Mesotrione, an active ingredient found in Syngenta's Callisto, Lumax, Lexar and Halex provides excellent preemergence control of black nightshade, lambsquarter, pigweeds, annual smartweeds, velvetleaf and waterhemp. It also provides excellent postemergence control of these weeds and several others. It is labeled for preemergence and in come cases postemergence in corn. Mesotrione is an HPPD inhibitor or a group 27 herbicide that causes bleaching in susceptible weeds. Purdue University's own weed guide gives a good tolerance rating for mesotrione for corn. In rare cases it has been reported to carry over into soybean.

Mesotrione carryover into soybean examinedIn the 2004 Weed Control update we reported on a few cases of mesotrione carryover injury to soybean in North East Indiana [1]. In these cases injury was most prevalent on sandy soils with low organic matter and low CEC. The soil pH was often reported to be low, below 6.0. The conditions leading up to carryover are not completely understood. Mike Owen of Iowa state university suggested that they occurred in areas where overlap could have taken place added to this that the summer of application was dry and they experienced a cold wet spring [2].

Mesotrione is broken down in the soil by microbial activity into an end product of CO2. Any soil quality that would inhibit microbial activity, such as dry conditions or changes in pH, could potentially lead to a decrease in the breakdown of the herbicides. Of course, doubling up on rates due to overlapping or improper calibration of spray equipment would also lead to a higher potential for carryover. In 2008, Darren Robinson of the University of Guelph reported that the addition of atrazine increased injury in horticulture crops planted one year after a mesotrione application.

In cases where mesotrione carryover is suspected, symptoms observed on soybean are bleaching of the tips and margins of new growth (figure 1). This bleaching often bleaches out the veins or causes a mottling effect (figure 1). In cases where drift is the suspected delivery method, bleaching of the veins and mottling is not as evident. In a carryover situation the introduced herbicide is taken up from the soil and if the herbicide is soluble in water it moved into the leaf to accumulate in the tips and margins. Oddly enough, in some cases the leaves may have a slight strapping effect (figure2). Strapping is typically connected to growth regulator herbicide injury, but in these cases of possible mesotrione carryover new growth may show a strapping effect. In some of the cases it has not always been clear if the injured soybean may have also come in contact with a growth regulator; however, in all cases a HPPD inhibitor was used the previous year and there are no reports of growth regulator herbicides causing a bleaching effect.

Mesotrione carryover into soybean examinedIn most cases soybean appear to grow out of this and yield is not greatly impacted. I could not find any reports indicating yield was impacted by mesotrione carryover. In the cases reported in 2004 no yield effects were reported.  

In a study conducted by Bryan Young at the Southern Illinois University,  mesotrione was applied over the top of soybean [3]. A 7 oz/A application of Callisto provides 0.2 lb ai/A of mesotrione. In the above study mesotirone was put out at 0.5, 1.5, 5, 15 and 45% of the 0.2 lb ai/A rate. All rates were reported to cause injury. Injury reported were “bleaching, necrosis, and malformation.” Suggesting that the strapping symptomology mentioned above is not completely unsubstantiated. The 15 and 45% rates induced 31 to 66% injury respectively, 28 days after treatment. At 56 days after treatment injury was 6% or less from all rates except the highest. At the lower rates soybean was able to bounce back having no yield impacts when compared to a non treated check. The two highest rates with 31 and 66% injury at 28 days after treatment were also reported to decrease yield 11 and 22% respectively.

Mesotrione and other HPPD inhibitors are valuable tools for weed management in corn. Callisto’s rotation restriction for planting soybean is 10 months and for the most part this is sufficient to assure that the herbicide has broken down enough to have no impact on the following years soybean. Oddly enough, nature and human error has a way of providing eloquent scenarios that can lead to products that typically do the job they were intended for with no implications to bite us a bit.

Reference:

1. Herbicide Update. 2004. Bill Johnson, Glenn Nice and Tom Bauman. Purdue University  www.btny.purdue.edu/weedscience/2004/articles/update04.pdf

2. Mesotrione Carryover to Soybean. 2004. Mike Owen. Iowa State University. http://www.ipm.iastate.edu/ipm/icm/2004/7-12-2004/mesotrione.html.

3. Soybean (Glycine max) Response to Foliar Applications of Mesotrione. 2003. Bryan G. Young, Julie M. Young, Joseph L. Matthews. Weed Technology, Vol. 17, No. 4 pp. 651-654