Utility of Residual Herbicides in No-till Double-Crop Glyphosate Resistant Soybean Production

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Utility of Residual Herbicides in No-till Double-Crop Glyphosate-Resistant Soybean Production

Timothy L. Grey, 115 Coastal Way, Crop and Soil Sciences Department, Coastal Plain Experiment Station, University of Georgia, Tifton 31794

Abstract

Field studies were conducted from 2002 to 2004 to evaluate weed control and yield of wheat and glyphosate-resistant soybean grown in rotation using narrow-row residual herbicide systems. The experiment was a randomized complete block design with a split-plot arrangement. Wheat herbicide treatments were the main effect with subplots of soybean herbicide treatments of pendimethalin preemergence (PRE), imazethapyr PRE, clomazone PRE, and glyphosate early postemergence (EPOST) applied alone or in sequence with glyphosate postemergence (POST), and a nontreated control. Wheat herbicide treatments did not affect soybean weed control. Control of tall morningglory increased when a PRE residual herbicide was included with POST applications of glyphosate compared to sequential applications of glyphosate alone, but control was inconsistent across locations. Sicklepod, large crabgrass, Florida beggarweed, and wild poinsettia control was 82, 88, 91, and 89%, respectively, or greater when residual herbicides were followed by a sequential application of glyphosate. In contrast sicklepod, large crabgrass, Florida beggarweed, and wild poinsettia control was 81, 80, 83, and 88%, respectively, or less when residual herbicides were used alone. By mid-season, narrow-row soybean had complete canopy closure and interference from weeds was minimal. Soybean yield was increased up to 380 kg/ha when a residual herbicide was included with a glyphosate application.

Introduction

In the southeastern US, wheat and soybean are often planted as part of a double-cropping system. Both crops receive some level of herbicide application for weed control. Wheat cultivars tolerant to metribuzin are utilized as this herbicide controls winter annual dicot and grass species (8). Glyphosate-resistant soybean are then planted as a second crop. This cropping system is widely used and can be profitable to farmers producing two crops in one year (5,10)

Some weed species, specifically morningglory (Ipomoea spp.), have some level of tolerance to glyphosate (1,19). Recent resistance developments to glyphosate by Amaranthus species (3) have enhanced the need for proper crop stewardship and pesticide use. There is now an emphasis on the use of soil-residual herbicides in order to introduce alternative modes of action for weed control. Research has indicated that use of residual herbicides can also be advantageous in the glyphosate systems by providing a longer period of time for glyphosate application (17).

A great deal of research has been conducted with glyphosate alone and in combination with other herbicides (1,2,7,11,13,16,19,20). Weed control is typically dependent on the species present and its susceptibility to glyphosate.

Reduction in row width can affect weed management programs, grain yield, and net returns for both traditional and herbicide-resistant soybean (13,14,15,20). The proportion of soybean planted to row widths of 47 cm or less has increased (12). Glyphosate-resistant technology for soybean now provides growers with an economic and effective means of weed control particularly in narrow-row plantings where cultivation is difficult. This technology now affords an opportunity to reduce inputs and increase net returns by utilizing narrow-row spacing (4).

Research has indicated that weeds are suppressed in narrow-row soybean due to the shading from the crop canopy and reduced herbicide inputs (5,13,15). Delaying the application of herbicides to control weed species present prior to soybean canopy closure can reduce weed interference (14) and because of glyphosate-resistant soybeans, there is a greater opportunity to control larger weeds (9). However, weed tolerance and variable control can increase if glyphosate application is delayed or applied past the label recommendation to older or larger Ipomoea spp. (9,15). This increased tolerance with Ipomoea spp. was observed and established before the advent of glyphosate-resistant crops (6,18). In 2005, Georgia reported a case of glyphosate-resistant Palmer amaranth (3) which will shift weed control in this region from heavy reliance on glyphosate to multi-layered control tactics.

Metribuzin tolerant wheat has potential for use in rotational scenarios in the southeastern US cropping production practices (8) while providing residual weed control for the following crop. The development and use of metribuzin tolerant wheat in combination with glyphosate-resistant soybean could aid in pest management by limiting weed shifts and resistance development while providing increased income with the production of two crops in one year. Therefore, field studies were conducted over two years to evaluate weed control and yield of double-crop glyphosate-resistant narrow-row soybean with PRE residual herbicides alone and in combination with POST applied glyphosate following wheat.

