Thursday, February 24, 2011

SCOUTING THIS GROWING SEASON: Weeds (Extension News Release)

Its time to talk about my favorite topic in pest management, weed control! Actually I should say management, control denotes a reactionary measure. We want to be proactive when it comes to taking care of our weeds. This is especially true when it comes to tough to handle weeds like marestail. Knowing a little about our weeds lifecycle can also help us plan a program to manage it.

Let’s use marestail as a case study. Here we have an ugly weed that can begin to emerge as early as August and continue emergence late into the fall. Those plants then overwinter and as wheat breaks dormancy, so does marestail. This is complicated by stragglers that emerge in the spring time. Marestail throws us another curve beginning in late April by switching from being a small basal rosette to bolting into the tall plant we typically see peaking through the soybean canopy. As if that wasn’t bad enough we also have populations in Ohio that are resistant to glyphosate and ALS herbicides.

How can we use this knowledge against marestail? Simple: attack early, attack often, and attack with everything we’ve got before it gets four inches tall. Marestail management should begin in the fall with an application of glyphosate + 2,4-D or Canopy + 2,4-D. Come back in the spring with a broad spectrum product like Ignite 280 SL or glyphosate with some extra kick like 2,4-D or Sharpen. Now there is a caveat with Sharpen, once we use it the label restricts using another PPO herbicide for 30 days. That eliminates another key element to our program, PRE herbicides with residual activity. Out of all of our best PREs Canopy DF is the only product that does not contain a PPO. So it will work with Sharpen, but add metribuzin. If you are not using Sharpen in your burndown other products lines like Valor and Authority have great residual control of marestail utilizing PPO herbicides. Another proven spring program is paraquat + 2,4-D + metribuzin. It is possible to have a one pass burndown + PRE treatment one week before soybean planting. Just make sure to use the appropriate rate of 2,4-D. That of course can be found on the label.

Now can we use POST herbicides to control marestail? Reading over research by Dr. Loux’s crew shows that utilizing Ignite 280 SL POST on marestail can be beneficial IF we utilized fall control, a good burndown with a PRE, AND marestail is four inches tall or less. The point of marestail management is to keep it under control until the soybean canopy closes. Marestail is a poor competitor if it emerges under a closed canopy.

The other weed we can’t seem to control is giant ragweed. In soybean many of the same spring burndown programs we use for marestail control work for giant ragweed. However we need to use PRE product containing the ALS herbicides chlorimuron or cloransulam-methyl. These can also be found mixed in with some of our PREs for marestail. We do have better POST programs for giant ragweed with Ignite 280 SL or glyphosate, if you are not fighting glyphosate resistance. However to make these programs work giant ragweed should be four to six inches tall for Ignite or six to ten inches tall for glyphosate. In corn adding atrazine to other broadleaf herbicides can typically control giant ragweed.

In general we want to observe a few simple rules in weed control. First, we want to start clean with a good burndown. By slacking off here you may set yourself up for an automatic 16% yield loss from marestail. Second, we want to stay clean until the soybean canopy closes by utilizing PRE and POST herbicides. To speed canopy closure you can drill beans or plant in 15s rather than 30s. Third, we want to scout and evaluate. Monitor what weeds are in your field throughout the season and see what weeds escape your program. Programs may need slight tweaks here and there or a major overhaul after being implemented for many years. For more information on weed control consult the 2011 Ohio and Indiana Weed Control Guide (bulletin 789) or contact Justin Petrosino at (937) 548-5215 or

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