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a USDA-ARS, Crop Genet. 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 Sci. Res. Unit, P.O. Box 350, Stoneville, MS 38776
* Corresponding author (lheatherly{at}ars.usda.gov)
Received for publication June 18, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase • GR, glyphosate resistant • WMS, weed management system
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Glyphosate-resistant cultivars offer producers the flexibility to control a broad spectrum of weeds in soybean with no concern for crop safety (Reddy, 2001). Cost of weed control using a postemergence management program for GR cultivars should be less, even with the greater cost for seed of most GR cultivars (Reddy et al., 1999; Heatherly et al., 2002b). This could translate to increased profits if yields from GR cultivars are equal or nearly equal to those from non-GR cultivars. Use of GR cultivars should preempt the use of tillage and pre-emergent herbicides for weed management. The flexibility of using either nonglyphosate herbicides or glyphosate on GR cultivars increases management options for weed control when GR cultivars are used. Nonglyphosate herbicides applied to GR soybean in monocrop or corn (Zea mays L.)–soybean rotation systems do not adversely affect GR soybean (Nelson and Renner, 1999; Webster et al., 1999).
Glyphosate inhibits 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) and thus blocks aromatic amino acid synthesis (Amrhein et al., 1980). While GR soybean cultivars contain resistant EPSPS, Bradyrhizobium japonicum, the principal N-fixing bacterium for soybean, does not contain a resistant enzyme. Thus, glyphosate applied to GR soybean may interfere with the symbiotic relationship (King et al., 2001). In controlled-environment experiments, Reddy et al. (2000) found that 1.12 kg a.i. ha-1 glyphosate reduced number and fresh weight of soybean nodules in the absence of N but had no effect with added N. In greenhouse, growth chamber, and field studies, early applications of 
1.68 kg ha-1 glyphosate generally delayed N2 fixation and decreased soybean biomass and N accumulation (King et al., 2001). However, plants had recovered by 40 d after emergence. In growth chamber studies, N2 fixation was more sensitive to water deficits in GR plants treated with glyphosate. In a 1-yr field study, soybean biomass was reduced by glyphosate applied to GR soybean at one of two locations; the location with the reduction had limited soil water. Yield was not consistently affected by glyphosate application. Conditions and treatments (like glyphosate) that adversely affect the symbiotic relationship may influence the sensitivity of N2 fixation to water deficits. King et al. (2001) surmised from their results that glyphosate applied to GR soybean not only delayed N2 fixation, but also increased sensitivity of N2 fixation to soil water deficits. Their conjecture was based on one year's field results from one location. Nelson and Renner (1999) and Elmore et al. (2001) found that glyphosate has no negative effect on GR soybean growth, development, or yield in the field.
Soybean grown on most soils does not respond to preplant N fertilization (Johnson, 1987; Varco, 1999; Hoeft et al., 2000). The exceptions cited by Johnson (1987) were applications made to soils that were somewhat poorly drained, low in organic matter, and/or strongly acid below the plow layer. Ferguson et al. (2000) summarized work from Nebraska that had positive responses to preplant N applications about half the time and determined that it was not possible to predict soybean response to N fertilizer based on soil properties. The situations with positive responses often either had very low residual N, low N mineralization capability, or soil pH so low that it inhibited nodulation and N fixation. In these cases, 56 to 112 kg N ha-1 increased yield potential. Kansas scientists found that soybean planted into large amounts of wheat (Triticum aestivum L.) residue responded to 11 to 22 kg ha-1 starter N because inorganic N is temporarily immobilized by soil microorganisms decomposing the straw (Whitney, 1997). They also found that soybean planted on recently leveled soils may respond to 33 to 45 kg N ha-1 because of low soil N. In most cases, N fertilization of soybean is an unnecessary expenditure (Varco, 1999; Hoeft et al., 2000). In addition, concentrations of N surrounding soybean roots can delay or impede nodulation (Gibson and Harper, 1985) and thus reduce N fixation.
Soybean, especially when not irrigated, provides relatively low gross return with a small margin for profit in the midsouthern USA (Heatherly and Spurlock, 1999; Williams, 1999). This small profit margin dictates that all inputs associated with production must be evaluated with respect to their likelihood of increasing profitability and that yield losses due to controllable pests such as weeds must be prevented within economic constraints. The objective of the producer is to control weeds adequately to maximize profits; however, inputs used for weed management in soybean represent a significant cost (Buhler et al., 1997; Johnson et al., 1997; Reddy and Whiting, 2000; Heatherly et al., 2001, 2002a). Weed management expenditures are almost always made before the onset of drought and without knowledge of ensuing moisture status for subsequent crop and weed development. This presents a challenge, especially in nonirrigated production systems that often result in low yield in the midsouthern USA.
