Oh how things change. A couple of years ago it was hard to find a weedy soybean field, while today they are easy to find while driving across the state. In fact, I recently had the opportunity to drive to West Lafayette, Indiana, and I can tell you Nebraska is not the only state struggling with weed control. In fact, fields across Iowa, Illinois and Indiana all had weed-control issues.
One thing in common across these states is the weeds that were present; marestail, waterhemp and some pockets of palmer amaranth. These three weeds are giving us the biggest challenges with weed control in eastern Nebraska, as all three have developed resistance to glyphosate, the active ingredient in Roundup.
So what should we be doing now, given that it is too late to spray?
First off, get out and scout your fields. It’s pretty easy to see the weeds above the canopy in bean fields, but take the time to quantify the number of weeds and see if you can figure out why they weren’t controlled by your weed-control program. If weeds are small, they may have come up after your postemergence trip, while very large weeds may have been missed by tillage or your burndown and were too big for control by your post trip.
While scouting, be sure to identify what weeds you have, especially when determining whether you have waterhemp or palmer amaranth. The easiest way to do this right now is to check the flower heads of the weeds. Palmer amaranth will have much longer seed heads than waterhemp and the female plants will have pointy, sharp seed heads, which waterhemp does not have.
Be very diligent in checking for palmer, as we have had a few new reports in areas where the weed has not been found before. In the region, I can find palmer amaranth in pockets south of the Platte river, and in the valley north of the river. In northern Platte and Colfax counties there have been a few scattered fields found in the last couple years, so scout your fields and, if possible, walk those fields to reduce seed production for next year. Note which fields have palmer, and consider harvesting those fields last or clean the combine between fields to reduce the spread of seed.
If your fields have waterhemp or palmer, make plans now to try something different next year. Remember the quote: “The definition of insanity is doing the same thing over and over again, but expecting different results.” If glyphosate didn’t give you good control this year, it most likely isn’t going to give you good control next year, so make a change. That might mean switching to a different herbicide trait, or layering residual herbicides with each pass you make across the field. For more information on control, the best resource we have is EC130, or what we always call the "weed guide."
The other problem weed that you can see right now is marestail. The battle with marestail is lost for this season, as they are going to seed right now, releasing their cottony seed into the wind. Our best bet for good control of this weed is to start making plans for fall control, either through herbicides in no-till systems or with tillage in other systems. Being a winter annual, marestail is easy to control after harvest and can be sprayed all the way up to hard freeze as long as you can get a couple days with temperatures above 50 degrees. Herbicide options can range from cheap programs of just 2,4-D and dicamba, or can be more expensive when you include residual herbicides. Everyone I have visited with who has done fall spraying has been satisfied, and I’ve had quite a few people tell me they are planning to try it this fall.
So in summary, get out, see how your weed-control program worked, and make changes to stay ahead of the weeds.