Waging war against pesky pigweed

Each growing season, produc­ers face multiple challenges when trying to plant their crops. Too much or not enough rain. Changes in chemical trade names and new premixes. New equipment, software and “smart” technologies to consid­er. On a positive note, commodity prices for grain and cereal crops have been on the rise, a relieving tune when compared to prices over the last seven years. I hope this is a good year for our growers’ pocket­books.

Weeds, on the other hand, care little about market trends and capitalize on our missed opportuni­ties and periods when stress levels are high. The pigweeds—Palmer amaranth and waterhemp, namely— have made headlines and likely are the focal point of many conversa­tions among growers and retailers in recent years. A keyword search for Palmer amaranth and water­hemp on the North Central Weed Science Society website revealed 30% of its published poster and paper abstracts in 2020 included one or both of these weeds, which means significant research contin­ues on both species. I don’t really see that trend changing anytime soon. Note that the members of the North Central Weed Science Society not only include Missouri but also Colorado, Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Nebraska, North Dako­ta, Ohio, South Dakota, Wisconsin, Wyoming and the Canadian Prov­ince of Ontario, and those search results indicate that these weeds are significant pests in many areas outside the Show-Me State.

Populations of waterhemp have become resistant to herbicides that contain the active ingredients glyphosate and lactofen. Some of the first reported cases date back to around 2008. More recent surveys (2015-2017) conducted by Dr. Kevin Bradley’s team at the University of Missouri in Columbia show the majority of waterhemp collected from Missouri fields were resistant to chlorimuron and glyphosate, and one field had plants not controlled with 2,4-D, atrazine, chlorimuron, fomesafen, glyphosate or mesotrione.

Palmer amaranth doesn’t seem to be as widespread as waterhemp in Missouri but certainly has a foothold farther south. For most of 2020—and at least the first half of 2021—the world has focused on mitigating the spread of COVID-19 and its lethal variants, but at the border of our southeast trade territory another issue has arisen: reports of Palmer amaranth control failure with glufosinate in two northeast counties in Arkansas. As a weed scientist by training, this saddened me to read. My intent is not to scare or signify the fight is hopeless but to inform readers of what is going on in our backyards. We are in a battle to isolate these populations as much as possible to prevent pollen and seed spread and to protect the technologies that we have left.

Knowing what we know about these species, we still have options. Protecting corn and soybeans from weed competition four to six weeks after emergence is necessary to minimize yield loss. Soil residu­al herbicides applied at planting followed by in-season overlapping residuals, including a Group 15 herbicide (such as Anthem, Cinch, Dual II Magnum, Outlook, War­rant and Zidua), have shown to be effective in managing these species before they emerge from the soil. Add an effective post-emergence herbicide (tailored to the soybean trait platform) for those weeds not controlled pre-emergence.

For various reasons, many Mis­souri farmers have planted cover crops. Fortunately, high biomass cover crops such as cereal rye make it difficult for small-seeded weeds to establish and penetrate through the residue layer. Sufficient closure of the seed furrow is critical to maximize soil-to-seed contact and achieve uniform crop emergence. Some challenges exist with cover crops, no doubt, and may not be suited for areas where soils poorly drain. Terminating the cover crop when it’s larger may prevent soil residual herbicides from contacting the soil. Soil-applied herbicides must be in the soil solution to be taken up by germinating seeds. However, planting conditions may deteriorate when cereal rye is termi­nated early because the once living cover crop is no longer extracting moisture from the soil.

Going forward, I’m optimistic that technology and adapting our production practices to exploit weaknesses in the plant’s biology will aid in managing these prob­lematic weed species. Visit with the agronomists at your MFA affiliate or feel free to contact me if you want more info on weed-control options for this year’s crop.

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