Yellow nutsedge does not compete well if we are in good conditions for early season crop vigor. If I can ever get cotton as tall as the nutgrass, we can out grow it. Purple is common, but yellow is dominant, and a little easier to manage, especially as long as we have MSMA, which we may be losing. Nutsedge typically seems to follow vegetables, as watermelon and lima bean farmers have more problems than long term cotton and peanut folks. I don't know specifics, whether it's cause or effect, but southern root knot nematode pressure is worse in nutesedge fields, and might just be a coincidence, but don't believe so. Many similar weeds I am sure are excellent alternative hosts for nematodes.

As far as problems weeds, our number one grass is Texas panicum, but there is some crabgrass around.  Our worst small seeded broadleaf weeds are pigweed (hey, includes glyphosate resistand Palmer Amaranth) and Florida beggarweed.  Others broadleaves are sicklepod, cocklebur, coffee senna, tropic croton, wild poinsettia, and bristly starbur.  Morninglory species include smallflower (really more closely realted to kudzu than the ipomea morningglories), entireleaf, ivyleaf, and some purple moonflower. 

We are really worried about the glyphosate resistant Palmer and its impact on continuing all of our conservation tillage production systems.  Other problem weeds in conservation tillage environments are Florida pusley and tropical spiderwort. 

Throw in some problem areas of bermudagrass, and you see it's pretty diverse here. Land that won't grow a weed won't grow a crop.  Let that battle continue.