture and the hogs right. There are lots of books out there to read but I prefer to read the research studies done by college agricultural science departments. Look around at your local schools to see if they have a grazing management program and ask to see their reports. Perhaps drive out and talk to the students and instructors. But be careful, lots of livestock management programs are just teaching how industrial ag does it.
Best of luck!