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

Abstract

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.