Dry Weather -- Fall Herbicide Treatments and Burndown vs Early POST in Wheat

Author’s note – I wrote this before this week’s rain, which is usually the way it works with articles about dry weather. 

Winter annual populations in many fields seem to be low to fairly nonexistent at this time due to the dry end of summer conditions we have been experiencing.  Growth (or regrowth) of perennial weeds is also less vigorous than in wetter years.  This has ramifications for both fall herbicide treatments ahead of corn or soybeans, and decisions about the need for burndown treatments in no-till wheat.  The bottom line on fall herbicide treatments for corn or soybeans – don’t be in a rush to apply. There is really no risk of less effective control by delaying treatment, even through mid to late November.  Herbicides are more effective on dandelion after a frost in late October or November anyway.  It’s important to allow perennials such as Canada thistle to reach a height of at least 8 to 12 inches to obtain maximum herbicide effectiveness and the resultant reduction in population.  Delaying application until late in the fall obviously has to be weighed against the possibility that it will turn wet at some point, rendering fields unsuitable for traffic, but fields are dry enough that substantial rain would be required for this to occur.  Waiting until early November could also allow for a more informed decision on whether fall treatment is actually necessary.  One possibility is that winter weed populations will be low this year, and scouting fields in November prior to application would provide better information on this.  Where scouting at that time fails to turn up many winter weeds, the better decision may be to delay application until spring, which makes better use of residual herbicides and can reduce overall weed management costs.

This same thinking can be applied to winter weed management in no-till wheat.  A preemergence application of glyphosate can control winter annuals fairly well in a typical year when their emergence is well under way by late September.  However, we have also effectively controlled winter annuals and dandelion by omitting the preplant burndown treatment, and instead applying POST herbicides in November, using mixtures such as dicamba plus Express (or the equivalent generic product).  The November treatment can be considerably more effective for control of control of dandelion compared with the late September glyphosate application - see related C.O.R.N. article from 2008:

http://corn.osu.edu/newsletters/2008/article?issueid=257&articleid=1590

The current lack of winter annual weeds or actively-growing dandelions in many fields lead us to suggest that this may be the year to skip the preplant burndown in wheat, and instead apply POST herbicides later in fall (or in spring if weed populations in November do not merit treatment).  This will allow time for rain and subsequent emergence of those weeds that are primed to germinate when soil moisture is sufficient, and for the resumption of active dandelion growth.  This decision should obviously be made on a field by field basis based on scouting at the time of wheat planting, and fields with substantial weed populations at planting should be considered candidates for preplant burndown treatment.  One of the key decisions here is whether the weed population at the time of planting is substantial enough to suppress wheat establishment before POST herbicides can be applied.  Labels for many wheat herbicides specify that they should not be applied in the fall until wheat has 2 to 3 leaves, which does allow substantial time for growth of weeds that have already emerged.

For no-till wheat fields that require a preemergence burndown treatment, glyphosate is often the most cost-effective option.  Sharpen is labeled for preemergence use in wheat at rates up to 2 oz/A, and it can be added to glyphosate to improve activity on marestail and other weeds.  Gramoxone is also effective on small winter annual weeds.  We often get questions about the preplant use of 2,4-D or dicamba in wheat.  This subject was covered well in the latest issue of the Penn State Field Crop News, found at this link:

http://fcn.agronomy.psu.edu/2010/fcn1028.cfm