Figure 1. Blue mustard flowers earlier in the spring than other mustards and requires earlier herbicide application for effective control. Blue mustard often starts on field edges and works its way into the field over time.

September 25, 2009

Dry conditions in late summer followed by rainfall at, or shortly after, wheat seeding are now stimulating the germination of winter annual weeds. Stories in this week's CropWatch address fall control of winter annual broadleaf and grass weeds.

Common broadleaf winter annual weeds in winter wheat include blue mustard (Figure 1), tansy mustard, tumble mustard, field pennycress, and shepherd's-purse. Unfortunately, many growers are unaware of these weeds in their fields until they start to bloom in the spring. By this time, control is difficult and most of the crop damage has already occurred.

To be effective, winter annual broadleaf weeds need to be controlled by late winter or very early spring, before the plants begin to bolt, or stems elongate.

If winter annual broadleaf weeds are present in the fall, they can be controlled with any number of sulfonylurea herbicides including Ally®, Amber®, Finesse®, or Peak®. Some wheat varieties are more sensitive than others to fall application, but we have not screened the current varieties for this sensitivity. Adding 2,4-D can improve the level of weed control, but be sure the wheat has at least four tillers before applying 2,4-D or serious crop injury may occur. This injury may not be noticeable until the following spring when wheat heads become trapped in the boot as they try to emerge from the stem.

In the spring, blue mustard is perhaps the most difficult of the winter annual broadleaf weeds to control because it bolts very early. To be effective, herbicides typically need to be applied to blue mustard in late February or early March. Early April applications of 2,4-D usually provide excellent control of tansy mustard and the other winter annual broadleaf weeds, but provides only fair control of blue mustard. If timed correctly, 2,4-D (8 oz/acre of LV4 ester or 16 oz/ac of 4 lb/gal amine) provides low-cost and effective control of these weeds. As previously stated, wheat should have at least four tillers before applying 2,4-D or serious crop injury may occur.

Adding a sulfonylurea herbicide, such as Ally or Amber, to 2,4-D may improve control, particularly after these plants have bolted.

Drew Lyon
Extension Dryland Cropping Systems Specialist, Scottsbluff
Robert Klein
Extension Western Nebraska Cropping Systems Specialist, North Platte