"In general, postemergent applications should be made before weeds get taller than 2-4 in., because that's when competition can begin to impact yields," says WinField Regional Agronomist Steve Barnhart. "Plus small weeds are a lot easier to control than larger ones."Because every field is different, there isn't a one-size-fits-all approach to turn to when it comes to planning postemergent herbicide applications. Therefore, growers need to consider three basic weed-management principles when looking to keep their acres clear of competition:

  • Protect yield potential
  • Minimize development of weed species shifts
  • Minimize development of weed resistance

"These basic principals can be accomplished by using a combination of prevention, growing competitive crops and utilizing timely mechanical and chemical control," says WinField Regional Agronomist Steve Barnhart. "Alternative herbicides that use modes of action different than glyphosate can be effective in reducing the chances of developing weed resistance."

Timing is crucial because applications need to be made before weeds get too tall and competitive, and corn hits its critical growth phase of 25-40 days after planting, when weed competition can take the biggest bite out of yields.

"In general, postemergent applications should be made before weeds get taller than 2-4 in., because that's when competition can begin to impact yields, plus small weeds are a lot easier to control than larger ones," Barnhart says. "Weed density also affects the height at which weeds need to be controlled. The more dense the weed population, the sooner a postemergent herbicide application will be required to protect yield potential."

The ideal timing for postemergent treatments changes quite a bit. "Producers who make preemergent foundation herbicide applications have improved flexibility built into their weed-control program, which can allow for a much wider application window for postemergent herbicides to be applied compared to total postemergent weed control programs," Barnhart says.

Corn is more tolerant to many herbicide applications as a small seedling compared to later stages of growth. Avoid increasing the potential for herbicide damage by always reading the labels of the herbicides used and staying within the labeled application window.

Due to the complex nature in the way that glyphosate kills weeds, resistance has not developed as rapidly as with the single "site-of-action" ALS herbicides. That being said, weed shifts, weed tolerance and weed resistance to wide spread use of glyphosate continues to be of a concern and is a growing issue. According to WeedScience.org, 22 species are known to have developed resistance to glyphosate, including annual bluegrass, common ragweed, giant ragweed, hairy fleabane, Italian Ryegrass, Johnsongrass, kochia, marestail/horseweed, palmer amaranth (pigweed), rigid ryegrass, waterhemp, morningglory and others.

Tankmix partners, whether added to a pre-emergent or postemergent application, should be based on the weeds in a specific field, as there isn't an all-in-one tankmix herbicide that will control all weed species. Herbicides that have soil residual activity can be added to postemergent applications to help prevent weed growth for another two to four weeks.

"For example, although the various formulations of some herbicides are usually soil-applied, they can be tank-mixed with early postemergent glyphosate applications to provide residual control of many grasses and many small-seeded broadleaf weeds," Barnhart says.

Efficacy obtained by various glyphosate or glyphosate tankmixes can be enhanced by selecting the proper adjuvant.