Managing Glyphosate Failures

July 15, 2011

By Bob Hartzler and Mike Owen, Department of Agronomy

In the past week we have received numerous calls from farmers, agricultural chemical dealers and industry representatives regarding waterhemp and horseweed/marestail surviving glyphosate applications made in late June and early July. Although there are numerous reasons why a herbicide application might fail at controlling weeds, we are certain that a significant percentage of these failures are due to the presence of glyphosate-resistant biotypes in the field.

The common question is what can be done to rescue the situation in the field. Unfortunately, at this time of the year there are few options. If glyphosate failed earlier to control the weeds, it is unlikely that a repeat application will do any good in controlling the surviving weeds. The PPO inhibitors (Reflex, Cobra, Phoenix, UltraBlazer, etc.) are the other postemergence option available for waterhemp in soybean. However, the label restrictions regarding weed size are long past and thus these herbicides are unlikely to provide affective control. There also is a potential for serious crop injury with the high temperature forecast for the coming week.  Furthermore, the harvest interval restrictions for each of these products should be reviewed.

Although not popular with the majority of growers, mechanical control is really the only available option to manage escaped and/or herbicide-resistant waterhemp and horseweed/marestail at this time. If only scattered plants are present in the field, hand-weeding the field would be worth the effort since this will slow the establishment and spread of resistance within the field. If the presence of surviving waterhemp and horseweed/marestail is more widespread, a trip back in time using a cultivator is really the only option to reduce the problems with these escapes. 

Bob Hartzler and Micheal Owen are professors of agronomy and weed science extension specialists with responsibilities in weed management and herbicide use.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 15, 2011. The information contained within may not be the most current and accurate depending on when it is accessed.

ICM News
Crop: 
Authors: 

Bob Hartzler Professor of Agronomy

Dr. Bob Hartzler is a professor of agronomy and an extension weed specialist. He conducts research on weed biology and how it impacts the efficacy of weed management programs in corn and soybean. Dr. Hartzler also teaches undergraduate classes in weed science and weed identificatio...

Micheal D.K. Owen University Professor and Associate Chair of Agronomy

Dr. Micheal D. K. Owen is a University Professor and associate chair in the Department of Agronomy and an extension weed specialist for Iowa State University Extension and Outreach. He is also an adjunct professor in the Department of Vegetable Protection at Esculea Agricola Panamericana at Zamor...