Two-Pass Weed Management with Preemergence and Postemergence Herbicides in Glyphosate-Resistant Soybean

There is little information on the efficacy and profitability of two-pass weed control strategies in soybean when a preemergence (PRE) residual herbicide is followed by glyphosate applied late postemergence (LPOST) under Ontario, Canada environmental conditions. Ten field trials were conducted during 2011-2013 in Ontario, Canada to determine the level of weed control, yield and net returns of various preemergence/postemergence programs in glyphosate-resistant soybean. Crop injury was 2% or less with the herbicides evaluated except for chlorimuron + flumioxazin (PRE) and pyroxasulfone + flumioxazin (PRE) which caused 4% and 7% visible injury in soybean, respectively. A single application of glyphosate resulted in variable weed control (73% 98%) while the sequential application of glyphosate provided excellent weed control (98% 100%). The control of all weeds 8 WAA after the LPOST glyphosate application was equivalent regardless of the PRE herbicide applied (96% 100%). Soybean yield was equivalent to the weed free control regardless of the PRE herbicide applied. Soybean yield was lower than the sequential application of glyphosate with chlorimuron or pyroxasulfone/flumioxazin PRE fb glyphosate LPOST. Generally net return with the two-pass programs was equivalent to the sequential application of glyphosate. Net returns were lower than the sequential application of glyphosate with chlorimuron or s-metolachlor + flumetsulam followed by glyphosate LPOST. Based on these results, a sequential application of glyphosate or a two-pass program of a preemergence residual herbicide followed by glyphosate LPOST are the preferred weed management programs in glyphosate-resistant soybean. The two-pass programs have the potential to reduce selection pressure for glyphosate-resistant weeds. Corresponding author.


Introduction
Soybean [Glycine max (L.) Merr.] is an important crop in the province of Ontario, Canada that has been grown since 1893 [1].Currently, soybean is the largest cash crop grown based on hectarage in the province surpassing corn [2].In 2012, soybean growers planted nearly 1.05 million hectares and produced 3.4 million tonnes of soybean with a farm gate value of more than $1,750,000,000 [3].Intensive agronomic practices, including effective integrated weed management strategies are needed for sustainable, long-term production of this important field crop.
Most of the soybean grown in Ontario is glyphosate-resistant which has provided growers with additional weed management options with economic advantages [4]- [6].Glyphosate-resistant soybean was first introduced in Canada in 1997 and the market share has increased steadily over the years.In 2012, about 72% of the soybean hectares in Eastern Canada were planted to glyphosate-resistant cultivars and the percentage is expected to increase in the future [5] [7].Glyphosate provides soybean growers with an efficacious herbicide option for the control of annual, biennial and perennial weeds, and a cost-effective weed management tactic that does not result in unacceptable risks to the environment [4].
The weed management strategies that can be utilized in glyphosate-resistant soybean include one application of glyphosate applied early (EPOST) or late postemergence (LPOST), a sequential application of glyphosate applied EPOST and LPOST, an EPOST tankmix application with glyphosate, and a sequential application of a preemergence (PRE) residual herbicide followed by glyphosate applied LPOST [8]- [11].Relying exclusively on glyphosate for weed management may result in the selection of glyphosate-resistant weed biotypes [11]- [13].Selection pressure can be reduced with tankmixes or sequential applications that utilize more than one herbicide mode of action [11].
Studies have shown that a single-pass herbicide program with no residual activity can result in weed escapes and poor control of late emerging weeds [14] [15].The sequential application of PRE herbicide such as s-metolachlor + metribuzin, metribuzin + imazethapyr, cloransulam-methyl, or sulfentrazone followed by glyphosate LPOST have potential to improve weed control in glyphosate-resistant soybean [8] [9] [14]- [18].
There is a demand on growers to maintain herbicide stewardship and produce economically sustainable crops that are competitive in the global market [19].To achieve this goal, data on weed control, crop yield, and profitability of various weed management strategies are required to help identify the most advantageous herbicide program.Research has shown that growers must consistently get acceptable weed control and an increase in profitability for them to implement any new weed management strategy.The viability for various weed management programs needs to be assessed based on profit margins over weed control costs.
There is little information on the efficacy and profitability of two-pass weed control strategies in soybean when a PRE residual herbicide is followed by glyphosate applied LPOST under Ontario, Canada environmental conditions.Therefore, the objectives of this study were to determine the level of weed control, yield and net returns of various PRE/POST programs in glyphosate-resistant soybean.

