Fall Weed Control Options for Winter Wheat

Fall Weed Control Options for Winter Wheat August 31, 2016

Downy brome in wheat
Figure 1. Downy brome emerging in a soon-to-be-planted wheat field in October 2015.
Russian thistle in wheat
Figure 2. Recently seeded winter wheat field near Scottsbluff with uncontrolled Russian thistle.

Current market conditions, especially low crop prices, have Nebraska wheat producers considering options to reduce input costs. Although herbicide costs may seem prohibitive, it’s important to consider the long-term implications of limiting or eliminating the use of herbicides in crop production systems.

First, weeds that are left unmanaged following wheat harvest use valuable nutrients and water needed for the following year’s crop. Unmanaged weeds also produce seeds to replenish the soil seed bank.

A study conducted in Tribune, Kan., by Kansas State University researcher Alan Schlegel demonstrated this by looking at two wheat stubble heights and herbicide timings (as well as no post-harvest weed control). Results in Table 1 show decreasing corn yield following wheat when the initial weed control is delayed or not applied at all.

Evaluating Weed Control in the Previous Year

Producers can start by reviewing the previous year’s herbicide applications, determining what worked well, what went wrong, and identifying any areas of particular fields that might need special attention to control some problem weeds.

Poor weed control could be related to a thin crop stand, planting date, crop rotation, weed size, dust, environmental conditions at time of spraying, plugged nozzles, improper application of herbicides and tank-mix partners, or herbicide-resistant weeds, among other things.

Weed management for this season’s winter wheat production should have been started two years ago after the wheat crop was harvested, during the after-harvest fallow period, corn growing season, and summer fallow. If growers proactively managed weeds during this time, weed management for the 2016-17 winter wheat growing season should be less challenging.

Table 1. A winter wheat study conducted by Kansas State University at Tribune, Kan., showed the impact of wheat stubble height and weed control timing on dryland no-till corn production in 2001 (A. Schlegel).
Weed control timingStubble Height 7.5 inches*
Corn BU/AC
Stubble Height 15 inches*
Corn Bu/ac
Initial spray in July 44 59
Initial spray in mid-August 25 38
No post-harvest weed control 13 40
*60 bu/ac wheat prior to corn crop

The vast majority of seeds from the most troublesome weed species in winter wheat typically don’t last more than two years in the soil seedbank, and as long as these species were not allowed to reproduce during corn and fallow periods, the weed pressure within a field should not be very high.

Starting Clean

Similar to other crops, it is recommended to plant winter wheat into a weed-free seed bed. Once wheat has been planted and emerged, the available options for weed control are dramatically reduced.

Two major groups of weeds compete with winter wheat: winter annuals that emerge in the fall, such as downy brome, jointed goatgrass, feral rye, marestail, and blue mustard; and summer annuals, which start emerging in the spring.

Winter annual weeds are the most challenging to control because they start to grow at the same time as the crop, if not earlier. Management of summer annual weeds should not be difficult because by the time these species are emerging, the winter wheat crop should be closing canopy and naturally suppressing these weeds.

Remember to read and follow herbicide labels to avoid any potential injury from herbicide carryover. Table 2 lists some herbicides and their rotation restrictions.

Getting a Jump Now on Winter Annual Weeds

Controlling winter annual weeds in late fall is the key to success. Winter annual weeds can begin emerging as early as August and continue to emerge through early March. By emerging so early in the cropping year, winter annuals can become tolerant to herbicides by the time spring applications are made. With any winter annual weed, growers should consider an effective burndown program to start “clean.” And for post-emergence, keep in mind that the best time to apply herbicides is in the fall, when weeds are still small and relatively easy to control.

Table 2. Herbicide rotational restrictions for planting winter wheat.
Herbicide

Rotation Restriction
to Winter Wheat

2,4-D Ester 30 days
2,4-D Amine 30 days
Dicamba None
Glyphosate None
MCPA 0-3 months,
depending on rate
Starane 3 months
Stinger None
Widematch None

Limited post-emergence options are available for marestail control in the fall, as most synthetic auxin herbicides cannot be applied before tillering. However, dicamba is labeled in winter wheat for application from planting up until nodes are visible in the spring. Dicamba will provide control of both ALS- and glyphosate-resistant marestail.

For control of downy brome in the fall, Maverick, Olympus, and Powerflex all provide good control. Beyond is also a good option if Clearfield wheat was planted. The effectiveness of all herbicides labeled for downy brome control in winter wheat can be quite variable from year to year, with performance of fall-applied herbicides ranging from moderate to excellent. While downy brome control with any herbicide can be quite variable from year to year, multiple studies from UNL and other universities have demonstrated consistently greater control from fall application compared with spring applications. Spring application of such herbicides can be highly influenced by weather conditions, weeds stage of development, and growth rate.