Fall Herbicide Treatments Weighed

By Jane Fyksen, Crops Editor
Thursday, October 5, 2006 11:01 AM CDT

As combines roll through corn and soybeans this month, spraying equipment might be close behind.

Fall applications are becoming more common as growers attempt to stop winter annuals from gaining a foothold come spring.

One big reason fall applications are gaining momentum is no-till, says Glenn Nice, Purdue University weed scientist in Indiana. In no-till, it's a "bad idea" to plant directly into a bed of weeds, so growers either have to consider a burndown in the spring or a fall herbicide application, he notes.

Winter annual weeds have become more an object of concern because of some mild winters in recent years, making it easier for them to survive. What's more, Mother Nature soon adapts to cropping system changes, thus no-till growers are finding weeds in their fields after harvest, such as smallflower buttercup, shepherd's purse, pennycress, smooth hawksbeard, one or more chickweed species and perhaps a perennial like dandelion or white cockle.

Research continues to show fall burndown programs to be effective so that planting next spring can proceed full speed ahead.

Nice thinks growers should weigh the pros and cons of a fall weed-management strategy, however. "One of the pros...is that you have a clean seedbed in the spring. If the winter cooperates, you might even be able to plant without having to worry about making a burndown application in the spring," he remarks.

"Another advantage...is the planting window the following spring," he says of wet springs that have pushed growers out of their fields for longer periods of time. "This allows a lot of weeds to build up and germinate in the early spring. A fall-applied herbicide with some residual activity can increase the window of when you would have to be in the field to plant."

A mat of vegetation in the spring can slow drying and warming of the soil and delay planting. Wet springs make it tough to apply timely spring burndowns, delaying planting further. In a few cases, planting has to be done into a mat of winter annuals and/or summer annuals coming up in the spring, or growers use aggressive tillage to dry the soil, resulting in added cost, soil compaction and fields prone to erosion if hay rains fall soon after tillage.

Nice notes there's also some research suggesting that a field heavy with winter annual weeds can attract pests. Some winter annuals can serve as alternate hosts to soybean cyst nematode. Purdue research is being conducted now to see what the optimum timing of fall applied herbicides is to interfere with nematode/weed interaction.

A fall program isn't for everyone, though. "If you're in a highly erodible area, it's probably a good idea to leave some vegetation on the field over the winter to reduce the movement of topsoil," says Nice. "Also, if the winter does not cooperate with you, you may still have to put down a burndown application anyway." That'll add to your cost and time inputs.

Nice says winter weather not only effects the germination patterns of winter annuals but the persistence of the herbicide in the soil if a residual is used. Winters that are wet and mild can increase microbial activity, thus speeding breakdown of herbicide in the soil.

"One other thing to keep in mind is that a lot of your fall-applied systems would not control giant ragweed in the spring. Often, if you use a product that does not have any residual activity, you're still going to get germination of lambsquarter and chickweed and so forth. Even with a residual, there's a good possibility that the residual activity will not be present at the time that giant ragweed starts to germinate," Nice concedes.

In other words, a nonresidual herbicide or a residual herbicide that doesn't provide good residual activity on common lambsquarter and giant ragweed in the fall will control winter annuals but allow the soil to warm more quickly in the spring. Thus summer annuals will emerge earlier in the spring.

Most fall burndown herbicides only kill weeds present now - not those that'll germinate next year. Also note that soybean fields are generally less likely to have winter annuals or dandelions than cornfields.

Research has demonstrated that dandelion can be more sensitive to effect herbicides when applied after a light frost. However, this would make for a small window of application in many years and possibly jeopardize control.

In a few cases, using a specific product will lock you into a specific crop next year. For instance, Canopy EX requires that soybeans be planted the following year, says Nice. Simazine 4L requires that only corn be planted the following year.

"One final 'con'...is that some producers winterize their spraying equipment before any chance of overnight frosts. The concern of having fluids freeze overnight in the equipment's plumbing could lead to extra time and money costs," he adds.

Nice says several fall-applied herbicides have fairly large application windows, allowing weed size to be the timing criteria for application. Basis or Canopy EX can be applied anytime after harvest to before the ground freezes when weeds are less than three inches tall or three inches in diameter, but before they bloom. Princep can also be applied after harvest to weeds less than two inches tall or wide. When winter annuals get bigger. Tank mixing 2,4-D or glyphosate is common. These also have wide windows of application.

All herbicide programs should be applied before the ground freezes though, Nice warns. This is for two reasons. Winter annuals have essentially shut down at that time and any application would have little impact. The other possibility is off-site movement of herbicide is greater when applied to frozen ground.

So what should growers use on some of the major winter annuals? The use of 2,4-D is common, more than likely combined with something else to provide broader spectrum control. Alone, 2,4-D isn't highly effective on common chickweed, Nice notes. In most cases, adding Basis, Canopy EX (before soybeans), Express, glyphosate, Simazine (before corn) or Valor increases control of winter annuals. Glyphosate products are often used in tank mixes due to its broad spectrum weed control and efficacy on many perennials.

