Sounds like ragweed.
Do you mean round-up resistant when you spray it when it's small or big? Anything big is going to have a much harder time being killed with round-up. On the round-up issue, some tidbits.... go heavier rates, 2-4 qts if any questions or just switch to spraying Liberty at 35-40 oz (roughly). Add Ammonium sulfate and crop oil. Spray when hot. Spray SMALL PLANTS actively growing. Those things will all help get better kill. FYI.
Sorry, in answer to killing ragweed in Switch..... Are you talking about NOW, earlier in season, in spring, when?
Several options..... 2,4-d. mix in crop oil and ams if you wanted. Roasted.
If switch is well established, you can run pre-emergents over it (ideally before it's out of dormancy) such as Plateau (which will free a growing plant & I'd suggest in situations where switch is already very thick). Oust is another option, Dbltree's forum has more on that. Oust makes me nervous but can read on that. You can continue Atrazine and swap Round-up for Liberty. 2,4-d in addition to any of this. By now, one of your other great aids & probably time is: FIRE.
Just like ANYTHING, there's usually option after option and most will vary depending on your situation (when you wanna kill, how good the switch stand is, is there anything else in there you want to save, do you really have Round-up resistance, etc, etc). Good luck!