Evaluating Weed Control and Yield Effects of Residual Herbicide

Field experiments were conducted in 2002-2003 and 2003-2004 at the Bledsoe research farm at Williamson, GA on a Cecil sandy clay loam (clayey, kaolinitic, thermic, Typic Hapludult) with 71% sand, 13% silt,16% clay, less than 2% organic matter, and pH 6.1 to 6.5, and in 2002-2003 at the Southwest Branch Experiment Station at Plains, GA on a Faceville sandy loam (clayey, thermic, Typic Kandiudults) with 72% sand, 12% silt, 16% clay, containing 1.5% organic matter and pH of 6.2.

The experiment was a randomized complete block design with a split-plot arrangement that were replicated four times. Wheat herbicide treatments (metribuzin alone and nontreated check) were the main effect with subplots of soybean herbicide treatment.

Wheat plots were 16.5 m wide by 7.6 m long. In autumn of each year, seedbeds were conventionally tilled using a disk harrow followed by a rotary tiller. Wheat planting dates were 26 October 2002 for Williamson, 2 November 2002 for Plains, and 1 October 2003 for Williamson, with the metribuzin tolerant cultivar ‘AGS 2000’ at 134 kg/ha with emergence 5 days after planting. Metribuzin was applied at 0.27 kg ai/ha to wheat on 16 January 2003 at Williamson, 2 February 2003 at Plains, and 7 December 2003 in Williamson. Metribuzin was applied to all studies when wheat was in Feekes stage 3 (tillering) with a tractor mounted compressed air sprayer calibrated to deliver 187 liter/ha. Wheat was harvested with a small-plot combine and final yield was adjusted to 13% moisture.

After wheat harvest, ‘DP 4690 RR’ were planted in Williamson on 5 June 2003 and 27 May 2004, and ‘DPX 7870 RR’ in Plains on 2 June 2003. Soybeans were planted into standing wheat stubble 1.5 cm deep in 38-cm rows at 70 kg/ha with emergence 5 days after planting. Irrigation was applied as needed at all locations. The soybean sub-plots included a nontreated control, imazethapyr at 0.07 kg ai/ha PRE, clomazone at 1.4 kg ai/ha PRE, pendimethalin 0.56 at kg ai/ha PRE, and glyphosate at 0.84 kg ae/ha early postemergence (EPOST) one week after planting (WAP) with a tractor mounted compressed air sprayer calibrated to deliver 187 liter/ha. The previously described herbicide treatments were applied in combination with postemergence (POST) glyphosate at 0.84 kg ae/ha three weeks after planting. All glyphosate applications were the potassium salt formulation (Roundup Weathermax).

For wheat, there were few naturally infested weeds so no weed control data was recorded. For soybean, naturally infested dicot weed species ranged in size from cotyledon to 4 leaves, large crabgrass was spike to 4 leaves, and soybean was in the V3 stage of growth for the EPOST application. Dicot weed species ranged in size from cotyledon to 7 leaves, large crabgrass was spike to 7 leaves, and soybean was in the V5 stage of growth for the POST application.

Soybean stand counts were taken one week after emergence. Visual estimates of percent weed control were recorded one week after the POST applications and again within two weeks prior to harvest on a scale of 0 (no control) to 100 % (complete control). Soybean injury was evaluated one week after emergence on a scale or 0 (no injury) to 100% (crop death). Soybean was harvested using a small-plot combine and final yield was adjusted to 13% moisture.

For all treatments, data was combined for analysis. Wheat yield, soybean stand, percent soybean weed control, and soybean injury and yield were subjected to analysis of variance and means were separated with appropriate Fisher's Protected LSD at P < 0.05 level of probability.

Wheat/Glyphosate-Resistant Soybean Rotation and Residual Herbicides Systems

The two-way interactions between wheat herbicide treatment and soybean herbicide treatment were not significant for any variable either year. Therefore, data for the main effects of wheat herbicide treatment were combined and analyzed across soybean herbicide treatment, and data for the main effects for soybean herbicide treatment were combined and analyzed across wheat herbicide treatments. There were no significant treatment-by-year and location interactions for soybean stand, soybean injury, large crabgrass, Florida beggarweed, wild poinsettia, or soybean yield. Therefore, data were combined for these parameters. Significant treatment-by-year and location interactions prevented the combining of tall morningglory and sicklepod control so these data are presented by location and year.

As the two-way interaction for wheat herbicide treatment allowed for the combining of data across soybean herbicide treatment (n = 216), wheat yield was not significantly different (P = 0.2995) with 3882 and 3795 kg/ha for the nontreated and metribuzin treated wheat, respectively. These data indicate that AGS 2000 was tolerant to metribuzin and with no effect on yield, as has been previously noted (8).

