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© 2003 Plant Management Network.
Accepted for publication 30 October 2003. Published 5 December 2003.

Weed Removal Timings in No-Till, Double-Crop, Glyphosate-Resistant Soybean Grown on Claypan Soils

Reece A. Dewell, Postdoctoral Research Assistant, and William G. Johnson, Assistant Professor, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907; Kelly A. Nelson, Research Assistant Professor, Greenley Agronomy Research Center, P.O. Box 126, Novelty, MO 63460; Jianmei Li, Research Specialist, and Jimmy D. Wait, Research Associate, Department of Agronomy, University of Missouri, Columbia 65211

Corresponding author: Reece A. Dewell. radewell@purdue.edu

Dewell, R. A., Johnson, W. G., Nelson, K. A., Li, J., and Wait, J. D. 2003. Weed removal timings in no-till, double-crop, glyphosate-resistant soybean grown on claypan soils. Online. Crop Management doi:10.1094/CM-2003-1205-01-RS.

Abstract

Field experiments were conducted in central and northern Missouri to evaluate the influence of glyphosate application timing on weed control, weed biomass production, and soybean (Glycine max (L.) Merr.) grain yield in no-till, double-crop, glyphosate-resistant soybean planted after winter wheat (Triticum aestivum L.) harvest. Glyphosate treatments included 0.75 lb ae/acre applied before planting, when weeds were 4-, 8-, or 12-inch tall, and 1.5 lb ae/acre applied when weeds were 24-inch tall. Removal of weeds with glyphosate at 0.75 lb ae/acre at the 8- and 12-inch timings resulted in weed biomasses and grain yields similar to the weed-free control at all six site-years. This research indicates that the optimal weed removal timings in no-till, double-crop, glyphosate-resistant soybeans grown on claypan soils were the 8- to 12-inch application timings.

Introduction

Double-cropping soybean after winter wheat harvest is a common practice in the southern cornbelt and southern USA. Since double-crop soybeans are typically planted directly into standing or burned wheat stubble, and precipitation is less frequent at this time of the growing season, soil-applied herbicide efficacy is less reliable (13). Thus, typical production practices for double-crop soybean involve no-till planting after wheat harvest, and managing weeds with postemergence herbicides (5,13). Research has shown that traditional postemergence herbicides perform similarly in full-season and double-crop soybean production (2,3,12). However, prior research did not evaluate the use of glyphosate in glyphosate-resistant soybeans. The combination of reduced tillage and predictions that over 80% of future soybean acres will include glyphosate-resistant varieties, indicate a need for research addressing the appropriate use of glyphosate in no-till, double-crop soybean production systems.

The efficacy of glyphosate on several annual weeds in full-season, glyphosate-resistant soybean under conventional-till systems is well documented (8,9,11,14,16,17). In these studies, somewhat variable control of pitted morningglory (Ipomoea lacunosa L.), ivyleaf morningglory (Ipomoea hederacea L.), common waterhemp (Amaranthus rudis Sauer), and prickly sida (Sida spinosa L.) was reported at various application timings and plant heights. These late-emerging weeds are typically present in full-season and double-crop soybean fields that have not developed an effective crop canopy (personal observation, authors). Effective herbicide programs, combined with competitive canopy closure and limited crop response, are needed to provide the necessary weed-free interval to achieve full yield potential (1,4,6,7), especially if herbicides without residual soil activity are used. Research investigating the use of glyphosate and glyphosate-resistant soybean in no-till, narrow-row, double-crop soybean production systems will be useful in providing effective weed management recommendations.

Several studies have evaluated weed removal timings in full-season soybean and some research has evaluated glyphosate application timings in double-crop soybean (10,15). However, at the initiation of this study, no research in the Midwest had provided recommendations for double-crop soybean producers regarding optimum timing for weed control. The objective of this research was to evaluate the influence of glyphosate application timing on weed control, weed biomass production, and soybean grain yield in no-till, glyphosate-resistant soybean planted after winter wheat harvest.

