North Carolina leads the way for Upper Southeast wheat acreage with another big and promising crop in the ground this year. But increasing problems with resistance to ryegrass-controlling herbicides is a big threat to the crop.

North Carolina State University Weed Scientist Wes Everman and a team of graduate students conducted a statewide study last year to measure the types and prevalence of weed resistance in Italian ryegrass in wheat.

As expected, there was widespread resistance to Hoelon, a long-time standard post-emergence treatment for ryegrass. Hoelon is an ACCase inhibitors, a family of herbicides that is widely used under other trade names in a number of crops grown in the Southeast.

Axial is a different type of ACC-inhibitor, but in the statewide study, the North Carolina State researchers found cross resistance to Hoelon and Axial.

“In some areas of the state, we don’t have a postemergence option to control Italian ryegrass in wheat,” Everman stresses. “We have two families of products:  ACCase inhibitors and ALS-inhibitors that control ryegrass, but are dogged by resistance issues."

Hoelon and Axial are the most commonly used ACC-inhibitors, and Osprey and Powerflex are the most commonly used ALS-inhibitors. Everman says to his knowledge there are no new postemergence herbicides on the horizon for controlling Italian ryegrass in wheat.

ACCase inhibitors, as the name implies, restrict the development and movement of ACCase enzymes. Each of these enzymes is fundamentally different, providing an easy-to-use pathway to block. However these biological differences are a classic case of being part of the cure and the problem. They provide a pathway to kill ryegrass but also provide a comparably easy mechanism for resistance to develop.

ALS-inhibiting herbicides inhibit acetolactate synthase (ALS), the enzyme common to the biosynthesis of branch-chain amino acids. The mode of action and long residual activity of this family of herbicides is believed to be the primary reasons for widespread weed resistance across a number of crops.

Right now, the most dependable products, or those with the least resistance problems, are preemergence herbicides.

Valor and Fierce, Everman says, are still good preemergence products. Axiom and Prowl H20 provide suppression, as will Zidua, if it gets a label, as is expected. Once these preemergence and spike materials break down, there are no truly reliable options for postemergence control, the researcher contends.

Weeds found resistant to all postemergence herbicides

The statewide wheat weed resistance study, he stresses, provided some expected and some highly unexpected results. “To find that only two of the sites we tested across the state did not show resistance to Hoelon was not surprising,” he says.

On the other hand, heavy resistance problems in the southeast part of the state to Osprey and Powerflex were surprising. “We didn’t find as widespread a problem with resistance to ALS-inhibitors (Powerflex and Osprey) in wheat in the Piedmont or the northeast part of the state, but resistance to all the available postemergence herbicides did show up statewide,” Everman says.

“The really scary thing is that we found five populations of weeds in wheat that are resistant to every one of the postemergence herbicides we have to control Italian ryegrass.  In a situation like that, once ryegrass comes up, you’re stuck with it,” he stresses.

These findings led the North Carolina State University researchers to begin looking for alternative ways to manage herbicide-resistant ryegrass. By chance, Everman was putting in a test for the Northeast Ag Expo and had a choice of no-till or conventional tillage plots to plant his tests. 

“For no good reason, I chose both, and that really opened our eyes later to some opportunities to use cultural practices to help manage post-emergence Italian ryegrass in wheat,” he says.

The North Carolina State researchers planted wheat in both no-till and conventional sites in conventional 7.5 inch rows and in narrow (3.75 inches) rows. It was no surprise, he says, that the narrow rows produced the higher wheat yields.

“We had very little ryegrass on these test sites—not even enough to rate the plots, so these became yield studies. They repeated the tests on grower fields with known high populations of ryegrass, but again had little weed pressure.

“We can’t make recommendations on one-year data under such low weed pressure, but our observations are clear. On the narrow row fields we definitely saw less ryegrass,” he stresses.

“Last year we had cool, wet weather, and I think that held ryegrass back. It seems like we are seeing later and later flushes of ryegrass, and this is where the narrow rows may pay off the most,” Everman contends.

The North Carolina State researchers used different combinations of all the pre-plant and spike stage herbicide families used to control ryegrass in wheat in both the no-till and conventional tillage plots.

"Again, these results are sufficient to make sweeping recommendations on—these were primarily demonstration plots, but our observations were clear. Across the different pre-plant and spike stage applications, the no-till wheat was covered up with ryegrass. The tilled plots, with the same herbicide treatments had some ryegrass, but noticeably less ryegrass than the no-till plots," he said.

“Across the board for all the treatments, we got a 30 bushel per-acre yield increase on the tilled versus the no-till plots. This test was on a poorly drained soils, so I believe some of the yield advantage from tillage is to be expected, but I also believe some of the yield advantage came from less competition with ryegrass in the tilled fields,” Everman said.