Control of common waterhemp with S-metolachlor plus fomesafen and competitiveness of protox-resistant common waterhemp

Abstract

Field experiments were conducted near Manhattan, KS in 2005 and 2006 and Sabetha, KS in 2005 to determine the efficacy of S-metolachlor tank mixed with fomesafen on common waterhemp in soybean. Preemergence treatments included S-metolachlor + fomesafen at 0.91 + 0.22, 1.21 + 0.28, 1.52 + 0.36, and 1.82 + 0.43 kg ha-1 and S-metolachlor + metribuzin at 0.55 + 0.14 kg ha-1. These treatments were applied alone or followed by a postemergence glyphosate application at 0.88 kg ha-1. Ratings were taken 2, 4 and 8 weeks after treatment. The study showed that S-metolachlor + fomesafen gave excellent early season control of common waterhemp at both Sabetha and Manhattan. S-metolachlor + fomesafen at the 1.52+0.36 kg ha-1 rate gave greater weed control than S-metolachlor + metribuzin. A separate study was conducted to determine the competitiveness and fitness of a protox-resistant common waterhemp biotype. Protox-resistant and protox-susceptible biotypes of common waterhemp were grown under noncompetitive and competitive arrangements in the greenhouse. In the noncompetitive study a single plant of both biotypes was planted in 15-cm-diam pots. Photosynthesis, leaf area, and plant biomass were measured 10, 20, 30, and 40 day after transplanting (DATP). In general, photosynthesis rate and plant biomass was similar between biotypes. However, the protox-resistant biotype had higher leaf area then the susceptible biotype at 20, 30, and 40 DATP. Under competitive conditions, a replacement series study, photosynthesis, leaf area, plant height, and plant biomass were measured 7, 14, 21, and 28 DATP. In general protox-resistant and –susceptible common waterhemp values were similar 28 DATP. Relative crowding coefficient values 28 DATP were 0.86, 0.89, 1.09, and 1.13 for photosynthesis, leaf area, plant height, and plant biomass, respectively. Suggesting, protox resistance did not change the ability of common waterhemp to grow normally under competitive conditions.