Mesotrione carryover in soybeans
by Micheal D. K. Owen
June 30, 2004 -  Iowa State University Agronomy Extension Weed Science has received a number of questions and viewed several samples of soybeans demonstrating symptoms of mesotrione carryover.  Mesotrione is a component of Callisto and Lumax herbicides marketed by Syngenta Crop Protection for weed control in corn.  Mesotrione causes the death of plants by blocking the biosynthesis of plant pigments and is a very effective residual herbicide for the control of a number of important broadleaf weeds.  Callisto is mesotrione alone and  Lumax is a prepackage mixture of S-metolachlor , atrazine and mestrione.  Both are registered for preemergence and postemergence applications, although most of the Lumax is applied preemergence and most of the Callisto is applied postemergence. The symptoms observed are depicted below.  Please note that drift picture is not the focus of this article.

Mesotrione provides excellent residual control and breaks down readily in the soil provided there is sufficient moisture.  Another important consideration is the rate of application, the timing of application, and soil characteristics likely influences the rate of degradation.  Another possible consideration is the interaction of the atrazine included in Lumax with the mesotrione.  Typically the mesotrione rate for a soil application of Callisto is approximately twice the rate when applied postemergence and is 0.24  and 0.1 lbs active ingredient/A.  The maximum rate of mesotrione in Lumax is 0.2 lbs active ingredient/A.  Most of the carryover injury has been observed along field margins, boom overlaps, turn rows and point rows (thus application rate is likely higher than labeled.   It is important to recall that last summer was relatively dry and thus did not afford the best opportunities for degradation.  The fact that this spring was cool and wet caused the soybeans to be under early stress, and made the mesotrione residues available for uptake.  Based on the samples we have received and the discussions we have had with the Field Crop Specialists, and various clientele, we do not anticipate that mesotrione carryover will significantly impact the 2004 soybean crop.  However, in isolated fields, specific areas within those fields may experience some loss of yield.  It would be wise to avoid subsequent herbicide applications (e.g. PPO inhibitors) which could contribute to greater problems.

The concern for mesotrione drift should not be a great and the injury that we have observed is more cosmetic than agronomically important.  However, the occurrence of mesotrione drift does suggest that greater "drift management" should be implemented.

        Mesotrione injury - note tips of the trifoliate to the right.                                                             Mesotrione (Lumax) carryover                       

Mesotrione drift

Prepared by Micheal D. K. Owen, extension weed management specialist, Department of Agronomy, Iowa State University