Distribution of Herbicide Resistant Shattercane and Johnsongrass Populations Across Nebraska and Kansas.

Traditional breeding technology is currently being used to develop grain sorghum germplasm that will be tolerant to acetolactate synthase (ALS)-inhibiting herbicides. Over-reliance on ALS inhibitors for weed control during the 1980’s and 1990’s resulted in the evolution of resistance to ALS-inhibitors in shattercane, a weedy relative of sorghum prevalent in Nebraska.  The objective of this study was to assess the baseline presence of ALS resistance in populations of shattercane and johnsongrass, another weedy sorghum relative. The populations were obtained from numerous locations, including where known resistance occurred in the past in Nebraska. In the fall of 2013, seeds from 190 shattercane and 58 johnsongrass populations were collected from northern Kansas, western Missouri, and southern Nebraska. In the summer of 2014, a field experiment was conducted near Mead, NE to evaluate the status of herbicide resistance in the aforementioned populations. Fifty shattercane and sixty johnsongrass seeds from each population were sown at 2 cm depth in 3 m rows using a cone planter. Treatments consisted of four herbicides applied at their labeled rate: nicosulfuron, imazethapyr, glyphosate and clethodim. Herbicide treatments were applied when most shattercane, johnsongrass and sorghum plants were at the five- to seven-leaf stage. The study for each species and herbicide was conducted separately (total of 8 studies). ALS-tolerant and susceptible sorghum were included in each study as controls. The experiment was arranged in a randomized complete block design with 4 replications of each population. Plant mortality (%) and visual injury data were collected at 21 days after treatment. For shattercane, 7 and 10 populations showed signs of resistance to nicosulfuron and imazethapyr, respectively, with 5 populations showing signs of cross-resistance to ALS inhibitors (resistance to both nicosulfuron and imazethapyr). For johnsongrass, 3 and 11 populations showed signs of resistance to nicosulfuron and imazethapyr, respectively, with 2 populations showing signs of cross-resistance to ALS inhibitors. The level of resistance highly varied across populations. Glyphosate and clethodim controlled all shattercane and johnsongrass populations screened in this study. Most of the ALS-resistant populations were collected from south-central and eastern Nebraska and north-east Kansas. Even though ALS inhibitors have not been widely used to control shattercane and johnsongrass since the commercialization of glyphosate-tolerant crops, the resistance trait is still present in locations where resistance was reported in the 1990’s, indicating the lack of a strong fitness cost associated with ALS-resistance in weedy sorghum populations. The results of this research will be helpful to indicate regions where shattercane and johnsongrass should be carefully managed prior to and during the introduction of ALS-tolerant sorghum.