It has been suggested that trifloxysulfuron might increase the efficacy of asulam for control of johnsongrass. Container and field studies were conducted to determine the efficacy of POST applications of trifloxysulfuron and asulam for johnsongrass control in sugarcane. Asulam was applied at 460 and 920 g ai/ha to container-grown johnsongrass plants, with and without 8 g ai/ha of trifloxysulfuron. Combinations of asulam and trifloxysulfuron generally reduced johnsongrass height, rhizome length, and biomass more than when either was applied alone. Results suggested that combinations of asulam and trifloxysulfuron were synergistic in their control of johnsongrass biomass 8 wk after treatment. In a sugarcane field heavily infested with rhizome johnsongrass, asulam was applied at 1,800, 2,800, and 3,700 g/ha with and without trifloxysulfuron at 16 g/ha. Asulam plus trifloxysulfuron generally controlled johnsongrass more effectively than either herbicide alone. The control of johnsongrass with asulam at 1,800 g/ha resulted in an increase in sugar yield of more than twice that in the nontreated control. Sugar yield increased further when asulam was applied at 2,800 g/ha or combined with trifloxysulfuron, but application of trifloxysulfuron alone did not increase yield. Combinations of asulam and trifloxysulfuron might slow the spread of rhizome johnsongrass enough to allow an increased number of ratoon crops before sugarcane fields need to be replanted.

In Argentinean crop fields, weed control is mainly achieved by intense use of glyphosate as a nonselective and/or selective herbicide. Glyphosate use is very high as more than 95% of the 16 million ha soybean crop consists of glyphosate-resistant cultivars, always treated with this herbicide. From initial success, inconsistent glyphosate control of Johnsongrass, an invading C4 perennial grass of soybean crops, has become evident to producers from northern Argentina over the last 3 yr. Prior to this, glyphosate provided good control. This study evaluated the nature of these recurrent glyphosate failures in Johnsongrass. Experiments conducted with Johnsongrass plants obtained from seed and rhizome phytomers collected from fields with intense glyphosate use history showed that these populations showed differential survival and biomass productivity when glyphosate treated than Johnsongrass plants obtained from similar propagules collected from field sites with no history of glyphosate use. This empirical evidence establishes that the Johnsongrass survival in glyphosate-treated transgenic soybean fields from northern Argentina is due to evolved glyphosate resistance.

During the summer fallow period of the sugarcane production cycle, glyphosate in conjunction with frequent tillage is used to destroy sugarcane regrowth and reduce perennial weed infestations. For tillage to be reduced or eliminated in fallowed fields, weed control must be maintained and sugarcane must be completely destroyed so as not to interfere with the subsequent planting operation. Field studies were conducted to evaluate glyphosate rates and formulations for control of sugarcane, bermudagrass, and johnsongrass. Glyphosate (isopropylamine salt) applied in April at 1.68, 2.24, and 2.80 kg ai/ha controlled 15-cm sugarcane at least 95% 42 d after treatment (DAT). Control of 25- and 40-cm sugarcane was maximized at 1.68 kg/ha (91 and 86% control, respectively). In another study, 25-cm sugarcane was controlled equally with isopropyl amine and potassium salt glyphosate formulations. Bermudagrass control 40 d after glyphosate was applied at 1.12 kg/ha was 86% and increased to 98% when the same rate was applied sequentially. In fallowed sugarcane fields, conventional-tillage, reduced-tillage, and no-tillage programs were implemented from mid-April through mid-August to evaluate weed control and economics. When a glyphosate application was substituted for a tillage operation, bermudagrass and johnsongrass control was increased compared with the conventional tillage alone program, but differences in sugarcane and sugar yield among the various programs the following year were not observed. Based on 2006 costs, elimination of a single tillage operation reduced cost $18.49/ha and addition of glyphosate (2.8 kg/ha plus application cost) increased cost $43.47/ha. Total cost for the conventional tillage–alone fallow program was $110.94/ha; where herbicide was used in the reduced-tillage and no-tillage programs, total cost was $19.47 to $77.38/ha more.

Field experiments were conducted to evaluate weed control provided by glyphosate, glufosinate, and MSMA applied alone or in mixture with residual and nonresidual last application (LAYBY) herbicides. Herbicide treatments included glyphosate early postemergence (EPOST) alone or followed by glyphosate, glufosinate, or MSMA late-postemergence (LPOST) alone or tank-mixed with one of the following LAYBY herbicides: carfentrazone-ethyl at 0.3 kg ai/ha, diuron at 1.12 kg ai/ha, flumioxazin at 0.07 kg ai/ha, fluometuron at 1.12 kg ai/ha, lactofen at 0.84 kg ai/ha, linuron at 0.56 kg ai/ha, oxyfluorfen at 1.12 kg ai/ha, prometryn at 1.12 kg ai/ha, or prometryn + trifloxysulfuron at 1.12 kg ai/ha + 10 g ai/ha. Residual herbicides were also applied alone LPOST. Weeds evaluated included barnyardgrass, broadleaf signalgrass, coffee senna, entireleaf morningglory, hemp sesbania, ivyleaf morningglory, johnsongrass, large crabgrass, Palmer amaranth, pitted morningglory, prickly sida, redroot pigweed, sicklepod, smooth pigweed, spiny amaranth, and velvetleaf. Treatments containing MSMA provided lower average weed control compared to those containing glyphosate or glufosinate, and residual herbicides applied alone provided inadequate weed control compared to mixtures containing a nonresidual herbicide. Across 315 of 567 comparisons (55%), when a LAYBY herbicide was added, weed control increased. The most difficult to control weed species at all locations was pitted morningglory. Barnyardgrass and hemp sesbania at the Mississippi location and hemp sesbania at the Louisiana location were collectively difficult to control across all treatments as well.