![]() While herbicide resistance might be to blame-the issue has become a major problem in agricultural weeds in recent decades-it's not the only reason glyphosate may have loosened its grip. The dataset for glyphosate can only show patterns, not explanations. Because if we didn't, we had a pretty good idea of where this was going to end up," Hager said. "Having already seen loss of control with ALS- and PPO-inhibitors, we eventually reached the point where we felt it necessary to come out with some very specific recommendations for glyphosate. Far from common practice at the time, the recommendation was largely ignored. His 2008 recommendation aimed at avoiding glyphosate resistance included guidance to use a pre-emergence herbicide at the full rate. colleagues, he has been cautioning against reliance on any single chemistry for 15 years. "The most we saw for any weed species was a 4.4% loss of control per decade, compared to 31.6% loss for glyphosate alone." "Adding a pre-emergence herbicide effective against the target weed species significantly improved control and reduced variability of glyphosate over time," Landau said. In the end, the dataset represented trial data from 11 institutions.Īfter documenting patterns of glyphosate control and variability over time, Landau re-ran the analysis for plots in which a pre-emergence herbicide had been applied before glyphosate. He also narrowed the target weeds to seven major players: annual and giant ragweed, horseweed, lambsquarter, Palmer amaranth, velvetleaf, and waterhemp. In cooperation with 24 institutions throughout North America, Landau compiled a massive database representing nearly 8 million observations from 1996 to 2021.įor the current study, Landau winnowed the data down to fields that tested glyphosate annually, alone or in combination with a pre-emergence herbicide. When they decided to look at the history of the country's most widely used herbicide, the team knew it would be more powerful to access data beyond Illinois. ![]() herbicide evaluation trials to look at the effect of climate change on weed control in Illinois corn and soybean fields. Most land-grant universities have multiple herbicide evaluation trials running statewide each year, with some continuously operating since the 1970s.Ī couple of years ago, Landau, Hager, and Williams mined historical data from U. These carefully managed trials test new and existing herbicides against numerous common and troublesome weed species. These patterns were derived from annual herbicide evaluation trials conducted at land-grant universities, usually in conjunction with their respective Extension services. But over time, acceptable weed control became rarer, often deteriorating below 50%, 30%, and worse." "For example, glyphosate provided nearly 100% control of a given species in most plots in the mid-1990s. "When glyphosate-tolerant crops were first adopted, weed control was high in every environment however, year after year glyphosate performance became less consistent," said co-author Marty Williams, an ecologist with the USDA-ARS and affiliate professor of crop sciences. In addition to loss of control, glyphosate efficacy became more variable over time. "Nature did exactly what we were trying to help people avoid: it adapted," said co-author Aaron Hager, professor and faculty Extension specialist in the Department of Crop Sciences and Illinois Extension, part of the College of Agricultural, Consumer and Environmental Sciences (ACES) at U. Within a decade, weeds were up to 31.6% less responsive to glyphosate, with further linear declines as time went on. "Our analysis represents one of the largest cumulative measures of how weed communities have adapted to the simplified weed management tactics adopted at an unprecedented scale throughout North America," said Chris Landau, postdoctoral researcher for USDA-ARS and first author on the paper.Īlthough glyphosate provided superior weed control in the early years, most of the weeds in the dataset showed signs of adaptation to the chemical in just two to three years. and Canada, the researchers show a significant and rapid decline in glyphosate control for all seven major weed species they examined. A new PNAS Nexus study led by scientists from the USDA Agricultural Research Service (USDA-ARS) and the University of Illinois Urbana-Champaign takes a retrospective look at glyphosate efficacy after the engineered crops were commercialized.Īmassing data from annual herbicide evaluation trials at land-grant universities across the U.S.
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