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Mesotrione is a selective herbicide used for preemergence and postemergence weed control in corn. A better understanding of the interactions between mesotrione and soil insecticides is needed to avoid risk of serious corn injury. Field trials were conducted to examine corn injury from mesotrione applied postemergence as affected by insecticide type, application method, and application rate. Corn injury from mesotrione applied postemergence at 0.19 lb ai/acre was 34, 19, and 13% in corn treated with terbufos, chlorpyrifos, and tefluthrin, respectively. In a separate study, corn injury was greater in corn treated with terbufos than clorpyrifos. Injury from mesotrione increased when terbufos was applied at two times the typical rate. Corn injury was higher when terbufos was applied in-furrow versus T-band. In each study, corn recovered rapidly from mesotrione injury with no reduction in corn yield. The extent of corn injury observed in these trials indicate that the combination of foliar-applied mesotrione following soil-applied terbufos should be avoided. Introduction Herbicide-insecticide interactions have the potential to cause corn (Zea mays L.) injury and yield loss. For example, the use of organophosphate (OP) soil insecticides in combination with sulfonylurea herbicides results in corn injury and yield losses up to 54% (2,5,6,8,9,10). OP insecticides reduce the rate at which the cytochrome P450 enzyme metabolizes sulfonylurea herbicides, resulting in a higher concentration of the herbicide remaining in the plant longer than if no insecticide was used (1,4). The type of soil insecticide, as well as the application method, affects the extent of corn injury from sulfonylurea herbicides. Previous research indicated that soil-applied terbufos followed by a postemergence (POST) application of a sulfonylurea herbicide caused more severe corn injury than other soil-applied OP insecticides followed by a POST application of a sulfonylurea (2,3). In-furrow applications of terbufos resulted in more severe corn injury from sulfonylurea herbicides than band applications of terbufos (2,10). Uptake and translocation of the insecticide within corn by plants may be the primary factor contributing to injury from the interaction of an OP insecticide and sulfonylurea herbicide. The availability of soil-applied terbufos for uptake by corn is influenced by soil pH, type, organic matter, and moisture (8,9). Mesotrione was registered in 2001 for preemergence (PRE) and POST weed control in corn. It is a member of the benzoylcyclohexane-1,3-dione herbicide family. Mesotrione is a chemical derivative of the herbicidal phytotoxin leptospermone, produced naturally by the bottle brush plant (Callistemon citrinus) in Australia (7). Mesotrione competitively inhibits the enzyme Phydroxyphenyl pyruvate dioxygenase (HPPD) (12). Corn is tolerant to both sulfonylurea herbicides and mesotrione as a consequence of selective metabolism involving the cytochrome P450 enzyme (7,13). An understanding of 3 November 2008 Crop Management the potential interaction between mesotrione and soil-applied OP insecticides is needed to avoid risk of serious corn injury. The objectives of this research were: (i) to characterize the interaction of mesotrione and soil-applied insecticides in the field; and (ii) to determine the effect of insecticide, application method, and application rate on corn injury from mesotrione. General Procedures for Two Field Trials in Two Years Field experiments were conducted in 2002 and 2003 in East Lansing, MI. Experiments were conducted as a randomized complete block design with four replications. Individual plots were 3-ft wide and 35-ft long, consisting of four rows spaced at 30 inches. Corn was planted with a White 5100 Seed Boss (Agco Corp., Duluth, GA) four-row, forced-air planter with attached granular soil insecticide applicators. Insecticides were applied at the time of corn planting, either in-furrow or in a 7-inch T-band ahead of the press wheel. Herbicides were applied with a tractor-mounted, compressed air sprayer. The sprayer was equipped with 8003 flat-fan nozzles spaced 30 inch apart and delivered 20 gal/acre at 30 psi with a ground speed of 3.5 mph. Boom height was set at 18 inch above the soil surface or plant canopy. POST mesotrione treatments were applied in a water carrier with crop oil concentrate and 28% liquid nitrogen fertilizer at 1 and 2.5% (v/v), respectively, at the 4to 5-leaf corn stage. Since the goal of the study was to evaluate crop injury, the experimental area was kept free of weeds through the use of PRE and POST herbicides. In 2002, atrazine and mesotrione were applied PRE over the entire study area at 1.5 lb ai/acre and 0.19 lb ai/acre, respectively. The study also received a POST application of glyphosate at 0.38 lb ai/acre and ammonium sulfate at 2% w/w /ha. In 2003, one study (study 1) was also