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Agronomy Update

Controlling Winter Annual Weeds

Published on Thursday, October 13, 2016

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.  

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!  

Reasons to Control Fall Emerging, Winter Annual Weeds Now:

  1. Improve next spring’s seedbed. When there is a mat of winter annuals killed in the spring, soil temperatures at corn and soybean planting depths are often cooler due to reduced light penetration and water evaporation from the soil surface. Dr. Kevin Bradley at the University of Missouri-Columbia has observed increases in soil temperatures (five degrees on ground going to corn and eight degrees on ground going to soybeans) where winter annual weeds were controlled versus a check treatment with no winter annual weed control.   

  2. Live weeds can compete with emerging seedlings for water and nutrients, interfering with planter performance. There is some research, however, which shows that live weed or plant growth actually helps dry out the soil ahead of planting when soils are too wet.

  3. Winter annual weed cover can host and protect undesirable pests and attract early black cutworm female moth flights.

  4. Some annual weeds are known hosts for soybean cyst nematode (SCN). Two good SCN hosts are henbit and purple deadnettle.

  5. Logistics. Although winter annuals can be killed in early spring, they are usually larger which requires greater application rates. Often times there is a limited window of good weather to get this accomplished in the spring.

  6. Kill resistant marestail (horseweed). This “driver” winter annual weed can cause reduced soybean yields and is resistant to glyphosate and ALS chemistries. We have large populations of resistant marestail on many no-till fields in Missouri and they are much easier to kill in the fall and early spring when they are in the rosette stage and before stem elongation (bolting) in later spring.

Management of Winter Annual Weeds

  1. Tillage is a great way to control weeds. A decision to work the ground must be made with consideration for soil erosion potential and the potential oxidation of accumulating organic matter.  

  2. Chemical control is effective and can be done in the fall rather inexpensively with application of glyphosate, 2,4-D, and the addition of one of the many available residual chemistry options. I like 1 qt. of simazine on acres going to corn, while adding 2 oz. of metribuzin can be very effective on acres going to soybean. There are many other chemical options to consider but remember:

Thinking Outside of the Box

Rotating to a winter annual crop like wheat or barley means you are covering the ground with a crop you intend to harvest and use. A winter grain crop can be very good for the soil, reduce undesirable weed species, and allow for frost seeding a clover hay crop if you have animals. Cover crops can also contribute anywhere from 20 to 40 lb./A. of nitrogen to a following corn crop. On ground where dirt work like terrace construction or drainage installation is required, a winter annual grain crop allows an open summer window to get the dirt work done.

Planting a cover crop allows YOU to decide what winter cover you will have on your acres. Various cover crop species have varied functions that can be beneficial to soil health, reduce soil erosion, cycle nutrients from deeper in the soil to the upper root zone, and out-complete undesirable and resistant weed species.

On soils with significant slope and high erosion risk where you are unable to rotate to wheat or plant a cover crop, let nature’s cover crop like henbit do its thing. Leave it with your residue to cover and protect soil from loss. Plan on spring termination of the weed as early as possible, use of insecticide at planting to control seedling pests, light seedbed preparation, and row cleaners on the planter.

If you have any questions, please feel free to reach out.


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David Hughes

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