Combines are a notoriously effective method of making a bad weed problem even worse, says Iowa State University (ISU) Extension weed specialist Bob Hartzler.

“While a small amount of seed may leave the field with grain, most weed seed that enter the combine are either spread throughout the field or carried to other crops,” he notes.

That means a small pocket of herbicide resistant waterhemp or other weeds can grow wider every year as the weed seed is broadcast out the back of the combine.

One way to limit this constant weed creep is through harvest weed seed control (HWSC), a technique pioneered in Australia where farmers battle widespread herbicide-resistant annual ryegrass populations.

Michael Walsh, weed research director at the University of Sydney, says...