Zero tolerance needed with waterhemp, Palmer amaranth
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.
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