For the 2022 growing season, Kentucky bluegrass (KBG) seed growers in Oregon will have access to sulfosulfuron (Outrider, from Valent USA) through a recently approved 24(c) Special Local Need registration, EPA SLN No. OR-220002. This label allows use in seedling and established Kentucky bluegrass stands (in Oregon only), for control or suppression of roughstalk bluegrass and other grass weeds. Outrider is an ALS-inhibitor herbicide (Group 2), that has potential to control a number of key weeds that were historically controlled with now-unavailable herbicide primisulfuron (Beacon).
Development efforts focused primarily on control of roughstalk bluegrass in fall-planted seedling KBG, a problem that is largely unique to Central Oregon production. For this use, a fall application of 0.5oz/ac Outrider in the fall when KBG has at least 2-3 tillers is probably the most conservative option in terms of balancing crop safety and weed control. In heavily infested fields, a split application of 0.38 oz/ac both fall and spring is probably a better salvage option. This gives better control of dense roughstalk bluegrass populations, but at the cost of some crop injury and yield reduction. The amount of yield loss observed in trials (10 to 20% over 3 years of trials) is still less than the losses expected from cleaning roughstalk bluegrass out of contaminated seed lots in most cases.
Photo 1. Roughstalk bluegrass control in seedling Kentucky bluegrass stand. Nontreated check on left. Plot to right treated with Outrider at 0.38 oz/ac in fall followed by 0.38 oz/ac in spring. Both applications included 1%v/v MSO + 1.5 lb/ac AMS. Roughstalk bluegrass visible as reddish/purplish heads within green Kentucky bluegrass crop. Crop injury from split Outrider application visible as slight stunting and growth stage delay in treated plot relative to check.
In spring planted stands of seedling KBG, Outrider may offer control, or at least suppression, of warm season annual grasses such as witchgrass and green fescue. Here, applications of 0.38 to 0.5 oz/ac Outrider made after KBG has at least 3 tillers has shown excellent crop safety. My initial recommendation would be to make applications once the first flush of target warm-season grasses reaches 2-4 leaves. Unfortunately, trials to date have not provided a firm picture of Outrider efficacy on these species, so I would recommend trying it on a limited area for the first application on your targeted weeds in order to better understand efficacy on these weeds. As the Federal Insecticide, Fungicide, and Rodenticide Act generally allows use of pesticides on a registered site for weeds not listed on the label, this use would be a legal, registered commercial use of Outrider.
In established KBG, Outrider can be substituted for Beacon in herbicide programs for control or suppression of downy brome (cheatgrass). Such a program should start with a pre-emergence herbicide application (e.g. pendimethalin or a tank mix of pendimethalin and dimethenamid) immediately prior to watering back to initiate fall regrowth.
When the first cheatgrass has emerged and has 1-2 leaves, apply Outrider at 0.67-0.76 oz/ac in combination with appropriate surfactants, and a tank-mix partner herbicide such as oxyfluorfen. Timing is critical for best control: if cheatgrass has more than 2 leaves, efficacy will be greatly reduced. Similarly, pre-emergence applications are most likely to result in suppression, not control. Finally, if rotation considerations allow, a follow-up application of terbacil at the end of the fall growing season is recommended for heavy cheatgrass populations.
Photo 2. Downy brome control in 2nd year irrigated Kentucky bluegrass. Herbicide program in white outlined plot: 48 oz/ac Prowl H20 immediately prior to watering back September 1st, followed by 0.76 oz/ac Outrider + 20 oz/ac Goal 2XL + 1% v/v MSO + 1.5 lb/ac AMS at 2-3 leaf cheatgrass in early October. Plots pictured in early May following fall application. Uncontrolled, heading cheatgrass plants evident in nontreated plot to left, and in large patch immediately behind trial.
If you are a KBG grower in the PNW and used Beacon to control weeds in a way that hasn’t been described yet – either for different weed species or at different application timings – please tell us the specifics in the comments. As development research with Outrider in KBG is still recent, there are a number of use patterns common with Beacon that haven’t yet been evaluated for Outrider. Knowing your needs will help to guide and prioritize further research needs (and hopefully, eventual registration of more uses in KBG across the PNW).
Cautions and Disclaimers
While some trials have been done in dryland KBG production systems by both WSU and U of I, neither WA nor ID has yet registered Outrider for use in KBG. Initial results from these trials have indicated that Outrider will likely have an eventual fit in these systems, pending registration. Further development work is still needed to define best use parameters in dryland systems. For KBG growers in Oregon, there are several use precautions worth highlighting here, but make sure to consult the 24(c) SLN label for the complete list prior to purchasing any product. First, in trial use, the registered use rates of Outrider have been consistently closer to the edge of crop tolerance than they are for Beacon, so be very careful to avoid any errors that could result in higher than labelled application rates (eg., spray overlaps, poorly calibrated equipment, etc.), as excessive crop injury is much more likely in these situations with Outrider than with Beacon. The currently labelled KBG growth stage is up to 2nd node emergence in spring for both seedling and established stands, so late-season applications for seed-head suppression are not registered. (Additionally, zero data is available on the crop safety of such applications, so unacceptable crop injury is likely to result as well.) Crop safety of Outrider in trials has exhibited variability on the basis of environmental conditions around the time of application, particularly cold or freezing temperatures. Accordingly, do not apply Outrider when temperatures will fall below 30° F for at least 48 hours before and 72 hours after the application, or unacceptably high crop injury is likely to occur. Finally, Outrider has a lengthy rotational (plantback) restriction to many other crops, ranging from 3 to 22 months for listed rotational crops. See the wheat section of the main Section 3 label on the container for full details. This characteristic will preclude use in annual KBG production systems (particularly in the Columbia Basin and other areas where high-value, broadleaf specialty crops are grown), and should be carefully considered for compatibility with rotational plans even in multi-year production systems.