Herbicidal Activity and Translocation of Glyphosate in Cyperus Rotundus l.

Zandstra, Bernard Henry
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Purple nutsedge (Cyperus rotundus L.) plants grown in the greenhouse were treated with 4 kg/ha glyphosate [N-(phosphonomethyl) glycine] or 2.2 kg/ha paraquat (1,1'-dimethyl-4,4'-bipyridinium ion). Glyphosate greatly reduced fresh weight of leaves, number of sprouts per original tuber, and number of sprouts per new tuber. Paraquat reduced fresh weight of leaves about as well as glyphosate, but was not as effective in reducing germination of tubers. Glyphosate at 2 and 4 kg/ha was compared in the field to paraquat, dicamba (3,6-dichloro-o-anisic acid), and MSMA (monosodium methanearsonate) for purple nutsedge control in repeated applications over 8 months. After two applications glyphosate had greatly reduced the number of plants. The other herbicides were not as effective in reducing number of shoots. At 5 months, after the field was rotovated and herbicides reapplied, glyphosate and MSMA gave better control of purple nutsedge plants than the other herbicides. Glyphosate and MSMA also reduced germination of tubers from treated plants. After five applications, glyphosate and MSMA reduced germination by 70%, and dicamba reduced germination by 43%. Paraquat did not reduce germination. Since glyphosate gave good control of purple nutsedge, further studies were conducted to determine the most effective rate of glyphosate, and the most susceptible age of purple nutsedge at application. Purple nutsedge was treated with glyphosate at 2, 4, 6, 12, or 24 weeks after field preparation. Glyphosate was reapplied at 2, 4, 6, or 12 week intervals, respectively, until no shoots emerged. Plants were counted in treated plots every 2 weeks, and tubers were dug every 4 weeks. These tubers were germinated to test viability by placing them in Petri dishes and incubating with 100 ppm N-6 benzyl adenine. All purple nutsedge plants treated at 12 weeks old were killed by glyphosate at 2 and 4 kg/ha, as evidenced by no regrowth of shoots, and almost no germination of tubers. Control of purple nutsedge in plots treated at 2 to 6 weeks old was less effective and several applications of glyphosate were needed to achieve good control levels. Application of glyphosate at 24 weeks killed purple nutsedge foliage, but new growth emerged immediately. Generally, application of glyphosate every 2 weeks reduced plant numbers more rapidly than every 4 or 6 weeks. Rates of 1, 2, and 4 kg/ha were equally effective in reducing number of plants after several applications, but 2 and 4 kg/ha were more effective with fewer applications. Two kg/ha gave as good control as 4 kg/ha. Viability of tubers from plants treated at 2 to 6 weeks old was higher than of tubers from plants treated at 12 weeks old. Applications of glyphosate at 24 weeks did not reduce viability of tubers. The field was rotovated 10 months after the initial preparation, and the purple nutsedge allowed to regrow. Regrowth was rapid in all plots. However, all plots treated with glyphosate except those treated at 24 weeks, produced less regrowth than the controls. Succeeding experiments carried out in the greenhouse further examined the effects of age and stage of purple nutsedge growth on its control with glyphosate. Tubers from purple nutsedge plants grown in the greenhouse for 2 to 10 weeks did not germinate after foliar application of glyphosate. Some tubers from, plants 12 and 24 weeks old survived glyphosate application. 14C-glyphosate was used to study translocation of glyphosate in 1-to 6-week-old purple nutsedge plants. Translocation of –glyphosate from treated leaves increased from 5% of the amount applied at 1 day to 19% at 4 days after application. Specific activity of in tubers was greater than in leaves at all growth stages. With increasing plant age, specific activity decreased in both tubers and leaves. Also with increasing plant age, total translocated increased in tubers, and decreased slightly in leaves. Thin layer chromatography showed no evidence of glyphosate metabolism in purple nutsedge. These results indicated that stage of growth is an important factor in obtaining control of purple nutsedge with glyphosate. Purple nutsedge in the field was most susceptible 12 weeks after the field was prepared. Purple nutsedge grown in the greenhouse was most susceptible at 2 to 10 weeks after planting. Evidently the stage of growth, or physiological age of the plant, is more important than chronological age. Once purple nutsedge plants have flowered, senescence sets in, and effectiveness of glyphosate declines.
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