Weed Control Alternatives in Very Early-Maturing Mississippi Soybean

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Daniel H. Poston, Vijay K. Nandula, R. Matt Griffin, Delta Research and Extension Center, Mississippi State University, Stoneville 38776; and David R. Shaw and M. Cade Smith, Department of Plant and Soil Sciences, Mississippi State University, Mississippi State 39762

Abstract

Field experiments were conducted in 2000 and 2001 at Stoneville, MS to determine the most efficacious and economical weed control programs for MG III soybean and to determine the profitability of using a preharvest desiccant with this production system. The most consistent Johnsongrass and barnyardgrass control over years and rating periods occurred with herbicide programs that utilized sequential POST glyphosate applications or a POST glyphosate application following PRE herbicides. Late-season prickly sida control was at least 86% with all herbicide programs except POST programs chosen by MSU-HERB, a computer-based herbicide recommendation program. Greater than 90% hemp sesbania control 7 weeks after final POST application (WAFPA) was achieved with any program that included a POST herbicide. At least 71% pitted morningglory control 7 WAFPA was achieved in 2000 with all treatments. Rainfall contributed to heavy preharvest weed infestations in 2001 and greater than 70% pitted morningglory control was achieved only when residual herbicides were utilized. Soybean yield and net returns were generally not improved by using a preharvest desiccant in 2000 when moisture was limited following in-season herbicide applications and weed flushes at harvest were low. Soybean yield and net returns generally increased with the use of a preharvest desiccant in 2001. Residual herbicides reduced the need for preharvest desiccants in 2001. Sequential glyphosate tank mix programs with cloransulam + acifluorfen provided net returns equal to the best treatments regardless of year or preharvest desiccant.

Introduction

Traditionally in the lower Mississippi River valley, MG V, VI, and VII soybean [Glycine max (L.) Merr.] are planted in May and June into tilled seedbeds in wide row spacing (91 to 102 cm) (4). This system is often unprofitable because of late-season drought common in August and extensive rains hindering harvest in October and November (4). Consequently, MG IV soybean production has proven more economical on non-irrigated Mississippi soils (7). The early soybean production system (ESPS) is used in Mississippi to avoid late-season drought and take advantage of more ideal growing conditions early in the growing season (9). With this system, MG IV soybean is planted in April into nontilled soil (9). Wet field conditions and extensive machinery demands in April often prevent producers from planting MG IV soybean in April. MG IV soybean planted in May does not always completely avoid seasonal drought. Consequently, producers lose the advantage of drought avoidance associated with the ESPS. MG III soybean may allow producers to plant in May and still avoid drought. For example, MG III soybean planted in early-May will generally mature in August similar to MG IV cultivars planted in mid-April. Therefore, planting MG III soybean in May might serve as an alternative to the ESPS.

New weed flushes during soybean senescence are likely in MG III soybean that mature in August, especially in years with ample rainfall. Because leaf drop begins in August, weeds that were suppressed by the soybean canopy can resume growth in the warm temperatures, high humidity, and long days typical of this time period (3,6). The economic benefit of using a preharvest desiccant to address this problem has not been fully researched. When planting early-maturing soybean, a preharvest desiccant was needed when large amounts of green weeds were present at harvest (6). Many effective soybean weed control programs have been extensively evaluated in the mid-South using traditional planting systems (13), but few programs have been evaluated for soybean grown using the ESPS (10) or for May-planted MG III cultivars that mature in August and subsequently require a shorter growing season.

The objectives of this study were to determine the most efficacious and economical weed control programs for MG III soybean grown in Mississippi and to determine the effect of a preharvest desiccant on soybean yield and net return above weed management costs.

Field Studies

Field studies were conducted in 2000 and 2001 at the Delta Research and Extension Center, Stoneville, MS. Soil type was Sharkey clay (very-fine, smectitic, thermic Chromic Epiaquert) with a pH of 6.9 and 2.9% organic matter. A glyphosate-resistant MG III soybean (Dekalb CX367cRR) was planted in 19-cm rows on 2 May 2000, and 30 April 2001. Seed were treated with mefenoxam and fludioxonil at 3.75 and 2.5 g ai/100 kg seed, respectively, prior to planting. All tests were planted with a grain drill using a seeding rate of approximately 430,000 seed/ha following conventional tillage with a disc-harrow and/or a field cultivator.

