You should have gotten a much better kill on that grass with the Round Up. Round Up usually gets almost a 100% kill on grass. If you disced and plowed right after spraying, that may be the reason. Round Up is a systemic herbicide, which means that it is taken in through the green parts of the plant, and needs time to travel all through the plant (usually a week or so). If you disced, then you may have separated the root from the rest of the plant before the Round Up got there, and the un-dead roots simply took hold and started growing again. Most grasses propagate via their rhizomes (root like runners...this is what forms the sod "mat") as much as if no more than via seed, so you may have actually INCREASED the amount of grass if you chopped up the un-dead root with the disc, and the "planted" them with the plow.
Offer No Apologies.....
NRA Endowment Life Member