Study Establishment
A total of ten field trials were conducted in Southwestern Ontario at the Greenhouse and Processing Crops Research Centre, Agriculture and Agri-Food Canada, Harrow, at the Huron Research Station, Exeter, Ontario and at the University of Guelph, Ridgetown Campus, Ridgetown, Ontario during 2011, 2012 and 2013.The soils ranged from Fox sandy loam to Brookston clay loam with 33% -82% sand, 5% -41% silt, 15% -29% clay, and pH of 6.0 -7.9.Site preparation included moldboard plowing or disking in the autumn followed by two passes with a field cultivator with rolling basket harrows in the spring.
All plots were 3 m (4 soybean rows spaced 75 cm apart) wide and 8 m long at Harrow and Ridgetown and 10 m long at Exeter.Glyphosate-resistant soybean cultivars were seeded at a density of 500,000 seeds•ha −1 .Herbicides were applied using a CO 2 -pressurized sprayer calibrated to deliver 222 L•ha −1 aqueous solution at 210 kPa at Harrow, 200 L•ha −1 aqueous solution at 241 kPa at Exeter and 200 L•ha −1 aqueous solution at 207 kPa at Ridgetown.The boom was 1.5 m wide with four ultra-low drift nozzles (ULD120-02, Hypro, New Brighton, MN) spaced 50 cm apart.PRE treatments were applied 0 -7 days after seeding, EPOST treatments were applied at 1 -2 trifoliate leaf stage and the LPOST treatments were applied at 5 -6 trifoliate leaf stage.
Crop injury was evaluated visually 2 and 3 weeks after emergence (WAE), using a scale of 0 to 100% where a rating of 0 was defined as no visible plant injury and a rating of 100 was defined as plant death.Percent weed control was visually assessed 4 and 8 weeks after the LPOST herbicide application (WAA) using a scale of 0 to 100% where a rating of 0 was defined as no weed control and a rating of 100 was defined as complete control.Weed density and biomass (shoot dry weight) were evaluated at approximately 3 WAE (prior to LPOST glyphosate application) by counting and cutting plants at the soil surface in two 0.5 m 2 quadrats per plot and separating by species.Plants were dried at 60˚C to a constant moisture and then weighed.Soybean was mechanically harvested at physiological maturity using a plot combine at all sites.Soybean yields were adjusted to a 13.0% seed moisture content level.

Statistical Analyses
All data were subjected to analysis of variance and analyzed using the PROC MIXED procedure in SAS statistical software (Version 9.2.SAS Institute, Inc., Box 8000, SAS Circle, Cary, NC 27512).Variances were partitioned into the fixed effect of herbicide treatment and into the random effects of environment (year and location).When there were no significant interactions between environment and herbicide treatment the data were pooled and averaged.The assumptions of the variance analysis were tested by ensuring that the residuals were random, homogeneous, with a normal distribution about a mean of zero using residual plots and a Shapiro-Wilk normality test.All percentage data required an arcsine square root transformation.Yield data did not require transformation.All percentage data presented in tables are on the back-transformed scale.Treatment means were separated at the 5% level of significance using Fisher's Protected LSD test.