The test area was naturally infested with Johnsongrass (Sorghum halepense L.). A rotary seeder was used to disperse approximately 4.5 to 5 kg of an equal mix of hemp sesbania [Sesbania exaltata (Raf.) Rydb. ex. A. W. Hill ], pitted morningglory (Ipomoea lacunosa L.), prickly sida (Sida spinosa L.), and barnyardgrass [Echinochloa crus-galli (L.) Beauv.] seed across the test area each year to ensure consistent weed pressure throughout the test areas. A field cultivator with rolling baskets operating at a depth of 5 cm was used to incorporate the weed seed.

Herbicide treatments were applied with a tractor-mounted compressed-air sprayer delivering a spray volume of 140 liter/ha at a pressure of 227 kPa. Spray adjuvants were added as suggested by the herbicide label. All POST applications were made when weeds were 5- to 10-cm tall and actively growing and soybean was at V1 to V3 growth stage.

Three basic types of weed control programs were included: (i) glyphosate-based treatments suitable for transgenetic, glyphosate-resistant soybean; (ii) standard herbicide treatments that utilize herbicides other than glyphosate that are typically used on non-transgenetic soybean; and (iii) standard herbicide treatments suggested by the MSU-HERB software program for non-transgenetic soybean. The MSU-HERB program used in this study only prescribed standard postemergence herbicides. Glyphosate was not an option for the computer software. Treatments in the latter two programs described above are more likely to be used on non-transgenetic soybean. Herbicide treatments are listed in Tables 2 to 6.

Weed population estimates were determined for MSU-HERB recommendations in 1-m² areas in each replication. Estimates were averaged and converted from a 1-m² sample area to the 10.4-m² sample area required with the MSU-HERB program. Estimates were entered into the program and the resulting recommendations recorded. MSU-HERB recommendations were 175 g ai/ha clethodim followed by 9 g ai/ha chlorimuron in 2000 and 62 g ai/ha quizalofop followed by 13 g ai/ha chlorimuron in 2001. A nonionic surfactant was used at 0.25% (v/v) with chlorimuron; crop oil concentrate was used at 1% (v/v) with clethodim and quizalofop. A pre-harvest desiccant, paraquat at 0.28 kg ai/ha + sodium chlorate at 3.4 kg ai/ha + nonionic surfactant (0.25%, v/v), was applied 5 to 7 days prior to harvest.

Weed control and soybean injury were determined visually 2 and 7 WAFPA based on a rating scale of 0 to 100, where 0 = no effect and 100 = death of weed or crop. Harvestability was determined visually at time of harvest based on a rating scale of 0 to 5, where 0 = unharvestable due to excessive green weed pressure and 5 = very easy to harvest with no green weeds or other foreign matter present in the plot. A 1.9-m swath from the center of each sub plot was harvested with a small-plot combine. Foreign matter was screened from seed samples and yield was adjusted for foreign matter and moisture percent using standards available at a local elevator. Net returns above weed management costs were calculated using herbicide and application costs from Mississippi State University soybean planning budgets and using market prices or loan rates for soybean at harvest each year (1,2).

A split-plot treatment arrangement in a randomized block experimental design with four replications was utilized. Main plot factor was herbicide treatment and subplot factor was preharvest desiccant usage. Main plots were 6 m by 12 m and sub plots were 3 m by 12 m. Weed control data were analyzed as a randomized complete block study because these data were recorded prior to application of preharvest desiccants. All other data were analyzed as a randomized complete block with a split plot treatment arrangement. All data were analyzed using analysis of variance with sums of squares partitioned to reflect the split-plot treatment arrangement. Tests for treatment by year interactions were conducted and data were pooled across years when appropriate. Soybean yield, net returns, and harvestability were presented separately by year because of differences in environmental conditions between years. The slice option of the lsmeans function of SAS was used to assess the impact of a preharvest desiccant on individual herbicide treatments. Consequently, data were presented separately for with and without a preharvest desiccant. Means were separated using Fisher’s protected LSD test conducted at the alpha = 0.05 level.

Weed Emergence and Weather

In 2000, adequate early-season rainfall was followed by late-season drought (Table 1). In 2001, rainfall occurred throughout the growing season and weed flushes stimulated by August rainfall were apparent at harvest. The predominant weeds were barnyardgrass, hemp sesbania, Johnsongrass, pitted morningglory, and prickly sida. All weed species emerged before, during, and after soybean emergence and can be detrimental to soybean yield depending on density. Based on visual observations, weed flushes were primarily the result of newly emerged weeds and to a lesser degree the result of weeds that recovered from earlier herbicide applications.