Field experiments were conducted in 2000, 2001, and 2002 at the University of Missouri Bradford Research and Extension Center near Columbia and the Greenley Research Center near Novelty. Soil at Columbia was a Mexico silt loam (fine, smectitic, mesic, Aeric Vertic Epiaqualfs), with 7% sand, 73% silt, 19% clay, cation exchange capacity (CEC) of 13.7 meq per 100 g, pH 6.8, and soil organic matter (SOM) content of 2.7%. Soil at Novelty was a Kilwinning silt loam (fine, montmorillonitic, mesic, Vertic Ochraqualfs), with 32% sand, 36% silt, 32% clay, CEC of 13.5 meq per 100 g, pH 6.0, and SOM content of 2.5%. Soils in this region are highly eroded, have relatively poor internal drainage characteristics, and consist of a shallow layer of silt (0- to 6-inch deep) over a clay layer several feet thick. Field operation procedures are presented in Table 1. Soybean seeds were planted in 15-inch rows at both locations at a rate of 175,000 seeds per acre at Columbia and 200,000 seeds per acre at Novelty. Weather data are shown in Tables 2 and 3.

Table 1. Dates of various field operations at Columbia and Novelty, MO in 2000, 2001, and 2002.

¹ Soybean varieties: Pioneer 94B01 (Columbia-2000), Morsoy 3660N RT (Columbia-2001 & 2002), and Asgrow 3801 (Novelty - all three years).

² Glyphosate applications to 12-in tall weeds were not made at Columbia in 2001 and 2002.

Table 2. Average air temperature from April through October at Columbia and Novelty, MO in 2000, 2001, and 2002.

Table 3. Total precipitation from April through October at Columbia and Novelty, MO in 2000, 2001, and 2002.

Experimental Design and Data Analysis

Each site was a randomized complete block with seven treatments and four replications. Plot size was 10 ft by 35 ft at Columbia and 10 ft by 40 ft at Novelty. Treatments consisted of weed-free and untreated checks; glyphosate (0.75 lb ae/acre) applied before soybean seeds were planted, or after soybean planting when weeds were 4-, 8-, or 12-inches tall; and glyphosate at 1.5 lb ae/acre applied when weeds were 24-inches tall. Due to unforeseen circumstances, the 12-inch-tall glyphosate application timings were missed at Columbia in 2001 and 2002. Therefore, results from this treatment are based on only 4 site-years of information. All treatments were applied with CO₂-pressurized backpack sprayers equipped with extended range flat fan nozzles at 15 gallons per acre (GPA) at Columbia and 20 GPA at Novelty.

Weed biomass from a 10.8-ft² area in each plot were collected 3 to 7 weeks before soybean harvest (Tables 4 and 5). Plant biomass was dried at approximately 104°F for 3 days. Soybean yields were adjusted to 13% moisture. All data were subjected to analysis of variance and means separated with Fishers Protected LSD at the 0.05 level of significance. Data were analyzed separately by year and location due to differences in predominant weed species and growing conditions (weather data for both sites are shown in Tables 2 and 3).

Table 4. Grass weed biomass (dry weight), three to seven weeks prior to harvest at Columbia and Novelty, MO in 2000, 2001, and 2002.

¹ Glyphosate applications to 12-inch-tall weeds were not made at Columbia in 2001 and 2002.

Table 5. Broadleaf weed biomass (dry weight), three to seven weeks prior to harvest at Columbia and Novelty, MO in 2000, 2001, and 2002.

¹ Broadleaf weeds were not present at Novelty in 2002.

² Glyphosate applications to 12-inch tall weeds were not made at Columbia in 2001 and 2002.

