Waterhemp and Palmer amaranth: Concepts for control
What will your weed control program be for 2013 soybeans? My Dad would say, “You and a weed hook.” However, today there are many alternatives that can be used before manual labor, but for some fields, a crew with hoes and weed hooks may be necessary, where weeds have developed resistance to many herbicides.
As you attend agronomy meetings this winter, weed specialists will likely have the attention of everyone in the room when they suggest alternatives for controlling waterhemp and Palmer amaranth. Their ability to escape an herbicide shower and reproduce their resistant genes has been a vexing problem for either you or at least someone you know well.
Iowa State University weed specialist Bob Hartzler says late in 2012 farmers responded to a survey at his agronomy meetings about resistance to 5 groups of common herbicides. He says, “In the survey, 60 percent of farmers reported that Group 2 (ALS inhibitors, such as Classic and Pursuit) resistant waterhemp was not present in fields they managed or they were unsure of its presence.
Industry representatives were somewhat more aware of Group 2 (ALS inhibitor) resistance, with 38 percent saying resistance was widespread and 42 percent reporting it was isolated in their territories. Both groups reported that glyphosate resistance was more common than Group 2 resistance.”
However, Hartzler says over 90 percent of waterhemp populations are resistant to ALS inhibitors, where possibly less than 20 percent of waterhemp populations are resistant to glyphosate, but that number is increasing rapidly. His point is that Group 2 herbicides were introduced in the 1980’s, became quite popular and by the mid-1990s Pursuit was used on more that 75 percent of soybeans in Iowa. That resulted in the widespread development of resistance to ALS inhibitors, and was one of the reasons that farmers adopted Roundup Ready beans in nearly wholesale fashion when they were introduced in the mid-1990s. Hartzler says most farmers have forgotten that waterhemp was becoming resistant to Pursuit, and “The loss of this Herbicide Group to manage a major weed problem shows us that similar things could happen with other herbicides if we do not manage weeds appropriately.”
Today’s growing problem is with the spread of Palmer amaranth into the Cornbelt. It has been a problem for farmers in southern states and seeds that are being germinated in Purdue laboratories have found the ability to survive 20lb ae/acre of glyphosate, or the equivalent of 7 gallons per acre of generic glyphosate.
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