treated with a POST application of bentazon + atrazine (0.75 + 0.75 lb ai/acre) and crop oil concentrate (1% v/v) to control escaping broadleaf weeds. In 2003, atrazine was applied PRE at 1.5 lb ai/acre over both study sites. Root ratings for corn rootworm (Diabrotica virgifera virgifera LeConte) damage were not taken in the trials because the studies were conducted in rotated fields, in an area lacking the rotation-resistant variant. Data from the experiments were subjected to analysis of variance and means were separated using Fisher’s protected LSD test (a = 0.05). Interactions of Mesotrione and Soil-applied Insecticides 2002 field study. An initial screen was conducted to determine if there was crop injury when mesotrione was combined with two commonly used OP soil insecticides. The study was conducted at the Agronomy Farm at Michigan State University (MSU) on a Capac loam (fine-loamy, mixed, mesic Aeric Ochraqualf) with 2.8% OM and a pH of 6.1. The study was planted on 7 May with the glyphosate resistant hybrid ‘Dekalb 44-46’ at 72,900 seeds/ha at a depth of 2 inches. The soil insecticide treatments were: chlorpyrifos (Lorsban 15G, OP) (Dow AgroSciences LLC, Indianapolis, IN) t-banded (1.31 lb/acre, a 1× rate); terbufos (Counter 20CR, OP) (BASF AG, Global Engineering, Ludwigshafen, Germany) t-banded (1.30 lb/acre, a 1× rate); and terbufos in-furrow (1.30 or 3.90 lb ai/acre, a 1× or 3× rate), respectively. Soil insecticide was applied only to the third and fourth row of each plot. Mesotrione was applied POST (0.10 lb ai/acre) to all four rows of these four treatments on 7 June. A fifth treatment had terbufos applied at 3.90 lb ai/acre to the third and fourth rows of each plot with no post application of mesotrione, allowing for an evaluation of possible injury from a high rate of soil insecticide alone. Prior to the POST application of mesotrione, corn injury was evaluated visually on 6 June for injury from the PRE application of mesotrione. Corn injury was then evaluated visually at 3, 5, and 14 days after the POST (DAPO) application on a scale of 0% (no injury) to 100% (complete death). Corn injury symptoms on the plant included chlorosis (whitening of tissue) and some stunting. Injury symptoms became less apparant with new corn growth (Fig. 1). Corn was harvested on 10 October with a smallplot combine in row 2 for post-mesotrione injury and in row 3 for postmesotrione plus insecticide injury. 3 November 2008 Crop Management Injury from mesotrione interactions with soil insecticides. No injury was observed from a 3× rate of terbufos alone, without mesotrione. Also, no injury was observed from the PRE application of mesotrione to insecticidetreated rows of any treatment. Minimal (5%) corn injury was observed 3 and 5 DAPO to rows treated with mesotrione alone applied POST. This injury declined to 0% by 14 DAPO. The decline in corn injury was related primarily to the emergence of new leaves exhibiting no injury symptoms. There was little additional injury when mesotrione was applied POST to corn treated with chlorpyrifos showing 6 to 7% injury through 14 days of rating (Table 1). However, the POST application of mesotrione in combination with terbufos infurrow resulted in statistically significant injury within 3 DAPO. Mesotrione injury to corn with a labeled rate of terbufos (1.30 lb ai/acre) in-furrow was significantly greater than injury to corn with a T-band application. This difference was evident until at least 5 DAPO. Corn injury was greatest with mesotrione POST following a 3× rate of terbufos in furrow (3.90 lb/acre), reaching 49% at 5 DAPO. Table 1. Crop injury 3, 5, and 14 days after a POST (DAPO) application of mesotrionew to corn previously treated with different OP soil insecticide rates and application placement, 2002. w Mesotrione applied POST at 0.10 lb/acre with crop oil concentrate and 28% liquid nitrogen fertilizer at 1 and 2.5% (v/v), respectively. Entire study area treated with atrazine plus mesotrione PRE at 1.5 lb/acre and 0.19 lb/acre, respectively. x Soil insecticides were applied at planting in-furrow (IF) or T-banded (T). z Crop injury rated visually on a 0 to 100% scale. Within each column, numbers followed by different letters indicate significant differences at P < 0.05. Injury from mesotrione alone POST was no longer evident by 14 DAPO, while corn treated with mesotrione POST following OP soil insecticides showed significant injury 14 DAPO (Table 1). In general, injury symptoms decreased over time, but were still up to 27% at 14 DAPO. Despite the crop injury, yield did not differ significantly among treatments (data not shown). Fig. 1. Corn injury from a 2× rate of terbufos (2.60 lb ai/acre) applied infurrow at planting followed by a 1× rate of mesotrione (0.10 lb/acre) applied POST (2003). Soil insecticide Rate (lb ai/acre) Application placementx Percent Crop Injuryz, DAPO |
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