Source: Dr. Mark Loux, OSU Extension

We’re running about a month behind in many cases, and with respect to weeds we are a month later than normal in implementing herbicide programs.  The most important thing to know about this is that we are well into the period of summer annual weed emergence, most of which occurs between early May and the end of June, which overall shortens the period of weed control that we need and allows earlier application of POST herbicides.  There are some advantages to this – here’s what it means for those fields just planted or that will still be planted within the next couple weeks:

Because we are this late, the burndown has become a major part of what is usually our in-season herbicide program, and is taking care of a good portion of the summer annuals that residual and POST herbicides would usually control.  The big glaring issue at this time is nasty burndown situations, and we provided some suggestions for this in previous articles.  Lots of pretty yellow fields due to cressleaf groundsel.  Keep in mind that this and other winter annuals that have flowered are ending their life cycles, so they have died or started dieing on their own anyway.  Focus should be more on the large giant ragweed, lambsquarters, marestail, etc that are present.  We are also late enough that waterhemp is part of the burndown mix in come fields.  Don’t skimp.

Herbicide programs do not have to last as long in crops planted late.  When we plant in early May, we need an herbicide program that controls weeds from then until the end of June or so, which is in part why we use residual herbicides and frequently apply POST herbicides 5 to 6 weeks after planting.  Applying the POST too early in a crop planted in early May introduces the risk of poor control of weeds that emerge soon after that application, before the crop is developed enough to control them on its own.  This is much less of an issue with a late-planted crop.  Since summer annual weed emergence tapers off as we move through June, the POST herbicides can be applied much sooner after planting without sacrificing control.  Planting soybeans this late can therefore allow earlier POST applications when weeds are small – more like 3 weeks after planting.  This can help minimize carryover concerns with fomesafen, and also provide a wider window to look for the right conditions to apply dicamba (see below).

In studies of reduced-rate POST applications that we conducted a couple decades ago, planting soybeans in late May or early June allowed us to use earlier POST applications (e.g 21 days after planting) at lower rates and still maintain control.  There were just fewer weeds emerging after planting and the duration of weed emergence after planting was also shorter.  It was not possible to achieve this in early-planted soybeans – we needed either two applications at reduced rate or a later application at full rates for control.

One issue with later POST sprays is the potential for herbicide carryover from products such as mesotrione and fomesafen (Flexstar, etc), among others.  Fomesafen carryover has been rare in the state but risk increases with later applications, especially if rainfall subsequently becomes limiting.  Where glyphosate and fomesafen are being combined in a late POST application to control ragweeds, it may become necessary to replace the fomesafen with lactofen (Cobra/Phoenix) as applications move into July.  Our research indicates that the lactofen products are less effective than fomesafen in this mix by about 10 to 20%.

Another concern would be POST applications of dicamba on Xtend soybeans shifted later into the hotter weather that occurs as we move from June into July.  The consensus of the weed science community is that both of the approved dicamba formulations have potential to move via volatilization, and the risk of this would increase with increasing temperatures and increased frequency and duration of inversions.  The current long-range forecast also indicates a trend for hotter than normal temperatures as we move into mid-summer.  Current labels allow application through 45 days after planting or prior to the R1 stage, whichever occurs first.  For soybeans planted early, the 45-day limit is often the main determinant, but later planted soybeans progress through growth stages more rapidly so the R1 stage may be the more frequent limitation.  Movement of dicamba onto other types of soybeans later in summer also has increased potential to reduce yield, since long-term effects of exposure to dicamba are more severe when soybeans have flowered.  Bottom line here is that there is less weather and time suitable for dicamba application with late-season applications, and movement and injury that does occur can have more substantial impact.  Planting within the next couple weeks and looking for the right conditions to spray starting about June 21 would provide more flexibility with regard to weather and weed size than deciding to wait until about July 10 to spray when it’s hotter and weeds are already large.

Can residual herbicides be omitted in late-planted soybeans?  Maybe.  Reduced weed populations could make this more feasible, but we really hesitate to recommend it.  Omitting residuals is never the right thing to do in fields with a history of weed control problems or high weed populations, or those with waterhemp and Palmer amaranth.  One advantage of omitting residuals would less risk of antagonism with burndown herbicides in mixtures.  Applying certain soybean residuals in June can increase risk of carryover.  The effectiveness of current soybean herbicide-tolerance trait systems makes this more of a possibility, but lack of residuals generally increases risk of problems and selection for resistant weeds, and makes timing of POST herbicides more critical.  The latter point is important because with a compressed season, applicators can be required to cover a lot of acres within a short period of time.  Keeping residuals in the program allows for more flexibility overall.

In late-planted corn, residual herbicides may be effective enough to reduce need for POST herbicides.  Or the residuals could be applied early POST, after the rush to plant is over (keeping rotation guidelines in mind).  Some corn fields are already in this situation, planted without any herbicide applied yet.  The table below shows restrictions on POST use of residual corn herbicides (source – U of Illinois).  This information can also be found in the herbicide descriptions in the Weed Control Guide , and is also summarized in this PSU newsletter article and this table from the MSU weed control guide.  Reminder that use of 28% UAN as a spray carrier is prohibited for POST application of herbicides with the exception of Degree Xtra.