If you have large yards for your chickens, don't use an herbicide, and if you are letting them forage, try using vinegar. It is a indiscriminate broad spectrum herbicide. I put it around the outside of the fence of my gardens. It does kill, I trim down the mesquite twigs coming out of the ground (will do the for years after the ground is cleared) and pour 1/2C or more on the root crown sending up the growth. It is 80% effective, the wood/root in the ground no longer tries to grow trees. Other than that, I know nothing about organic herbicides, and I won't use Round Up or any other chemical poison. The migraines are not worth it.