Many winter wheat fields have rather large broadleaf and grassy weeds growing at this time. A number of these fields received harvest aid treatments. If weeds are cut off with the combine at harvest, you will need to let them regrow before herbicide application.
Timely control of weeds following winter wheat harvest can limit soil moisture loss to weeds and prevent the deposit of more weed seeds in the soil, two factors that can benefit the next crop's yield. In addition, timely control of volunteer wheat is essential in reducing the spread of wheat streak mosaic disease.
Figure 1. The effect of winter wheat crop residue levels on weeds in corn following winter wheat treated with herbicides post harvest. In general, 1 bushel of wheat produces 100 pounds of crop residue or 60 bushels of winter wheat equals 6,000 lb of crop residue.
The effectiveness of post harvest weed control is influenced by production practices used with the previous wheat crop, such as
- winter wheat variety selection,
- fertilizer practices,
- row spacing,
- planting date,
- seeding rate, and
- weed control in the growing wheat.
Other factors include:
- weed size,
- cutting off weed tops with the combine,
- crop rotation,
- temperature when spraying,
- rain the day of spraying,
- weed seed distribution, and
- streaks caused by sprayers, terraces, dust, straw, and chaff.
The amount of residue from this winter wheat crop affects how the next crop will compete with weeds.
Effective Cultural Practices To Aid Weed Control
Many options besides increasing herbicide rates are available for weed control after wheat harvest. Vigorous winter wheat stands will compete better with weeds. To achieve maximum control:
- Prepare a good firm seedbed.
- Control weeds in a timely manner.
- Fertilize if needed.
- Seed properly.
- Plant at the optimum time.
- Select a competitive winter hardy winter wheat variety.
Controlling weeds in the growing wheat offers the best chance of reducing weed population and vigor after harvest.
Weeds under stress are difficult to control, however, this may be less of a problem this year as many areas have excellent soil water. It's a general rule that you can wait up to 30 days after harvest to spray wheat grown as part of a three-year rotation. If the wheat was planted without an 11- to 14-month fallow period, spray it within 15 days of harvest. Examine each field separately and adjust your treatment schedule accordingly. This year some fields will need to be sprayed before 15-30 days. The key is to prevent weeds from using soil water and producing weed seeds.
As with all weed control, it's essential that you closely watch for weed developments and spray at the proper time to achieve maximum control. Most labels state that weeds must be treated before they are 6 inches tall. If weeds are under severe drought stress, wait for rain and spray about a week later.
Split treatments, which have a good history of effectiveness, should be especially beneficial this year. In Kansas, there was a 20-bushel increase in corn yields the next year for treatments applied in July vs. mid-August. When using a split treatment, apply the glyphosate products with companion herbicides (adding surfactant, if needed, plus ammonium sulfate) as the first application in July or early August. Some glyphosate products include sufficient surfactant while many products require more. Be sure to check the product label.
For all glyphosate brands, add ammonium sulfate (spray grade) at 17 lb per 100 gallons of spray solution. (The ammonium sulfate is the first item put into the spray tank after the water.) Ammonium sulfate is especially helpful when stress conditions are present. Liquid ammonium sulfate, with or without a drift retardant, also is available. It's difficult to recognize weed stress so it's wise to always add ammonium sulfate.