Thank you! I had that growing in my pasture and could not identify it. It started as just a handful of plants and the chickens loved it, so I let it be. Then it spread very rapidly and it had to go. It took me three years of pulling, digging and spraying to finally be rid of that stuff.
Edited to add- Just looked it up and found this "A giant ragweed plant can produce more than 60,000 seeds if allowed to fully mature. "
No wonder it spreads so fast!