In the past week Iowa State University Extension 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, ISU Extension agents 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.