This year's severe drought increases the likelihood of herbicide carryover on fall-seeded crops, states the University of Missouri agronomist Kevin Bradley. "While it is difficult to predict exactly when or where herbicide carryover injury might occur, there are several factors that will influence the likelihood of herbicide carryover occurring to these crops. These include the type of herbicide applied, the rate of herbicide applied, the time during the season that the herbicide was applied, the soil pH, and most importantly the amount of rainfall received since the time of the initial herbicide application," he says.
In fields where corn was the previous crop, he cites triazine herbicides as the greatest concern in terms of herbicide carryover injury to wheat. "These include atrazine or any of the many prepackaged herbicide mixtures that contain atrazine as one of the active ingredients (Bicep II Magnum, Degree Xtra, Guardsman Max, Harness Extra, Lumax, Lexar, etc.). It is important to note that atrazine or any of the atrazine-containing products DO NOT allow wheat or forage grasses to be planted in the fall following a spring application, although in some years and in some areas of Missouri certain farmers choose to plant wheat following their corn crop. With the extreme drought we have experienced this year, any wheat planted after a corn crop that has been treated with atrazine this season will be at risk for atrazine carryover injury.
In fields where soybeans were the previous crop, the likelihood of carryover injury to wheat is lower, but still possible in a year with as little rainfall as we have experienced, he says. "There are generally fewer residual herbicides applied in soybean, but that trend is changing. Also, as a result of our glyphosate-resistant waterhemp problem throughout the state, the herbicide fomesafen , which is the active ingredient in Flexstar, Flexstar GT, Rhythm, and Prefix, has now become a very common post-emergence herbicide of choice in soybean. Fomesafen-containing products have a four-month wheat replant interval and in areas that have received little to no rainfall following application, fomesafen carryover injury to wheat or other forage grasses can be a concern this year," he states.
The rate of herbicide applied and the timing of the herbicide application are other factors that influence the likelihood of herbicide carryover injury to wheat or other rotational crops, Bradley observes. "Simply put, the higher the rate of herbicide applied and the later the herbicide application was made, the greater the chance that some of the herbicide will remain to cause carryover injury to wheat. For example, if Flexstar or Flexstar GT applications were made later in the season to control glyphosate-resistant waterhemp in soybean, these sites are more at risk for herbicide carryover this fall," he says.
"In fields where there is a high degree of concern and/or uncertainty about herbicide carryover, one way to obtain more information is to conduct a soil bioassay. This is a simple test that can be done to ensure you don’t waste a lot of money by planting an entire field and then observe that the entire stand is injured as a result of herbicide carryover. To conduct a soil bioassay, gather several soil samples from across the field in question several weeks before you intend to plant your fall-seeded crop at that location, then take those soil samples and mix them together and place the soil in some kind of greenhouse flats or pots. Plant your wheat or forage grass seed into these pots and wait for the seedlings to germinate in order to observe any signs of herbicide carryover injury that may be present. In order to have a comparison, it will be important to follow this same procedure at the same time with soil from a location where you know there are no concerns with herbicide carryover," he states.
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