As the growing season progresses, keeping ahead of weeds is important. Effective weed management is one of the more important factors affecting crop productivity. With the number of herbicide-resistant weeds continuing to increase, this is becoming a greater challenge. Research has shown early season weed control (before V3) has the greatest impact on yields. This does not mean mid-season weed control is not significant as late emerging weeds are still able to produce seed and can have a substantial impact on the spread of herbicide-resistant weed populations the following year.

As with early season management, many of the same principles apply for effective herbicide applications:

• scout fields routinely;
• use multiple herbicide modes of action; and
• apply the labeled herbicide rate to weeds at the recommended sizes.

In addition, conditions that favor crop growth, such as optimal fertility, irrigation management, and early crop canopy, will help with weed control. Finally, managing field borders can be critical to preventing the infestation of herbicide-resistance weeds into new fields.

Even the best weed management plans sometimes fail due to circumstances outside our control. Weeds like marestail, waterhemp, and Palmer amaranth can be particularly difficult to control mid-season. For each of these weeds, an aggressive strategy to manage escaped weeds is critical. If the area of escaped weeds is relatively small, a target herbicide application or hand rogueing is the best option to prevent the weeds from infesting a much larger area the following year. Extra effort in year one when the problem is relatively small will save a lot of time and money in subsequent years.

For larger areas there are a number of effective herbicide options in corn such as Acuron, Laudis, or Diflexx Duo. In soybean herbicide options are much more limited. When coupled with traited seed, Liberty, or Xtendimax can be effective at controlling these weeds postemergence, and in a Roundup Ready system Warrant Ultra or Flexstar GT are good options. It's not too early to plan how to improve weed control in fields with a history of difficult-to-control weeds.  A good preemergence herbicide program, use of narrow row-spacing, and even cover crops, when used as part of an integrated management plan, can improve control of herbicide-resistant weeds.

The Nebraska Guide for Weed, Disease and Insect Management has helpful information for selecting effective herbicides depending upon the weed species present in your field. When making postemergence herbicide applications, crop safety is an important consideration as is the potential for off-target injury to a neighboring field. When making in-season applications, consider crop rotation restrictions as fall-planted-cover-crops or spring rotational crops may be affected, depending on the herbicide selected.

The table linked in the box, excerpted from the 2019 Guide for Weed, Disease, and Insect Management, lists crop growth stage restrictions for corn and soybean residual herbicides that can be applied postemergence.

This was originally published in Nebraska Farmer magazine.