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a USDA-ARS, Crop Genetics and Prod. Res. Unit, P.O. Box 343, Stoneville, MS 38776
b Dep. of Agric. Econ., P.O. Box 9755, Mississippi State, MS 39762
c USDA-ARS, Southern Weed Science Res. Unit, P.O. Box 350, Stoneville, MS 38776
* Corresponding author (lheatherly{at}ars.usda.gov).
Received for publication August 14, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DT, deep tillage • ESPS, early soybean production system • GR, glyphosate-resistant • MG, maturity group • POST, postemergent • PRE, preemergent • ST, shallow tillage • WMS, weed management system
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Redvine is a perennial, woody dicot vine that occurs extensively in crop and non-crop lands in the lower Mississippi River alluvial flood plain. Redvine is difficult to control because it can propagate from a deep and extensive root system (Elmore et al., 1989a, 1989b). Shallow tillage (
15 cm) is often ineffective for its control because new flushes of shoots emerge from large, long woody rootstocks that are not affected by such tillage. In fact, Koskinen and McWhorter (1986) predicted increased populations of perennial and biennial weeds such as redvine from using reduced tillage systems. It is surmised that deep tillage (subsoiling usually >30 cm deep) in the fall can physically break up the network of rootstocks, and root segments that are brought to the surface will be destroyed by exposure to ambient conditions in the winter and early spring. Thus, deep tillage of clay soils can be considered for managing problem perennial weeds such as redvine.
Some herbicides [e.g., acifluorfen (sodium (5-[2-chloro-4-(trifluoromethyl)phenox]-2-nitrobenzoate), glufosinate (glufosinate–ammonimum), and paraquat (1,1'-dimethyl-4,4'-bipyridinium dichloride)] effectively remove top growth of perennial weeds, but have little effect on the rootstock. Destruction of foliage is temporary and partial, and new sprouts subsequently arise. Glyphosate, a nonselective systemic herbicide, has activity on redvine. In greenhouse studies, glyphosate at 3.36 kg a.e. (acid-equivalent) ha–1 controlled redvine 98% (Reddy, 2000). In field studies, however, control was <86% when two sequential applications of 840 g a.e. ha–1 were applied to GR soybean (Reddy and Chachalis, 2000). The label use rate specifically limits single (1.68 kg a.e. ha–1) and sequential (840 g a.e. ha–1) in-season applications to <2.52 kg a.e. ha–1 in GR soybean. Thus, effective control of redvine in transgenic soybean requires glyphosate applied at rates that are higher than those used for normal in-season weed control. The challenge, then, is to develop an economical strategy to manage redvine in soybean production systems that exploit the benefits of deep fall tillage and GR soybean cultivars.
Wesley and Smith (1991) performed deep tillage on a Tunica silty clay soil in the fall in Mississippi following soybean harvest when the soil profile was dry. They measured significant yield increases from soybean planted in May that was not irrigated in years when drought occurred during the growing season, and determined that net return was greatly increased from this practice (Wesley et al., 2000). The increased production was associated with increased moisture content in the soil, presumably because of greater infiltration and storage of winter rain resulting from deep tillage. This work has been used to promote deep tillage of dry clay soils in the fall in the midsouthern USA.
Studies on Sharkey clay (very-fine, smectitic, thermic Chromic Epiaquert) in Arkansas (Popp et al., 2001) and Mississippi (Wesley et al., 2001) showed average yield increases of 580 kg ha–1 and 365 kg ha–1, respectively, and average increases in net return of $96 and $71 ha–1, respectively, from fall deep tillage. In the Arkansas study, yields following fall deep tillage were significantly greater than those from conventional tillage even though drought was not severe. The Mississippi study used estimated deep tillage costs that were $17 to $20 ha–1 more than those for a treatment that received only shallow tillage (10 cm). Heatherly and Spurlock (2001) and Heatherly et al. (2002a) determined that profits from producing soybean following deep tillage of Sharkey clay were significantly greater than those from conventional tillage only when plantings were made in April vs. May and later. In their study, costs associated with deep tillage were $29 to $42 ha–1 greater than those for a conventional shallow tillage system (fall tillage with a disk harrow and/or a spring-tooth harrow) because of expense associated with deep tillage and one extra shallow tillage operation to smooth the soil surface following deep tillage. In extremely dry years (yield levels <1000 kg ha–1), deep tillage provided no yield or economic benefit (Heatherly et al., 2002a). On a Coastal Plain loamy sand soil in South Carolina, Frederick et al. (2001) measured a 12% yield increase from deep tillage just before May planting of soybean that was not irrigated compared to no deep tillage (2415 vs. 2160 kg ha–1).