Hemp Sesbania Control

Hemp sesbania control through harvest was easily achieved with most herbicide programs regardless of year (Table 2). Hemp sesbania control 2 WAFPA in 2000 ranged from 73 to 98%. The most efficacious treatments 2 WAFPA were glyphosate + cloransulam + acifluorfen followed by glyphosate, pendimethalin + sulfentrazone + chlorimuron followed by any POST application, and MSU-HERB recommendations applied POST. However, control ratings improved over time as soybean canopy closure improved and hemp sesbania control 7 WAFPA was at least 88%, regardless of treatment. Hemp sesbania has been reported to decrease soybean light interception 29 to 68% and to reduce soybean yield 30 to 48%, with competition for light as the primary competitive factor (12). In 2001, all treatments except one application of glyphosate as well as pendimethalin + sulfentrazone + chlorimuron applied PRE controlled hemp sesbania at least 93% at 2 WAFPA. Control 7 WAFPA was at least 89% with all treatments except pendimethalin + sulfentrazone + chlorimuron applied alone, which controlled hemp sesbania 78%. No reemergence of hemp sesbania was observed in either year despite differences in environmental conditions.

Pitted Morningglory Control

The level of pitted morningglory control was significantly impacted by the differences in environmental conditions observed in 2000 compared to 2001 (Table 2). Control 2 WAFPA in 2000 was 90 to 95% with glyphosate + cloransulam + acifluorfen followed by glyphosate, pendimethalin + sulfentrazone + chlorimuron followed by a POST treatment, and the MSU-HERB recommendation with the addition of dimethenamid. By 7 WAFPA in 2000, two and three applications of glyphosate with and without the addition of cloransulam + acifluorfen or dimethenamid, and also treatments including pendimethalin + sulfentrazone + chlorimuron followed by a POST application controlled pitted morningglory at least 87%. Sequential POST glyphosate applications and PRE herbicides were more beneficial in 2001 when ideal growing conditions brought about by consistent rainfall events prevailed during most of the growing season. All treatments controlled pitted morningglory at least 89% 2 WAFPA except for single applications of glyphosate alone, glyphosate + cloransulam + acifluorfen, and glyphosate + dimethenamid. Consistent rainfall in 2001 resulted in heavy pitted morningglory emergence after the single POST applications had been applied. Sequential applications of glyphosate have previously been found to provide better overall weed control compared to a single application (18). Control ratings 7 WAFPA were generally lower in 2001 than in 2000, due in part to pitted morningglory emergence and flushes brought about by heavy rainfall events during the time period when soybeans were senescing. By 7 WAFPA, only pendimethalin + sulfentrazone + chlorimuron PRE followed by a POST herbicide controlled pitted morningglory at least 80%. Control with other treatments ranged from 38 to 74%. The most consistent pitted morningglory control occurred with programs that utilized PRE herbicides followed by glyphosate or chlorimuron POST. Pitted morningglory control was at least 80% with these treatments, regardless of year or rating period.

Johnsongrass Control

Control both years 2 WAFPA was at least 89% with treatments containing glyphosate or an MSU-HERB recommendation (Table 3). Environmental factors in 2000 and 2001 affected late-season control resulting in a year by treatment interaction for the 7 WAFPA data. The most consistent Johnsongrass control was observed with sequential POST glyphosate applications or pendimethalin + sulfentrazone + chlorimuron PRE followed by glyphosate POST. These treatments provided at least 82% Johnsongrass control regardless of year or rating period. In 2000, drought resulted in a lack of late-season weed emergence while late-season rains in 2001 caused additional weed emergence and growth after herbicide applications were made. In 2000, control 7 WAFPA with treatments containing at least one POST application ranged from 63 to 99% compared to 30 to 88% in 2001.

Barnyardgrass Control

In 2000 at 2 WAFPA, barnyardgrass control was at least 96% with treatments containing multiple applications of glyphosate, pendimethalin + sulfentrazone + chlorimuron followed by glyphosate, and MSU-HERB + dimethenamid, and glyphosate + dimethenamid (Table 3). In 2001 at 2 WAFPA, all treatments controlled barnyardgrass at least 85%. In 2000 and 2001, addition of dimethenamid to glyphosate and MSU-HERB recommendation improved barnyardgrass control from 78 to 85% to 91 to 96% and from 86 to 92% to 97 to 98%, respectively. Therefore, season-long annual grass control may be achieved with single POST herbicide applications that include a residual grass herbicide. This would be especially important to producers with fields infested primarily with annual grasses because the cost of one POST application could potentially be eliminated. In addition, similar results may be achieved with dimethenamid rates considerably lower than those used in this study, thereby reducing overall treatment cost. For example, half rates of dimethenamid applied POST could potentially provide 8 weeks of control of some problem grasses (16). However, this level of control may only be achieved in soybean planted to narrow rows. Barnyardgrass ratings were not taken 7 WAFPA either year because Johnsongrass was the predominant weed in the study 7 WAFPA.

