True 2,4-D formulations have more active ingredient and are thus more effective. If using pure 2,4-D make sure to use amine formulations in home use environments, its less volatile. All do well in Montana but limited by marketing options. Sunflower is very well adapted but due to light test weight its bulky to ship. Yes, as long as weeds are actively growing.
Most weeds stay green longer than crops since they are more deep rooted.
In dry range conditions avoid spraying when very hot and dry as products like Tordon photo decomposes quite rapidly. If your near ground water the best is 2,4-D amine. If the water table is 10 to 25 feet deep Curtail is preferred and if the water table is deeper than 25 feet Tordon is preferred. Need to pre-bait to improve performance. Call Jim Knight at 994-5579 for additional suggestions.