Weed science odds and ends
"Control emerged summer annual weeds in fields not yet planted prior to planting," said Hager, particularly in fields containing emerged waterhemp that will be planted to soybean. If the field will be planted with a non-GMO or glyphosate-resistant soybean variety and the emerged waterhemp is resistant to glyphosate, PPO, and ALS inhibitors, there are no herbicides that can control the waterhemp after the soybean emerges.
Waterhemp began to emerge at Urbana during the second week of March, and there have been reports of emerged waterhemp as tall as three to four inches across areas of central Illinois. Established weeds can be quite competitive with the emerging crop plants.
Horseweed (marestail) is growing rapidly in no-till fields that have not yet been treated with a burndown herbicide. "Select the appropriate herbicide or herbicide combination and application rate to ensure good control of these plants," Hager said.
"If you suspect glyphosate-resistant horseweed, consider adjusting your burndown herbicide program before making an application to increase the likelihood of achieving complete control before planting," he added. Whether or not a horseweed population is resistant to glyphosate, it is advisable to control horseweed before the plants are more than four to six inches high.