I'm not a big fan of Roundup (I say send kids through the field cutting weeds with a bean hook like I did growing up; it builds character), but they have a legit argument that spraying post-emergence causes less runoff than tilling weeds under before planting, so you retain more soil and have less chemicals running into waterways.

It requires less because it’s a broad spectrum herbicide but the crops tolerate it. You don’t need much so not very much runs off. Needing to do multiple applications of multiple types of herbicide is definitely going to cause more runoff and more downstream effects.