The field has had Roundup, I took the pics before it had taken any effect.
It shold have been topped first and then the regrowth sprayed off. Roundup on those big cabbage that are left won't do much.
No doubt the field will be full of cabbage white butterflies soon, as it was last year too. They will get an insecticide at some point shortly after it is replanted, but that is for aphids mostly.
It might be planted with cabbage again, not sure yet, but it was cabbage the year before. Doesn't seem to cause any problems and we have been doing it for decades. Maybe it is because they are usually overwintered so little about insectwise, then generally cut and gone before they get going again in the spring.
Not all the fields look like this by any means, some are almost completely weed free and the rest are perfectly acceptable. For some reason every year we have a field or two that gets bad, and usually it is at my place. We grow 40+ acres every year, which is over a million plants, so a bit like this now and then isn't the end of the world.
Not long back we had over 70acres of them, but have reduced the area as the market can't stand that many.