Weed management systems for soybean generally involve two basic approaches: use of preemergent followed by postemergent herbicides, and use of postemergent-only herbicides. Herbicides applied only postemergent can be used effectively to control early-season weeds (Reddy et al., 1999; Heatherly et al., 2002b, 2003a, 2003b) in midsouthern USA soybean plantings. Economically feasible weed control strategies using preemergent and postemergent herbicides in nonirrigated ESPS plantings following shallow and deep fall tillage have not been determined.
Clayey (silty clay loam, silty clay, clay) soils occupy more than 3.65 million ha or about 50% of the land area in the lower Mississippi River alluvial flood plain in the midsouthern USA. These clay soils crack when dried and swell when wetted, and have poor internal drainage when wet. Sharkey and Tunica are prominent clayey series, with the Tunica soils having coarser-textured materials starting at about 60 to 75 cm below the upper clay layers. Much of the area occupied by clayey soils in the region is cropped to soybean, and redvine and johnsongrass are prominent perennial weeds. The objective of this work was to assess perennial weed control in and compare yields and economic returns from April plantings of MG IV and MG V GR and non-GR soybean grown using two weed management systems without irrigation following shallow and deep tillage of clay soil in the fall. The reason for conducting this research was based on the premise that fall tillage and in-season weed management systems might act synergistically to effectively control perennial weeds and enhance soybean yield and economic return. Economic analysis of 3 yr of results was conducted to assess and compare the profitability of weed management systems in the two tillage environments.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Experiments were conducted in a randomized complete block design with a split-plot factorial arrangement of treatments and four replicates within each tillage environment. In early October of 1999 and subsequent years, one-half of the area (same area each year) was deep-tilled (DT) with an implement having curved tines spaced 1 m apart, and one-half of the area (same area each year) was shallow-tilled (ST) using a disk harrow and/or spring-tooth cultivator. The deep tillage was done approximately 0.4 to 0.45 m deep. The shallow tillage was done approximately 10 cm deep. Rainfall during the 30 d preceding deep tillage was 29 mm in 1999, 66 mm in 2000, and 21 mm in 2001; thus, soil was relatively dry preceding each year's deep tillage. Shallow tillage (two passes on ST and three on DT) with a disk harrow and/or a spring-tooth cultivator was conducted after completion of deep tillage each year. Weather data in Table 1 were collected about 4 km from the experimental site.
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Weed management systems were selected along the following premises. First, uncontrolled weeds will reduce soybean yield; therefore, no weedy check was included. The intent in this experiment was to ensure that both weed management systems controlled weeds until canopy closure. Second, the inclusion of economic analyses in this study dictated that both weed management systems be practical and realistic. Also, there was no intent to determine how weed management systems related to an economically unattainable or unfeasible weed-free environment. Therefore, a weed-free check was not included. Finally, the intent was to assess the effect of using the two accepted approaches for weed management in soybean, which are a system with a preemergent component and a system that relies solely on postemergent-only control. Based on these premises, the eight weed management systems were: (i) MG IV GR cultivar with weed control using PRE nonglyphosate herbicides followed by POST applications of glyphosate; (ii) MG V GR cultivar with weed control using PRE nonglyphosate herbicides followed by POST applications of glyphosate; (iii) MG IV GR cultivar with weed control using POST applications of glyphosate; (iv) MG V GR cultivar with weed control using POST applications of glyphosate; (v) MG IV non-GR cultivar with weed control using PRE plus POST nonglyphosate herbicides; (vi) MG V non-GR cultivar with weed control using PRE plus POST nonglyphosate herbicides; (vii) MG IV non-GR cultivar with POST weed control using nonglyphosate herbicides; and (viii) MG V non-GR cultivar with POST weed control using nonglyphosate herbicides. Herbicides applied to each weed management system across ST and DT were the same and were applied at the same time in ST and DT within each year.
