You can’t walk every row of every field looking for signs of glyphosate-resistant weeds.

Extension weed scientist Jeff Stachler suggests focusing first on parts of the field that have high weed populations or stressful growing conditions. Stressful environments, like salt pockets, low spots that flood or droughty hilltops, intensify selection pressure on weeds, thereby increasing genetic diversity. “Areas of the field that allow for greater genetic diversity of weeds are where you’re likely to see resistance first.”

Fence lines and weedy field borders are another place where problem weeds like waterhemp thrive, says Minnesota crop consultant Darrol Ike, AgroInfoMN, Delano. Often, these areas don’t get a full dose of herbicide, either, further intensifying selection pressure. As you’re scouting, Ike adds, be alert to the stage of weed growth so you know how many days you have to eliminate weeds before they flower.