I think I know the weed you are talking about Kooty, we get them here on the farm in S. MN on the old cattle feed lot. Super tall, big broad leaves, when they die they look like tall cane poles? If so, they are quite persistent if you dont get on them right away. If it were me planting, I would work the soil, clear old vegetation clumps if need be, plant and then spray to kill any small emerging weeds that didnt get worked up with the soil. I know it sounds silly to spray what looks to be mostly black dirt, but it has worked great for me on new plots. Then once the corn is like 10″ tall go spray again. Planting early in the season instead of waiting should help you get the corn an even start with the weed growth, then the round up will suppress the weeds. I would also suggest, as Bob did, to still add some fertilizer. Probably wont need as much due to natural fert you have, but adding some will help. Just what id do any ways.