Fast-start spring

June-like weather in March gave weeds a huge head start this spring. Soil temperatures in March matched what Iowa normally has in late April and early May. A mild winter and warm spring create greater weed challenges, especially for no-tillers. Winter annuals can turn fields green by late March. And weeds such as marestail grow faster and reach stages difficult to control much sooner than in a “normal” spring.

Tillage can control early weeds. “But on no-till fields, you rely on herbicide management,” says Kyle Wendt, who farms with brother Dave near Mitchellville in central Iowa. “Applying a preemergence corn herbicide earlier than we normally do is needed in a year like this to hold weeds in check so they don’t get away from us.”

Key Points

To avoid competition, early weeds needto be controlled by burndown herbicides.

This also prevents weed seed production and helps control early-season pests.

Use full rates of preemergence herbicides for longer-lasting weed control.


Do you make a split application, applying half the preemergence herbicide rate early and the rest later? Or apply the full rate early? People use different strategies, depending on weed species in the field and the herbicide you prefer, says Wendt. Some apply a full rate of preemergence herbicide earlier than normal, adding whatever else is needed to clean up winter annuals. Some add glyphosate with a preemergence product. Others use a half rate of preemergence early, and come back later with a postemergence herbicide.

Weeds vex soybeans

Soybeans often have more weed pressure. Bean herbicides work better with a full-rate preemergence and 2,4-D to burn off winter annuals. If you have grass along with winter annuals, add a little glyphosate and come back with a post herbicide.

Applying burndown herbicides in early April can help control winter annuals and early-spring weeds, says Iowa State University Extension weed specialist Bob Hartzler. Also, earlier-than-usual burndown treatments minimize the risk of needing 2,4-D at higher rates.

Many farmers include preemergence herbicides with early-spring burndown treatments. While this may provide a clean seedbed at planting and crop emergence, longevity of weed control will be shortened significantly. How much depends on the herbicide rate and the time and weather between application and planting. Rates of many preemergence herbicides have been reduced due to reliance on post products. Hartzler says, “If applications are to be made a few weeks earlier than normal, carefully evaluate product rates to maximize the contribution of the preemergence herbicide to residual weed control after crop emergence.”

Preemergence herbicides are a key part of resistance management, but they need to be used in a way that controls the target weed species. Reduced rates or very early applications of preemergence herbicides will greatly decrease their effectiveness on late-emerging weeds, such as waterhemp, or large-seeded species like giant ragweed. “Many preemergence herbicides specify split application,” says Hartzler. “A portion of the product is applied early; the rest is applied at or shortly after planting. This approach could be beneficial this year when an extended period of weed control may be needed due to early applications.”

Insects also appeared a month earlier than usual this spring. ISU entomologist Erin Hodgson says seed treatments should be effective to help control early-season pests, but cautions some insects may require a foliar insecticide application. “Pay close attention to your fields during crop emergence and establishment,” she adds.

Scouting app available

A new crop scouting application is available. Developed by ISU, the ScoutPro app is for smartphone, iPad or other tablet devices to identify weeds, insects and diseases, while also creating crop scouting reports. Go to www.scoutpro.org.

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early WARM-UP: Winter annuals and certain other weeds pose a greater problem in an unusually warm spring. Like other farmers planting no-till corn, Kyle and Dave Wendt have had to adjust their preemergence herbicide timing and burndown strategy.

This article published in the April, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.