Winter Annuals, Giant Ragweed, Foxtail Barley, and Reach Back

May 6, 2009
ICM News

By Micheal Owen, Department of Agronomy

Winter annual weeds have really begun to grow with a vengeance and the recent wet conditions have kept most of the field work to a minimum. Given that most of the corn is already planted, I am hopeful (albeit not confident) that these weeds have been effectively eliminated from the fields. If not, do not delay as they become considerably more difficult to control once they get some growth momentum. In particular, marestail (aka.Horseweed) is extremely tough to manage once it grows beyond the rosette stage. Other winter annuals are also more difficult to manage as they mature.

Simple perennials such as dandelions are also doing well this year. If you have simple perennieal or winter annual weed problems, move quickly and move effectively. I suspect that many of the fields that are intended for soybeans are getting extremely overgrown. Do yourself a favor and manage these weeds before you plant and do not presume to wait, as two things are likely to happen if you take this recourse; 1) you will lose yield potential and 2) your weed control will not please you. Act soon in order to best protect your crop yield potential and manage these weeds most effectively.

Giant ragweed is becoming a more frequent visitor to Iowa fields. This weed is one of the first to germinate (in some of our plots, it is already pretty large) and becomes very difficult to control with most POST products with the exception of 2,4-D and other auxinic herbicides.  POST HPPD inhibitor herbicides and triazines will do a nice job IF the giant ragweeds are small. If you plan to use glyphosate, do not skimp on the rate and apply sooner rather than later.

Foxtail barley is also becoming an occasional problem in many no-tillage fields. This perennial bunchgrass is very difficult to control in the spring. Generally the best you can do is to set it back with whatever treatment you spray (glyphosate and some HPPD inhibitors have activity on the weed) but I am not aware of any good spring-applied control tactic. The best treatment is glyphosate in the fall or aggressive tillage.

Reach back is a term used to describe residual herbicides that fail to control weeds when they emerge but with subsequent rain will move into the roots and ultimately kill the weeds as the herbicide moves upwards in the xylem (water pipes of the plant). While this phenomenon does actually occur, I strongly suggest that you not count on it. There are too many factors that must come together at the same time for "reach back" to work. If weeds emerge through a soil-applied herbicide program, do not delay by waiting for the reach back effect; move aggressively to manage the escaped weeds as quickly as you can. In this case, time actually is money — the longer the weeds grow with the crop, the more yield you lose, never to be regained — regardless of how dead you kill the weeds. 


Micheal Owen is a professor of agronomy and weed science extension specialist with responsibilities in weed management and herbicide use. Owen can be reached by email at or by phone at (515) 294-5936.



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