C.O.R.N. Newsletter 2010-30

We have published numerous articles on fall herbicide treatments in the C.O.R.N. newsletter over the years, and I’m not sure that anything I write here can greatly improve on some of those articles.  Here are the links to two articles from 2007 that cover the subject fairly well:  

“The ABC’s of fall herbicide treatments”

http://corn.osu.edu/newsletters/2007/article?issueid=204&articleid=1251

“Should residual soybean herbicides be applied in the fall?”

http://corn.osu.edu/newsletters/2007/article?issueid=204&articleid=1252

Revisiting some of the main points of these articles briefly:

1.  You should have a reason for applying herbicides in the fall, the primary reasons being control of an existing infestation of winter annuals or marestail, or control of biennials (wild carrot, poison hemlock) or perennials (dandelion, quackgrass, Canada thistle) that are most susceptible to herbicides in the fall.  It’s possible to justify making a fall application to prevent some of these problems also, but low populations may be adequately controlled with spring burndown treatments.  The primary value of fall herbicide treatments is control of weeds that have emerged by the time of application, which typically results in a weed-free field next spring, at least until sometime later in April.  This can be accomplished with about $4 to $12 worth if herbicide.  

2.  There is a core group of herbicides that will control emerged weeds when applied in the fall, which includes the following:

Soybeans next spring
Canopy EX or DF + 2,4-D
Glyphosate + 2,4-D
Metribuzin + 2,4-D (excluding dandelions)

Corn next spring
Basis + 2,4-D
Glyphosate + 2,4-D
Simazine + 2,4-D (except dandelion)

It’s possible to use other treatments in the fall, such as dicamba + 2,4-D, Autumn + glyphosate or 2,4-D, or Express + 2,4-D, but these are not more effective than the treatments listed above (and can be less effective on dandelion).  Glyphosate + 2,4-D is the most effective for control of most perennial and biennial weeds, and glyphosate can be added to the other treatments to accomplish this.  Combinations of 2,4-D plus Canopy or Basis have been the most consistently effective on dandelion.  

3.  Among all of the herbicides mentioned above, only Canopy products provide substantial residual control of annual weeds that emerge in spring or early summer.

Within the labeled rate range, any Canopy rate is usually effective for control of emerged weeds in fall, but increasing the rate can improve the control of summer annual weeds that emerge the following May/June (but not always).  Our research shows that it is largely a waste of money to apply residual soybean herbicides other than Canopy in the fall with the goal of controlling weeds that emerge after soybean planting the following spring, because they all largely dissipate during winter to the point of providing little to no activity the following spring.  Basis and simazine treatments provide limited residual control of spring-emerging weeds, primarily early-emerging winter annuals. The choice of herbicide programs for in-season weed control in the following year’s corn should largely not be based on what was applied in the fall.  In other words, a comprehensive herbicide program is needed in corn regardless of whether herbicides were applied the previous fall, with the exception that the preemergence herbicides may not need to have much burndown activity in fall-treated fields.

In most soybean fields it is a big mistake to apply all of the residual herbicide in the fall, with the goal of making only post emergence glyphosate applications the following year.  It is possible to do this with Canopy products in fields with low populations and no glyphosate resistance issues, but most fields benefit from use of residual herbicides in the spring, along with some additional burndown herbicide if needed.  This program (fall residual followed by POST glyphosate next year) is frequently doomed to fail in fields with marestail problems.  Most of the marestail in the state is now glyphosate-resistant, and many populations are resistant to ALS inhibitors also.  This means that it is impossible to control marestail after soybeans have emerged, unless the field is planted with LibertyLink soybeans.  

In those fields requiring a fall herbicide treatment for management of other winter annual weeds, dandelion, etc., where marestail also occurs, it is essential not to apply all of the residual herbicide in the fall.  The majority of the residual herbicide should be applied in the spring, to maximize control of marestail that emerges in May and June.  The most effective residual herbicides include two modes of action, to ensure effectiveness on ALS-resistant marestail.  Examples:  Envive, Valor XLT, Gangster, Sonic, Authority First, Canopy DF + metribuzin.  We suggest one of the following approaches:

1.  Apply a combination of glyphosate and 2,4-D in the fall, followed by application of residual herbicide in the spring prior to soybean emergence.  At the time of soybean planting, the field may be infested with marestail that emerged earlier in spring, so include effective burndown herbicides (2,4-D, Gramoxone, glyphosate, or Ignite or some combination as appropriate based on herbicide resistance, plant size and time until soybean planting) to control emerged plants.

2.  Apply 2,4-D with Canopy DF or EX at fairly low rates (e.g. 1 oz of EX or 2 oz of DF) in the fall, followed by application of residual herbicide in the spring (with burndown herbicides if the residual from fall does not hold marestail through planting).  It is possible to follow the fall Canopy application with a spring application of a chlorimuron-containing herbicide, as long as the total does not exceed maximum labeled rates of chlorimuron for the soil type.

3.  Apply 2,4-D with metribuzin in the fall, which should control most emerged winter annuals, but is weak on dandelion.  Follow with application of residual herbicide in the spring (with burndown herbicides if the residual from fall does not hold marestail through planting). 

The idea here is to apply a herbicide treatment in the fall that adequately controls emerged weeds, provides some residual if desired, but does not break the bank and allows use of the majority of the residual herbicide in the spring.  Option 1 has become lower in cost recently, due to declines in the price of glyphosate.  Options 2 and 3 fit because the residual herbicides can control emerged weeds when combined with 2,4-D without the need for glyphosate, and they provide some residual.  Canopy certainly provides substantially longer residual than metribuzin, but use of metribuzin preserves the option to plant corn the following spring. 

Where the marestail is resistant to ALS inhibitors as well as glyphosate, fall application of Canopy will fail to provide control of spring-emerging marestail.  This will usually not be improved with application of a premix such as Valor XLT/Envive, Authority First/Sonic, etc., since the second component of these products does not provide much control of spring-emerging weeds when applied in the fall.  Money spent on the more expensive fall approach would be better spent on a more effective spring burndown program if necessary.  Our research shows that the best approach where marestail are resistant to both glyphosate and ALS inhibitors:  Apply glyphosate + 2,4-D in the fall and follow with broad-spectrum residual herbicides in spring combined with enough burndown herbicide to ensure complete control of marestail.  This can be a challenge in the parts of the state where wet soils in spring can result in late planting and big marestail.  But applying more residual herbicide in the fall won’t solve this problem, and we have burndown options that can work on big marestail (Ignite + Sharpen, Ignite + metribuzin, glyphosate + Sharpen, etc.).  Reminder that there is a fact sheet available on marestail management in soybeans, which is available on our website and also at the OSU and Purdue weed science websites.  Link: https://agcrops.osu.edu/specialists/weeds/marestail/marestailfact09_000.pdf/view