Kill Those Pesky Winter Annual Weeds

Kill Those Pesky Winter Annual Weeds

Here are some timely tips on controlling winter annual weeds in fields that are to be planted to corn and soybeans.

With the warm March weather this year, winter annual weeds will be developing a bit ahead of normal. The following links are to two articles by Bob Hartzler, an ISU Extension weed management specialist on dealing with winter annual weed control in corn and soybeans. The first article is "It's Time to Scout to Winter Annual Weeds" at  and then be sure to read "Managing Winter Annual Weeds" at

2011 Herbicide Guide for Iowa Corn and Soybean Production

Kill Those Pesky Winter Annual Weeds

This is a very helpful publication to help sort through herbicide choices, says Brian Lang, ISU Extension field agronomist at Decorah, in northeast Iowa. It includes herbicide rating charts, brief listings of newer herbicides for 2011, herbicide modes of action and more. Get a free copy at:

Biennial thistle control in pasture—needs to be done soon

If you did not apply herbicide to control biennial thistles in pastures last fall, you can still get good biennial thistle control with herbicide if applied before thistles start to bolt, says Lang.  An article discussing biennial thistle management is posted at  

Biennial thistles are much easier to kill when in the rosette stage, so applications should be made in the fall or early spring before bolting initiates (Figure 1 of the web article). Effective herbicide options include: 1 to 2 qts of 2,4-D LVE (4 lb ae/gal); 1 qt 2,4-D LVE + 0.5 pt Banvel/Clarity; 0.2-0.3 oz Ally; 2 to 4 pt Grazon P&D; 2 to 3 qt Curtail and 3 to 5 oz Milestone. With the exception of Ally, all of the above products are plant growth regulator herbicides. Many of these herbicides are off-patent and sold under a variety of trade names and in different combinations.

Grazon P&D contains picloram (Tordon), and is a classified as a "Restricted Use Product" due to its persistence and mobility. Fall treatments are very effective against established rosettes, and can be made after several frosts have occurred. Spring treatments should be made prior to bolting since thistle tolerance increases greatly after bolting has initiated. Grazon P&D and Milestone provide more consistent control of thistles that have bolted than other treatments, but treatment costs increase since higher rates are required. "A potential advantage of spring treatments is they can control both second year rosettes and first year seedlings, whereas fall treatments typically only control rosettes that established earlier in the spring," says Lang.

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