Farm Horizons, August 2013

Giant ragweed control in soybeans

By Dave Schwartz
Certified crop advisor, Gold Country Seed

I walk many soybean fields during the summer, looking for pests and doing stand counts. The past two years, I have noticed one weed in particular that appears to be increasing in numbers.

Giant ragweed, also called Kinghead, has been around as long as I can recall, so it is not a new weed, but it is becoming more common in soybean fields in Meeker County and counties that border Meeker.

In some fields in southern Minnesota, researchers have found plants that are resistant to glyphosate. In fields that I have walked, I think it is more a case of tolerance than resistance.

I thought it might be worthwhile to examine the biology of giant ragweed to get a better idea of what makes this weed so difficult to control.

It is called giant ragweed because it is big. In the eastern states of the Midwest, it has been found to grow more than 15 feet tall. In addition, its leaves are very large, so it can be very competitive in crops.

In university plots, corn yields are reduced 13 percent with two ragweed plants per 110 square feet. Soybean yields suffer 50 percent with one giant ragweed plant in plots of 110 square feet in size.

These annual plants produce large leaves opposite of each other with three to five lobes. Stems are coarse, and rough to the touch. Plants produce a much larger seed than most other weeds – one-quarter to one-half inch long.

Years ago, giant ragweed would germinate early in the season and was present more so on field edges and wasteland. This has changed as plants germinate later in the season, and they seem to thrive in soybean fields. One other characteristic of this weed that makes it unique is that it has a very rapid growth rate.

Giant ragweed seems to thrive with current management practices on many farms, including corn/soybean rotation, chisel plow, and glyphosate as the main herbicide mode of action.

Adding a residual herbicide applied early in the season will significantly improve control of giant ragweed. Corn growers have many residual options to choose from, including Atrazine, Lumax, 2,4-D, Dicamba, Callisto, and many more.

Soybean growers have fewer options available. The Iowa State 2013 Herbicide Guide lists Authority, FirstRate, Pursuit, and Valor as the only options that will provide fair to excellent control. For optimum weed control, I would encourage soybean growers in 2014 to use a residual herbicide followed up with full rates of glyphosate when broadleaf weeds are no more than 4 inches tall.

Farm Horizons: Main Menu | 2013 Stories

Herald Journal
Stories | Columns | Obituaries | Classifieds
Guides | Sitemap | Dassel-Cokato Home | Delano Home | HJ Home