Now, Shaw said, nine weed species in the United States have been confirmed as glyphosate resistant. These include: Palmer amaranth, common waterhemp, common ragweed, giant ragweed, hairy fleabane, horseweed, Italian ryegrass, rigid ryegrass and Johnsongrass.

A concerted effort is underway to identify how widespread the problem has become and what changes weed resistance is forcing farmers to make. “We are concerned that many farmers are going back to more tillage and away from conservation tillage systems,” Shaw said.

Consequently, the Weed Science Society of America, along with other partners, is developing education initiatives to help growers learn how to manage resistant weed species. They have developed a list of best management practices.

Crop rotation includes rotating to other chemistries and selecting crops with different growing seasons—wheat or canola instead of cotton, for instance.

Cultural practices such as changing planting dates or converting acreage to haying, grazing or burning may break the weed cycle.