Fall herbicide treatments. What else remains to be said?
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.