Iowa State University

A Case for Burndown Herbicides   
by Bob Hartzler

June 29, 2004 -  In my travels to Kanawha I encountered a no-till soybean field with islands of giant ragweed and velvetleaf.  The beauty of Roundup Ready soybeans are that they allowed this field to be cleaned up.  However, skipping the at-planting burndown application had a serious impact on the soybean stand, and most likely  yield, due to early-season competition.

Before
Large weeds in background are 3 ft giant ragweed,  in front of the ragweed is
a dense stand of 1 ft velvetleaf.  Amidst the weeds are soybean in the unifoliate stage.
A sprayer was just pulling into the field at the time of the photo.

After
Two weeks after the application.  Velvetleaf kill was near perfect, giant ragweed control was greater than 95%.

 ← Soybean in relatively weed free area of field
         (98,900 plants per acre)

                                        Soybean  in giant ragweed patch  →
                                        (37,600 plants per acre)

Skipping an at-planting burndown treatment in no-till soybeans may eliminate a herbicide application and reduce weed control costs.  The problem with this strategy is that it requires timely application of the initial postemergence treatment to prevent weeds from competing with the soybean.  When prolonged bad weather prevents getting into the field to complete field operations, serious problems such as seen above can greatly reduce returns per acre.   Providing the crop with an even start with weeds still makes sense in the age of herbicide resistant crops.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University