Eight field trials were conducted in 2013 and 2014 on farms located near Tilbury, Essex and Harrow, Ontario, Canada, to evaluate the efficacy of PP and POST glyphosate tank-mixes for GR giant ragweed control in corn. Giant ragweed populations were confirmed as resistant to glyphosate prior to study establishment [10] . The first set of trials determined GR giant ragweed control using various PP tank-mixtures while the second examined GR giant ragweed control using POST tank-mixes. Each experiment was arranged as a randomized complete block design with four replications. Plots measured 2 m wide by 8 m long. For the “PP Tank-mixes” trial, herbicides were applied following GR giant ragweed emergence but prior to seeding of corn; for the “POST Tank-mixes”, herbicides were applied following corn emergence. Applications were made using a CO₂-pressurized backpack sprayer calibrated to deliver 200 L aqueous solution ha⁻¹ at 240 kPa. Boom length was 1.5 m with four ultra-low drift nozzles (ULD 120-02, Hypro, New Brighton, MN) spaced 50 cm apart. A weedy and weed-free control was included in each replicate of each trial. Weed-free controls were sprayed prior to corn seeding with glyphosate (1800 g・ae・ha⁻¹) and 2-4, D ester (500 g・ae・ha⁻¹) or glyphosate (900 g・ae・ha⁻¹) plus saflufenacil/dime- thenamid-P (735 g・ae・ha⁻¹) and maintained weed-free by hoeing and hand weeding as required throughout the growing season. Details of soil characteristics, herbicide application dates and GR giant ragweed density and height are listed in Table 1.

PP tank-mix treatments included glyphosate (1800 g・ae・ha⁻¹) mixed with 2,4-D, atrazine, dicamba, dicamba/atrazine, mesotrione plus atrazine, flumetsulam, isoxaflutole plus atrazine, saflufenacil/dimethenamid-P, S- metolachlor/atrazine and rimsulfuron. Herbicide doses are listed in Table 2. POST tank-mix treatments included glyphosate and glyphosate tank-mixed with 2,4-D ester, atrazine, dicamba, dicamba/diflufenzopyr plus Agral 90% and 28% UAN, dicamba/atrazine, bromoxynil plus atrazine, prosulfuron plus dicamba and Agral 90, mesotrione plus atrazine and Agral 90, topramezone plus atrazine and Assist, tembotrione/thiencarbazone-methyl and glufosinate. Herbicide doses are listed in Table 3.

^aAbbreviations: PP: prior to corn seeding; POST: after corn emergence.

^aAbbreviation: WAA, weeks after herbicide application; ^bMeans followed by the same letter (a - e) within a column do not significantly differ according to Fisher’s Protected LSD at P < 0.05.

^aAbbreviations: WAA, weeks after herbicide application; ^bMeans followed by the same letter within a column (a - g) do not significantly differ according to Fisher’s Protected LSD at P < 0.05; ^cAdded Agral 90 (0.25% vol/vol); ^dAdded 28% UAN (2.5% vol/vol); ^eAdded Assist (1.25 vol/vol).

Giant ragweed control was estimated visually on a scale of 0% (no control) to 100% (complete plant death) at four and eight weeks after application (WAA). Corn injury was also documented on these dates (data not shown). Injury ratings were recorded on a scale of 0% to 100%, with 0% defined as no corn injury and 100% as complete corn death. Giant ragweed density and dry weight were determined at 8 WAA by counting and then cutting the plants at the soil surface from two 0.25 m² quadrats per plot. Plants were then dried at 60˚C to constant moisture and weighed.

Data were analyzed using PROC MIXED in SAS (SAS Institute Inc., Cary, NC, Version 9.3). Herbicide treatment was considered a fixed effect, while environment (year-location combinations), the interaction between environment and herbicide treatment, and replicate nested within environment were considered random effects. Significance of the fixed effect was determined using the F-test; random effects were tested using a Z-test of the variance estimate. Data was tested for normality and homogeneity of variance using PROC UNIVARIATE; normality was determined using the Shapiro-Wilkes test in addition to the normal probability plot. For all weed control ratings, the weedy control (assigned a value of zero) was excluded from the analysis. However, injury and weed control assessments were independently compared to the weedy and weed-free controls, respectively. To satisfy the assumptions of the variance analyses, weed control was arcsine-transformed; giant ragweed density and dry weight were log-transformed. Treatment means were separated using Fisher’s Protected LSD at a level of P < 0.05. Data compared on the transformed scale were converted back to the original scale for presentation of results.

Glyphosate tank-mixes, applied PP, containing dicamba or mesotrione plus atrazine provided the greatest control of GR giant ragweed (94% to 95%) and were statistically equivalent to the weed-free control (Table 2). Similarly, excellent control of GR giant ragweed was observed for treatments containing dicamba/atrazine, isoxaflutole plus atrazine, saflufenacil/dimethenamid-P and rimsulfuron which provided 91% to 93%, 84% to 89%, 87% to 91% and 91% to 93% control, respectively. In contrast, glyphosate plus atrazine, flumetsulam or S-metola- chlor/atrazine applied PP provided unacceptable control ranging from 26% to 54%. GR giant ragweed control was similar at 4 and 8 WAA with the exceptions of atrazine and flumetsulam for which control decreased by 10% and 14%, respectively.

