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Sponsored: Controlling Winter Annual Weeds

The time spent in the combine is perfect for scouting winter annual weeds while monitoring your harvest operations. A winter annual weed is just like it sounds… an annual weed with a life cycle that begins in the fall and may go dormant during the winter before maturing and dying in the spring. With this type of life cycle, winter annual weeds were less of a concern in past years because they weren’t considered to be competitive with a growing summer crop, and conventional tillage practices took care of them before planting. In the past decade however, winter annual weed pressure has become a significant problem for corn and soybean farmers who:

  1. Have a greater percentage of conventional and no-till acres
  2. Have had a significant reduction in use of soil residual herbicides during the “post-glyphosate only” era of soybean production. This is rapidly changing, however, as we increase our use of soil residual herbicides in our fight against weed resistance. That being said, we may see improvements in our winter weed control moving forward.

Winter annuals tend to favor either fall or early spring for emergence and growth as they all can emerge in both windows depending on conditions. Species favoring fall growth include chickweed, henbit, Carolina foxtail, and marestail. Those favoring spring include Virginia pepperweed, purselane, sheperdspurse, marestail, and field pennycress. 

Winter Annual Weeds
October emerging henbit and marestail in a corn field in the Missouri River bottom in Boone County.

Many farmers I talk to hope to apply a chemistry in the fall that not only controls their winter annual weeds but will also offer residual control for later-emerging winter annual and early spring-emerging summer annual weeds. Talk about killing two birds with one stone! Unfortunately, fall applications of residual chemicals to control summer annual weeds that emerge early before the next planting season is generally ineffective. The objective of a fall herbicide program should be to control winter annual weeds that have already emerged, while providing some residual control for later emerging winter annual weeds. You fall herbicide program should be completed at a low enough cost that the majority of your herbicide budget can be used the following spring to layer residual chemistries in an effective weed resistance management program.  

Today, winter annual weeds are emerging primarily in corn stalk ground. The increased use of post-applied soil residuals on our soybean acres delays germination and growth of winter annuals. In addition, soils have been relatively dry at the surface recently favoring harvest operations and delaying winter annual emergence. Recent rainfall should help spur increased germination of our winter annuals, so be on the lookout! 

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