Feral Rye Control with Herbicide-Tolerant Wheat

Clearfield Plus and CoAXium production systems are effective at controlling feral rye when application timing and herbicide rate are carefully considered. Although Beyond herbicide (used with Clearfield systems) only officially offers feral rye suppression, effective control can be achieved (Figure 1). Besides rate and timing, the addition of adjuvants is also important in both Clearfield Plus and CoAXium systems. In this trial, fall applications of Aggressor were tank-mixed with a nonionic surfactant (NIS) at 0.25% vol/vol in the fall or a methylated seed oil (MSO) in the spring. All Beyond applications were tank-mixed with NIS at 0.25% vol/vol plus urea ammonium nitrate (UAN) at 5% vol/vol.

 

In the 2020-21 trial, the factor that was most critical in the success of all herbicide treatments was application timing. End-of-season visual feral rye control increased at least 20% for both Aggressor and Beyond treatments when applications were made in the spring compared to fall. Spring applications were more effective as it is common for more than one flush of feral rye to germinate in a wheat growing season. Although an application after the final flush results in increased end-of-season control, wheat still competes with rye plants before the spring treatment. This can be an issue depending on feral rye density and how early the spring treatment is applied. The best option, if economically viable, is to make a fall and spring application of Aggressor or Beyond.

 

 

Figure 1. Feral rye control with CoAXium and Clearfield production systems.

 

Abbreviations: F, fall; S, spring; Agg, Aggressor herbicide, Bey, Beyond herbicide; fb, followed by; oz, ounces.

 

Metribuzin Use in Oklahoma Winter Wheat

Metribuzin, a photosystem II inhibiting herbicide, effectively controls many troublesome weed species in Oklahoma wheat (Figure 2). However, it is sprayed on few wheat acres due to crop tolerance concerns. Historically, winter wheat varieties have been linked to crop tolerance, but information on currently used varieties is limited.

 

Figure 2. Bromus tectorum control with metribuzin at Perkins. A) nontreated control B) metribuzin plus pyroxasulfone applied delayed preemergence.

 

Winter wheat tolerance to metribuzin tank-mixed with pyroxasulfone was evaluated at three locations in Oklahoma during the 2020-21 growing season (Fort Cobb, Goodwell and Perkins). Three factors were evaluated at each site: winter wheat variety, herbicide rate and application timing. Winter wheat varieties included Fusion AX, Showdown, Strad CL+ and Uncharted. Herbicide treatments consisted of two herbicide mixtures (2 or 4 ounces per acre of metribuzin plus 3.25 fl ounces per acre of pyroxasulfone) and a nontreated control. Herbicide treatments were applied preemergence (PRE) or delayed PRE (wheat spike).

 

The effect of variety was variable and often inconsistent; however, application timing and metribuzin rate consistently affected wheat response to metribuzin across locations. At Fort Cobb, applying metribuzin delayed PRE at the low rate of 2 oz/A resulted in similar biomass compared to the low and high rate applied PRE (Figure 3). However, at the high rate (4 oz/A) of metribuzin applied DPRE, there was a decrease in end-of-season biomass relative to all other treatments. Although it is typical to observe the highest levels of injury following earlier applications, it should be noted that temperatures before and after application and rainfall following application are important and will influence crop response. At Fort Cobb, adequate rains following both application timings helped to incorporate metribuzin herbicide into the wheat root zone, resulting in decreased wheat biomass. Cold temperatures following application also contributed to wheat injury as growing conditions were less than ideal for wheat plants to metabolize metribuzin. Immediate and plentiful rains following the delayed PRE timing likely caused the severe wheat biomass reduction observed at the high rate of metribuzin.

 

Figure 3. End-of-season wheat biomass following metribuzin applied preemergence (PRE) and  delayed preemergence (DPRE) at 2 or 4 ounces per acre. Abbreviations: NT, nontreated.