Actually, outcrossing is a rare source of resistance in weeds since few of current GE crops (eg corn, soybeans, cotton) have weedy relatives with which they are interfertile. Also, most crop species have been so thoroughly domesticated that they have very poor survival characteristics outside of an agricultural setting (they don't compete worth a darn), and these adverse characteristics also go along with any outcrossing. Canola/rapeseed may be an exception in that it has some weediness characteristics and interfertility with wild/weedy relatives. Of course a HT (herbicide tolerant) turfgrass would also likely have a similar risk.
The biggest single source of weed resistance to glyphosate or any other farm chemical is repeated use (or "overuse"), which is a very powerful selective pressure on the organisms that are exposed (this is equally true of insect resistance to pesticides). Mother nature constantly introduces genetic diversity via mutation and recombination so the rise of such resistance is a normal, expected outcome from a strong and prolonged selective force.
The presence of resistance genes in an organism only benefits it in the presence of that selective pressure...outside of the farm (or lawn) environment, the pesticide/herbicide resistance genes are not helpful at all, and may be deleterious.
BOB
