The 1999 growing season marked the initial commercialization of two new soil-applied corn herbicides in Illinois and 16 other states. These herbicides, Balance and Epic, both contain the new active ingredient isoxaflutole. This active ingredient is part of a new family of herbicides, known as the isoxazoles. This family of herbicides inhibits carotenoid biosynthesis, causing susceptible species to appear "bleached" or "whitened," similar to other carotenoid inhibitors such as Command (clomazone). |
Balance (isoxaflutole) is manufactured and marketed by Aventis CropScience (previously Rhône-Poulenc Ag Company) and Epic, a premixture of isoxaflutole and flufenacet, is manufactured and marketed by Bayer Corporation. Flufenacet is the grass component of the herbicides Axiom, Domain, and Axiom AT, which are also marketed by Bayer Corporation.
The commercialization of these two herbicides provided additional alternatives for early preplant and preemergence weed control in corn. Many of the comments heard from producers who used these two products in the 1999 growing season were that they provided good to excellent control of a number of the weed species that have been hard to control with other herbicide programs. These weed species included waterhemp, pigweed species, common lambsquarters, common ragweed, kochia, eastern black nightshade, and jimsonweed. One of the greatest strengths of these two products is velvetleaf control. These products also provided additional control of one of the tougher grass species, woolly cupgrass. But along with the rave reviews on the excellent weed control that was provided by these two products, some producers experienced problems with isoxaflutole injury to corn.
What were some of the factors that may have contributed to the incidences of isoxaflutole injury to corn? What can be done to reduce or prevent crop injury from isoxaflutole for the 2000 growing season?
Where Did Balance Injury Occur?
Environmental and soil conditions. In Illinois, there did not appear to be specific regions where more notable concerns from isoxaflutole injury occurred. In most cases, the injury appeared to be patchy in fields, and observations indicated that the most severe injury occurred in areas where applications may have been doubled, such as along headland rows.
Additionally, there were some cases in which Balance was not thoroughly dissolved and mixed in the spray tank, allowing for some areas in the field to receive greater amounts of isoxaflutole than others. Still other areas in fields, where injury may have been more severe, included regions where soils were lower in organic matter and coarser in texture. Additionally, the cooler, wetter growing conditions seen early in the 1999 season may have also increased the incidence of corn injury from isoxaflutole. The cooler weather and wetter soil conditions slowed overall corn growth, which may have also slowed the plant's ability to metabolize the herbicide.
Hybrid differences. Other instances of isoxaflutole injury during the 1999 growing season occurred in corn hybrid trials. In one of these trials, as many as 14 different corn hybrids were planted and treated with Balance, but only a few of these hybrids showed signs of herbicide damage. This observation suggests that there are probably differences in corn hybrid tolerance to isoxaflutole. This observation also has been supported by previous research conducted to examine corn hybrid sensitivity to isoxaflutole. In these studies, differences in corn tolerance were quantified by determining the herbicide rate required to injure corn and reduce plant height. Of four different hybrids evaluated, two were less tolerant to isoxaflutole. Differences in hybrid tolerance were determined to be primarily due to differential herbicide metabolism rates.
Early postemergence applications. Other instances where isoxaflutole injury occurred in 1999 may be attributed to equipment failures and excessive rainfall, which delayed applications. Some of these delays may have resulted in applications being made after corn began to emerge. Previous research has shown that Balance applied to corn foliage increases the instances of injury, especially when tank-mixed with herbicides such as Dual II, Surpass, and Harness. It is important to follow the label and not apply isoxaflutole to emerged corn.
Other factors. Other factors that may have contributed to corn injury from Balance this year include shallow planting and failure to close the seed slot. All of these cases allow the herbicide to come into contact with the seed, which can result in increased incidences of injury.
What Can Be Done for 2000?
These explanations may provide some answers to the widespread corn injury that occurred during the 1999 growing season. However, there were other cases where these factors do not appear to be the cause of isoxaflutole injury. So what happens in 2000? While we do not have any clear answers, if extraordinary environmental conditions actually explained the majority of Balance injury complaints last season, these conditions cannot be controlled from year to year. We do know that there are certain precautions that need to be taken when using these products, because they do have a narrow corn safety margin. First, producers need to be aware that some corn hybrids vary in their tolerances to isoxaflutole. Second, it is important to follow the label and use the recommended rate specified for a specific soil type. Third, make sure that corn is planted at least 1.5 inches deep and the seed slot is closed. Finally, do not treat emerged corn with Balance or Epic. A number of these precautions and restrictions are also addressed in the new Balance and Epic labels for the 2000 growing season. Most notable on the new Balance label are the new PRE-SLURRY or PRE-SOAK mixing instructions to ensure that the product is thoroughly dissolved and dispersed, the inbred sensitivity precaution statement, and the restructured use rate table (Table 9). The new Epic label also includes a restructured use rate table (Table 10), along with some other restrictions and precaution statements. These precautions include a corn hybrid sensitivity statement and specific instructions not to use Epic on high-oil corn, seed corn, popcorn, or sweet corn.--Christy Sprague and Aaron Hager