Soybean with weeds

Editor’s note: The following was written by Bob Hartzler, Iowa State University professor of agronomy and Extension weed specialist, and Meaghan Anderson, Extension field agronomist in central Iowa, for the university’s Integrated Crop Management News website June 11.


While good progress has been made toward getting crops in the ground, the adverse early spring conditions are likely to complicate weed management throughout the 2019 growing season.

The most important step in minimizing problems is to scout fields regularly to identify problems quickly and allow timely adjustments to management.

Performance of PREs

Pre-emergence (PRE) products used in fields planted in a “timely fashion” are likely to have shortened residual activity due to abundant rain, therefore increasing the importance of timely postemergence (POST) applications.

A different issue will be encountered in fields that were planted in the last week or two. These fields were planted during peak periods of weed emergence, and many areas have gone more than a week without rain to activate PREs. Many of these fields will require a POST treatment several weeks earlier than normal.

Crop development

A full crop canopy is a critical component of successful weed management. While last week’s growing degree days helped move corn forward, the slow development throughout May, along with early degradation of the PRE products, will make achieving full season control more difficult this year than most.

Delayed soybean planting is also likely to complicate weed management. Many fields were planted under less than ideal conditions that may result in uneven stands, creating gaps in the canopy. Soybeans planted in 30-inch rows may not form a full canopy, further favoring the survival of late-emerging weeds. The delayed or hindered crop canopy development will make timely application of an effective POST program more important than in most years.

The application window for many products will be reduced due to the crop advancing through growth stages more quickly than with normal planting dates. Dicamba products registered for Xtend soybean varieties must be applied prior to R1 stage soybeans (before first flowers). Liberty (Liberty Link) has the same restriction (up to R1), whereas glyphosate (Roundup Ready) or 2,4-D (Enlist) can be applied through the R2 stage.

Performance of POST

While we have reduced our reliance on POST products due to widespread herbicide resistance, POST products will be more important this year due to the environmental factors influencing PRE herbicides.

The best way to reduce risks is to scout fields earlier than would be typical, spray when weeds are smaller than label limits, and include a residual herbicide with the POST program to extend control later into the season. Waiting until weeds reach the maximum controllable size to begin applications and failing to include a residual herbicide will result in a greater risk of control failures, more weed survival and seed production, and greater opportunity for new resistance issues.

Delayed application results in the herbicide being applied closer to the establishment of the following crop, and in some cases might lead to problems.

Check rotational restrictions on products to ensure they won’t affect plans for rotational crops this fall or next spring. Herbicides for which late applications can increase carryover risk include fomesafen, imazethapyr and chlorimuron.

The impact of this spring’s cool and wet weather on weed control is not over. Weed management programs in many fields will need to be adjusted from what was planned in order to avoid costly control failures. Early scouting of fields is essential to determine how well PRE programs are working and allow timely adjustments to the POST program.