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.