The best time to treat for weeds is BEFORE you plant a crop. There are quite a few residual pre-emerge products available that greatly reduce the volume and types of weeds that'll show up later down the road. Before the advent of "RoundUp Ready" corn/soy beans, that was the number one way of treating for weeds. Now days, with glyphosate tolerant weeds beginning to show up, a good pre emerge treatment is coming back into vogue. If you watch closely, MOST of the "new" pre-emerge chemicals being used now are the very same ones that were popular BEFORE "RoundUp Ready" crops became all the rage. (Or combinations of 2 or more older formulations) Pre's are once again being used in many instances, even with RoundUp Ready crops, due largely to the numbers of RR tolerant weeds.
On the subject of "RoundUp Ready" crops, check into Liberty Link corn. I'm planting much of next years acreage with LL.
There are a lot of good post emerge weed control products (besides RoundUp) out there too, but MOST require a certain amount of product knowledge. They USUALLY treat for specific weeds. (or at the very least, PERFORM best on a short list of specific weeds) You have to know what you're battling, as well as knowing how to apply them in a manner that won't "ding" the crop in the process.
I've had my best results by using an effective pre applied with my glyphosate burn down (prior to no tilling crop), then a full rate application of RoundUp and a tank mix partner targeting the short list of RoundUp tolerant weeds common to this area, after crop is up (in RR crops of course) ,IN A HEALTHY STAND OF CORN/BEANS.... A good healthy crop will create a full crop canopy, cutting off sunlight to emerging weeds. If weeds are controlled in the initial stages of crop growth, there won't be much weed pressure once the crop is well developed.
Worth mentioning is, many of the GOOD weed control products will be "restricted use" chemicals. You'll have to get proper certification to buy and/or use them. Not THAT difficult, but yet another hoop the government makes us jump through.... RoundUp is NOT (yet) a restricted use product (but give the EPA time....

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Several of the varieties of RoundUp offer warranties. Monsanto will pick up part to ALL of the cost of product on second application if first (used under EXACT labeled directions) proves to be insufficient. That can reduce cost per acre significantly. The ONLY catch is, you have to be a "real" farmer and not doing food plots, ect....
Depending on who's numbers you look at, somewhere between 95% and 98% of the cash corn grown in the US employs chemical weed control of some sort. BT/GMO corn represents a large portion of chemically treated corn, but not ALL. Field crops were using chemical weed control before anyone had a clue "BT" crops were even possible. There is a market for specialty "organic" grain crops, but it is somewhat limited in demand. Organic corn gets a higher price, but AT a higher price. Mechanical weed control is expensive when done on a large scale, as well as the fact most TRUE organic corn has lower yields than it's non organic counterpart. It just isn't nearly as profitable in MOST cases. (The term "organic" gets tossed around, but the term is NOT used in it's proper context in most cases.)
Chemical weed control products were commonly used for 35+ years before RoundUp Ready crop science was heard of. RR crops have been on the market about 20 years now, and it looks like their "glory days" are fading away to an extent. Glyphosate tolerant crops were so simple and so inexpensive to grow, the technology took the industry by storm. They were the meal ticket for most farmers after the thrashing we took in the 1980's. Until the next "miracle" weed control technology comes along, looks like we may be forced to depend somewhat on the best of the technology of '60's, '70's, and '80's again.
Take it from an old farmer who spent WAY too many days in the sun riding a tractor, cultivating corn.....It CAN be done, but spraying is a FAR more productive, far less expensive, less time consuming method.