Dicamba, dicamba/atrazine, mesotrione plus atrazine, isoxaflutole plus atrazine, saflufenacil/dimethenamid-P, and rimsulfuron tank-mixes reduced GR giant ragweed density by 100%, 94%, 100%, 75%, 94% and 94%, respectively (Table 2). In contrast, glyphosate plus atrazine, flumetsulam and S-metolachlor/atrazine reduced GR giant ragweed density 31%, 38% and 44%, respectively. GR giant ragweed dry weight was reduced by 100%, 99%, 100%, 92%, 98% and 99% with glyphosate plus dicamba, dicamba/atrazine, mesotrione plus atrazine, isoxaflutole plus atrazine, saflufenacil/dimethenamid-P and rimsulfuron, respectively; whereas glyphosate plus atrazine, flumetsulam and S-metolachlor/atrazine reduced density only by 69%, 30% and 66%, respectively.

In other studies, poor GR giant ragweed control with glyphosate plus flumetsulam, applied PP, has been reported ranging from 12% to 66% [13] and 26% to 61% [14] , 8 WAA. Reference [17] similarly reported unacceptable giant ragweed control of 9 to 52% 8 WAA with atrazine applied PP in corn. While the current study demonstrated good control for glyphosate plus mesotrione and atrazine applied PP, reference [18] reported only GR giant ragweed suppression from the same treatment. Furthermore, reference [17] reported moderate, yet variable, control of giant ragweed ranging from 33% to 80% from mesotrione plus atrazine. In dicamba-tolerant soybean, up to 100% control of GR giant ragweed 8 WAA has been reported from dicamba PP application [19] . On the contrary, reference [20] reported poor control (30%) of GR giant ragweed with dicamba applied PP in dicamba-tolerant soybeans, however, control increased to over 80% when accompanied by a dicamba POST application. Moreover, reference [12] reported that either single or sequential dicamba applications, when timed to appropriately match the biology of the weed species, can be used as an effective integrated weed management tool for the control of GR giant ragweed.

At 4 WAA, none of the glyphosate POST tank-mixes provided acceptable control (24% to 80%) of GR giant ragweed (Table 3). Among the POST tank-mixes evaluated, glyphosate tank-mixed with 2,4-D ester, dicamba, dicamba/diflufenzopyr and dicamba/atrazine were most effective and provided 75%, 80%, 75% and 72% control of GR giant ragweed, respectively. While the remaining herbicides performed poorly, treatment with glyphosate alone and tank-mixed with atrazine and topramezone plus atrazine were least effective providing only 24%, 27% and 42% control, respectively.

In general, GR giant ragweed control was greater at 8 WAA than 4 WAA for the POST tank-mixes evaluated (Table 3). Glyphosate plus dicamba (94%) provided the best control of GR giant ragweed; control was equivalent to the weed-free control. Tank-mixes of glyphosate plus 2,4-D ester, dicamba/diflufenzopyr, dicamba/atrazine, bromoxynil plus atrazine and prosulfuron plus dicamba provided 84%, 89%, 86%, 83% and 78% control, respectively and were statistically equal to the best tank-mix, glyphosate plus dicamba. Although control increased by 8 WAA, the resultant level of control would not generally be considered commercially acceptable by a grower, with the exception of glyphosate plus dicamba.

Glyphosate plus dicamba, applied POST, reduced GR giant ragweed density 93% (Table 3). Similarly, glyphosate plus 2,4-D ester, dicamba/diflufenzopyr, dicamba/atrazineand bromoxynil plus atrazine reduced GR giant ragweed density by 93%, 87%, 87% and 87%, respectively. Glyphosate plus prosulfuron plus dicamba, mesotrione plus atrazine and glufosinate provided a moderate reduction in GR giant ragweed density (73% to 80%), while treatments containing glyphosate alone and glyphosate tank-mixed with atrazine, topramezone plus atrazine and tembotrione/thiencarbazone-methyl caused an unacceptable reduction in GR giant ragweed density (0% to 53%).

Glyphosate plus 2,4-D ester, dicamba, dicamba/diflufenzopyr, dicamba/atrazine, bromoxynil plus atrazine and prosulfuron plus dicamba reduced GR giant ragweed dry weight by 99%, 99%, 99%, 98%, 96% and 96%, respectively and were found to be equal to the weed-free control. Although glyphosate tank-mixed with mesotrione plus atrazine, topramezone plus atrazine, tembotrione/thiencarbazone-methyl and glufosinate provided poor control and density reduction, the reduction in density relative to the weedy control was moderate ranging from 89% to 95%. Glyphosate applied alone and in tank-mix with atrazine reduced GR giant ragweed dry weight by 55% and 72% respectively.

In other studies, reference [17] evaluated control of giant ragweed using various POST herbicides in corn and found dicamba/atrazine most effective, ranging from 87% to 94%, while atrazine control was as low as 46%. In dicamba-tolerant soybean, up to 100% control of GR giant ragweed has been reported for dicamba applied POST [19] and for sequential early- and late-POST dicamba applications [20] . Up to 90% control of GR giant ragweed has been reported for glufosinate under greenhouse conditions [21] ; however, any obstructing vegetation that may be present under field conditions has the potential to decrease control as glufosinate is a contact herbicide and lacks in-plant mobility.