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ISSUE 7   June 14, 2001



Optimum weed control is achieved if weeds are sprayed at the right time. However, lack of uniformity in weed seedling emergence and, consequently, weed size results in the application of herbicides at times when perhaps control cannot be maximized. Inclement weather also presents additional challenges to producers particularly when they apply postemergence herbicides. Despite this, knowing when weeds emerge and how fast they grow can offer producers a means by which to target weed control practices.

WeedCast was developed by Dr. Frank Forcella (USDA-ARS, Morris, MN) and other weed scientists in the North Central states, and was first released in 1997. The software was developed to assist farmers, consultants, industry, and students to easily forecast seedling emergence and growth of common annual weeds of crops in the North Central region of the United States. You can download WeedCast, free-of-charge, from the USDA Morris, MN homepage: http://www.mrsars.usda.gov. WeedCast predictions should be treated as guides or rules-of-thumb in situations where knowledge of weed biology is necessary for making weed management decisions. The predictions represent the best estimates based upon current understandings of the biology of 17 weed species. More species will be added as soon as relevant information is gathered.

WeedCast is driven primarily by temperature and moisture, two of the most important resources necessary for germination of seeds and seedling growth. The best estimates of seedling emergence and growth are obtained using data for specific locations (max and min temps, rainfall, soil type, tillage system, previous crop, and soil wetness/dryness).

WeedCast predictions (emergence percentages and seedling size) can be used as a basis for making managerial decisions i.e. whether to control weeds, wait a while, or not. Research conducted by scientists within the North Central states has shown that foxtails are controlled best within several crops when you cultivate crops twice, at 15% and at 30% predicted foxtail emergence, using a rotary hoe. In other research conducted at NDSU in 2000, it was shown that kochia growing in field pea was best controlled when postemergence herbicide was split-applied, at 30% and at 60% predicted kochia emergence.

George Kegode
NDSU Weed Ecology/Biology




Many are concerned with weed control and crop response from herbicides applied the cool conditions prevalent for the last several weeks. Below is a summary.

Ideal temperatures for applying most POST herbicides are between 65 and 85 F. Most weeds are killed slowly below 60 F. Some herbicides may injure crops if applied above 85 F. Avoid applying volatile herbicides such as 2,4-D ester, MCPA ester and dicamba during hot weather, especially near susceptible broadleaf crops, shelterbelts, or farmsteads.

Temperatures following herbicide application determine crop safety or injury. Cold temperatures cause concern for crop safety and possible reduced weed control. Plants degrade herbicides by metabolism, but plant metabolism slows during cool or cold conditions, which extends the amount of time required to degrade herbicides in plants. Rapid degradation under warm conditions allows crop plants to escape herbicide injury. However, cold temperatures during and after herbicide application increases degradation time of the active ingredient and may result in crop injury. Continued cold night and day temperatures cause metabolism to shutdown in plants, preventing herbicide degradation. However, herbicides may be sprayed following cold night-time temperatures if day-time temperatures warm to at least 60 degrees.

Wild oat is a cool season grass and is more sensitive to products containing fenoxaprop (Dakota, Tiller, Cheyenne, and Puma) during cool rather than warm or hot conditions. Green and yellow foxtail are warm season grasses and may shutdown under cold conditions resulting in reduced control. Many broadleaf weeds are warm season plants and are controlled better during warm/hot conditions under active plant growth with sufficient moisture.

ACCase inhibitors, such as Achieve and fenoxaprop (Dakota, Tiller, Cheyenne, Puma), may cause crop injury and give greater weed control when cold temperature follow application. Fenoxaprop is more active on grass weeds during cold temperatures. Risk of crop injury is much greater from fenoxaprop in cool/cold conditions.

Other herbicides, such as Assure II, Poast, Fusilade DX, and Select provide better grass control in warm weather when grasses are actively growing.

Cold temperatures following application of ALS herbicides may increase crop injury with little effect on weed control. Pursuit and Raptor on beans; Accent, Matrix in Steadfast, Basis, Basis Gold and Accent Gold, and Harmony GT in Basis in corn; Lightning on Clearfield corn; FirstRate on soybeans; Bladex in corn; Sencor in legumes and potatoes; and bromoxynil in grass crops all have shown significant corn and wheat leaf burn when freezing temperatures follow application.

Basagran, Cobra, Flexstar, Liberty, paraquat, Stampede, and Ultra Blazer may not cause crop injury when cold temperatures follow application but less weed control may result.

2,4-D, dicamba, MCPA, Stinger, Starane, glyphosate (resistant crops) have adequate crop safety and provide similar weed control, but weed death may be slowed when cold temperatures follow application.

Recommendation for applying fenoxaprop based herbicides, ALS herbicides, Bladex, and Sencor is to delay application until daytime temperatures exceed 60 degrees F and after active plant growth resumes.

Adjuvants may also affect crop safety and weed control. Oil additives may increase risk of crop injury but may be necessary for greatest weed control. Refer to each herbicide label for specific information on adjuvant use during stress conditions. Use an oil additive if risk of crop injury is acceptable for those herbicides that allow use.

Richard Zollinger
NDSU Extension Weed Specialist




Hail can often increase the severity of weed competition since the hail may open the crop canopy and provide the weeds with a competitive advantage. Postemergence herbicide treatments should not be delayed after hail due to concerns about increased crop injury. The hail will reduce leaf area and thereby reduce herbicide uptake. Hail may reduce weed control and crop injury if remaining leaf area is minimal. So, assuming that the hail left enough crop to be worth saving and assuming weeds are a problem, the best choice postemergence herbicide treatment should be applied as soon as possible after the hail when the crop is in the proper growth stage. Consider that hail sometimes makes growth staging difficult and the crop may be more physiologically mature than it appears.

Alan Dexter
Extension Sugarbeet Weed Specialist


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