Prickly Sida Control

In 2000, all treatments controlled prickly sida at least 90% except those utilizing MSU-HERB recommendations (Table 3). In 2001, all of the treatments controlled prickly sida 86 to 91%, excluding the treatments that included MSU-HERB recommendations. The MSU-HERB recommendations were made when the weeds reached 5 to 10 cm in height. The major weeds influencing the MSU-HERB program were hemp sesbania, pitted morningglory, and Johnsongrass that emerged in large numbers and before the majority of prickly sida plants. Subsequently, MSU-HERB recommendations were not targeted for prickly sida populations that were overshadowed by other weeds at the time of application, and herbicides recommended were not efficacious on this weed. A likely solution to this control problem would have been the addition of flumetsulam to the MSU-HERB recommendation. Flumetsulam was reported to be effective on prickly sida as early as 1992 (11). A follow-up herbicide application would have been required to control prickly sida under these circumstances if a producer chose to rely solely on the MSU-HERB recommendation, despite the fact that the problem could have been easily remedied with a tank-mix partner. This illustrates one weakness of the computer-generated recommendations used in this study.

Soybean Injury

Soybean injury ratings are summarized in Table 4. Heavy rainfall events just after planting in 2000 contributed to high levels of injury from PRE herbicides. Soybean injury of 13 to 33% occurred where pendimethalin + sulfentrazone + chlorimuron was used. Injury to soybean from pendimethalin and sulfentrazone has been reported earlier (8,17). The treatment utilizing the MSU-HERB recommendation alone, which was quizalofop followed by chlorimuron, caused 23% injury, and injury was increased to 43% when dimethenamid was applied in combination with chlorimuron. Injury persisted throughout the growing season and delayed canopy closure. In 2001, injury was generally greatest (8 to 10%) where pendimethalin + sulfentrazone + chlorimuron was used in combination with chlorimuron POST or where dimethenamid was used. A similar level of soybean injury (9 to 10%) with dimethenamid was reported when applied early-POST, but this has not shown to decrease soybean yield (15,16).

Soybean Yield

Within years, no herbicide treatment by preharvest desiccant interactions occurred either year (data not presented). This would indicate that treatments were the same relative to each other regardless of desiccant use and that the best treatments without a desiccant were still the best treatments with a desiccant. However, part of the intent of this research was to assess the impact of using a preharvest desiccant with specific herbicide programs. Therefore, soybean yield with and without a preharvest desiccant are presented separately (Table 4).

Soybean yield was relatively low in 2000 when late-season drought prevailed, with yields ranging from 480 kg/ha with the nontreated control to 2370 kg/ha with the best treatments (Table 4). Although soybean yield was low in 2000, yields with all herbicide treatments exceeded the Mississippi state soybean average of 1480 kg/ha (14). It should be noted that the state yield average also included yield data from both irrigated and non-irrigated fields. These findings suggest that late-season drought can be successfully avoided by planting MG III soybean in early-May.

Soybean yields were similar for 11 of 14 herbicide programs in 2000 regardless of desiccant use (Table 4). This suggests that a wide variety of herbicide programs can be used successfully with this early production system. Herbicide programs that failed to produce yields similar to the highest-yielding programs were pendimethalin + sulfentrazone + chlorimuron PRE, pendimethalin + sulfentrazone + chlorimuron PRE followed by chlorimuron POST, and MSU-HERB POST. Soybean yields with pendimethalin + sulfentrazone + chlorimuron PRE followed by chlorimuron POST increased to levels equal to the best treatments only when a preharvest desiccant was utilized. Reduced yields in plots treated with pendimethalin + sulfentrazone + chlorimuron PRE or pendimethalin + sulfentrazone + chlorimuron PRE followed by chlorimuron POST may be associated with less-than-optimum Johnsongrass control or increased herbicide injury compared to other treatments. Preharvest desiccants generally did not influence soybean yield in 2000. Yield increased significantly with the use of preharvest desiccant only when glyphosate alone was applied POST as a single application. Little benefit from preharvest desiccants was expected because limited weed resurgence occurred prior to harvest and weeds that were present were desiccated by drought.

