how long after round up beans start growing before i can spray them for weeds and grass coming up in the plot?

Its important to kill the weeds before they get to 4 inches. I usually end up spraying mine a second time later in the summer. If you dont kill them early they will stunt your beans growth. There is some really good info on planting, fertilizers etc... if you google Eagle seeds. They are a company out of Arkansas. Also I always use the generic round up as its cheaper.

so it doesn't matter how small the bean plants are the roundup won't hurt them?

Thats correct, you can apply round up any time. What do you have for application?
I use atv sprayer for small plots and tractor with 55 galon sprayer on larger plots.

i have the same equipment. these are smaller plots so i will be using the atv. how much material are you using per gallon of water?

I just follow the instructions on the Round up container. If im spraying before I till a plot up I mix it a bit stronger.

Ditto - I use a 1% solution by volume and keep the ground speed so that I apply about the right amount of Glysophate to the area. It does take me 2 apps to keep the weeds clear. Usually at about 6" height and again around 2'. I do about 8 acres RR beans and 3 acres RR corn. I hit the corn only one time at about 1' height. I do not have a high platform sprayer. Beans don't get going all that well due to being hammered. Corn has made fine crops every season. I leave the corn standing and plant a green field around the corn. Works out pretty good for a while, but the ears are all gone more or less by early November.