Other Soil-Incorporated Herbicides Still Needed
The advent of no-till and conservation tillage greatly changed herbicide use in sunflower. Prior to the first Spartan (Sulfentrazone) label in 1999, farmers were voicing the need for herbicides that did not require mechanical incorporation for activiation. Prowl (Pendimethalin) was the only choice at that time. However, it had limitations as a ‘stand alone’ herbicide, unable to control all of the broadleaf and grassy weeds that growers encountered. The need for mechanical soil incorporation reduced soil moisture in the critical top two inches.
Treflan (Trifluralin) and Sonalan (Ethalfuralin) were two common soil-applied herbicides prior to Spartan. Both need shallow incorporation. Eptam (EPTC) is another similar product requiring incorporation that controls most grasses and some broadleaf weeds.
As sunflower moved further west along the U.S. Highway 83 corridor from Canada to Texas, these herbicides no longer fit the need to conserve moisture via reduced or zero tillage. Producers in this low rainfall area could not tolerate drying out the top soil layer.
Dual Magnum® (S-metolachlor) was labeled on sunflower in 2004. It provides more flexibility since it does not need to be mechanically incorporated to be activated. However, the product does need rainfall or irrigation for incorporation — and tillage incorporation improves consistency of control. Dual’s limitation in the northern states is the lack of control of wild mustard and wild oat.
Rates for these soil-applied products relate to soil texture. For Dual Magnum, rates are generally higher on fine-textured soils with a high organic matter, while some other herbicides require a lower rate on light soils.
These products continue to serve important markets in areas where conventional tillage predominates. They also have value as tank mixes. Richard Zollinger, NDSU extension weed specialist, likes Prowl in a Spartan tank mix. He finds that Prowl gives better season-long weed control compared to the other soil-incorporated products. He also finds that Dual does not perform as well in some of the northern areas due to high organic matter and fine-textured soils.
However, Dual tends to fit better in the High Plains. Phil Stahlman, Kansas State University weed scientist at Hays, recommends Dual as a tank mix with Spartan in his area. He finds better residual with Dual, and says a second benefit may be enhanced control of some of the difficult broadleaf weeds like Palmer amaranth. That, however, depends on a good rain event for incorporation.
Stahlman says the soil-incorporated herbicides other than Spartan have limitations for broadleaves. “These products are for grassy weed control. Using one of these products in combination with Spartan becomes an economic issue for the grower.” Stahlman likes the added grass control that either Dual or Prowl provide in no-till or reduced tillage in combination with Spartan. He does not see much use of postemerge grass products in central Kansas sunflower since growers tend to expect the sunflower plant’s aggressive growth to compete well with grassy weeds.
That may not be the case in the northern states where cool soil and air temperatures can delay early sunflower growth. Central North Dakota producer Tim DeKrey does not use either Dual or Prowl with his Spartan. He uses Spartan Advance right after he plants (handling grasses at that time), then follows up with a postemerge grass herbicide.
So what’s the future of these soil incorporated herbicides? The simple answer is that they are important weed control tools for sunflower in a toolbox that is pretty thin to begin with. There are very few new developments in herbicides these days, with glyphosate dominating the weed control market. Sunflower growers want to hang on to every herbicide they have — and that includes the older soil-incorporated ones. — Larry Kleingartner
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