Waterhemp and Palmer amaranth have reputations as tough-to-control weeds, and that reputation has only increased as weed shifts have evolved and they have developed increasing resistance to glyphosate. Today when these weeds are present one-pass weed control is nearly impossible.  

According to Penn State Extension, “A weed shift is the change in the composition or relative frequencies of weeds in a weed population…or community…in response to natural or human-made environmental changes in a system.”

Bob Hartzler, Extension agronomist with Iowa State University, explained what kinds of circumstances cause these weed shifts.

“Each member of the weed community has different optimum growing conditions and responds differently to control practices,” Hartzler said. “The community within a field is a direct result of current and past management practices.”

Mismanagement of glyphosate is commonly blamed for causing weed shifts in addition to the development of herbicide resistance in key weeds. 

900,000 Seeds Per Plant vs. You

Waterhemp and Palmer amaranth are two of the most commonly discussed weeds when it comes to weed shifts and resistance because they reproduce rapidly and prolifically. A single plant can produce hundreds of thousands of seeds.

“These are two very aggressive weed species in terms of growth habitat and seed production,” said Jim Bloomberg, product development manager, Bayer CropScience. “If left alone and untreated, a single Palmer amaranth plant can produce up to a million seeds. The seeds germinate from early spring all the way through the first killing frost, so you’ve got a very long germination window.”

That long germination window means no chemistry will last long enough to control all the flushes of waterhemp and Palmer amaranth.

“You really need to use multiple products and rely on multiple herbicide application timings,” Bloomberg said. “You need to have zero tolerance for these weeds –- they can’t get big enough to reproduce.”

Not only are herbicide choices one way to manage tough-to-control weeds, but stewardship and tillage practices come into play too.

“Even equipment use and practices can contribute to the spread of weeds and weed shifts,” Bloomberg said. “For example, people can run the combine through one field in which waterhemp or Palmer amaranth were present during the season. The seeds are still in the soil that clings to the combine tires, and is transferred to the next field.

That next field may not have had waterhemp or Palmer amaranth previously, but one or both will likely show up in those fields the next year.”

Keep Time on Your Side

Some growers might get overwhelmed by the number of factors that could play into the spread and management of these weeds. But Jeff Springsteen, senior product manager, Bayer CropScience, said the best place to start comes down to two words: zero tolerance.

“A zero tolerance policy means that you don’t even give these weeds a chance to go to seed,” Springsteen explained. “You just can’t let these plants go to seed and produce new weeds because once that gets started, it’s very difficult to slow down.”

It’s Time to Resist Resistance

“Growers need to exhibit meticulous stewardship when it comes to their herbicide programs,” said Springsteen. “A lot of growers have the expectation that companies will come out with new chemistries, as they did through the 80s and even into the early 90s. What they don’t realize is that the newest class of herbicides is at least 20 years old.”

Springsteen further explained that contrary to what some growers think, crop protection companies do not have any new modes of action currently in the pipeline.   

“We are all looking, but even if a new mode of action was discovered today it would take eight to 10 years before it became commercially available to growers,” Springsteen said. “And that’s a big if. With no new modes of action coming to the market anytime soon, agricultural chemical companies are focusing on how to correctly use existing chemistry and combining multiple modes of action into premixes.”

“That’s exactly why zero tolerance is needed,” explained Bloomberg. “We need to be judicious in our use of the currently available herbicides.”

Since its introduction in 2009, Corvus herbicide typically has been very successful at providing true one-pass control of weeds in corn, Springsteen said. But today, he said, there is no such thing as a herbicide that can provide weed control in a single pass if waterhemp and Palmer amaranth are present.

“Corvus is just as effective as it always has been. It’s just that the weed spectrum has changed–-and is continuing to change–-very quickly,” Springsteen said. “The chemistry has not changed and still provides one-pass weed control in most fields if waterhemp and Palmer amaranth are not present.”