Mark VanGessel, Extension Weed Specialist; firstname.lastname@example.org
In last week’s issue of Weekly Crop Update I explained why we need to consider fall herbicide treatments for small grains. When splitting nitrogen applications in the spring, neither one of the timings are a good for herbicide application when trying to achieve spraying small weeds that are actively growing and achieve good coverage.
1. Weeds are more susceptible in the fall.
2. Fall applications match better with weed development.
3. Weed emergence is primarily in fall.
4. Fall herbicide applications are not influenced by temperature as much as spring applications.
5. Coverage is better with fall applications.
6. Spreads out the workload.
For no-till fields, a non-selective herbicide needs to be used prior to planting. However, we do not have effective herbicides labeled for preemergence applications, so it is important that the field be scouted to ensure the crop is at the proper stage for herbicide application.
A few products can be used shortly after the crop has emerged. Axiom and Prowl H2O can be used at crop emergence (Axiom at the spike stage and Prowl H2O at 1 leaf stage); however they need to be tankmixed with other herbicides or followed by postemergence herbicides to provide a broad spectrum control.
Products that provide postemergence control include: Harmony, Harmony Extra, Starane Ultra, Osprey, PowerFlex, Axial XL. Others labeled with a limited fit include metribuzin, Finesse, Maverick, 2,4-D or dicamba.
Control of specific problem weeds:
Annual bluegrass: fall applications of Osprey are the most consistent. Fall application of PowerFlex is also good. Maverick is a last resort type treatment in the spring (Maverick requires use of STS soybeans).
Annual ryegrass: fall applications of Osprey, PowerFlex, or Axial XL work extremely well. Spring applications of PowerFlex and Axial XL are options, but neither can be applied in nitrogen without reducing the amount of nitrogen applied.
Roughstalk bluegrass: Osprey or PowerFlex perform well on this species.
Speedwells: We have had limited trials with the speedwell species, but fall treatments seem to be most consistent. Harmony Extra has little to no effect on this species, PowerFlex in the spring was rated as fair to good; and slightly better than Osprey (fair). Research at Virginia Tech has shown good results with Finesse postemergence, but this treatment requires the use of STS soybeans. Initial results with metribuzin show some utility for speedwells.
Jagged chickweed: This is another species we have limited trials for, but fall applications seem much more effective than spring treatments. Osprey, Harmony Extra, and PowerFlex seem to work well when applied in the fall.
ALS-resistant chickweed: This species is on the move with more reports each year. Harmony Extra, Osprey, and PowerFlex are all ALS herbicides (Group 2) and have no activity on this biotype. Rather, Starane Ultra or metribuzin in the fall have been the best treatments.
ALS-resistant horseweed: Another species with no trials. Starane Ultra lists horseweed as a species it will suppress. We do know that 2,4-D will control horseweed in burn-down situations, but we have not looked at low rates of 2,4-D in wheat for crop safety and effectiveness.
One common weed that is not controlled with fall applications is wild garlic. But this weed needs to be treated with Harmony Extra (or similar products) in the early spring, about the time we apply the second nitrogen application. We need to think of wild garlic (a late emerging perennial) separately from the annual weeds mentioned above.
A rotation to vegetables is an issue with many of these herbicides, including Osprey, PowerFlex, Finesse, Maverick, and metribuzin. Starane Ultra is a 4 month rotation to most crops. As you can see there is no one program that will provide control of all of our problem species. In most situations, a fall treatment will outperform a spring application, and you need to select the herbicide(s) based on the problem weeds you have in your field.