Horseweed (marestail) is an annual weed that often becomes a problem in continuous notill fields, although it has also been a problem in some tilled fields in recent years. Horseweed can follow a winter annual or summer annual life cycle. While the majority of the horseweed emerges in the fall, it can also emerge in spring and early summer. We have observed horsweed emergence throughout the entire year in some fields. Horseweed does not mature until late summer, so unlike many other winter annuals that mature in late spring, horseweed competes directly with corn and soybean growth during the growing season and interferes with harvest also.
Horseweed is more easily controlled when small in the late fall or early spring. As the horseweed becomes larger in the spring and early summer, it becomes more difficult to control. The activity of glyphosate on horseweed varies with environmental conditions. Some populations of horseweed in Ohio appear to be resistant to ALS inhibitors (Classic, FirstRate, Beacon, etc) and herbicides with this site of action will be ineffective for horseweed control. Where ALS resistance is suspected, use of Roundup Ready soybeans is suggested to ensure that an effective postemergence option is available. However, glyhosate-resistant horseweed has been found in the eastern United States, and overuse of glyphosate for control of horseweed could result in the development of glyphosate-resistant populations in Ohio.
Noncrop/Fallow Areas. The most effective and economical control is a preplant application of 2,4D ester (1 to 2 pints/A), or a combination of 2,4-D ester plus glyphosate (0.56 to 1.125 lb ae/A). Higher rates of glyphosate applied without 2,4-D can control horseweed up to 12 inches tall. Application of any of these treatments in the fall will also greatly reduce horseweed populations the following spring.
Corn. Most postemergence treatments containing a growth regulator herbicide (2,4-D, Marksman, dicamba) will suppress horseweed sufficiently until the corn is wellestablished. Beacon, Spirit, Exceed, and Hornet will control seedling horseweed, but should be mixed with low rates of 2,4-D or dicamba for larger plants. Liberty (24 to 34 ounces/A) or Liberty ATZ (32 to 40 oz) will control small horseweed when applied postemergence in Liberty Link corn.
Soybeans. Application of 2,4-D alone (1.5 to 2 pints/A) in late fall or early spring will control small horseweed plants. As horseweed increase in size, the addition of glyphosate, Canopy XL, Python, FirstRate, Guantlet or Sencor will be necessary. All of these except glyphosate also provide residual control of later-emerging horseweed, along with Valor. The activity of 2,4-D plus glyphosate can be reduced under cold tmperatures. Canopy XL, Python, FirstRate and Guantlet will not control ALS-resistant horseweed. Another effective option, especially for ALS-resistant horseweed, is a mixture of Sencor (8 oz) plus Gramoxone Max (1.25 pt/A) plus 2,4-D ester (1 pt/A), which may be more consistent than glyphosate/2,4-D on large plants under cold temperatures. Classic (1/2 to 3/4 oz/A), FirstRate (0.3 oz/A), and Synchrony STS (1/2 oz/A) are labeled for postemergence control of horseweed that are 2 to 5 inches tall, but control with these products is becoming more variable across Ohio due to ALS-resistance. Glyphosate (Roundup Ready soybeans) will suppress or control horseweed, but rate should be adjusted for horseweed size. Plants emerging from seed will be more easily controlled than plants that have overwintered.
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