Distribution of Herbicide-Resistant Shattercane and Johnsongrass Populations in Sorghum Production Areas of Nebraska and Northern Kansas

Overreliance on acetolactate synthase (ALS)-inhibiting herbicides for weed control during the 1990s resulted in selection of ALS-resistant shattercane [Sorghum bicolor (L.) Moench ssp. drummondii (Nees ex Steud.) de Wet ex Davidse] biotypes in Nebraska. The objective of this study was to assess the baseline presence of ALS-resistance in 190 shattercane and 59 johnsongrass [Sorghum halepense (L.) Pers.] populations collected across northern Kansas, northwestern Missouri, and southern Nebraska in 2013. In 2014, a preliminary field experiment was conducted to evaluate the presence of herbicide resistance in the aforementioned populations. Treatments consisted of four herbicides (clethodim {2-[1-[[(E)-3-chloroprop-2-enoxy]amino]propylidene]-5-(2-ethylsulfanylpropyl)cyclohexane-1,3-dione}, glyphosate [N-(phosphonomethyl) glycine], imazethapyr [5-ethyl-2-(4-methyl-5-oxo-4-propan-2-yl-1H-imidazol-2-yl)pyridine-3-carboxylic acid], and nicosulfuron {2-[(4,6-dimethoxypyrimidin-2-yl)carbamoylsulfamoyl]-N,N-dimethylpyridine-3-carboxamide}) applied at labeled rates. Clethodim and glyphosate controlled all shattercane and johnsongrass populations evaluated. Putative imazethapyr and nicosulfuron (ALS-inhibiting herbicides) resistant populations were further exposed to a dose-response study under greenhouse conditions. Five shattercane and five johnsongrass populations were confirmed resistant to imazethapyr. Four shattercane and three johnsongrass populations were confirmed resistant to nicosulfuron. All ALS-resistant shattercane and johnsongrass populations were collected in Nebraska except for one johnsongrass population, resistant to nicosulfuron, that was collected in Kansas. Acetolactate synthase-resistance persists, even though ALS-inhibitors have not been widely used to control shattercane and johnsongrass for more than 15 yr, indicating the lack of a strong fitness cost associated with ALS-resistance. Therefore, shattercane and johnsongrass should be properly managed before and during the commercialization of ALS-tolerant grain sorghum [Sorghum bicolor (L.) Moench ssp. bicolor] (expected in 2017), especially in regions where ALS-resistance has been confirmed.