Consideration of Pyroxasulfone Herbicides for Suppression of Annual Ryegrass in Wheat
Travis Legleiter—Assistant Extension Professor, Weed Science
Italian or annual ryegrass (Lolium perenne L. ssp. multiflorum ) has been a major weed issue for Kentucky wheat growers for many years. The introduction of ALS-inhibiting herbicides that had selectivity on ryegrass brought about a great tool for con-trolling ryegrass in wheat post-emergence. The addition of pinoxaden (Axial XL) to the ryegrass post-emergence market only added to the arsenal of products for controlling this troublesome weed. Although, now we face many circumstances in which that arsenal has been almost completely depleted due to herbicide resistance. ALS-herbicide resistance is widespread across the state and at least one population of pinoxaden resistant ryegrass has been identified with multiple other suspected populations being identified. The depletion of the post-emergence herbicide options enforces the need for a new strategy for controlling ryegrass in wheat.
Those farmers who are facing ALS-resistant ryegrass and potentially pinoxaden-resistant ryegrass should focus on the use of pyroxasulfone based residual herbicides. These products provide preemergence control of ryegrass only and do not control emerged ryegrass so timing is critical. Another consideration of application timing is the risk of potential crop injury. The labeled timing of each product varies but ranges from 14 day preplant to early wheat post-emergence. Products currently available for use in wheat in Kentucky are Zidua, Fierce, and Anthem Flex. The labeled application timings for each product are outlined in Table 1.
As mentioned above there is a risk of injury to wheat with the pyroxasulfone products. The risk of injury depends on soil type, planting conditions, and environmental conditions. Fields with coarse soils and low organic matter are at greater risk of pyroxasulfone injury than those with medium and fine texture soils with higher organic matter. Wheat should be planted at least 1-inch-deep to avoid injury and poor planting conditions such as cloddy seed beds or unclosed furrows increase risk of injury due to direct exposure of seed to the herbicide. Heavy rainfall and saturated soils following pyroxasulfone application can concentrate the herbicide in the seed furrow and also cause significant injury. Avoidance of these environmental conditions can significantly reduce the risk of wheat injury.
The risk of pyroxasulfone injury to emerging wheat is greatest when applications are made preemergence and prior to wheat germination, thus the majority of Kentucky labels are restrict-ed to delayed preemergence applications. This timing is from when 80% of the wheat plants have at least a ½ inch long shoot until spiking. This timing may only span a couple of days and can be very difficult to time. The other fallacy with this timing is that often ryegrass is emerging with wheat and the pyroxasulfone application fails to control those already emerged or emerging ryegrass plants, so a postemergence herbicide must be included to control those emerged or emerging plants.
Most of the pyroxasulfone labels also allow for early postemergence timings that pose the least amount of risk for wheat injury. Although again any ryegrass that has emerged prior to this application will not be controlled by the pyroxasul-fone. Farmers may consider split applications in which they apply part of their ryegrass residual preemergence and follow that with the remainder of residual applied early postemer-gence prior to any ryegrass emergence. Refer to table 1 for maximum allowable cumulative rate of each product per year.
Currently only Fierce is allowed to be applied Preplant to wheat in Kentucky and requires at least 14 days between ap-plication and planting to reduce risk of Injury. Zidua and An-them Flex currently are restricted to delayed Pre and Early POST applications in Kentucky, although supplemental labeling for Anthem Flex preemergence could potential be available for the 2018 wheat planting season.
While there is risk of crop injury with the pyroxasulfone prod-ucts the benefit of suppressing ryegrass emergence can out-weigh these risk in many cases. This is especially true for those farmers who are dealing with ALS and pinoxaden-resistant ryegrass populations. In numerous studies conducted at the University of Kentucky pyroxasulfone has shown superb sup-pression of ryegrass when applied appropriately and prior to ryegrass germination.