Within each weed management system for GR and non-GR cultivars, use of herbicides and their combinations was dictated by expected weed populations (PRE + POST) or actual populations (POST). Selection of postemergent herbicides for the non-GR cultivars was based on weekly assessment of the presence and size of particular weed species in plots of each weed management system. The objective was to minimize weed competition within the constraints of each individual weed management system. Preemergent herbicides were applied immediately after planting each year. In each year, rainfall of at least 13 mm occurred within 10 d of preemergent application. Preemergent herbicides and postemergent broadleaf herbicides were applied in 187 L ha–1 water, whereas postemergent grass herbicides and glyphosate were applied in 94 L ha–1 water. Herbicides were applied using a canopied sprayer (Ginn et al., 1998a) for over-the-top applications (to prevent drift to adjacent plots of different systems) or a directed sprayer (Ginn et al., 1998b) for applications underneath the developing soybean canopy.
Herbicides (Table 2) were broadcast-applied each year at labeled rates with recommended adjuvants and in recommended tank mixes. Rates for preemergent herbicides applied to both GR and non-GR cultivars were a premix of metribuzin {4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one} at 450 g a.i. ha–1 plus chlorimuron ethyl [ethyl 2-{{{{(4-chloro-6-methoxypyrimidin-2-yl)amino}carbonyl}amino}sulfonyl}benzoate] at 75 g a.i. ha–1 applied in 2000 and 2001, and imazaquin {2-[4,5-dihydro-4-methyl-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} at 137 g a.i. ha–1 applied in 2002. Rates for postemergent herbicides applied to non-GR cultivars were: premix of 560 g a.i. ha–1 bentazon [3-(isopropyl)-1H-2,1,3-benzothiadiazin-4-(3H)-one 2,2-dioxide] and 280 g a.i. ha–1 acifluorfen [sodium [5-{2-chloro-4-(trifluoromethyl)phenoxy}-2-nitrobenzoate]; premix of 560 g a.i. ha–1 bentazon plus 280 g a.i. ha–1 acifluorfen plus 140 g a.i. ha–1 clethodim [(E)-2{1-{{(3-chloro-2-propenyl)oxy}imino}propyl}-5-{2-(ethylthio)propyl}-3-hydroxy-2-cyclohexen-1-one]; fomesafen {5-[2-chloro-4-(trifluormethyl)phenoxy]-N-(methylsulfonyl)-2-nitrobenazmide} at 213 g a.i. ha–1; fluazifop {(R)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoate} at 213 g a.i. ha–1; sethoxydim [2-{1-(ethoxyimino)butyl}-5-{2-(ethylthio)propyl}-3-hydroxy-2-cyclohexen-1-one] at 213 g a.i. ha–1; clethodim[(E)-2{1-{{(3-chloro-2-propenyl)oxy}imino}propyl}-5-{2-(ethylthio)propyl}-3-hydroxy-2-cyclohexen-1-one] at 105 g a.i. ha–1; and a tankmix of 2,4-DB {4-(2,4-dichlorophenoxy)butyric acid, dimethylamine salt} at 224 g a.i. ha–1 plus metribuzin at 280 g a.i. ha–1 applied as a directed spray underneath the soybean canopy.
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The degree of weed control was assessed after soybean leaf senescence each year to measure the season-long effect of weed management systems that were intended to provide complete weed control. Control of individual weed species was visually estimated based on weed cover in each plot on a scale of 0 (no weed control) to 100% (complete weed control). Because the extent of weed cover present in plots was related to the effect of each weed management system, the weed cover estimates were used to compare the varying weed management systems. This is similar to the method used by Heatherly et al. (2003b) to estimate weed control in GR and non-GR soybean systems at the end of the growing season. Weed control data were subjected to analysis of variance using PROC MIXED (SAS Inst., 1998) to determine significance of main effects and any interactions among main effects. Means were separated at the 0.05 level of probability using Fisher's Protected LSD test. Estimates of weed cover in 1999 were used to determine initial redvine population levels.
A field combine modified for small plots was used to harvest the four center rows in plots on 11 and 21 Sept. 2000, 14 Sept. and 4 Oct. 2001, and 7 Sept. and 2 Oct. 2002. Seed from all plots were cleaned by the harvesting machine. Thus, correction for foreign matter content in seed was not necessary in any year. Harvested seed were weighed, moisture content was determined, and weights were adjusted to 130 g moisture kg–1 seed. Yield data from 1999 are not used in the yield analysis since the entire site had been deep-tilled preceding the 1999 growing season.