Yields were higher in 2001 and ranged from 620 kg/ha in nontreated plots to 3450 kg/ha with glyphosate + cloransulam + acifluorfen followed by glyphosate followed by a preharvest desiccant (Table 4). The Mississippi state yield average in 2001 was 2220 kg/ha (14). Soybean yield with most herbicide programs exceeded this yield average. Pendimethalin + sulfentrazone + chlorimuron PRE was the only programs that did not produce yields greater than the state average. However, preharvest desiccants were required with some herbicide programs to match or exceed the state average.

Three POST applications of glyphosate, glyphosate + cloransulam + acifluorfen followed by glyphosate POST, glyphosate followed by glyphosate + cloransulam + acifluorfen POST, pendimethalin + sulfentrazone + chlorimuron PRE followed by glyphosate POST, MSU-HERB POST, and MSU-HERB + dimethenamid POST were the only herbicide programs that resulted in yields equal to the best treatments regardless of whether of preharvest desiccant was used (Table 4). Five other treatments produced yields equal to the best treatments only when used in conjunction with a preharvest desiccant. Soybean yield increased with the use of a preharvest desiccant for all POST glyphosate programs that did not include dimethenamid POST or pendimethalin + sulfentrazone + chlorimuron PRE. Therefore, residual herbicides reduced the need for a preharvest desiccant in 2001.

Net Returns

In 2000, highest net returns regardless of desiccant use generally occurred with POST only glyphosate-based herbicide programs and with MSU-HERB POST (Table 5). Soybean yields with glyphosate followed by glyphosate + dimethenamid POST, pendimethalin + sulfentrazone + chlorimuron PRE, pendimethalin + sulfentrazone + chlorimuron PRE followed by glyphosate POST, and MSU-HERB + dimethenamid POST were equal to the best treatments only when the added cost of a preharvest desiccant was not incurred. Net returns did not increase with the use of a desiccant in 2000. In fact, net returns decreased with four herbicide programs in 2000 when a preharvest desiccant was applied.

Glyphosate + cloransulam + acifluorfen followed by glyphosate POST, glyphosate followed by glyphosate + cloransulam + acifluorfen POST, pendimethalin + sulfentrazone + chlorimuron PRE followed by glyphosate POST, and MSU-HERB followed by dimethenamid POST were the only herbicide programs in 2001 that generated net returns equal to the best treatments regardless of desiccant use (Table 5). Four other POST-only programs produced similar returns only when a desiccant was used. Increases in net returns associated with the use a preharvest desiccant occurred with all POST-only glyphosate-based programs except where dimethenamid was included. As with soybean yield, residual herbicides reduced the need for preharvest desiccants. However, the added cost of these programs did not always result in the highest net returns. These finding do illustrate that non-glyphosate herbicide programs like MSU-HERB POST can be profitable with this short-season production system. It is important to note, however, that net returns from non-glyphosate programs equal to the best treatments in this study were achieved only with total POST programs rather than with PRE followed by POST programs. The profitability of PRE followed by POST programs might be improved by using reduced rates of soil-applied herbicides. This was not addressed in this study.

Harvestability

Following timely August rains in 2001, new flushes of weeds emerged as soybean was senescing. Pitted morningglory tended to cause the greatest harvest interference. Vines encircled the soybean plants and wrapped around the plot combine’s table auger. Consequently, desiccants were more beneficial in 2001 than in 2000 (Table 6). The preharvest desiccant increased harvestability with 8 of 15 treatments in 2000 and 13 of 15 treatments in 2001.

Based on findings from this research, several weed control options that are currently used in later-maturing soybean can be used profitably in early-maturing soybean. Multiple applications of glyphosate or glyphosate tank-mixes are likely to provide consistent broad-spectrum weed control across years. Residual herbicides may reduce the need for preharvest desiccants and may be needed to provide season-long control of problem weeds like pitted morningglory. The benefit of a preharvest desiccant can be greatly influenced by environmental conditions and weed pressure at harvest. A preharvest desiccant can increase net returns if weed pressure warrants application. The potential economic benefits of a preharvest desiccant may be closely linked to level of pitted morningglory control just prior to harvest. Yields were equal or above the Mississippi state soybean average both years. This indicates a potential benefit for planting MG III soybean in Mississippi on nonirrigated soil as a drought-avoidance tool.

Acknowledgment

The authors thank Russell Coleman for technical support and Dr. Patrick Gerard for assistance with statistical analyses. The Mississippi Soybean Promotion Board provided support for this research. Gratitude is also expressed to all graduate students and summer employees that assisted with this project.

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