Graduate Thesis Or Dissertation


Interaction between glyphosate and certain s-triazine herbicides Public Deposited

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  • Investigations were made in the field, greenhouse, growth chamber, and laboratory to: (a) observe the general activity of N- (phosphonomethyl)glycine (glyphosate); (b) determine the extent of its interaction with certain s-triazine herbicides; and (c) find an explanation for such an interaction. Greenhouse and growth chamber studies substantiated other observations that glyphosate is very active as a foliarly-applied herbicide. It had very little crop selectivity in these studies. Glyphosate stimulated the transpiration rate of wheat plants shortly after treatment. The stimulation disappeared as visible plant injury appeared. Glyphosate did not affect root-shoot ratios in wheat, measured on a dry weight basis over time. It did not seem to cause direct damage to cell membranes, since no significant early leakage of electrolytes from leaf sections floating in glyphosate solutions was observed. No significant differences were found in injury ratings or growth of wheat plants treated with different available formulations of glyphosate in the greenhouse. When glyphosate was applied to quackgrass in the field in combination with 2-chloro-4, 6-bis(ethylamino)-s-triazine (simazine), no clear interaction was observed, but simazine reduced glyphosate activity on quackgrass in the greenhouse. Antagonistic interactions were observed between glyphosate and simazine on quackgrass, corn, and beans, and between glyphosate and 2-chloro-4-(ethylamino)-6- (isopropylamino)-s-triazine (atrazine) on corn in greenhouse studies. Antagonism also was observed when glyphosate was applied to corn in combination with 2-chloro-2', 6'-diethyl-N-(methoxymethyl)-acetanilide (alachlor) and (2, 4-dichlorophenoxy)acetic acid (2, 4-D) which were not formulated with clay materials. The interaction of glyphosate with alachlor was similar to that provided by simazine or atrazine, while the interaction with 2, 4-D was temporary, disappearing after 1 week. In all cases, the interaction was overcome by an increase in application rate of glyphosate. When glyphosate and simazine were applied to corn and beans, reduction in glyphosate activity was observed only when the two herbicides were applied together as a mixture. Simultaneous application of simazine on different leaves of the same plants or to the soil while glyphosate was applied on the foliage did not produce any visible interaction. In subsequent experiments, an equal rate of the inert ingredients used in a commercial formulation of simazine also reduced the activity of glyphosate to a similar extent as when mixed with the commercial product containing the active ingredient. This finding tends to exclude the possibility of major physiological involvement in the interaction, and suggests a physical and/or chemical interaction between glyphosate molecules and those of simazine and the inert ingredients as a primary cause. In further laboratory studies, about 10% of the glyphosate was removed from solution by mixing with the simazine formulation or inert ingredients and centrifuging. Significantly more glyphosate was adsorbed by the suspended materials when the spray mixtures became more concentrated. Less glyphosate was recovered by washing dried pellets derived from the centrifugation of the spray mixtures in comparison with the amounts of glyphosate recovered by washing wet pellets. Supernatants of the spray mixtures applied to beans caused less plant injury than glyphosate alone. Addition of extra surfactant to these supernatants did not improve their activity on bean plants. Results of these studies are consistent with the hypothesis that physical and/or chemical interaction between glyphosate and other herbicide products in the spray tank and on the leaf surface is the primary cause of the observed reduction of glyphosate activity on test plants. Other possible explanations were not disproved and should be investigated further.
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