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ONE RUNNING joke Bill Johnson has been hearing is that waterhemp was really bad this year in the Eastern Corn Belt because it was jealous from all the attention being paid of late to its pigweed relative, Palmer amaranth.
But it’s no laughing matter — waterhemp continues to be one of the most troublesome weeds in soybeans for no-tillers, says the Purdue University weed scientist. Over the last decade, waterhemp has developed resistance to ALS-, PPO- and HPPD-inhibiting herbicides, glyphosate and 2,4-D — in some cases, developing multiple resistance.
A systems approach that includes multiple modes of action, layered residuals, intensive scouting, switching to narrow rows and using cover crops is necessary to keep waterhemp from spreading and hurting yields.
Few weeds have demonstrated the ability waterhemp shows to develop resistance. The simple reason is that it’s a dioecious species that carries seeds on the female plant, and the male and female plants have to cross pollinate to survive, Johnson says.
“They naturally have a tremendous amount of genetic mixing anyway, which increases probability of picking up a resistance gene from the male plant,” he explains. “You also have a weed that emerges over a long period of time, so it can receive continuous selection pressure almost all year.”
Waterhemp thrives in wet fields and emerges later in the field than other summer annual weeds. It can be hard to differentiate from other pigweed species prior to later stages of development. Waterhemp cotyledons are more egg-shaped, with longer…