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Wild Oat (Avena Fatua L.) Resistance to Diclofop in the Red River Valley

J.B. Frie, R.K. Zollinger, F.A. Manthey, and B.R. Durgan
1993 NCWSS Proceedings

Abstract. Diclofop provides selective, postemergence control of wild oat and other grasses in small grain crops. Recently, isolated sites of wild oat resistant to diclofop have been identified in the Red River Valley (eastern North Dakota and western Minnesota). The objective of this research was to determine the magnitude of diclofop resistant wild oat in the Red River Valley. The research was a cooperative effort involving North Dakota State University and the University of Minnesota.

Possible sites for wild oat seed collection were determined by a survey sent to growers, a random sampling of wild oat across the Valley, and cooperation with local crop consultants and crop production specialists. Twenty individual wild oat plants were sampled from 99 locations throughout the Valley. Most wild oat seed were dormant which resulted in a low percentage germinating under natural conditions. To break dormancy, seeds were subjected to a treatment of cold temperatures and piercing. Seeds from 11 locations were germinated in petri dishes, planted into susceptible and resistant wild oat biotypes were also planted. Wild oat plants in the 3-leaf stage were treated with diclofop at 454 g/ha. Plants were evaluated 21 days after treatment.

Resistance to diclofop ranged from 0 to 100 percent from all plants tested. Plants from locations that had known to be exposed to diclofop expressed high susceptibility to the herbicide. Plants from nine locations with histories of high diclofop use expressed varying degrees of resistance to diclofop. However, at other locations, 40 to 100% of wild oat plants were resistant to diclofop. Plants from only one location expressed 100% susceptibility to diclofop.

Most wild oat plants that expressed resistance to diclofop were from fields with a small grain and sugar beet rotation. Diclofop is used extensively in the Red River Valley for wild oat control in small grains. Sethoxydim is the only postemergence herbicide labeled for grass control in sugar beet. Both diclofop and sethoxydim are ACCase inhibiting herbicides. Repeated use of herbicides with the same mechanism of action increases the risk for resistance to develop. The Red River Valley of North Dakota and Minnesota is at high risk for the development of wild oat resistance to diclofop and sethoxydim.