Profitability Analysis
The profitability analysis is based on the level of profit margins over weed control costs, measured as gross income less herbicide and application costs.Gross income for each treatment was calculated as the yield multiplied by the cash price of soybean on October 1 of 2011, 2012 and 2013 at Chatham, ON and the herbicide costs for each treatment are based on the herbicide prices reported by AGRIS (AGRIS Co-operative Ltd., 835 Park Avenue West, Chatham, ON N7M 5J6, Canada) in 2011, 2012 and 2013.Application costs are determined based on cost of production data reported by the Ontario Ministry of Agriculture, Food and Rural Affairs (Field Crop Budgets, Publication 60, updated annually; Ontario Ministry of Agriculture, Food and Rural Affairs, 1 Stone Road West, Guelph, ON N1G 4Y2, Canada).All other costs of production are assumed to be constant across treatments, thus they are not considered in the analysis.Pairwise comparisons were made between treatments to test for significant differences in average profit margins between treatments.These pairwise comparisons are made across all locations and years as well as for each location in each year.

Weed Control after In-Crop Glyphosate Application
Data for weed control at 4 and 8 WAA were similar; therefore only data for 8 WAA are presented (Table 3).
a POST application of glyphosate compared to a single-pass application of glyphosate POST [16].

Crop Injury and Yield
At 2 weeks after emergence (WAE) chlorimuron + flumioxacin fb glyphosate (PRE fb LPOST); s-metolachlor + metribuzin fb glyphosate (PRE fb LPOST); s-metolachlor/metribuzin fb glyphosate (PRE fb LPOST); flumioxazin fb glyphosate (PRE fb LPOST); and pyroxasulfone + flumioxazin fb glyphosate (PRE fb LPOST) caused 4%, 2%, 2%, 4%, and 7% injury in soybean, respectively (Table 4).However, there was no injury with other treatments evaluated.At 3 WAE, no significant visible injury resulted from the herbicide treatments evaluated except for pyroxasulfone + flumioxazin which caused 3% injury in soybean.Other studies have also shown transient crop injury with pyroxasulfone + flumioxazin in soybean [20]- [24].Reduced weed interference by the herbicide treatments evaluated resulted in an increase soybean yield of 1.2 to 1.6 t•ha −1 compared to non-treated weedy control (Table 5).There was no adverse effect on soybean yield with glyphosate applied EPOST, LPOST and the sequential glyphosate applications (EPOST fb LPOST) compared to the weed free control (Table 5).Soybean yield was equivalent to the weed free control regardless of the PRE herbicide fb glyphosate applied.Soybean yield was lower than the sequential application of glyphosate with chlorimuron or pyroxasulfone/flumioxazin fb glyphosate (Table 5).Other studies that have shown lower soybean yield with PRE application of pyroxasulfone/flumioxazin, flumioxazin, pyroxasulfone, s-metolachlor + metribuzin, flumioxazin + imazethapyr + metribuzin, dimethenamid-p + imazethapyr + metribuzin, and s-metholachlor + metribuzin + chlorimuron compared to the weed-free control [20].In other studies, soybean yields were not significantly different between the glyphosate EPOST alone or tank-mixed combination with imazethapyr, sequential applications of glyphosate, s-metolachlor + metribuzin followed by glyphosate and flumetsulam/s-metolachlor followed by glyphosate treatments [18].Swanton et al. [25] also found no significant differences in soybean yield between sequential applications of glyphosate and glyphosate + imazethapyr.

Profitability Analysis
Profitability analysis indicated that weeds decreased profit margin 688 CAN $ ha −1 compared to the weed-free control (Table 5).Herbicide treatments increased profit margin 535 to 677 CAN $ ha −1 compared to the non-  treated weedy control (Table 5).There was no significant difference in profit margin between glyphosate applied EPOST (1815 CAN $ ha −1 ) or LPOST (1797 CAN $ ha −1 ).The sequential glyphosate application EPOST fb LPOST had a profit margin of 1907 CAN $ ha −1 which was higher than a single application of glyphosate although results were not statistically significant (Table 5).Generally net return with the two-pass (PRE fb POST) programs was equivalent to the sequential application of glyphosate (EPOST fb LPOST).However, net returns were lower than the sequential application of glyphosate with chlorimuron or s-metolachlor + flumetsulam PRE followed by glyphosate LPOST (Table 5).

