By Mark Loux, Extension Weed Scientist
The number of new herbicide introductions has slowed down over the past couple years, and most of the “new” products are actually just a recombination of existing active ingredients. The 2016 edition of the “Weed Control Guide for Ohio, Indiana, and Illinois” contains information on all but a few of the most recent products. A pdf of the 2016 guide is available at http://u.osu.edu/osuweeds/, along with videos that summarize new products and technology (hard copy of the guide available by mid-December).
One of the big questions remaining about 2016 herbicide programs is still, what will happen with Xtend and Enlist soybeans and the associated herbicides? Dow has an approved product label for Enlist Duo, but not all the necessary export clearances yet for the Enlist soybeans. Monsanto is apparently still working on everything — export clearances and dicamba product label approvals — so who knows where we will be by April of 2016.
One of the persistent questions about new products this year has been, “How much better is Acuron on giant ragweed compared with Lexar?” This question was driven in part by a higher price for Acuron, although apparently this price differential no longer exists.
Both Acuron and Lexar contain S-metolachlor (site 15), atrazine (site 5), and mesotrione (site 27). Acuron also contains bicyclopyrone, another site 27 inhibitor that contributes activity on larger-seeded broadleaf weeds such as giant ragweed and cocklebur. The rate of atrazine is lower in Acuron compared with Lexar, but this is presumably offset by the addition of the bicyclopyrone.
Acuron was sold in 2015 but was not listed or rated in the weed control guide until this latest edition. (See this article from Iowa State.)
Lexar is already a very broad-spectrum herbicide premix, and the two products have essentially identical ratings on grass and broadleaf weeds across the board in the guide.
One exception is giant ragweed, where Acuron has rating of “8+” and Lexar has a rating of “8.” The assignment of effectiveness ratings in the guide is the result of discussion among all of the authors. There was just enough research experience among all of us to conclude that Acuron can at times be a little more effective on giant ragweed than Lexar, and deserved a slightly higher rating.
OSU weed scientists would have been happy to keep the rating an “8” because we don’t believe any residual herbicide deserves a higher rating on giant ragweed. It’s a large-seeded weed that can germinate from fairly deep in the soil profile and it emerges from March through early July, which overall limits how effective residual herbicides can be on it. So while a number of residual herbicides can provide 100% control of smaller-seeded broadleaf weeds, such as lambsquarters, pigweed and smartweed, this is near impossible to achieve for giant ragweed.
We were not convinced that there is a consistent difference in control between Acuron and Lexar, but we agreed to the “8+” rating for Acuron in the interest of world peace and collegiality, and maintaining harmony with our weed control guide co-authors from Purdue and University of Illinois.
Here’s why none of this really matters though. Effective control of giant ragweed almost always requires a combination pre and post herbicide treatments.
There are several good residual herbicide programs for corn with substantial early-season activity on giant ragweed. Acuron is one of these, along with Lexar, Lumax, and mixtures of atrazine with Corvus, SureStart, Instigate or Balance Flexx. None of these will completely control even a low population of giant ragweed, but when there’s not much giant ragweed to begin with, the number of escapes can be low enough that following with post herbicides is not economical. It’s a numbers game really, so as soon as areas of higher ragweed density start to occur, the number of escapes increases and even the most effective pre herbicide program will not be sufficient without a post follow up. The fact that one of these herbicide programs might be a little better than another then becomes irrelevant, because without a post treatment they all fail to be effective enough at the end of the season.
Giant ragweed populations tend to be somewhat “patchy” in fields also, based on things like soil properties, drainage and proximity to infested fencerows. So rather than having a low population uniformly dispersed across a field, there is more likely to be widespread areas with almost no ragweed and smaller areas where the population is still moderate to high.
Remember that it’s much easier and economical to get effective giant ragweed control in corn compared with soybeans, especially if the ragweed has any level of resistance to glyphosate and ALS inhibitors. Using the appropriate program in corn for ragweed pays dividends in the following year(s) of soybeans. Giant ragweed does not produce a lot of seed and the seed has a relatively short life in soil, so it’s possible to greatly reduce the population over several years with effective programs.
In fields with more than a uniformly sparse population of giant ragweed, factor the cost of a post treatment into the planning, rather than using the whole budget on pre herbicides. Using one of the more effective pre treatments isn’t a bad idea — just don’t continue to increase money spent on pre herbicides in hopes of getting by without a post treatment.
And budget for something besides just glyphosate in the post treatment, since we are continuing to select for glyphosate resistance in our giant ragweed populations due to glyphosate overuse in corn and soybeans. Likewise, if the pre herbicide program comes with the guarantee of a “free” post treatment to control escapes if necessary, insist that the post treatment be something other than just glyphosate. Otherwise we’re all just continuing to shoot ourselves in the foot.