Ignorance might be bliss for some situations in life, but not when it comes to guaranteeing effective weed control for profitable corn yields, caution several Midwestern extension weed scientists.

“Good weed control starts by knowing what the predominant weed species problems are for each field,” says Aaron Hager, University of Illinois Extension weed scientist. “If you don’t know what your worst weeds are, then you won’t know how to choose the best product, or mix of products, to control them.”

Learning from last season’s mistakes can be the key to successful control for the future, particularly when it comes to managing herbicide resistance issues, says Mike Owen, Iowa State University Extension weed specialist. “For any hard-to-control weed, using a diversity of weed management tools is very important, but first you need to know the herbicide history of a field and whether any weed populations have developed herbicide resistance,” Owen says. “So scouting, record keeping and advanced planning are the places to start.”

Most major weed and herbicide resistance problems in corn vary by location — across the Corn Belt, within a state, and even within different fields on the same farm, says Lowell Sandell, extension weed scientist, University of Nebraska-Lincoln.

Sandell ranks velvetleaf, waterhemp, horseweed (marestail), green foxtail and giant ragweed as the five worst weeds (the most competitive weeds with corn, with the largest weed populations in fields) for the eastern part of Nebraska. For the western part of the state, he ranks common lambsquarters and kochia as the fourth- and fifth-worst weeds, ahead of foxtail and ragweed.

Neither Owen nor Hager rank velvetleaf among the three worst weeds for their states. For Iowa, Owen ranks giant foxtail first, common waterhemp second, giant ragweed third, horseweed (marestail) fourth and velvetleaf fifth. For Illinois, Hager ranks these weeds from first- to fifth-worst: waterhemp, giant ragweed, foxtail, morningglory, and fall panicum and other annual grasses.