Weed killer

A heavy rate should kill them. You will want to be careful with the crop, especially soybeans. If you drift onto non-Enlist soybeans you will kill them with the crossbow, and even with Enlist beans your rate would be high enough in a spot-spray situation that you'd likely kill them or injure them. You will be better off with corn, but still make sure not to drift too much.

I like to drive in the beans and spray out towards the ditch when the beans are about 4-6" tall. I'm on 15' rows so I run quite a few over, but at that growth stage they pop right back up. This helps with the drift vs spraying from the road into the field.

If you are the landowner and not the farmer, I would check with your tenant to be sure what you are doing will work with him. He will appreciate you being pro-active with it and will probably help you with all of it.
If I do it in the Fall after harvest would it have any effect on crops in the Spring?
 
If I do it in the Fall after harvest would it have any effect on crops in the Spring?

That should work just fine with no damage. Best get on them when they are saplings like you mention, when they get bigger the chainsaw is your only option!
 
Have kids and make them pull weeds. Quarantine has done wonders for my lawn. Pretty sure when my son was under the age of 8 he certainly did some watering and potentially fertilizing. And yes I live in town.
 
I have two long farm driveways that I end up mowing. I need something to put in my atv sprayer that kills everything. roundup?

32oz Roundup and 1.5 pint Amine/10 gallons H2O will get rid of everything. That said, I'd rather mow driveways than try to keep up with weeds. At least you have a mowing schedule and it's cheap compared to spraying a driveway a bunch of times and it being clean for about 4 total weeks of the year. Plus, they wash out.
 
Question from the OP that hasn't been answered - do any organic weed killers actually work?

My gf hates roundup, a lot. So I can't use that for patio, sidewalk, driveway (it's her place, I am a kept man lol).

I've read a mix of dish soap, vinegar, and salt (or borax) will kill weeds but not permanently. Anyone with experience on this? I'm not planning to do it in the lawn, just in areas where I want zero vegetation at all. Can I salt the earth?

Don't tell her what you are using
 
I use Trimec to kill creeping Charlie & other broadleaf. Can get it at Fleet Farm for $25 a gallon but I caught it on sale several weeks ago for $17.50. Should last me 5 years