The role of glyphosate for weed management in glyphosate tolerant cotton

Through the cooperation of Monsanto Agricultural Products Co. and seed producers, a cotton cultivar which possess the ability to tolerate POST over-the-top applications of glyphosate (N-(phosphonomethyl)glycine) is being developed. Glyphosate is a broad spectrum, non-selective herbicide used for control of many broadleaf and grass weeds. While glyphosate possesses activity in many broadleaf weeds, control of morningglory species (Ipomoea spp.) is not always successful.;Field experiments were conducted to identify means of managing morningglory species in glyphosate tolerant cotton. Morningglory control was generally not increased with glyphosate tank-mix applications over that obtained from glyphosate alone. The combination of glyphosate with some other broadleaf-controlling herbicides resulted in decreased control of annual grasses. Increasing the application rate of glyphosate successfully increased morningglory control over that provided by tank-mixtures. Control of morningglory species with glyphosate was generally no greater than that provided by currently used herbicides. While control of morningglory species was successful early in the season when weeds were small, late season control of larger morningglory was not successful. If glyphosate applications were delayed until morningglory reached the four- to six or seven- to 10-leaf stage, late season control was no greater than approximately 60 and 40% respectively.;Sequential applications of herbicides are often successful in controlling difficult to control weeds. The effects of glyphosate applied sequentially to morningglory at various timings was investigated in the greenhouse and laboratory. Glyphosate was initially applied to pitted morningglory (Ipomoea lacunosa L.) at 426 g ae/ha and followed by the same rate either 0, 3, 7, 14, or 21 days later. Greatest biomass reduction was obtained when the sequential glyphosate application was applied within 7 days of the initial application. Similar laboratory experiments were conducted to study the effects of sequential glyphosate applications on the absorption and translocation of glyphosate in pitted morningglory. At least 76% of the glyphosate that was absorbed remained in the treated leaf, with no detectible differences among sequential timing or sampling time. Absorption of sequential applications was greatest if the application was delayed until 5 or 10 d after the initial application which was similar to the 1 DAT harvested at 24 hr. Except for the 1 DAT sequential application, absorption was not increased as the harvest time was delayed from 12 to 24 hr. Initial application rate appears to be more important than timely sequential applications of glyphosate when targeting morningglory species.