When it comes to understanding if weed shifts or hard-to control weeds are a problem, Mick Minchow of Waverly, Neb., turns to his ag retailer agronomist because he trusts him to give him an honest evaluation of the current situation.

“Locally, we have a really good agronomist, and we’ll sit down and have lunch every once and awhile. We’ll go over products that he’s witnessed and seen that I haven’t seen…He’s just my ears on things, on things that I don’t hear that I really appreciate,” said Minchow.

The worry about herbicide-resistant weeds showing up influences decisions that Minchow is making. Palmer amaranth and waterhemp are two of the main weeds discussed these days across the nation.

These weeds require “zero tolerance,” which means not letting a single weed go to seed because a healthy Palmer amaranth can produce up to 900,000 seeds, and that spells trouble. Even a Palmer or waterhemp seed that germinates late at the edge of the field or hidden somewhere in a corn field can produce seed. These two species in the pigweed family will grow and produce seed until a freeze.

“Even though you might only have a two-inch plant that came up in the fall, it can still produce seed. That is the real challenge. A lot of times in the southern United States where they harvest early, you are potentially going to have another flush of weeds after harvest to re-infest the acre where you did a great job of controlling weeds all the way through harvest,” noted Jim Bloomberg, crop development manager for corn and soybeans with Bayer CropScience.

Letting these weeds survive hidden around or in a corn field is not acceptable if a farmer wants to keep herbicide-resistant weeds from taking over a field and farm. It is in the soybean year of a rotation that the weeds are obvious.

“Palmer amaranth is spreading across the United States and now is established in the South, as well as, several states in the Northeast, Minn., Ind., and most recently was detected in western Iowa. It survives the winters quite well,” Bloomberg said.

Zero tolerance for weeds in corn is imporant

“Waterhemp is primarily situated in the Midwest, but each year we have been seeing more and more waterhemp sticking through soybean fields’ canopy, and it is probably glyphosate resistant,” he also said. “The Midwest is starting to look a little bit like the Mid-south in terms of weed control and resistance problems cropping up.”

A pre-emergence application of Corvus has worked well for Minchow in growing corn, and he has not had to make a post-emergence application in every field. He only hopes using multiple modes of action and multiple applications of products whenever a weed is discovered will keep him from having a problem with waterhemp and Palmer amaranth.

“Corvus offers excellent control of most weeds when applied as a one pass program, but for situations where waterhemp and Palmer amaranth are present,” Bloomberg explained, “a two pass program will be needed due to the extended germination window of these two weed species.”

He added that as part of this two pass program, Bayer CropScience recommends the herbicides used contain several differing but effective sites of action for control of these species.

“By using differing site of action herbicides in these programs, this should help slow the development of herbicide resistance,” Bloomberg said. For example, if Corvus is used as the pre-emergence option, a grower can come back with a postemergence program containing Laudis or Capreno. In the interest of helping manage and prevent weed resistance from developing, these postemergence options should also contain another effective site of action herbicide such as an auxin or atrazine.