Fall Weed Management, is it Right for You?

Glenn Nice and Dr. Bill Johnson, Department of Botany & Plant Pathology, Purdue University

In the past, you might have been reading this article about fall tillage, but today with no-till corn and soybean common practice the use of herbicides in the fall has replaced tillage.  The use of herbicides in the fall has become a strong market for many of the companies selling herbicides.  There are several products labeled to be used in the fall (Table 1 lists a few).  Some products, such as Canopy Ex, simazine, or Autumn can provide residual control into the spring with some foliar activity; while other herbicides, such as glyphosate, Gramoxone Inteon, and 2,4-D provide burndown punch, but give little to no residual activity.  In many cases, the two (residual activity with a burndown product) are often combined to provide effective control of emerged weeds and residual control for those early spring germinators.

Why Use a Herbicide in the Fall?

Winter annuals have appeared to be on the increase over the past several years.  This is due to several things including, but not limited to, the adoption of no-till, milder winters, and the decrease in the use of residual products as a whole.  A mat of vegetation in the spring can delay planting.  The soils dry and warm slower in the spring when vegetation is present.  In Indiana, spring burndowns are often delayed due to wet weather making the use of a fall applied program a benefit.  Research being conducted at Purdue University and other universities have also found that some winter annuals not only attract pests in the spring, but some of the winter annuals are serving as hosts to soybean cyst nematode (SCN).  Purple deadnettle and henbit have been identified as strong alternative hosts for SCN; while field pennycress was identified as a moderate host; chickweed, shepherd’s purse, and smallflower bittercress have been identified as weak hosts.

Many of the biennials and perennials that appear to be on the rise are controlled more effectively in the fall than in the spring.  In the fall, perennials are often translocating sugars down into the underground structures to survive the winters.  A herbicide that is translocated, for example glyphosate or 2,4-D, would also be translocated to these underground structures.  Two weeds that are becoming increasing problems in areas of Indiana are dandelion (perennial) and cressleaf groundsel (annual).  Two biennials that we have received more calls about in the past couple of years are musk thistle and poison hemlock.  Control with herbicides is often more effective when these weeds are in the rosette stage in the fall before they bolt.

Fall applied programs may not be for everybody.  If you are on highly erodible land it might not be a good idea to have a vegetation free field over the winter months.  Many growers in this situation will plant cover crops that will help in maintaining top soil and help suppress weed development.  Using a fall applied program does not guarantee that you will not need a burndown in the spring.  If the winter months are wet and warm, microbial activity will increase breaking down herbicides faster decreasing the amount of herbicide available in the spring.  A warm winter will also decrease winter annual seedling mortality and/or increase the possibility of germination during the winter or early spring.  Furthermore, early germinating weeds such as common lambsquarter and giant ragweed will take advantage of the clean field and warmer soil to emerge earlier in the spring, thus needing a burndown.  In some cases, as with Canopy EX or simazine, the use of a fall applied herbicide will lock you into what you can plant the following year (Table 1).

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The use of a fall applied program on the left; no fall applied program on the right, South East Purdue Ag. Center