Weeds in a Wet Year

May 22, 2019
ICM News

The delays in planting get the headlines (for good reason), but another issue with prolonged wet periods is the inability to manage weeds and cover crops in a timely fashion. The following is a brief review of some of the problems that may be encountered.

Large winter annual weeds or cover crops

Effective management of vegetation present at planting is critical, and as termination is delayed the weeds/cover crops will become more difficult to kill. While glyphosate is better at controlling larger weeds than other products, it is essential to take steps to maximize its performance under these conditions. Factors that may reduce glyphosate activity include tank mixes with other herbicides, especially those that cause rapid foliar injury, and the use of UAN (28% or 32% N) as carrier. Be sure to use recommended rates of AMS, increase spray volume in fields with dense vegetation, avoid applications toward twilight, and most importantly, don’t reduce glyphosate rates.  2,4-D is often included with glyphosate to enhance control of certain broadleaf species, be sure to follow the planting interval restriction (seven days for 0.25 lb 2,4-D LVE/A in most situations) when this product is used.

Application of preemergence herbicides delayed following planting

The rush to get fields planted last week undoubtedly resulted in sprayers not keeping up with the planters. As soon as fields dry it will be important to determine whether crops and weeds have emerged, and then identify appropriate treatments.

Most corn preemergence herbicides can be applied after the crop has emerged, but there is a wide range in how much activity the products have on emerged weeds. In fields with emerged weeds, determine the need to include additional herbicides to control the established weeds. Corn that emerges in the cool wet conditions will likely be under stress, reducing herbicide tolerance. If fields have low densities of small weeds it may be better to delay application a few days to allow the corn vigor to improve, but in fields with high weed densities the weeds may pose a greater threat to the crop than the risk of herbicide injury.  

Unlike in corn, preemergence options for soybean are greatly limited when the crop has emerged, emphasizing the importance of getting the herbicide on as soon as conditions allow for field operations to resume. Products containing flumioxazin (Valor, Fierce, etc.), saflufenacil (Sharpen, Optill, etc.), sulfentrazone (Authority products), and metribuzin (Boundary, Canopy, etc.) cannot be applied after soybean have emerged. In fields where the crop emerges prior to herbicide application, the best option is to apply an appropriate postemergence product as soon as the field is fit and include a Group 15 herbicide to provide control into the season. Addition of a Group 2 herbicide (e.g. imazethapyr, cloransulam-methyl) or fomesafen (HG 14) could improve residual control of certain broadleaf species.  

Slow development of crop due to cool conditions

While some areas were able to get corn planted in a somewhat timely fashion, the below average temperatures have slowed emergence and development of the crop. The slow canopy will require preemergence herbicides to provide longer control than in more typical springs. The longevity of preemergence control will also be reduced in fields receiving above average rain. Thus, fields that were planted and sprayed in a timely fashion should be scouted early enough to allow remedial treatments to be made if the preemergence program breaks down early.

Switching planting intention from corn to soybean

If planting is delayed further some farmers may consider switching fields intended for corn to soybean. If herbicides for the corn crop were applied this spring, or last fall, the option to change crops may be eliminated. Check labels of any applied products to determine the rotational interval for soybean.

Managing weeds in prevented planting acres

Continued planting delays or flooding may result in some fields qualifying for prevented planting payment. It is as important to have a weed management plan for these fields as for other fields since allowing weeds to survive will result in a dramatic increase in weed pressure for future years. Planting a suitable cover crop to provide significant ground cover into a weed-free seedbed is the most important step in managing weeds in these fields. 

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on May 22, 2019. The information contained within may not be the most current and accurate depending on when it is accessed.

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Bob Hartzler Professor of Agronomy

Dr. Bob Hartzler is a professor of agronomy and an extension weed specialist. He conducts research on weed biology and how it impacts the efficacy of weed management programs in corn and soybean. Dr. Hartzler also teaches undergraduate classes in weed science and weed identificatio...