Response of Several Weed Species to Low Volume Glyphosate Application Methods

Abstract

Various low volume applicators were evaluated for controlling several perennial weed species in Hawaii. White thunbergia (Thunbergia fragrans Roxb.), purple nutsedge (Cyperus rotundus L.), guava (Psidium guajava L.) and guineagrass (Panicum maximum Jacq.) were the weed species used. The wiper applicator (10% and 25% v/v) provided the best overall weed control. The Magicwand (5% and 10% v/v) and the brush applicator (5% and 10% v/v) provided similar control to that of the conventional application method (1% and 2% v/v). The failure of the Magicwand and brush applicator to provide better control of purple nutsedge and thunbergia than the conventional method was probably due to inadequate herbicide coverage. Guava was the most tolerant weed species; none of the plants were killed. Glyphosate at rates of 0, 0.5, 1.0, 2.0 and 4.0 kg a.e./ha was applied in diluent volumes of 45, 90, 180 and 560 L/ha by the conventional method to the weed species listed above. Decreasing the diluent volume (360 L/ha to 45 L/ha) of a given glyphosate rate (usually the lower rates) caused an increase in glyphosate activity. Again, guava was the most tolerant of the 4 weed species. A study was conducted to determine the effect of glyphosate (MON-0139; 0, 1, 2, 4% w/w) and surfactant (MON-0818; O, 0.1, 1, 10% w/w) concentrations applied in different drop numbers to a specified leaf pair of thunbergia and guava. For thunbergia, the experiment showed that 1-1 uL drop of a 4% w/w glyphosate solution was more effective in reducing shoot fresh weight than 4-1 uL drops of a 1% w/w glyphosate solution, but this was not reflected in regrowth shoot fresh weight. Addition of surfactant also enhanced glyphosate activity, but glyphosate concentration in the drop was the more important limiting factor. With guava, only visual ratings showed a result comparable to that of thunbergia. Other parameters did not reflect any trends. This was probably due to the high degree of tolerance of guava to glyphosate.