Weed control through the years has been talked about in terms of whether weed escapes have the potential for economic yield damage, but the situation has changed with herbicide-resistant weeds.
“We can still consider that economic damage criteria with some weeds like a few scattered velvetleaf or volunteer corn, but with herbicide-resistant weeds it’s a different today,” say Trace Waddington, an experienced ag retail agronomist in Illinois.
Some of the key weeds such as waterhemp and Palmer amaranth that have become resistant to glyphosate produce hundreds of thousands of seeds per plant and spread like wildfire from one year to the next.
Often soybeans fields are where weed cleanup was done, and weeds in corn fields that germinated mid-season were more or less ignored. But ignoring them in corn isn’t a wise decision with these herbicide-resistant ones.
Waddington noted, “If weeds are showing up in a drownedout area, you can easily understand, but if there is a weedy area and there has been some fairly good ground canopy cover by the corn, then that is a different issue. Those weeds that get started might not turn into really tall weeds, but are still there and can produce viable seeds.”
For these reasons, Waddington sees the need for residual herbicides with more than one mode of action being used in corn in a late post-emerge pass through a corn field.
As Jim Bloomberg, crop development manager for corn and soybeans with Bayer CropScience, said, “Weeds such as waterhemp and Palmer amaranth, which in many cases are no longer controlled by glyphosate, germinate season long, but farmers need to maintain a season-long clean field. They need to use multiple modes of action in their fields to keep them clean.”
Bayer doesn’t see its own chemistry, or any one company’s chemistry, as the total solution for all situations. “Capreno herbicide used in a post-emerge application is an example, Bloomberg said.
“Capreno offers excellent one-pass season-long weed control for most weeds. “It has two modes of action, but we also recommend mixing glyphosate with it because glyphosate is still very effective on most weed species. If ‘driver weeds’ are present in the fields, however, (waterhemp and Palmer armaranth) we also recommend growers put in some atrazine and even sometimes dicamba. In other words, farmers need to have from a minimum of two to maybe five differing and effective modes of action in their herbicide programs during the year.”
Waddington agreed by saying that weed control still needs to be handled case by case. He also agreed with a point that Bloomberg constantly emphasizes—zero-tolerance weed control or no driver weed left in a field.
Farmers who ignore weeds at the edge of a field or the end rows are looking for trouble, Waddington suggested.
“Farmers have said they missed a few weeds on the end rows. I don’t think they were missed. I think they might be resistant ones. In either case, a few weeds one year means a lot more next year,” Waddington said. “It would be an excellent idea to hand weed those missed or escaped weeds. We are not talking about walking whole fields, but a few acres on the end rows. Whether they are resistant or escapes, you remove them before they go to seed.”
Those end-row weeds can quickly be spread by a combine more than by nature.
Farmers have to recognize the weed populations at the edge and the middle of their fields and develop cultural and herbicide weed control programs. Waddington said, “Farmers need to be encouraged to do more scouting or have assistance with scouting throughout the season.”