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Northern Illinois Agriculture

University of Illinois Extension
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What we have learned from weeds

Posted by Russel Higgins - Weeds

Harvest has begun at the NIARC, early planted soybean are being harvested to allow the planting of winter wheat, weather permitting we should keep on harvesting from this date on. As crops matured in northern Illinois and dropped leaves on corn and soybean plants prior to harvest, producers have the opportunity to visually evaluate the effectiveness of their herbicide program in a challenging weed control year in 2012. Dry conditions, weeds "hardening off", applications taking place on weeds that were oversized, resistance or combinations of issues resulted in fields that no longer fit the "weed free" standard many farmers have grown to expect in the glyphosate era. For those who have listened to extension weed specialists and incorporated a soil applied herbicide in their program, the weedy fields may have come as a surprise and disappointment. Adding insult from Mother Nature, the soil applied herbicides that did not necessarily perform as expected following application could still be present at levels that may injure 2013 plantings due to limited rainfall. Despite frustrations, we can learn from and use 2012 as an educational tool to assist the decision making process for our 2013 weed management program. A basic tutorial on how soil applied herbicides function and what happens to the product after application will help temper our expectations.

  • Weeds are most vulnerable at the seedling stage. Soil applied herbicides are most effective when they are in place prior to seedling emergence. If weeds exceed 2-3 inches, they often fail to be controlled by the soil applied herbicide.
  • Ideal placement of soil-applied herbicides is the top 2 inches of the soil surface. This places the herbicide in close contact with most emerging and actively growing weed seedlings. This can be achieved via water (rainfall, irrigation) or tillage. Moving the herbicide too deeply in the soil profile can dilute the herbicide concentration and results in poor weed control.
  • The potential for crop injury is generally considered to be greater for soil-applied herbicides in comparison to postemergence herbicides.As a general rule, smaller (younger) crops do not have the herbicide tolerance found in mature plants. Overapplication and overlapping can be critical. Accurate application rates are very important for weed control and crop safety.
  • Soils with a higher cation exchange capacity, those with a higher organic matter or clay content will require higher rates of most soil applied herbicides because of the soils inherent absorptive properties.
  • Herbicide adsorption varies with soil pH, soil organic matter content, and climate
  • Herbicide carryover and crop injury are more likely in sandy and coarse soils where less herbicide is adsorbed to the soil.

To address evolving issues in weed control in northern Illinois a number of trials are underway at the Northern Illinois Agronomy Research Center. Specific studies include

  • Herbicide systems for weed control in Roundup\Liberty Link Corn
  • Pre and Post herbicides for weed control in corn
  • Weed control systems for Roundup Ready soybean
  • Tank mix and application timings for Giant ragweed control in soybean
Control ratings and yield information will be available in the near future following compilation of harvest data. Another opportunity for a valuable scouting operation that should take place during harvest or in harvested fields exists. Take note of areas or fields that had heavy weed pressure this year and give nature its due. Identify weed species and if the weeds reached maturity assume (many) viable seeds were produced. Expect pressure in that field and make herbicide selections to accommodate that pressure.

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