Graduate Thesis Or Dissertation

Glyphosate activity as affected by spray drop size and leaf coverage

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  • Bean shoot parts that respond to glyphosate (N- (phosphonomethyl)glycine) in ways useful for bioassay were determined by applying glyphosate doses of 3.8 to 60.3 ug ae per plant to the simple leaves when the first trifoliolate leaflets were about 1 cm long. Dry weights of parts that were almost fully enlarged at treatment were greater in treated than untreated plants after two weeks. Maximum weight increase was found with the 15.1 ug dose. Growth of younger shoot parts was reduced by glyphosate, showing linear reductions with log dose from 3.8 to 30.2 ug. Growth reduction of young shoot parts was therefore concluded to be a better measure of sublethal glyphosate activity than reduction of total shoot growth. Using the above assay, growth of bean plants from a controlled environment was evaluated after treatment with a 3.0 mM glyphosate solution applied in uniform drops of 138, 430, and 1230 um diameter. The largest drops were less effective than both smaller sizes, and no difference in activity between the two smaller sizes was found. Bean plants grown outdoors in pots responded similarly to a 12.2 mM glyphosate solution applied in drops of 138, 240, 430, 740, and 1230 um diameter. There were no activity differences between the four smallest sizes, and the 1230 um size was less effective than all others. To ascertain effects of leaf coverage on glyphosate activity, 1.0 ul drops of glyphosate solution were applied to the simple leaves of bean plants and physically spread with the tip of a small, glass rod to cover areas of different size. Herbicidal activity was reduced with increased drop spread on plants grown in a controlled environment, but not with plants grown outdoors. Cuticular adsorption of glyphosate was assumed not to be a factor in reducing activity because isolated leaf cuticles from beans grown both outdoors and in a growth chamber were shaken in an aqueous solution of ¹⁴C-glyphosate at 25 C and showed no adsorptive tendency throughout an 83-hour period. The hypothesis that glyphosate applied in low volumes is more effective because of reduced leaf surface contact is not supported by drop size data, and is supported only by results from controlled environment treatments showing less activity from increased drop spread.
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