Martin- I frequent the Ohio Agricultural and Research Development Center’s web site for tips on this issue. According to a news item on that site, it is useful to practice the following: Use either tillage or pre-plant burndown herbicides to make sure your crop is planted into a clean seedbed, make sure to choose an herbicide program that caters to the weeds in your field, diversify the use of herbicide sites of action, use other herbicides in conjunction with the glyphosate and make sure to appropriately manage post-emergence glyphosate applications. I also would say it is best to use a combination of both pre- and post-emergence herbicide applications when dealing with weeds such as giant ragweed or perennial weeds.

I found this brochure on Purdue's Web site - Management of Giant Ragweed in Roundup Ready Soybean Fields with a History of Poor Control. http://www.btny.purdue.edu/weedscience/2006/GiantRagweed06.pdf

It lists specific product recommendations from Ohio State and Purdue. Is this helpful? What have you been doing for giant ragweed?

I would like to add that because of the risk of attaining resistant weeds from a neighbor’s field, it is especially important to develop and follow a resistance management plan.  Glyphosate is the most important herbicide that has been developed in the last 100 years, but you have to use more than just glyphosate.  

It is also important to note that there aren’t new herbicides coming down the pipe right now.  Syngenta is committed to doing research and finding new herbicides, but the easy herbicides have been found.  And after a new herbicide is found it’s going to take another ten years to get it to the market, which is what makes preventative measures so important right now.  Because glyphosate is such an important herbicide, we need to preserve it.

Until a resistant population in a field exceeds 30% research says that most growers won’t recognize it, and what concerns me is what we’re not seeing right now. Once a resistant population exceeds 30% the problem becomes much more difficult to combat, which is why you want to catch the problem before it happens.  Pigweed, for example, has a broad genome; just one plant produces half a million seeds.  In order to prevent resistance, you want to include other modes of action and use residual products.  You want to do anything you can to get these weeds out of the field before they produce seed.  Planning ahead with a resistance management program is the best defense we have against resistance right now.

One tool Syngenta offers is the Resistance Management Solutions Module. Growers can assess their situation on a field-by-fiend basis and determine what would work best for their individual situation. Check it out here: http://www.resistancefighter.com/solutions

By Chuck Foresman, manager of weed resistance strategies, Syngenta