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Fall-Applied Soybean Herbicides: To Spray or Wait to Spray?

October 23, 2000
Recently, the concept of fall applications of certain soil-applied soybean herbicides has generated some interest and raised several questions about this weed-management practice. What are advantages and disadvantages of fall herbicide applications? Are there benefits to this practice? How about any serious limitations? Is fall a good time of the year to be thinking about weed management?

First of all, not every soil-applied soybean herbicide is labeled for fall application. And while the idea of fall-applied soybean herbicides is new to most people, some herbicide formulations (trifluralin, for example) have been labeled for fall applications for many years. The "new" thrust for fall-applied soybean herbicides originates in the idea that if a herbicide is applied to no-till ground in the fall, this may help to reduce or eliminate the growth of winter or early-summer annual weed species. Winter annual weed species such as purple deadnettle, henbit, chickweed, horseweed (marestail to most folks), and a number of mustard species can add both color and headaches to no-till soybean fields during the spring. The prevalence of these winter annual weed species has increased over the past couple of years and is probably due to extremely mild winters and the increased use of certain postemergence herbicides that lack significant soil residual activity. These mild winters have essentially extended the growing season, allowing for increased growth of these winter annual species. Dense populations of winter annuals can physically interfere with soybean planting and in some cases even reduce soil drying to a point which delays soybean planting. So the idea behind fall applications is this: If a herbicide with soil residual activity can be applied in the fall before most of these species germinate, it may help reduce the amount of vegetation that needs to be dealt with in the spring prior to soybean planting, maybe even eliminating the need for a burndown herbicide application. While the idea may sound good in theory, the actual results may or may not be as good as expected. Several factors will come into play that will determine how well this system will work.

Location in the State

Anyone who has traveled north to south or south to north in Illinois during the early spring months realizes there is a "gradient" with respect to when spring "green-up" occurs. Growth of winter or early-summer annual weed species in no-till fields will begin much earlier in the southern portions of the state than in the central or northern portions. As a general observation, weed growth is usually much further along in the southern portion than in the central/northern portions when a burndown herbicide application is made. So it’s not too surprising that much of the interest in fall-applied soybean herbicides occurs from about Springfield south. It’s not atypical in this region of the state to deal with weeds anywhere from a few inches to more than a foot in height when burndown herbicide applications are made. Soils typically take longer to dry following precipitation in the southern third of Illinois than the "darker" soils of central/northern Illinois, which can frequently result in additional delays in making burndown applications. Even though fields may be too wet to travel across, the weeds continue to grow.

Herbicide Selection and Application Rate

Currently, there are several soil-applied soybean herbicides labeled for fall application (yes, there are also several corn herbicides labeled for fall application, but we’ll cover these at a future time). Some of the current soil-applied soybean herbicides labeled for fall applications include Canopy, Canopy XL, Sencor, Sencor + Python, Steel, and Backdraft. With each of these herbicides there are still many things to be learned related to fall applications (which herbicide controls which winter annual species, when will be the best time to apply during the fall, will spray additives need to be included, etc.). In particular, the question of application rate is one that will be of significant importance, as will knowing which winter annual weed species will predominate. Many of these herbicides may also recommend the addition of 2,4-D, glyphosate, or some other herbicide to help control any emerged vegetation at the time of application. If needed, what rate of these tank-mix partners will be needed?

Concerns to Ponder

One of the major concerns with fall-applied residual herbicides is that the herbicide will be applied on average at least 6 months before soybeans are planted. Rainfall events and snowmelt may have a major impact on how much of the herbicide remains by the time soybeans are planted. These precipitation events, combined with degradation from soil microorganisms, increase the probability that herbicide activity will be reduced by the time of soybean planting. This may not be as large a concern if fall applications are targeted more to reduce the amount of winter annuals present at planting than to guaranteed control of summer annual weed species well into the growing season. Fall-applied residual soybean herbicides also limit the choice of what crop can be planted in the spring, since several of these herbicides have crop rotational restrictions.

Have we conducted any research on fall-applied soybean herbicides?

Yes; we have one year of data at this point in time. We initiated an experiment during the fall of 1999 at four locations to examine the efficacy of fall-applied soybean herbicides. The locations we selected were Dekalb, Urbana, Brownstown, and Altamont, which represent a good north-to-south gradient, as well as some diversity in weed species. At these four locations fall herbicide applications were made in mid-November. The herbicides that we included were Canopy, Canopy XL, and Sencor each at two rates, with and without Roundup Ultra + 2,4-D. Roundup Ultra + 2,4-D (1.5 pt + 0.5 pt) was also applied alone to see how this treatment would work without a residual herbicide. As we expected at the outset, results with these fall applications were quite variable.

As previously mentioned, there can be quite a difference in winter annual weed growth from south to north in the state. One result that really coincides with this observation was that at the northernmost location (Dekalb) there were no weeds present at the time of application in the fall or even at soybean planting, including untreated plots. This suggests that areas in the northern part of Illinois, with cooler soil and air temperatures in the spring, are probably not well suited for fall soybean herbicide applications for winter annual weed control because these weeds tend to be not as much of a problem north of I-80 compared to the southern regions of the state.

However, we had some good results from fall applications at Urbana, Brownstown, and Altamont. At planting, fall-applied Canopy (7.0 oz/A) was the most consistent soil-applied herbicide for controlling common chickweed, purple deadnettle, and winter annual mustard species (Table 1). Henbit control was excellent and was not different between any of the herbicide treatments. Many of the winter annual species were present when the fall applications were made, and even fall application of Roundup Ultra + 2,4-D provided greater than 80% control of common chickweed and mustard species at soybean planting. Both rates of fall-applied Canopy, and the highest rate of Canopy XL (6.8 oz/A), also controlled common lambsquarters, and common ragweed by soybean planting (Table 2). Even though we observed some positive results this year, conclusions are usually hard to come to with only one year of data. We plan to repeat this research during the fall of 2000.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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