Conclusions
Based on results of this study, herbicide efficacy is specific weed species and the choice of PRE herbicide is dependent on historical field records.S-metolachlor/metribuzin provided poor control of velvetleaf, pigweed and ragweed.Chlorimuron and flumioxazin provided poor control of ragweed, barnyard grass and foxtail.Saflufenacil/dimethenamid-p provided poor control of lambsquarters, barnyard grass and foxtail.Saflufenacil/imazethapyr provided poor control of barnyard grass and foxtail.Imazethapyr + metribuzin provided poor control of barnyard grass.
A single application of glyphosate resulted in variable weed control (73% -98%) while the sequential application of glyphosate provided excellent weed control (98% -100%).Glyphosate was the great equalizer as the control of all weeds 8 WAA after the LPOST glyphosate application was equivalent regardless of the PRE herbicide applied (96% -100%).
Soybean yield was equivalent to the weed free control regardless of the PRE herbicide applied.Soybean yield was lower than the sequential application of glyphosate with chlorimuron or pyroxasulfone/flumioxazin PRE fb glyphosate LPOST.Generally, net return with the two-pass programs was equivalent to the sequential application of glyphosate.Net returns were lower than the sequential application of glyphosate with chlorimuron or s-metolachlor + flumetsulam followed by glyphosate LPOST.
This study concludes that the most efficacious and profitable weed management programs in glyphosate resistant soybean are a sequential application of glyphosate or a two-pass program of a preemergence residual herbicide followed by glyphosate LPOST.The two-pass programs have the potential to reduce selection pressure for glyphosate-resistant weeds and therefore they have glyphosate stewardship benefits.

Table 1 .
Mean density (no.m −2 ) and biomass (g•m −2 ) of various weeds in response to weed management strategies approximately 3 weeks after emergence (prior to the in-crop application of glyphosate) in ten field trials conducted at Exeter, Harrow and Ridgetown, ON, Canada during 2011 to 2013 a,b .
a Abbreviations: ABUTH, velvetleaf; AMARE, redroot pigweed; AMBEL, common ragweed; CHEAL, common lambsquarters; ECHCG, barnyardgrass; and SETVI, green foxtail.b Data were averaged for environments.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05).Treatment 4 was removed as there was no herbicide applied at this point.Treatment 5 was also removed since at this point in the season it was a repeat of Treatment 3.

Table 2 .
Mean control (%) of various weeds in response to weed management strategies 3 weeks after emergence (prior to the in-crop application of glyphosate) in ten field trials conducted at Exeter, Harrow and Ridgetown, ON, Canada during 2011 to 2013 a,b .
a Abbreviations: ABUTH, velvetleaf; AMARE, redroot pigweed; AMBEL, common ragweed; CHEAL, common lambsquarters; ECHCG, barnyardgrass; and SETVI, green foxtail.b Data were averaged for environments.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05).Treatment 4 was removed as there was no herbicide applied at this point.Treatment 5 was also removed since at this point in the season it was a repeat of Treatment 3.

Table 3 .
Mean control (%) of various weeds in response to weed management strategies 8 WAA (after in-crop application of glyphosate) in ten field trials conducted at Exeter, Harrow and Ridgetown, ON, Canada during 2011 to 2013 (data averaged over environments) a .

Table 4 .
Mean soybean injury with various weed management strategies used in ten field trials conducted at Exeter, Harrow, and Ridgetown, ON, Canada during 2011 to 2013 a,b .Data were averaged for environments.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05).

Table 5 .
Mean soybean yield and profit margin of weed management strategies used in ten field trials conducted at Exeter, Harrow, and Ridgetown, ON, Canada during 2011 to 2013 a,b .
a Data were averaged for environments.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05).b Abbreviations: EPOST, early postemergence; LPOST, late postemergence; fb, followed by.