Moscow, Idaho — University of Idaho researchers have good news for Idaho farmers seeking to reduce their reliance on glyphosate herbicide, hoping to prolong the popular agricultural chemical’s efficacy in the face of resistant weeds.
Chandra Maki, of Colfax, WA, is finishing her master’s thesis evaluating alternatives to glyphosate for ridding fields of weeds and volunteer plants prior to planting fall grains. Maki has found several options that offer equivalent weed control as glyphosate without harming subsequently planted grain crops.
Glyphosate, the active ingredient in Roundup herbicide, is highly effective and has long been the go-to product for farmers when they clean their fields prior to planting other crops, such as wheat or barley. However, Roundup is also used in season to control weeds in fields planted in Roundup ready sugarbeet, canola, alfalfa and corn varieties, which are bred to withstand glyphosate applications. While most herbicides target broadleaf weeds or grasses, growers enjoy strong control of both groups with glyphosate.
But growers risk selecting for resistant weeds when they use the same herbicide repeatedly rather than applying a broad variety of products.
Maki and her advisor, Albert Adjesiwor, a UI Extension weed specialist based at the Kimberly Research and Extension Center, began a two-year project to study glyphosate alternatives in 2021. The Idaho Barley Commission and the Idaho Wheat Commission evenly split the cost of the $30,000 study. Many of the treatments, while being more costly than using glyphosate alone, controlled well above 90 percent of weeds.
“What excites me about this is having the ability to give options and having the data to back it up – showing treatments won’t affect yield and you will get good weed control,” Maki said. “If you have herbicide resistance, especially to glyphosate, that’s going to cost you more money down the line.”
Maki and Adjesiwor have presented their findings at several conferences, to the Idaho Wheat Commission and in newsletters sent to growers. They plan to publish an Extension bulletin on the research this fall. Glyphosate-resistant kochia, cheatgrass and Italian ryegrass have already emerged in Idaho. Palmer amaranth, a pigweed that’s resistant to glyphosate and other chemistries, was also recently discovered in the state, and Adjesiwor is keeping an eye out for resistant water hemp.
“When I got here, one of the things the small grain producers were interested in, they were seeing increasing cases of glyphosate resistance,” Adjesiwor said. “They are looking for alternative herbicides to apply for weed control before they plant their small grains so they don’t have to put so much pressure on glyphosate.”
Fall trials were hosted in Kimberly, Aberdeen and Rockland in the 2021 and 2022 growing seasons and produced consistent results at all locations and during both years. Like glyphosate, the herbicides the researchers tested don’t remain in the soil for long and are therefore less likely to harm crops planted shortly after application.
The study evaluated performance against six types of weeds – common lambsquarters, kochia, hairy nightshade, redroot pigweed, green foxtail and barnyard grass.
Adjesiwor considers any product or combination of products offering better than 90 percent weed control to be worth reporting to growers. By itself, Gramoxone herbicide, which contains the active ingredient paraquat, controlled 91 percent of weeds. Reviton herbicide, which contains the active ingredient tiafenacil, was also comparable to glyphosate when used alone. The combination of Gramoxone and Sharpen, which contains the active ingredient saflufenacil, was the top performer, controlling nearly 99 percent of weeds.
“Overall, it performed better than the highest dose of glyphosate,” Maki said of the combination.
Next, Adjesiwor plans to study the effectiveness of the herbicides at reduced dosages which he anticipates will enable growers to cut costs of applications without sacrificing much weed control.
Joan Campbell, a principal researcher in the Department of Plant Sciences, tested the same herbicides in Moscow prior to spring planting this season.
Albert Adjesiwor
UI Extension Weed Specialist
208-423-6616
aadjesiwor@uidaho.edu