Source: Ohio State University Extension

By Mark Loux, Weed Scientist

According to our weather guru, there is no close precedent for a summer like this in the last 100 years, and I can’t recall a year with this much mid-season rain in my almost 30 years here. This has obviously caused immense problems with post-emergence herbicide applications.

There are many fields with large giant ragweed plants that still require treatment, should field conditions become suitable for traffic again. Even the best herbicide treatments are not likely to completely control all of the large giant ragweed, but they can be at least partially effective.

Additional goals of herbicide treatments at this time are: 1) prevent plants from getting even larger; 2) prevent or reduce seed production; and 3) prevent harvest problems and losses. When selecting herbicides for late-season herbicide applications, keep in mind the restrictions on maximum crop stage, pre-harvest intervals, and crop rotation restrictions. For some of the key giant ragweed herbicides, these are as follows:

Classic — Apply 60 days before maturity. 9 months to corn, 3 months to wheat.

Cobra — Apply 45 days before harvest. No rotation restrictions.

FirstRate — Apply 65 days before harvest. 9 months to corn, 4 months to wheat.

Flexstar, Flexstar GT (including generic equivalents) — Apply 45 days before harvest. 10 months to corn, 4 months to wheat.

Glyphosate — Must be applied prior to the end of the R2 stage. No rotation restrictions

Liberty — Prior to soybean bloom. No rotation restrictions.

We suggest using maximum-labeled rates of any herbicide(s) applied for control of ragweed this late in the season. Other application parameters — adjuvants, nozzles, volume — should also be adjusted to maximize herbicide activity on large weeds.

Obtaining effective control of marestail in wheat stubble has been an issue for several years. This situation can include marestail plants that are cut off by the combine, which then regrow, and smaller, younger plants that were “lurking” lower in the wheat below the cutter bar.

We conducted a study last summer to determine the effect of herbicide and application timing on control, in a field that mostly had the latter type of plant. In a nutshell, herbicides applied on July 25 were overall more effective than those applied on Aug. 7 (and we assume that effectiveness continues to further decline for applications later in August). 

The commonly used combination of glyphosate and 2,4-D adequately controlled marestail at the first application (not all plants dead but prevented seed production), but some plants survived and produced seed at the later application. Similar results occurred for the combination of glyphosate and dicamba.

Other treatments, which consisted of various herbicides mixed with Sharpen, Liberty, or Gramoxone, largely prevented seed production at both application timings. We assume that control of larger plants previously mowed off by the combine would be more difficult.

Overall, the results of this study indicate that earlier is better than later with regard to marestail control following wheat harvest. Preventing marestail seed production is really the key goal of wheat stubble treatments, and it’s possible for this to be achieved even if some plants are still alive late in summer.