|Abstract or Summary
- 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
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.