By Chris Proctor

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 effect on yields.

However, this doesn't mean midseason weed control is not significant, as late-emerging weeds are still able to produce seed and can have a substantial effect 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 in-season:

• Scout fields routinely.

• Use multiple herbicide modes of action.

• Apply the labeled herbicide rate to weeds at the recommended sizes.

Table 1. Crop growth stage restrictions for residual herbicides applied postemergence in corn.

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

Even the best weed management plans sometimes fail because of circumstances outside of our control. Weeds such as marestail, waterhemp and Palmer amaranth can be particularly difficult to control midseason.

For each of these weeds, an aggressive strategy to manage escaped weeds is critical. If the area of escaped weeds is relatively small, a targeted 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 1 when the problem is relatively small will save a lot of time and money in subsequent years. For larger areas, there are several effective herbicide options in corn, such as Acuron, Laudis or Diflexx Duo.

Table 2. Crop growth stage restrictions for residual herbicides applied postemergence in soybean.

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.

When thinking about improving your control in fields with a history of difficult-to-control weeds, it's a good idea to start thinking ahead. A good preemergence herbicide program, use of narrow row-spacing and even cover crops, when used as part of an integrated management plan, can be successful in controlling herbicide resistant weeds.

The Nebraska Guide for Weed, Disease and Insect Management has helpful information for selecting effective herbicides depending on 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, it also is good to consider crop rotation restrictions as fall-planted cover crops or spring rotational crops may be affected depending on the herbicide selected.

The accompanying tables (also found on page 175 of the Guide for Weed, Disease, and Insect Management) list crop growth stage restrictions for residual herbicides that can be applied postemergence in corn and soybeans.

Proctor is a weed management Extension educator at the University of Nebraska-Lincoln.