Weed Management in Winter Wheat Stubble and Under Drought Conditions

Weed Management in Winter Wheat Stubble and Under Drought Conditions

Table 1. Control of volunteer wheat (4 bu/ac) with atrazine over an extended period, based on research conducted at UNL's West Central Research and Education Center.
  % Control

Atrazine Rate

July 16 Sept. 15
0 0 0
1 46 99
2 84 100
3 93 100

Many winter wheat stubble fields are relativity weed-free because of the drought.  Some fields, however, may have weed patches where wheat density was low. If you don't plan to treat the whole field soon, spot-treat these areas. 

In areas where there's been enough precipitation for broadleaf and grassy weeds to actively grow in the winter wheat stubble, control measures should be initiated. These weeds include volunteer winter wheat.

Control weeds before they set seed and use a lot of soil water. Weeds such as kochia and Russian thistle are day-length sensitive and begin flowering in late July and early August. Viable seed will be produced shortly after flowering and timely control will reduce next year's weed problems and conserve soil water.

Select herbicide treatments according to the weeds present in the field. See the latest edition of EC130 Weed Management Guide for recommendations. Keep in mind that you may have herbicide-resistant weeds in the field and will need tank mixes to control the weeds.

Table 2.  Relative damage to tomatoes caused by vapors from 2,4-D. Ratings were taken 24 hours after exposure and range from 1 (no effect) to 6 (severe damage). (Source:  The Responses of Tomato Plants to Vapors of 2,4-D and/or 2,4,5-T Formulations at Normal and Higher Temperatures, by A. David Baskin and E.A. Walker, published in Weeds.)
  Temperature and hours of exposure
  70-75°F 90°F 120°F
2,4-D Formulation 2 hr 16 hr 2 hr 16 hr 2 hr 16 hr
Butyl ester (High volatile) 3.5 6.0 5.8 6.0 6.0 6.0
Butoxyethanol ester
(Low volatile)
1.0 1.0 2.3 5.7 6.0 6.0
Dimethylamine (Non-volatile) 1.0 1.0 1.0 1.1 1.2 1.2

When applying herbicides at this time of year, be particularly concerned about particle drift and vapor drift (Table 1). While butyl esters are no longer used, low-volatile esters are and can be a problem with higher temperatures. Check for susceptible crops in the area before application to avoid drift problems.
Do not include atrazine in the tank mix at this time (Table 2). Atrazine residual is quite short under higher temperatures and will not provide adequate control of fall-emerged weeds or winter annuals if applied in July or August. Application in early September usually works for western Nebraska and late September in eastern Nebraska. A warm September will push these dates back later.

Under drought conditions weeds respond by thickening their leaf cuticle and reducing vegetative growth. Moisture-stressed weeds are more difficult to control because of reduced herbicide absorption and low physiological activity. Under these conditions, usually the full labeled rate of glyphosate is required along with the recommended amount of ammonium sulfate and other additives. Many of the herbicides tank-mixed with glyphosate will need to be used at higher rates, but do not exceed the label rates. And, always check rotation restrictions before selecting herbicides.

Robert Klein
Extension Western Nebraska Crops Specialist