Glyphosphate Loses Status as Premier Weed Killer

For decades, corn and soy farmers have heavily relied on one herbicide: glyphosate. Crops bred to resist glyphosate have been extremely successful, with over 90% of corn and soy hectares planted with glyphosate-resistant varieties by 2014. But as Christopher Landau and colleagues document, the chemical was not quite the "silver bullet" it was promised to be. With an entire industry using the same chemical—the US and Canada alone apply more than 130 million kg annually—evolutionary selection pressures on weed plants have been intense. Since 1996, there have been 354 confirmed cases of glyphosate resistance in 57 weed species around the world. The authors compiled herbicide evaluation trials from 24 institutions across the US and Canada from 1996 to 2021. The analysis was restricted to the seven species that had more than 50 observations from three or more locations: Abutilon theophrasti Medik. (Velvetleaf), Amaranthus palmeri (Palmer amaranth), Amaranthus tuberculatus (waterhemp), Ambrosia artemisiifolia L. (common ragweed), Ambrosia trifida L. (giant ragweed), Chenopodium album (common lambsquarters), and Erigeron canadensis (horseweed). Over time, weed control declined and variability of control increased for glyphosate used after crops emerged from the soil. However, fields treated with both post emergence glyphosate and an effective pre-emergence herbicide applied before crops sprouted did not show control declines or variability increases. According to the authors, the result highlights the need for diversity in weed management programs to provide high and consistent weed control.