I just wanted to start out this week’s column by thanking everyone who attended my first event here in Darke County on Tuesday night. We had a great turn out, with a group that facilitated discussion and shared information. I think I learned about as much as the attendees did. For those of you who were working or voting last night, I wanted to write up a brief summary of what others learned Tuesday night.
We are dealing with two very hard to control weeds in soybeans. Giant ragweed and marestail are becoming more and more prevalent in our soybeans because our practices of late have selected for them. ‘Selected for’ is an ecological phrase that means what we are doing as producers gives these weeds the right conditions to flourish. The two major contributing factors to these weeds becoming a problem are pushing for earlier planting dates and heavy dependence on glyphosate for control in soybeans.
These weed populations are seeing and increased level of glyphosate resistance. Resistance is the ability of a plant to survive normal and even higher application rates of an herbicide. Resistance is something that occurs naturally. Each plant in a species (like marestail) is genetically different, just like people. Some plants have resistance for the herbicides encoded in their genes before they even emerge from the seed. This becomes a problem when we select for them. Applying glyphosate as a burndown, and post emergence herbicide kills all the weeds that are susceptible to the herbicide. This leaves the resistant plants to survive and produce seed. We see a shift in the population from susceptible to resistant. Resistance in a weed like marestail is especially scary because the weed seeds can spread by wind up to 200 miles. Even if you take care to rotate your herbicides, if your neighbor doesn’t his seed may blow into your field or vice versa.
The other problem with these weeds is that producers are pushing for earlier planting with soybeans, well into April. Giant ragweed and marestail have extended emergence well into July. The earlier we plant the longer the soybeans are subject to new flushes of weeds. As an end result, producers need to make more post emergence applications of herbicides to soybeans. This is especially true to keep the soybeans weed free for the first 6 to 8 weeks of growth to maximize yield potential. Even after those 6 to 8 weeks, the weeds can still cause yield loss.
So how do we deal with these weeds? We practice integrated weed management. This involves cultural methods of control, cleaning your equipment between fields to prevent spreading seed, rotating your crops to allow the use of different herbicides and different herbicide timings on the weeds and rotating your herbicide modes of action. A good program for marestail is the use of a fall burndown with glyphosate tank mixed with 2,4-D. In the spring, a burndown application before planting with herbicides like Sharpen mixed with glyphosate or paraquat mixed with 2,4-D and metribuzin. Also a pre-emergence herbicide is needed to control marestail. Mark Loux, the OSU Extension Weeds Specialist, does not recommend post emergence applications to control marestail…it is a last resort. He doesn’t even recommend glyphosate or ALS herbicides; Ignite is your only choice. Giant ragweed can be controlled with a spring burndown with glyphosate and 2,4-D or Sharpen. Also a pre-emergence herbicide helps keep the weeds smaller and increase the time you have to apply your POST herbicides. Ignite has proven, through research, to control giant ragweed very well. The other key to controlling these weeds is to kill them before they get taller than 6 inches, because they are most vulnerable before then.
I have handouts left over from the talk in the carousel here at OSU Extension. These handouts have a full description of the weeds, how to control them and lists of herbicides that are effective. Also OSU Bulletin 789, “Weed Control for Ohio and Indiana” is a great reference for controlling most weeds in all of our crops in Ohio. If you would like to discuss this, or any other weed issue, contact me at OSU Extension, Darke County, by dropping by or calling 548-5215. Remember to get out and scout your fields and be a good land steward, so your neighbor doesn’t have marestail float into his field.
Justin Petrosino, Extension Educator, Agriculture and Natural Resources
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