Source: Crop Watch
University of Nebraska, Lincoln

Clearfield sunflower hybrids will be readily available for the first time in 2004. These hybrids have a tolerance to the herbicide Beyond™ and are used in the Clearfield sunflower system. Beyond™, an imazamox herbicide, was labeled for this use in 2003. Clearfield sunflower hybrids contain a gene that confers tolerance, not resistance, to the herbicide Beyond. Some slight crop injury (leaf yellowing and plant stunting) may occur after Beyond is applied to these hybrids, especially where overapplication occurs as in spray overlaps or field ends. Injured plants often recover quickly. Non-Clearfield hybrids will be killed if treated with Beyond. Clearfield sunflower were developed through a traditional breeding technique known as mutagenesis and are not considered to be genetically modified organisms. Beyond herbicide is applied early postemergence to sunflower with two to eight leaves at a rate of 4 oz per acre. Weeds should be actively growing at the time of application and broadleaf weeds should be less than 3 inches tall. Grass weeds should have no more than four to five leaves. A nonionic surfactant and nitrogen based fertilizer must be added to the spray solution for optimum weed control.

A Clearfield sunflower hybrid (center row) is tolerant to Beyond herbicide, but two conventional sunflower hybrids (two rows to the right of the Clearfield hybrid) are severely damaged by Beyond herbicide. The Express-tolerant sunflower hybrid (the row left of the Clearfield hybrid) shows a low level of cross-tolerance to Beyond herbicide, but still exhibits significant injury when treated with Beyond herbicide. (UNL IANR Photo)

It is recommended that a soil-applied grass herbicide, such as Prowl® or Dual Magnum®, be applied before Beyond is applied. Beyond will control many broadleaf weeds that are troublesome in Nebraska sunflower fields, including pigweed, kochia, Russian thistle, and nightshade. However, Beyond is an ALS-inhibitor and will not effectively control ALS-resistant kochia or Russian thistle. There is also some concern about the risk of transferring the gene conferring tolerance to Beyond from the commercial sunflower hybrid to weedy sunflower species such as common or prairie sunflower. This technology should be avoided in fields where weedy sunflower species are present.

In a dryland field study conducted at the High Plains Ag Lab near Sidney in 2001, all treatments containing Beyond herbicide provided excellent pigweed (tumble and redroot) and witchgrass control. Some injury was observed with all Beyond treatments. Most sunflower plants grew out of the injury within four weeks and no yield loss was observed.

In an irrigated study conducted near Scottsbluff in 2003, postemergence applications of Beyond caused the sunflower growing point to turn a light yellow, but plants recovered. Even though Beyond caused early season crop injury, sunflowers recovered by late June. Weed populations were dense and consisted of common lambsquarters, hairy nightshade, redroot pigweed, stinkgrass, and common purslane.

Preemergence application of Spartan plus Prowl herbicides provided excellent control of all weeds except hairy nightshade. Postemergence application of Beyond herbicide effectively controlled redroot pigweed and hairy nightshade. Combining a preemergence Prowl application with a postemergence Beyond application effectively controlled the weed spectrum present in this trial.

Although Beyond breaks down in the soil more quickly than some imidazolinone and sulfonylurea herbicides, there are still some crop rotation restrictions that need to be followed. Be sure to check the label when planning.

Drew J. Lyon
Extension Dryland Cropping Systems Specialist
Robert G. Wilson
Extension Weed Specialist