In light of the likely sizable increase in wheat acres that will be planted this fall we should take some time to consider herbicide resistance management in this crop. Herbicide-resistance issues are not just a concern in soybeans and cotton but in wheat as well.
The most problematic weed in wheat is Italian ryegrass which, like Palmer amaranth, has a long track record of developing resistance to many herbicides. In fact, from a global perspective, the ryegrass complex of weeds has been the most successful species to get around almost all herbicide modes of action.
In Tennessee, by the year 2000, most of our ryegrass populations had developed good tolerance to the herbicide Hoelon. Most folks rotated from Hoelon to the herbicide Osprey about 7 years back to control the Hoelon-resistant ryegrass.
In more recent years, other herbicides in the Osprey class (ALS-inhibitors) like Powerflex and Finesse have been widely utilized to control ryegrass. These herbicides did a good job for several years but now we are starting to find ALS-resistant ryegrass populations showing up in Tennessee. The counties in West Tennessee along the Kentucky border have the most Hoelon/ALS-resistant ryegrass populations we have found. Many now have moved to Axial to control the Hoelon/ALS-resistant ryegrass populations.
So far, we have been fortunate and Axial still looks to be providing good control. This will likely not last much longer. Axial is in the same class of herbicides as Hoelon so the probability that Axial resistance will develop in the next several years is very high.
So what are some ryegrass management suggestions in wheat to delay Axial resistance from developing?
The suggestions we have been preaching for management of Palmer amaranth in soybean and corn apply to ryegrass in wheat as well. Primarily, they include starting clean and have the crop emerge weed free, second be sure to utilize herbicides with multiple modes of action on the weed in question and third be sure to use cultural practices to help control the weed.
Starting the crop clean is often easier said than done. Ryegrass started emerging some time back and herbicides like paraquat would be a good choice to control existing ryegrass.
In years past, I often recommended glyphosate for burndown of ryegrass but with the resistant biotype of ryegrass now present in the Mid-South, paraquat is the best choice.
Second, be sure to incorporate herbicides with different modes of action on ryegrass in your wheat management program. For the ryegrass that emerges with the wheat consider utilizing herbicides like metribuzin or Axiom (metribuzin plus flufenacet). Metribuzin will provide control of small ryegrass with a different mode of action than the Axial or ALS-inhibiting (Osprey/Powerflex/Finesse) herbicides.
Moreover, flufenacet in Axiom or even Prowl H2O will provide good residual control of ryegrass with a change-up mode of action than Axial or ALS-inhibitor herbicides. Then plan to use Axial or one of the ALS-inhibiting herbicides next spring to take out any newly emerged ryegrass. If you are counting with the plan outlined above we are utilizing three different herbicide modes of action on ryegrass -- not just one.
Finally, remember to utilize cultural practices. Tillage, where it can be used, is a good resistance management option prior to planting. Planting a good population of wheat seed per acre (ideally 1.2 to 1.4 million seeds per acre) will help wheat shade out weeds. Also relying on a sound fertility program and managing diseases as well as insects can help the wheat be more competitive with ryegrass.
Historically, wheat in the Mid-South is often managed in a low-input fashion. These low impute systems typically relied on one herbicide to control ryegrass. This has to change if we are going to raise wheat sustainably where ryegrass is an issue.
The good news is that we are seeing more folks put a lot more management into their wheat crop. This increased management ranges from improved disease, insect and weed management. This is a good change and should be even more adapted with the good wheat commodity price of today.