Pre-emergence Herbicides
Soil-applied herbicides available for use in sunflower are EPTC (Eptam), ethalfluralin (Sonalan), S-metolachlor (Dual Magnum), pendimethalin (Prowl H2O and others), sulfentrazone (Spartan) and trifluralin (Treflan and others).
If you know a local farmer, they might be using a herbicide that would work for you and hit your plot for you or maybe purchase a quart of hericide from them...chemical dealers might only sell 5 gallons (box of 2 2.5 gal). These might be restricted use chemicals, which means you must have certified training to purchase. Talk to the local grain elevator agronomist, they might be able to get you what you need or know someone that might help you out. I doesn't sound like the Pheasants Forever chapters are too plentiful in TX or I would suggest talking to their habitat guys. Good luck with this.
Do you care about habitat or you are planting Sunflowers to harvest?
Your question doesnt make much sense unless you have invasive cool season grasses or something like russian thistle and you're focused on whats good for upland birds.
In Texas, Beginning of September was so green and overgrown. The doves didn’t arrive/really fly good till end of September…. Everything was behind this year.