Mark Loux at Ohio State University also sees more no-tillers relying on fall herbicide treatments.

"First, the fall treatment has to control winter annual, biennial and perennial weeds that emerge in late summer or fall or are already present at the end of the previous crop's harvest. Weeds in this category include chickweed, annual bluegrass, purple deadnettle, marestail, wild carrot and dandelion, among others. These weeds overwinter and regrow in the spring, interfering with crop establishment and early-season growth, and they need to be controlled by a fall or early-spring herbicide application," Loux says.

"A secondary goal," he says, "is to prevent seed production by these weeds, which reduces future weed infestations. Fall herbicide treatments are by far the most effective method of controlling dandelions, poison hemlock and wild carrot, which are difficult to control in the spring."

Ohio State has conducted research on fall herbicide treatments for a number of years. He lists what he feels are the most effective treatments based on Ohio research. One new herbicide, Autumn, an ALS inhibitor labeled for fall application prior to corn was recently added to the list; Loux admits they have somewhat limited research on the effectiveness of Autumn compared to the other herbicides listed, but he says it appears to control many winter annuals and dandelions when mixed with either 2,4-D or glyphosate. Autumn doesn't provide residual control of weeds that emerge in the spring, he adds.

Effective fall herbicide treatments include the following, according to Loux:

Corn next spring - Glyphosate and 2,4-D; Simazine and 2,4-D; Basis and 2,4-D; and Autumn and 2,4-D or glyphosate.

Loux agrees that for the most effective dandelion control, delay application until after a frost. He generally recommends treating when dandelions are still mostly green though. Apply glyphosate-containing treatments with ammonium sulfate and additional nonionic surfactant if specified by the product label. Treatments that don't contain glyphosate should generally be applied with crop oil concentrate for best results, he says.

"Other herbicides - Valor, Scepter, Python, Aim, etc. - can be applied in the fall, but most of these will not effectively control winter weeds unless they are combined with one of the herbicide treatments listed above," Loux maintains.

"Effective control in the fall can be obtained with treatments costing $6 to $12 - excluding application cost - and we question the value of treatments that cost more than about $12," he states. "One of the reasons for this is that the use of a fall treatment, even one with residual activity, does not guarantee that only one herbicide treatment will be required in Roundup Ready soybeans the following year. Excess money spent on fall treatments results in less money available for weed control in the crop, where it usually is greatly needed."

"Our experience has been that the primary benefit of fall treatments is control of weeds that are present at the time of treatment - not residual control of weeds the following year," Loux continues. "The primary exception to this is Canopy EX, which provides a longer period of residual control of a greater number of weed species than other residual herbicides that can be applied in the fall."

"An effective fall treatment usually results in a field that is mostly free of weeds until about late April, and this goes for treatments without residual as well as those with residual," Loux says. "In other words, in late April, we cannot usually discern much difference between a fall treatment of glyphosate plus 2,4-D versus Canopy EX plus 2,3-D, even though the Canopy EX provides residual activity into the spring."

"However, the effect of the residual herbicide becomes much more apparent by the end of May when its activity on summer annual weeds comes into play," he adds.

"The issue here is not really control at the time of soybean planting, since any effective all treatment results in a relatively weed-free field at the end of April. This issue is how well the residual herbicide controls weeds after planting in order to build more flexibility into the postemergence application window. This flexibility can result in less risk of early-season yield loss from weed interference and result in a better chance of getting the postemergence herbicides applied to the right size weeds," says Loux.

"We also look to the residual herbicide to help control several weeds that glyphosate can be somewhat variable on, such as giant ragweed and lambsquarters," he states, noting it's the opinion of Ohio State experts that if glyphosate is being managed properly, it's typically going to be extremely difficult to get by with one postemergence glyphosate application unless the residual herbicide applied in the fall provides substantial weed control into late May. Even when that happens, a second postemerge application is often needed to get season-long control.

"Herbicides other than Canopy EX can provide residual control of certain weeds when applied in the fall, but they tend to control fewer weed species and/or be generally less effective than Canopy EX," Loux points out. "For example, Valor provides residual control of lambsquarters into early June but is less effective than Canopy EX and provides very little control of giant ragweed. Scepter provides very little control of lambsquarters or giant ragweed when applied in the fall, whereas it can provide substantial residual control of these weeds when applied in the spring."

"Gangster is labeled for fall application, and it may be fairly comparable to Canopy EX in its residual. Fall application of Gagster would still require the addition of glyphosate plus 2,4-D, which would result in a high-priced treatment," he says.

The bottom line on fall herbicide treatments? Choose an effective treatment from the list above, says Loux, and be cautious about adding residual herbicides that claim to improve control of weeds that emerge in the spring.

"The utility of most residual herbicides is maximized when applied in the spring prior to planting - not in the fall," he stresses, noting a better alternative is to use a non-residual herbicide program in the fall, such as glyphosate plus 2,4-D, followed by spring application of residual herbicides. This approach, he concludes, provides for the greatest flexibility in the postemerge application window and the most effective control of weeds like lambsquarters, marestail and giant ragweed.

 

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