Field experiments were conducted at 4 locations in 1993 to determine the effectiveness of reduced rates of postemergence herbicides in drilled no-till soybeans. Two of the locations were farmer-cooperators in Shelby and Ashland Counties. Other locations were the Logan County Farm and the OSU Research Farm in Columbus. 1993 was the first year of no-till at Columbus, while the other sites had been in no-till production for 6 to 23 years. Postemergence herbicide programs varied among locations, and were chosen based on the cooperator's previous experience and/or the weed history in the field. Herbicides were applied at rates equivalent to 1/4 of the labeled rate (1/4X) approximately 6 days after weed emergence (DAWE), 1/2X approximately 12 DAWE, or at the labeled rate when postemergence herbicide would typically be applied. At Logan County and Columbus, experiments included two planting dates and additional treatments consisting of 1/4X followed by 1/4X and 1/2X followed by 1/2X, with the second application made about two weeks after the first.
Giant foxtail was the predominant weed species at all locations. Infestations of common or giant ragweed, or common lambsquarters were sufficient at two locations to evaluate control. Foxtail control was always reduced at 1/4X, compared to later applications at the labeled rate. Application at 1/2X resulted in reduced control also, but only in early-planted soybeans (where experiments included two planting dates). Foxtail control was reduced to less than 90% in 1/4X and 1/2X treatments, but was not less than 80% except in early-planted soybeans at one location. Control of giant ragweed (at one location only) was reduced at 1/4X in early-planted soybeans, and at 1/4X and 1/2X in later-planted soybeans. Treatments consisting of sequential applications of 1/4X or 1/2X provided 95% or better control of giant foxtail and giant ragweed, which was similar to that from the labeled rate. Control of common ragweed and common lambsquarters was greater than 90% in all treatments. Soybean yield did not vary significantly with herbicide rate and application timing, but there was a trend at two locations for lower yield in the 1/4X treatments in early planted soybeans.
Our experience with reduced-rate postemergence programs in 1993 indicates some promise for this approach to weed control in no-till soybeans. It was evident that the 1/4X or 1/2X rates applied once will not always provide control equal to that from a labeled rate, but that the level of control may often be sufficient to avoid yield loss. This is somewhat dependent upon planting date; single applications of low rates should have a decreasing rate of failure as planting is delayed from early May into early June. While yield may not be reduced where single applications of low rates are applied, return of weed seed to the soil in treatments where control was reduced could increase weed control problems in future years. Weed control from sequential applications of 1/4X or 1/2X rates was equal to that at the labeled rate. We see little potential for producers to use sequential applications of the 1/2X rate, since this involves an additional trip across the field and no reduction in herbicide applied. Sequential application of 1/4X rates could be used by producers with little risk of failure, and this program reduces herbicide use by one-half. However, producers are still resistant to making an additional trip across the field due to time constraints. For that reason, this program may fit best in areas where herbicide reduction efforts are encouraged to preserve water quality, or where future government regulations may mandate herbicide reductions.
Extension Program Implementation:
Additional research is needed before a reduced-rate postemergence herbicide program can be recommended by OSU Extension, but we anticipate being able to make recommendations in 1995. Field plots were shown to producers at several field days in 1993, and there appears to be great interest in reduced-rate programs. Research results will be reported at the 1993 North Central Weed Science Society annual meeting in December, and the Ohio Association of Independent Crop Consultants annual meeting in early 1994.
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