Abstract
Italian ryegrass is consistently ranked as one of the most problematic weeds of winter wheat in the Southeastern United States. To determine the distribution of resistant Italian ryegrass biotypes, seed was collected from locations throughout North Carolina and screened with diclofop, pinoxaden, mesosulfuron, and pyroxsulam. Results identified evidence of resistance to diclofop at all locations sampled throughout the state. Resistance to mesosulfuron, pyroxsulam, and pinoxaden were confirmed in 11, 19, and five percent of sampled locations, respectively. Additionally, Italian ryegrass biotypes resistant to multiple and all herbicides tested were identified, eliminating POST herbicide application as an option for control. Adjusting tillage practices may be an option for sustainable weed management to maintain effective control and maximize crop yield. Companion studies were established in the Coastal Plain and Piedmont regions of North Carolina in 2013 and 2014 to evaluate the effect of tillage on Italian ryegrass efficacy with herbicides. Herbicide treatments consisted of pyroxasulfone PRE only, mesosulfuron, or pinoxaden and POST only applications of mesosulfuron plus pyroxasulfone or pinoxaden plus pyroxasulfone. Tillage treatments included no-till and conservation tillage. Treatments containing pinoxaden provided the greatest Italian ryegrass control, regardless of tillage system. The use of pyroxasulfone PRE controlled a higher percentage of Italian ryegrass in the Piedmont when compared to the Coastal Plain, which is believed to be due to multiple flushes during the growing season in the Coastal Plain. Herbicide treatment was still a significant factor in Italian ryegrass control, but Italian ryegrass seed head density was consistently lower in the no-till system. Tillage may be stimulating germination, allowing greater control with PRE herbicides. An integrated system of herbicides and tillage may allow for greater yield and reduce selection pressure on POST herbicides.
Highlights
Wheat (Triticum aestivum L.) is a staple crop grown worldwide (Gupta et al, 2008)
When a field was visited, samples were collected only when Italian ryegrass was visible above the crop canopy, meaning it is likely that these plants were escapes from previous herbicide applications
While pinoxaden has the same mode of action as diclofop, pinoxaden can control diclofop-resistant Italian ryegrass populations, and resistance has not been reported to be as common in similar surveys (Kuk and Burgos, 2007; Salas et al, 2013; Bararpour et al, 2018)
Summary
Wheat (Triticum aestivum L.) is a staple crop grown worldwide (Gupta et al, 2008). Growers around the globe produce over 26 billion bushels of wheat per year (United States Department of Agriculture, 2014). The United States produces ∼2 billion bushels per year, of which 44 million are produced in North Carolina (United States Department of Agriculture, 2014). Winter wheat is planted in mid- to late- November and harvested around June in North Carolina (Weisz, 2013). As in many agronomic cropping systems, an effective weed management program plays an important role in a successful wheat growing season. Weed interference can cause significant yield reductions in winter wheat (Liebl and Worsham, 1987; Wilson and Wright, 1990)
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