Soybean Yield

Soybean grain yields for the untreated checks were lower than weed-free checks at all site-years (Table 6), indicating that weed pressure was high enough to cause yield reductions. Soybean grain yield in the burndown treatment was lower than the weed-free check in three of six site-years. This shows that a burndown treatment alone did not consistently provide adequate weed control, and additional weed flushes were likely to occur. Yields from the postemergence glyphosate treatments and weed-free checks were similar in five of six, six of six, four of four, and four of six site-years for the 4-, 8-, 12-, and 24-inch application timings, respectively. Late emerging grass and broadleaf weeds reduced grain yield in the 4-inch timing plots at Columbia (2002). Both broadleaf and grass weed biomass were reduced following glyphosate applications to 24-inch tall weeds, but combining inconsistent yield performance with the 2× glyphosate rate, indicate that weeds should be removed before they reach 24-inch tall.

Table 6. Soybean yield (lb/acre) at Columbia and Novelty, MO in 2000, 2001, and 2002.

¹ Glyphosate applications to 12-inch tall weeds were not made at Columbia in 2001 and 2002.

Grass Weed Control

Grass weed pressure was highly variable (especially at Columbia), which made detecting biomass differences difficult. Giant foxtail was the most common grass weed across all site-years, with sporadic infestations of barnyardgrass, fall panicum, large crabgrass, shattercane, and volunteer wheat (at Novelty only) also present. At Columbia, glyphosate applications to 24-inch tall weeds in 2000 and 2001 were the only treatments to statistically reduce grass weed biomass compared to untreated checks (Table 4). This suggests that glyphosate was fairly effective in controlling large grass weeds. At this location, grass biomass was greatest with the burndown and 4-inch weed treatments (2002), which also correlated to the only glyphosate treatments with reduced soybean yields at Columbia. This indicates that additional grass weeds emerged following the early glyphosate applications, resulting in biomass values about 2× greater than the untreated checks. At Novelty, all post application timings reduced grass weed biomass values compared to untreated checks, indicating that new weed flushes did not occur at this site. Grass weed biomass in the burndown treatments at Novelty was primarily due to volunteer wheat, which emerged after the application. Emerged wheat seedlings in the remaining plots were controlled by all of the postemergence glyphosate treatments.

Broadleaf Weed Control

Broadleaf weed populations were more sporadic, and no single species was consistently present at all site-years. Species present at both locations included common cocklebur, common lambsquarters, common ragweed, common waterhemp, horseweed, ivyleaf morningglory, Pennsylvania smartweed, pitted morningglory, prickly sida, and velvetleaf. There were very few broadleaf weeds present at the Novelty site in 2001, and none present in 2002. Broadleaf weed biomass values were statistically lower than the untreated checks in all but three situations: burndown treatments at Columbia-2001 and Novelty-2000, and 4-inch weed timing at Columbia-2002 (Table 5). Two of these situations, 4-inch weed timing at Columbia-2002 and burndown at Novelty-2000 corresponded with reduced soybean yields. Soybean yield in the third situation, burndown at Columbia-2001, was the lowest yielding glyphosate treatment at that site-year, but not statistically significant.

Conclusions

This research indicates that glyphosate applications when weeds were 8- or 12-inch tall were the most consistent for reducing weed biomass and maintaining soybean grain yield potential. These timings resulted in no yield reductions, minimal reductions in soybean biomass (data not presented), and adequate control of both broadleaf and grass weeds.

In conclusion, optimal treatments for removing weeds in no-till, double-crop, glyphosate-resistant soybeans grown on claypan soils were broad, and can range from 8- to 12-inch weeds, compared to recommendations in full-season soybeans that typically occur with applications to 4- to 8-inch tall weeds (16). This is probably due to the lack of late emerging weeds associated with the later application timings of double-crop soybean, and agrees with previous research which indicated that postemergence glyphosate applications in double-crop no-till soybean could be made over a longer application window (15). In all site-years, there was a yield advantage to removing weeds compared to untreated checks, and the yield increase was greater than the cost of control.

Acknowledgments

Manuscript number 17267, published with the permission of Purdue Agricultural Research Programs.

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