There was no significant difference for the two-way interaction for soybean herbicide treatment by wheat herbicide treatment for any variable. Therefore, data for the main effects of soybean herbicide were combined and analyzed across wheat herbicide treatment.

Soybean stand ranged from 10 to 16 plants per meter of row across all treatments (data not shown). Soybean residual herbicides did not affect soybean emergence or stand establishment (data not shown).

Mid-season control of tall morningglory was not always improved when glyphosate was included with a residual herbicide (Table 1). For Williamson in 2003 and 2004, tall morningglory control did not exceed 85% for pendimethalin or clomazone alone or when used in combination with glyphosate POST. Glyphosate EPOST alone controlled tall morningglory no greater than 76% across all three experiments. Control of tall morningglory increased when a residual herbicide was included, but it was not always consistent. Previous research has indicated that increasing the rate of glyphosate increased control of ivyleaf (I. hederacea L.) (11), entireleaf (I. hederacea var. integriuscula Gray) (2,16), pitted (I. lacunosa L.) (15,16), and tall [I. purpurea (L.) Roth] (2) morningglories. Sequential treatments of glyphosate resulted in tall morningglory control ranging from 55 to 93%.

Sicklepod control with pendimethalin, imazethapyr, and clomazone alone applied PRE was less than 61, 60, and 81%, respectively across all locations (Table 1). When glyphosate was applied as a POST application following pendimethalin, imazethapyr, or clomazone, sicklepod control improved to greater than 82%. In contrast, sicklepod control with one application of glyphosate EPOST provided variable control (67 to 94%). Poor control by a single application of glyphosate was attributed to sicklepod emergence after the EPOST application. Applying a sequential glyphosate POST treatment in tandem with glyphosate EPOST provided good to excellent control at 88 to 96%. Thus, sequential applications of glyphosate did improve sicklepod control over a single application or a residual herbicide applied alone. Nice et al. (14) reported that early-season control of sicklepod is essential in total postemergence systems and that the shading affect of the canopy can effectively suppress late emerging sicklepod.

Pendimethalin, imazethapyr, and clomazone provided similar large crabgrass control at 73, 79, and 80%, respectively (Table 2). Sequential applications of glyphosate increased large crabgrass control over a single residual herbicide application to greater than 88%. Glyphosate EPOST provided control (74%) similar to the residual herbicides while adding a POST treatment improved control to 85%. Gimenez (7) reported large crabgrass was controlled by glyphosate alone.

Florida beggarweed control with pendimethalin, imazethapyr, and clomazone alone was 71, 73, and 83%, respectively (Table 2). Addition of glyphosate POST in combination with these residual PRE herbicides, improved Florida beggarweed control to 91% or greater. In contrast, glyphosate EPOST controlled Florida beggarweed 94% and the addition of a sequential glyphosate POST treatment did not significantly improve control (96%). In fact, anytime glyphosate was applied in this study either EPOST or POST, Florida beggarweed control was 91% or greater.

Wild poinsettia was controlled 69% with pendimethalin, 88% with imazethapyr, and 63% with clomazone (Table 2). The addition of glyphosate POST along with a PRE herbicide, or when used in sequential with glyphosate EPOST, improved wild poinsettia control to 89% or greater.

There were no significant differences in soybean yield among the herbicide treatments due to differences in weed control, except for imazethapyr alone (Table 2). This was attributed to the lack of season long sicklepod and Florida beggarweed control. By four weeks after planting, soybean canopy closure was observed.

Results from this experiment suggest that two crops can be grown effectively when wheat was double-cropped with glyphosate-resistant soybean. When there are limited winter annual weed populations, the use of metribuzin in wheat was not necessary. When PRE residual herbicides, glyphosate (EPOST and POST), and narrow-rows are used, effective weed control can be achieved. Similar conclusions have been made with full season glyphosate-resistant soybeans (2). Tall morningglory control was improved by the addition of glyphosate with any PRE residual herbicide, but there was inconsistency. Pendimethalin or clomazone PRE followed by glyphosate POST maximized soybean yield with sicklepod, large crabgrass, Florida beggarweed, and wild poinsettia control greater than 89% while tall morningglory control was greater than 72%. Sequential applications of glyphosate POST did improve sicklepod, large crabgrass, Florida beggarweed, and wild poinsettia control. Double-crop strategies that incorporate PRE residual herbicides, and glyphosate EPOST and POST in irrigated glyphosate-resistant soybean using narrow-rows in the southeastern US, will assist farmers in maintaining a potentially profitable endeavor. By utilizing residual herbicides with different modes of action, farmers have the ability to manage herbicide weed resistance.

Literature Cited

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