Since 2002, six weeds (horseweed, common ragweed, giant ragweed, Palmer pigweed, Johnsongrass, and Italian ryegrass) have been confirmed to be resistant to glyphosate in Arkansas. Of these, horseweed and pigweed have by far had the greatest impact on farming practices. In addition to glyphosate resistance, pigweed populations also exist that are resistant to the ALS and dinitroanaline families of chemistry. Ryegrass populations exist that not only tolerate glyphosate but also ALS and certain ACCase chemistries, causing difficulties in not only spring burndown, but also wheat ryegrass control. Glyphosate resistant crops were grown on approximately 100 percent of the corn, cotton, and soybean acres in Arkansas by the year 2008. In 2010, it was estimated that most all cotton acres and at least 50% of the soybean acres had become infested with glyphosate resistant Palmer amaranth. Due to resistance management practices that were implemented statewide from 2009-2012, a true crisis in weed control has been averted in the state.  Although pigweed is still both the most common and troublesome weed in both soybean and cotton, it is relatively under control. Many acres of cotton still require hand weeding for complete control, and farmers do still struggle in soybean with added input costs and time management. Pigweed control options and current management strategies will be discussed. In addition to pigweed in row crops, a similar resistance situation is evolving in rice with barnyardgrass. This speaks to an overall greater concern with the lack of discovery of new herbicide modes of action and the bottlenecks that it creates for certain cropping systems, where only very specific herbicide recommendations will actually control weeds, which can inevitably lead to more resistance.

Weed resistance in Arkansas: One crisis averted another begins?

246th National Meeting (2013)

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