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We have published articles about fall herbicide treatments in C.O.R.N. over most of the last five years or more.  Readers can check the archives for those articles, as our suggestions about fall weed control have not really changed much from year to year.  Maybe the best thing we can do here instead of regurgitating the same information is to list some of the questions we have received this year about fall treatments, with answers of course.

1.  Do I need residual herbicide in fall treatments to control weeds that might emerge later in the fall? 

For treatments applied after about mid-October – probably not.  For early October treatments, a case can possibly be made for the addition of some Canopy or metribuzin to control weeds that emerge later in October.  Although we believe that Sharpen has more utility in spring than fall, it also provides some residual activity, primarily at rates higher than 1 oz/A.

2.  Should I deemphasize use of glyphosate in the fall and use dicamba instead?

This is certainly one option to reduce glyphosate use somewhere in the herbicide program.  In our research, glyphosate has really been a more effective fall-applied herbicide than dicamba, although dicamba used in the right mixture is fine.  We never recommend glyphosate alone in fall applications, so there are usually additional sites of action in the mix with activity on the typical fall weeds.  For glyphosate-resistant marestail, the other herbicides would of course have to carry the load, but 2,4-D is effective on small marestail in the fall even by itself.  We have been hearing that dicamba is currently expensive and in short supply, and replacing it with glyphosate is one option to limit costs.

3.  What rates of 2,4-D and dicamba should I use in a mixture, or what rates of the premix? 

We usually start with a 2,4-D rate of 0.5 lb ai/A and add other herbicide(s) to that to develop a comprehensive treatment.  We would suggest at least 8 oz/A of a dicamba product in a mixture with this rate of 2,4-D.  Higher rates of both could be warranted for control of biennials and perennials.  For the premix products, select a rate that provides at these rates of 2,4-D and dicamba or higher.  This may mean that you end up with a higher rate of one or the other based on the ratio of the two herbicides in the premix. 

4.  Are there differences in effectiveness among 2,4-D/dicamba premix products due to the formulation (ester vs amine, type of salt, etc)? 

Not that we know of.  Most effective herbicide mixtures that are applied in fall eventually kill the weeds, and minor differences in initial activity are not important, since the treatment will weaken the weeds, and cold winter weather will finish them off. 

5.  Can I use dicamba alone? 

No, and do not use 2,4-D alone either.  Dicamba has been ineffective on several key species in the fall when applied alone, and appears to be more affected by cold weather than other herbicides.  When applied alone, 2,4-D misses chickweed, especially in corn stubble, and is weaker on deadnettle, henbit, and dandelion compared to mixtures of 2,4-D with glyphosate, Canopy, Basis, etc.

6.  My rep is recommending about half of the residual herbicide premix be applied in fall and then the rest in spring.  Does this make sense? 

Not usually.  Chlorimuron is really the only herbicide that had substantial residual activity into spring when applied in fall.  Where residual into spring is desired, we suggest the use of a relatively low rate of a Canopy/Cloak DF or EX, which works well because it is low cost, controls emerged weeds when mixed with 2,4-D, and still allows the use of effective rates of residual premix products in spring.  Cloransulam (FirstRate) provides some residual into spring, but substantially less than chlorimuron.  Using a premix product in the fall (eg Valor XLT, Sonic, Authority First) is not cost-effective, since the second component of the premix (Valor or Authority) provides essentially no residual weed control in spring (and residual into later fall is usually not that important – see question 1).  We would rather see the full rate of the premix product applied in spring, to try to maximize control of marestail that emerge into June.  Metribuzin also provides no residual activity into spring, but has enough activity on emerged weeds in fall that a mix of metribuzin (about 6 oz) + 2,4-D can be cost effective.  

7.  Seems like the cost of fall treatments being recommended is going up.  How much do I need to spend? 

The cost of fall treatments can go certainly up considerably when the approach in question 4 is taken.  We still believe that the cost of herbicides for fall treatment should be less than $15.  We doubt that spending more than this improves control or makes the entire weed management program for the 2014 season work any better in the end.

8.  I am going to run some type of ripper, vertical tillage, or similar tillage tool this fall.  Should I apply herbicides before or after the tillage? 

Before, when weeds are still intact.  Also- these tillage tools do not adequately control emerged weeds in the absence of herbicides.

9.  Corn plants are large this year and there will be a lot of fodder on the ground after harvest.  Should I delay herbicide application until later in fall where this occurs? 

Our research results, along with the comments we get from dealers and growers about this, have led us to believe that the fodder should not be a problem if the correct rates and carrier volumes are used.  Fall-applied herbicides seem to work regardless and marestail does not like to grow under heavy residue anyway.  We don’t see a problem with waiting until later in fall to treat these fields, except that should wet weather and field conditions develop, it might be impossible to do so.  Over the years we have heard from a number of growers who ran into this problem (partly because we have advocated early to mid November as a good time to apply), and these growers now apply sooner in fall to ensure that it gets done. 

10. I have a new flush of emerged palmer amaranth this fall. Should I control it prior to frost?

Tough question. These plants may produce seed, and farmers in the mid south are using fall-applied herbicides to kill them before they produce seed and increase the seedbank.  If you are in a strict no-till system, then an application of Gramoxone or 2,4-D + dicamba might be warranted for fields that have this issue.  However, frost will kill existing Palmer amaranth, and herbicides are not needed if the plants fail to produce viable seed prior to frost.

Mark Loux is a professor of Weed Management in Field Crops at Ohio University.

This article originally appeared in Ohio State University’s C.O.R.N Newsletter and has been reposted with permission.