Low Rates of Glyphosate for Management of Cyperus Rotundus 1

Abstract

Post-emergence applications of glyphosate (N-(phosphonomethyl) glycine) have not eradicated purple nutsedge (Cyperus rotundus L.) In the field. Possible reasons for this were investigated. Glyphosate at 1.0 and 2.0 kg ae/ha was applied to 6 week old purple nutsedge in the field. Tubers from different locations of the tuber system were sampled 3 weeks later and were sprouted in petri dishes at 23° C with 100 ppm w/w benzyl adenine for 10 weeks. This experiment was conducted four times. Least sprouting occurred in the newly developing tubers with 29% and 8% sprouting at 1.0 and 2.0 kg/ha gl3rphosate respectively. Most sprouting occurred in tubers from parent tuber chains of two tubers with 71% and 52% sprouting at the two rates respectively. Basal bulbs responded similarly with 68% and 46% sprouting respectively at the two rates. Single parent tubers had 53% and 32% sprouting respectively for the two rates. June and October applications of glyphosate were more effective in reducing tuber sprouting than March application. To estimate the proportion of different types of purple nutsedge tubers in the field, soil blocks from a 6 week old purple nutsedge stand were washed and the different types of tubers were categorized. The field contained an average of 5,000 tubers/m^ in the upper 30 cm of soil. Of this 51% were from the parent population of tubers, 33% were basal bulbs and 16% were newly developing tubers. Of the parent population, 30% were unsprouted tubers with no aerial connections. Forty six percent of the tubers in the parent population occurred singly. The rest were in chains of two or more tubers. Glyphosate at low levels was tested for its effects on purple nutsedge tuber sprouting. Pots with 6 week old purple nutsedge were sprayed with 0.0, 0.25, 0.5, 1.0 and 2.0 kg/ha glyphosate. Tubers were sampled 3 weeks later and sprouted in petri dishes for 10 weeks. Sprouting of tubers was reduced by glyphosate rates of 0.5 kg/ha or more. Tuber sprouting at 1.0 kg/ha of glyphosate Increased with Increased ratio of tuber fresh weight to leaf fresh weight. Field management of purple nutsedge in continuously cropped lettuce (Lactuca satlva L.) variety 'Manoa', continuously cropped green bean (Phaseolus vulgaris L.) variety 'Green Crop' and a rotation of the two crops under a conventional cultivation system was tested In a 13 month, six crop cycle experiment. Glyphosate at 0.5 and 1.0 kg/ha was applied post-harvest to crops. In addition, efficacy of glyphosate for management of purple nutsedge at 1.0 kg/ha under no-till was compared with the conventional cultivation system. Both the 0.5 and 1.0 kg/ha rates of glyphosate reduced purple nutsedge stand equally Irrespective of the crop. Furthermore, purple nutsedge in glyphosate treated plots was shorter than those in untreated plots. Purple nutsedge in no-till plots was shorter than those in the rotovated plots with 1.0 kg/ha glyphosate. At the end of the sixth crop cycle, purple nutsedge tuber populations in the glyphosate treated plots and handweeded plots were 70% lower than the unweeded control. Glyphosate at 0.5 kg/ha was as effective as 1.0 kg/ha or handweeding in reducing the purple nutsedge tubers. The fresh weight, dry weight and mean dally accumulation of dry matter of lettuce was reduced by purple nutsedge during the summer months. This was related to purple nutsedge plant height that increased with an increased air temperature. Therefore, control of purple nutsedge in lettuce 'Manoa' was necessary only during summer. The presence of purple nutsedge resulted in an Increase in the fresh weight of lettuce because it retained more water than in the weed-free hand weeded plots. Stand reduction of purple nutsedge with glyphosate at 1.0 kg/ha increased the fresh weight of lettuce from the second crop cycle. However, during September, when purple nutsedge had the highest plant height, lettuce yields were not increased in the 1.0 kg/ha glyphosate treatment plots. Under no-till, lettuce yield increased during the sixth crop cycle and decreased during the fourth crop cycle. Bean yield was not affected by purple nutsedge at any time. Therefore, the control of purple nutsedge in bean 'Green Crop' seemed unnecessary.