I agree with bill. if you are going to use significant tillage and plant immediately afterward, you won't have weeds when you plant. Tillage disrupts weeds, but brings more weed seed into the germination layer. They will be on relatively equal footing with your beans and corn at the start. Unless you have a specific problem with a gly-resistant weed, you simply wait until the newly germinated weeds are 2" - 3" tall and spray your crop with glyphosate. When weeds are young and supple like this, most are very susceptible to glyphosate. (Glyphosate is the generic form of Roundup). Roundup Ready crops are genetically modified to be resistant to glyphosate. So, most weeds are killed and your crop is not. There is hno license required for glyphosate.
This is the traditional way farmers planted beans and corn for many years. There are some issues with it. Tillage has a substantial negative impact on soil. If you are have prime farming soils, they can tolerate tillage abuse for many years and still produce well. The more marginal your soils are the sooner the cumulative impact of soils evidence themselves.
Unless you have a no-till drill or planter, you may not have any other option for large seeded warm season annuals like beans and corn. Two issues result from applying this method year after year. First is the soil issues I already talked about. The second is that some weeds have a natural resistance to glyphosate to some degree. Those with the most resistance won't die and the others will. The next generation of weeds will have the genetics from those that survived. Over time you end up with a field that has weeds that can no-longer be controlled by glyphosate.
You can read on other threads that Bayer now has a new genetic modification that makes beans and corn (and other crops) resistant to a new herbicide. Many farmers are now turning to this because the RR/Glyphosate combination no longer works for them, but it is expensive. This glyphosate resistance problem is just as big of an issue for no-till planting but no-till methods don't have the soil issues compounding things.
So, as Bill says, mounting a sprayer on a planter you are using after tillage has no real advantage.
With a no-till drill or planter, a cutting wheel up front to cut thorough debris, and opener to open a small trench in the soil, and some cultipacker or closing wheel to close the trench after the seed has dropped in. These drills are heavy and can plant through a heavy crop like winter rye. The Winter rye or, whatever the previous crop was, needs to be terminated so it does not compete with the planted crop. Folks do this with either a crimper, which breaks the shaft, or by spraying with glyphosate. So, in a case like this a sprayer on a drill would allow both to be done in one pass.
The reason I'm talking about no-till is that the same concept may be used for min-till. With minimum tillage, you may be able to use a regular planter or drill like a no-till drill. You might be able to get min-till accomplished with a disk if the soil conditions are right and you have it set very non-aggressively. My experience is doing it with a tiller. I lift it so high that it is only engaging the top inch of soil. This disrupts many of the weeds mechanically and mixes small amounts of soil with them. The soil microbes help them decompose faster. Enough weeds will survive this, if done correctly, they you still need to spray with glyphosate as you plant. This very light tillage has significantly less negative long-term soil impacts than deeper tillage and it is often sufficient for a regular planter or drill to get beans and corn planted deep enough.
Hope this helps,
Jack
