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Advances in Nutsedge (Cyperus spp.) Management with K-Pam®

Background. In the search for methyl bromide (MBr) alternatives, hundreds of trials have been conducted statewide to examine different combinations of soil fumigants in polyethylene-mulched tomato, pepper, strawberry, cucurbits, cutflowers, and other commodities. Although a great deal of progress has been achieved, currently there is no single compound to replace MBr. Instead, on-going research focuses not only on certain fumigant and herbicide active ingredients and combinations, but also on application techniques and formulations to improve efficacy on weeds.

Among the weeds, most of the attention has focused on nutsedge. Both purple (C. rotundus) and yellow nutsedge (C. esculentus) have the ability to emerge through the mulch film and cause yield and quality losses. A great deal of research has been conducted on tomato, where some potential alternatives are available. However, in other vegetables and ornamentals, these alternatives do not appear as promising as for tomato. Examples of these are the methyl isothiocyanate generators dazomet, metam sodium, and metam potassium (K-Pam^®). The first two compounds have been the subject of extensive scrutiny with mixed results on nutsedge control. Recently, various reports have suggested improved K-Pam performance on these weeds; however, K-Pam rates, application techniques and formulations are blamed for inconsistent results. Because of this situation, field research has been conducted since 2002 to address these concerns.

Water Application Volume and Flow. Research plots were established at the University of Florida Gulf Coast Research and Education Center (GCREC) in Bradenton to determine whether different water volumes and flow rates of the emulsifiable K-Pam formulation had an impact on nutsedge control. For these trials, 8 in. tall by 28 in. wide beds were pressed and covered with low-density polyethylene mulch. Two drip irrigation lines with emitters every 12 in were placed under the plastic. A single K-Pam rate of 60 gal/A was injected with 1 (»3000 ppm) and 2 A in (»1500 ppm) of water. The flow rates were 0.22, 0.34, and 0.45 gal/min/100 ft within each water volume. An untreated control was included. Nutsedge was counted at 2, 10, and 15 weeks after treatment (WAT). The results indicate that the application of K-Pam improved nutsedge control at 2 WAT. However, this effect disappeared at 10 WAT and beyond. There were no differences among water flow rates and volumes, indicating that speed of K-Pam injection did not improve efficacy. This finding suggested that K-Pam rates and concentrations had to be studied more closely to improve performance.

Application Rates. Field trials at the GCREC were carried out in similar fashion as described above. Application rates were: a) 30 gal/A applied with 0.5 A in of water (»3000 ppm); b) 60 gal/A in 0.5 A in of water (»6000 ppm); c) 60 gal/A in 1 A in of water (»3000 ppm); d) 120 gal/A in 1 A in of water (»6000 ppm); e) 120 gal/A in 2 A in of water (»3000 ppm); and f) 240 gal/A applied with 2 A in of water (»6000 ppm). Nutsedge was counted at 4 and 10 WAT. All the K-Pam treatments improved nutsedge control compared to the untreated check, but there were no significant differences among K-Pam treatments with respect to water volume. These trials confirmed that water volume alone was not an important factor to improve efficacy. Instead, water volume and rate play significant roles in the definition of the concentrations applied in the field. Those treatments with 6000 ppm, regardless of the water volume and rate, had better nutsedge control than those with 3000 ppm.

Concentration Levels. The effect of K-Pam concentration on nutsedge growth was assessed using similar methodology as explained previously. The applied concentrations were 0, 2000, 3000, 4000, 5000, and 6000 ppm. Nutsedge densities were determined at 4, 6, and 10 WAT. The application of 2000 ppm caused a sharp decrease in weed population, followed by slow density reductions thereafter (Figure 1). At 4, 6 or 10 WAT, there were no differences in efficacy between 5000 and 6000 ppm. This indicated that 5000 ppm appeared to be the critical K-Pam concentration to obtain nutsedge densities below 100 plants/10 ft row (»1 m²). The application of 3000 ppm gave poor performance in the trials. This concentration has been usually recommended based on 60 gal K-Pam/A applied with 1 A in of water. These trials consistently show that concentration is more important than rates for effective nutsedge control with metam potassium. 

( Bielinski M. Santos and James P. Gilreath, GCREC-Bradenton, Vegetarian 04-06)