In my last article, I wrote that weed control in conventional soybeans is more challenging because it requires controlling a broad spectrum weed problem with narrow spectrum herbicides. This means a typical conventional program requires both soil-applied herbicides and often several postemergence herbicides.
Before Roundup Ready, we did a lot of research on total postemergence weed control programs. It would seem that with the array of both grass and broadleaf herbicides available we could have done it.
In situations where growers were using preplant tillage, conventional row spacing and cultivation, it was possible in some cases. However, conventional tillage systems made it easy to use preplant incorporated herbicides and most were doing that.
Growers who wanted to use narrow row culture and conservation tillage programs were putting a lot of pressure on university guys like myself to come up with total postemergence programs that would work. What they really wanted was to plant and be able to make a couple of over-the-top treatments with a grass and broadleaf herbicide combination like we were later able to do in the Roundup Ready system.
There were several problems trying to do this successfully. First, we didn’t have very good herbicides for some weed species such as prickly sida, Palmer pigweed, and sicklepod, to name a few.
Another problem with the herbicides of the day (which are the same ones you will have to use now) is the timing is extremely critical. If they were not sprayed on most weed species by 14 days after emergence — and under extremely good environmental conditions — they failed and there were no good salvage treatments.
There were problems such as tank mix antagonism between the grass and broadleaf herbicides, but there were ways to overcome that one.
As we started using more ALS inhibiting herbicides in the mixes, carryover could be an issue with some of them, especially with rice grown on high pH soils.
Perhaps the biggest problem was that under heavy weed pressure (which described most fields) the weeds often would simply overpower the treatments even though everything had been done right.
Roundup Ready came along and essentially all of the problems described above went away. For the first time in my career, I could plant research plots using narrow rows — either conventional or no till — and get excellent control of almost any spectrum of weeds using two over-the-top treatments. We had just not been able to do that with conventional herbicide programs.
Therefore, if you try some conventional soybeans, my suggestion would be to look for situations where you can avoid as many of the pitfalls described earlier as possible.
The first suggestion is to put them in your cleanest fields. Grass weeds are much easier to control in conventional soybeans than many of the broadleaf weeds. The fields with mostly grass pressure would be good candidates.
Unless you know the fields have extremely low weed pressure, my recommendation would be to avoid trying to rely strictly on postemergence herbicides. If you must depend upon soil residual herbicides, I have a lot more confidence in preplant incorporated herbicides than pre-emergence herbicides.
In a conventional tillage system, incorporating trifluralin, Prowl or Dual perhaps with something like Scepter and following with a timely postemergence program can make a nice program. Even that program will consistently work better in conventional rows with cultivation than in narrow rows.
That does not mean every other type of conventional program is doomed to failure. Picking the right fields, picking the right herbicides and very early application timing are much more critical than in a Roundup Ready or now LibertyLink program.
If you just attempt to grow conventional soybeans like Roundup Ready or LibertyLink, and hit them over-the-top a couple times with a conventional herbicide tank mix and expect them to be clean, look out!
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