Estimates of total expenses (excluding charges for land, management, and general farm overhead) and returns were developed for each annual cycle of each experimental unit using the Mississippi State Budget Generator (Spurlock and Laughlin, 1992). Total specified expenses were calculated using actual inputs in each year of the experiment and included all operating expenses and machinery ownership costs, but excluded charges for land, management, and general farm overhead which were assumed to be the same for all treatment combinations. Machinery ownership costs for tractors, self-propelled harvesters, implements, and sprayers were estimated by computing the annual capital recovery charge for each machine and applying its per-hectare rate to each field operation. Operating expenses included those for: herbicides, adjuvants, seed, and labor; fuel, repair, and maintenance of machinery; hauling harvested seed; and interest on operating capital. Weed management expenses after planting were calculated for each system, and included charges for herbicides, surfactants, and application, and the extra cost for seed of GR cultivars (Table 3). All application charges included both operating expenses and ownership costs associated with tractors and sprayers. Costs for machinery and operating expenses were based on prices paid by Mississippi farmers each year.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Weed Management Expense and Total Expense
Cost of weed management for GR and non-GR cultivars was always less with POST-only than with PRE + POST application of herbicides (Table 3). The 3-yr average weed management cost for GR (includes extra seed cost shown in Table 3) and non-GR cultivars using POST was $75 and $126 ha–1, and for PRE + POST was $107 and $169 ha–1, respectively. Thus, weed management expense for non-GR cultivars was greater, even with a higher cost for seed of GR cultivars. Differences in total expenses (excluding charges for land, management, and general farm overhead) among WMSs followed the same pattern as the differences in weed management expenses (Table 3). Estimated expenses for DT averaged $324 to $422 ha–1, while those for ST averaged $270 to $372 ha–1.
Weed Control
In 1999 (first year following fall tillage), the fall tillage x WMS interaction was not significant for redvine control (Table 4). Redvine control averaged across fall tillage treatment ranged from 73 to 89%. In 2000 and 2001, the fall tillage x WMS interaction was not significant for redvine control at soybean maturity (Table 4). Thus, average redvine control values across fall tillage treatment are discussed for those 2 yr. In 2000, WMS did not significantly affect redvine control, which ranged from 75 to 92%. In 2001, redvine control in the MG V GR cultivar with PRE + POST weed management was greater than that in MG IV non-GR cultivars and the MG V non-GR cultivar with POST-only weed management. In 2002, the fall tillage x WMS interaction was significant. In the ST treatment, WMSs that had GR cultivars and glyphosate weed management resulted in greater control than did the MG IV non-GR cultivar or the MG V non-GR cultivar with POST-only weed management. The more complete canopy of the MG V non-GR cultivar that resulted from its longer growing season (Table 1), in combination with PRE + POST weed management, was effective in suppressing redvine in the ST environment. In the DT treatment, all WMSs had statistically similar redvine control.
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In addition to redvine, johnsongrass and pitted morningglory (Ipomoea lacunosa L.) became dominant weed species by 2002. In 2002, johnsongrass control was not significantly affected by the fall tillage treatment x WMS interaction. Average control of johnsongrass following 4 yr of the same WMSs applied to the same plots was statistically equal between PRE + POST and POST when GR cultivars and glyphosate were used (Table 5). Control was significantly less in non-GR cultivars compared with GR cultivars across fall tillage treatment. When non-GR cultivars were used, PRE + POST weed management controlled johnsongrass better than POST-only weed management; however, control was low in all WMSs with non-GR cultivars. This population shift over the years may have been due to lack of control of rhizome johnsongrass and inadequate control of seedling johnsongrass with PRE herbicides as well as failure to control late-emerging flushes with POST nonglyphosate herbicides in non-GR cultivars. Failure to control johnsongrass with POST herbicides in non-GR weed management systems could have been due to antagonism associated with tank mixtures of grass (clethodim) and broadleaf (acifluorfen, bentazon) herbicides. Vidrine et al. (1995) demonstrated that broadleaf herbicides applied in mixtures were antagonistic toward the activity of grass herbicides. However, the premix of bentazon + acifluorfen + clethodim used in 2000 and 2001 (Table 2) was a recommended product by the Mississippi State University Extension Service, and the estimated level of johnsongrass control by this product was rated 9 out of 10. Therefore, its use was expected to provide johnsongrass control. In 2002, sethoxydim and fluazifop were applied following application of the bentazon + acifluorfen premix (Table 2) to avoid the possibility that an antagonism between the broadleaf and grass herbicides in the premix would contribute to poor johnsongrass control.
