Iowa State University

Waterhemp Control with Amide Herbicides
by Bob Hartzler

March 31, 2003 -   Waterhemp continues to cause problems in Iowa corn production.   While many effective herbicides are available, the prolonged emergence pattern makes it difficult to achieve full- season control.  Researchers at the University of Illinois recently published results of research investigating several approaches to managing waterhemp ( Steckel, L.E., C.L. Sprague and A.G. Hager.  2002.  Common waterhemp control in corn with single preemergence and sequential applications of residual herbicide.  Weed Technol.  16:755-761.)  Several preemergence herbicides were applied either as a preemergence application (within 3 days after corn planting) or as a sequential treatment with 2/3 of the rate applied preemergence and the remaining applied as an early-post application (17-23 days after planting).  All herbicides were applied at the recommended label rate for the soil type at the specific location.  The experiment was conducted at five locations during 2000 and 2001, data are averaged across the five locations.

All of the preemergence treatments provided greater than 90% waterhemp control 28 days after planting (DAP), and there were no differences among the herbicides evaluated (Table 1).   However, there were significant differences in both visual ratings and densities when evaluated at 56 DAP.  The two encapsulated formulations of acetochlor (TopNotch, Degree) were the only herbicides to provide 85% or greater control at the late evaluation, and control with these products was better than that achieved with Harness, Frontier, Dual II or Prowl.  Sequential application of Harness, Frontier and Dual II  improved the control ratings at 56 DAP compared to the preemergence application, whereas there was no benefit sequential applications with the encapsulated acetochlor or Prowl treatments.  Only the Frontier and Prowl sequential treatments provided less control than the best sequential treatments (TopNotch and Degree).   All herbicide treatments provided corn yields equivalent to the weed-free control (data not presented), indicating that the waterhemp after the 28 DAP ratings were not competitive with corn.  Yield in the untreated control was reduced 13%.

Table 1.  Effectiveness of preemergence treatments on waterhemp control.

¹Sequential treatments in bold are significantly better than the same herbicide applied PRE.
Steckel, Sprague and Hager.  2002.    Weed Technol.  16:755-761.

The authors also investigated the same herbicides applied as their respective premix with atrazine (i.e. FulTime, Degree Xtra, Bicep II, etc.).  Results with atrazine premixes were similar to the herbicides applied alone, although the differences between treatments were diminished (Table 2).

Table 2.  Effectiveness of atrazine premix treatments on waterhemp control.

¹Sequential treatments in bold are significantly better than the same herbicide applied PRE.
Steckel, Sprague and Hager.  2002.    Weed Technol.  16:755-761.

Summary.  All herbicides provided excellent early season waterhemp control (>90% control at 28 DAP).  However,  differences in effectiveness were evident at the 56 DAP ratings.   The encapsulated formulations of acetochlor (TopNotch, Degree) provided the best full-season control and did not benefit from sequential applications.  The waterhemp control reported in this research is greater than I would expect from amide herbicides in Iowa.  I suspect performance was enhanced by planting dates later than those of most commercial farms.  At four sites corn was planted in early May whereas at the remaining site corn wasn't planted until late May.  Delayed planting favors herbicide performance since the corn canopy develops more quickly and waterhemp emerges closer to the herbicide application date.  These factors reduce the length of time that the herbicide must provide residual activity, and reduces the likelihood of late-season escapes.  While the results reported here are better than might be expected with mid to late-April planting dates, the relative performance of the herbicides should remain similar and thus the data provide a good comparison of products.

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