Field studies were conducted at the Main Experiment Station, Fayetteville, AR, in 1994 and 1995 to evaluate the effect of postemergence herbicides applied at late-season timings on sicklepod seed production and viability. AC 263,222, dicamba, glyphosate, glufosinate, and paraquat were applied at five rates (zero; labeled rate; one-half, one-fourth, and one-eighth the labeled rate) to three sicklepod growth stages (bud formation, flowering to 9-cm pods, and 15- to 30-cm pods). The greatest reduction in seed production occurred when herbicides were applied at the flowering to 9-cm pod growth stage. All herbicides applied at one-half the labeled rate to sicklepod at bud formation and at the flowering to 9-cm pod growth stage reduced seed production greater than 80%, except glufosinate. Regardless of herbicide, rate, or application timing, viability of remaining seed was ≥ 90% with all treatments.

Common cocklebur and sicklepod are troublesome weeds in soybean in the southern United States. A field experiment was conducted from 1991 through 1995 to determine (1) the influence of tillage (no-till and tilled after initial seed deposition) and intraspecific and interspecific interference on seed production potential, emergence pattern, and soil seedbank of common cocklebur and sicklepod, and (2) the dominant species after introduction into a weed-free field. Under intraspecific interference, 1,430 and 1,392 common cocklebur achenes m−2 and 1,827 and 5,435 sicklepod seed m−2 were deposited to the seedbank after 1 and 2 yr of seed production, respectively. For both species, approximately 11% of the initial seedbank emerged under tilled conditions the first year after deposition. Under no-till conditions, only 0.7% of common cocklebur and 1.6% of sicklepod emerged. The second year after deposition, common cocklebur emergence in no-till decreased to 0.25% of the initial seedbank, while sicklepod increased to 8% of the initial seedbank and remained higher than in tilled plots. Under tilled conditions, common cocklebur became the dominant species, and sicklepod became dominant under no-till conditions. Seedbank depletion was greater for both species under tillage. Three years after initial seed deposition, sicklepod seed was 100% viable but common cocklebur achenes were not viable. Under no-till conditions, common cocklebur was depleted in the seedbank but sicklepod was not. Thus, sicklepod poses a greater long-term weed problem than common cocklebur, especially under no-till conditions.