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Seed Yield and Net Return
Across-years analyses revealed significant interactions between tillage treatment and year and between WMS and year for both seed yield and net return. Also, weather patterns mentioned earlier and shown in Table 1 were different among the 3 yr. Therefore, individual-year results are discussed and related to data shown in Table 6.
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2001
As in 2000, the fall tillage x WMS interaction was not significant for either seed yield or net return. Weed management system significantly affected both variables (Table 6). The four WMSs that included MG V cultivars yielded the most and resulted in the greatest net returns. This apparently resulted from the above-normal rain in August (Table 1) that provided more water during seed fill of the MG V cultivars. Using GR or non-GR cultivars and PRE + POST or POST-only weed control made no significant difference when MG V cultivars were used. Glyphosate-resistant MG IV cultivars produced greater net returns than did non-GR MG IV cultivars. Using PRE + POST vs. POST-only weed control resulted in greater net returns when non-GR MG IV cultivars were used. As in 2000, the lack of a significant fall tillage x WMS interaction indicates that tillage environment had no significant effect on results.
2002
As in the previous 2 yr, the fall tillage x WMS interaction was not significant for either seed yield or net return. Weed management system significantly affected both variables (Table 6). Unlike the previous 2 yr, however, there was no advantage in yield or net return for either MG IV or MG V cultivars. When MG IV cultivars were used, glyphosate resistance had no significant effect on yield, but resulted in greater net returns because of the lower cost of weed control for the GR cultivar (Table 3). When MG V cultivars were used, glyphosate resistance resulted in greater yield and net returns. Use of PRE + POST vs. POST-only weed control did not significantly affect yield, but did result in lower net returns when the MG IV GR cultivar was used. As in the previous 2 yr, the lack of a significant fall tillage x WMS interaction indicates that tillage environment had no effect on results.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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40% in non-GR cultivars regardless of fall tillage treatment. When GR cultivars were used in either tillage environment, control of johnsongrass was 
93%. These results indicate that the extra expense incurred from using DT for perennial weed control is not justified when GR cultivars are used in this environment. This is counter to the premise of Koskinen and McWhorter (1986) that continued use of shallow or minimum tillage may result in increasing levels of perennial weed infestations, which was proffered before GR cultivars were in use. The heavy johnsongrass pressure in non-GR cultivars in 2002 was associated with lower yields and net returns from the WMSs with non-GR cultivars. The greater expense associated with use of PRE + POST compared with POST did not translate into increased yields, but resulted in lower profits in some cases regardless of tillage treatment. This finding supports those of earlier-cited studies. It is noted that the POST non-GR programs in 2001 and 2002 contained residual herbicides (fomesafen in 2001 and metribuzin in 2002; Table 2), and these would have been beneficial for late-season weed control in non-GR soybean. Total POST programs that would have relied on nonresidual herbicides may not have been as successful.
Direct comparisons between tillage treatments are not valid because replicates are subsamples of tillage treatment. However, trends did occur. In 2000 and 2001 when low and untimely rain coincided with MG IV reproductive development, average yields and profits from MG IV cultivars grown in DT were 2357 kg ha–1 and $101 ha–1, whereas those from ST were 1672 kg ha–1 and $20 ha–1. In 2002, when rain patterns were timely for MG IV reproductive development, this trend did not occur. When later-maturing MG V cultivars were used, there were no trends for differences in profits resulting from using different fall tillage treatments. These trends support the findings from earlier-cited reports.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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This article has been cited by other articles:
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  L. G. Heatherly, K. N. Reddy, and S. R. Spurlock Weed Management in Glyphosate-Resistant and Non-Glyphosate-Resistant Soybean Grown Continuously and in Rotation Agron. J., March 1, 2005; 97(2): 568 - 577. [Abstract] [Full Text] [PDF]
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