Because the PPO inhibitors are among the few postemergence herbicide options for resistant waterhemp in soybeans, you should avoid using them in corn, says Mark Bernard, an independent agronomist from New Richmond, Minn. “If you use a PPO in beans, then stay away from herbicides such as Sharpen (SOA 14) or Verdict (SOA 14, 15) the next year in corn.”

Fortunately, corn offers more opportunities to switch up herbicide families and still control waterhemp and common and giant ragweed — the main target weeds for Bernard’s southern Minnesota growers. Depending on your weed spectrum, he says, you could shift to products such as Hornet (SOA 2, 4), Harness (SOA 15), Sure Start/Triple Flex (SOA 2, 4, 15), or group 27 products, such as Callisto, Laudis or Impact.

Yet, even if you rotate crops annually, “it can be tough to change sites of action each year,” says Kevin Bradley, University of Missouri weed scientist. For example, common soybean residual herbicides such as Dual and Warrant have the same site of action — group 15 — as widely used corn herbicides, including TripleFLEX, Verdict and others.

But “If you use multiple sites of action each year and rotate crops, you are more likely to get adequate diversity,” Legleiter says.

The Britts put down Guardsman Max premix (SOA 5, 15) before planting corn, followed by an early postemergent application of glyphosate plus more atrazine (SOA 9, 5). They came back with a second postemergent pass of Armezon (SOA 27) to extend waterhemp control. Counting the burndown (SOA 9, 4), their corn weed program supplies five sites of action, four of which are effective on waterhemp.