Are glyphosate‐resistant weeds a threat to conservation agriculture? Evidence from tillage practices in soybeans

Conservation tillage in American soybean production has become increasingly common, improving soil health while reducing soil erosion and fuel consumption. This trend has been reinforced by the widespread adoption of glyphosate‐based weed control systems. Many weed species have since evolved to resist glyphosate, reducing its effectiveness. We provide evidence that the spread of glyphosate‐resistant weeds is responsible for significant reductions in the use of conservation tillage in soybean production. We estimate reduced‐form and structural probit models of tillage choice, using a large panel of field‐level soybean management decisions from across the United States spanning 1998–2016. We find that the first emergence of glyphosate‐resistant weed species has little initial effect on tillage practices, though by the time that eight glyphosate‐resistant weed species are identified, conservation tillage and no‐till use fall by 3.9 percentage points and 7.6 percentage points, respectively. We further find that when ten glyphosate‐resistant species are present, the predicted adoption rate of non‐glyphosate herbicides rises 50 percentage points, and that the availability of non‐glyphosate herbicides facilitates continued use of conservation tillage as glyphosate‐resistant weeds proliferate. Using a simple benefits transfer model, we conservatively estimate that between 2008 and 2016 farmers' tillage responses to the spread of glyphosate‐resistant weeds have caused water quality and climate damages via fuel emissions valued at nearly $245 million. This value does not account for climate damages due to carbon released during soil disruptions and is likely to grow as glyphosate resistance becomes more widespread and more farmers turn to tillage for supplemental weed control.