Fifteen years ago, University of Guelph’s Dr. Clarence Swanton helped revolutionize weed control with his pioneering research on the critical weed-free period for soybeans and corn.
Essentially, Swanton helped usher in the thinking that weeds that emerge with or shortly after the crop cause irreversible yield loss. He defined the critical weed-free period as 1st to 3rd trifoliate (V2-V3) for soybeans and 3 to 8-leaf in corn.
Fast forward to 2016 and Dr. Swanton is shaking up the weed control world once again with more revolutionary thinking. In this interview with Real Agriculture resident agronomist Peter Johnson, Swanton comments on new research about to be released that indicates soybean plants can actually sense the presence of weeds before the plant emerges.
“We’re just about to release some work where we show that as a soybean plant is coming up through the ground… it begins to know that there’s green tissue on the top of that surface. It picks up the signal through the pores of the soil and actually changes before it even emerges out of the ground,” explains Swanton. He adds that “the plants are actually communicating, they are reading their environment and they are actually changing their physiology and their morphology.”
Swanton says the original critical weed-free period was based on when researchers could detect yield impact. But this research focuses on the cell level of the plant and what’s happening before the plant emerges. He doesn’t yet know whether early cell level changes will impact yield or whether the plant could repair or compensate for injury or loss.
“Right now our data says very early on, prior to the critical period, we’re starting to detect physiological changes and so that argues well for the fact that you should start clean and stay clean.”
Swanton says the research may also have significant implications for glyphosate-tolerant crops. “Glyphosate takes ten days to brown out a weed and even though you spray today the impact of that weed is still being felt though signaling on the actual crop plant itself… so the longer it takes to die the more impact it has on the crop seedling.”
Swanton agrees with Johnson’s interpretation of the research and what it means for growers: “What it says is we have to have everything clean before the soybean emerges — those weeds need to be dead.”