Ryegrass Control in Wheat Begins at Planting

Wheat Science Newsletter - August 27, 2019 Volume 23, Issue 4

Travis Legleiter—Extension Weed Control Specialist

Italian ryegrass (AKA: annual ryegrass) continues to be a major weed in Kentucky wheat acres, and has arguably become tougher to control over the last several years. This spring in particular we observed numerous wheat fields in western and central Kentucky that had ryegrass escapes late in the season and at harvest. Many producers have traditionally depended on post-emergence herbicides for control of ryegrass including several ALS-inhibitors and more recently Group 1 herbicides such as Axial XL and the recently released Axial Bold. While these post-emergence products have shown great efficacy against ryegrass, and still do in some cases, the development of resistance to these sites of action have eliminated their utility on many Kentucky wheat acres. These resistance events include the confirmation of pinoxaden (Axial XL) resistant ryegrass in the state of Kentucky. The increasing in-stances of resistance to post-emergence herbicides requires wheat growers to rethink their strategy for controlling ryegrass in wheat, primarily rethinking the timing of ryegrass control.

While ryegrass escapes become most blatantly obvious late in the season the optimal time for controlling ryegrass in wheat, regardless of resistance within the population, is prior to or immediately after planting. A number of herbicides have been labeled for use as residual herbicide for control of annual ryegrass in wheat. Along the same lines as using PRE herbicides in our corn or soybean acres, the use of pre-emergence herbicides in wheat brings a great amount of utility in not only providing effective weed control, but also allowing for the use of alternative herbicide sites of action within the growing season.

The majority of pre-emergence herbicides labeled for ryegrass control in wheat contain the active ingredient pyroxasulfone. These products provide pre-emergence 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 pre-plant to early wheat post-emergence. Products currently available for use in wheat in Kentucky are Zidua, Fierce, and Anthem Flex. The labeled application timings, use rates, and maximum cumulative rate per year for each product are outlined in Table 1.

As mentioned above there is a risk of injury to wheat with 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 in-crease 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 pre-emergence and prior to wheat germination. Currently, Anthem Flex is the only herbicide that is allowed to be applied pre-emergence from planting until wheat spiking. This application timing is al-lowed through a local needs 24c label for Kentucky wheat acres. The 24c label comes with several restrictions including allowance on only on medium and fines soils with 2.5% or greater organic matter. A full list of restrictions for application of Anthem Flex applied pre-emergence in order to reduce crop injury risk are listed in Table 2. The University of Kentucky has evaluated this timing of Anthem Flex within these restrictions and has observed great utility in this application timing with limited crop injury risk. Fierce is the only product allowed to be applied pre-plant to wheat in Kentucky and re-quires at least 14 days between application and planting to reduce risk of injury.

The timing of Delayed Pre is allow with both Zidua and An-them Flex. This timing is from when 80% of the wheat has an at least ½ inch long shoot until spiking. Allowing wheat to germinate and produce a ½ inch shoot reduced the risk of crop injury, although 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 post-emergence herbicide must be included to control those emerged or emerging plants.

The Anthem Flex and Zidua labels also allow for early post-emergence 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 pyroxasulfone. Farmers may consider split applications in which they apply part of their ryegrass residual pre-emergence and follow that with the remainder of residual applied early post-emergence prior to any ryegrass emergence. Refer to table 1 for maximum allowable cumulative rate of each product per year.

While there is risk of crop injury with the pyroxasulfone products, the benefit of suppressing ryegrass emergence outweighs 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 suppression of ryegrass when applied appropriately and prior to ryegrass germination. Data combined across trials from the previous two growing seasons has shown that pyroxasulfone products applied 14DPP, PRE, or Delayed Pre provide 78% or greater control of ryegrass during spring evaluations. Early post applied products showed greater variability in control, but still provided 65% or greater control in the spring. As with all herbicide residual products, these herbicides are not intended to provide season long control, but rather are another tool in controlling difficult and resistant weeds. The use of pre-emergence herbicides also helps preserve our currently limited post-emergence herbicide options by reducing the selection pressure on the post-emergence applications.