Measuring the effect of glyphosate on GR cultivars will involve the use of nonglyphosate pre-emergent and postemergent herbicides on both non-GR and GR cultivars and glyphosate on GR cultivars. It will also involve the application of early-season N to both non-GR and GR cultivars that are grown under the same weed management system (WMS). The treatments used in this study address these criteria. This research was designed to determine if the perceived effect of glyphosate on the symbiotic relationship between N-fixing nodulating bacteria and GR soybean cultivars can be overcome with, or compensated for by, the addition of N soon after planting in the field. The objective was to compare the yield and economic return from GR and non-GR soybean cultivars where early-season N was applied before application of postemergent nonglyphosate and glyphosate herbicides in nonirrigated (low yielding) and irrigated (high yielding) environments in the midsouthern USA. Economic analysis of results was conducted to assess the profitability of two WMSs and added N. Seed yields and estimated costs and returns were used to generate budgets for the economic comparisons.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Separate nonirrigated and irrigated experiments were conducted using a randomized complete block design with four replicates each year. Treatments were arrayed in a split-plot factorial arrangement, with cultivar as the main plot and the combination of WMS and early-season N level as the subplot. Treatments were randomly assigned to plots at the beginning of the study period and remained in the same location thereafter to determine effects where the same WMS and N level were used continuously over a period of time.
Row width was 0.5 m, and seeding rate was 16 seed m-1 row, or about 50 kg ha-1 seed. Plots were 4 m wide (8 rows) and 22 m (irrigated) or 20.5 m (nonirrigated) long. All experiments were seeded into a stale seedbed (Heatherly and Elmore, 1983; Heatherly, 1999a) that had been tilled the preceding fall. Fall tillage consisted of chisel plowing 45 cm deep followed by shallow tillage (<10 cm deep) with a disk harrow and spring-tooth cultivator in 1998 and 1999 and shallow tillage with a disk harrow and spring-tooth cultivator in 2000. Glyphosate at 840 g a.i. ha-1 in 94 L ha-1 water was applied preplant to each experimental site each year to kill existing weed vegetation.
Non-GR and GR cultivars were used each year. They were non-GR ‘AP 4880’ in 1999 and ‘AP 4882’ in 2000 and 2001 and GR ‘SG 468’ and ‘DP 4750’ in 1999 and ‘DP 4690’ and ‘AG 4702’ in 2000 and 2001. Cultivars were chosen based on regional variety trial results, use patterns by producers, and recency of release. Cultivars were updated in 2000 to ensure that recently released, relevant cultivars that offered potentially improved performance were used. Planting dates were 17 May 1999, 28 Apr. 2000, and 2 Apr. 2001. Seed were treated with mefenoxam {(R)-2-[2,6-(dimethylphenyl)-methoxyacetylamino]-propionic acid methyl ester} fungicide at 0.11 g a.i. kg-1 seed before seeding each year.
Levels of N were 0 and 35 kg ha-1 surface-applied as granular ammonium nitrate (340 g N kg-1 material) using a granular fertilizer applicator on 7 June 1999, 15 May 2000, and 11 Apr. 2001. These applications were made within 14 d after emergence and before stage V2 and preceded all postemergent herbicide applications. Rainfall of >2 cm occurred 19, 5, and 1 d after N application in 1999, 2000, and 2001, respectively. Costs for the N and its application were $38.20 ha-1, $41.40 ha-1, and $48.96 ha-1 in 1999, 2000, and 2001, respectively.
Weed management systems each year were (i) pre-emergent broadleaf followed by postemergent broadleaf and grass weed management using nonglyphosate herbicides applied to both GR and non-GR cultivars (PRE + POST) and (ii) postemergent broadleaf and grass weed management using glyphosate on GR cultivars and nonglyphosate herbicides on non-GR cultivars (POST). Within each WMS, use of herbicides and their combinations (Table 1) was dictated by expected weed populations (PRE) or actual populations (POST). Expert opinion during the growing season was used to determine when weed populations within each WMS were sufficient to justify application of postemergent herbicides and what herbicides to use. The PRE + POST WMS for GR cultivars received nonglyphosate herbicides applied postemergence to determine the effect of N application on GR cultivars that had no glyphosate applied to them. Two applications of glyphosate applied sequentially to GR cultivars in the POST treatment is supported by results from previous research (Gonzini et al., 1999; Wait et al., 1999; Payne and Oliver, 2000; Swanton et al., 2000). The objective in each WMS was to use the rates of glyphosate or nonglyphosate herbicides most likely to minimize weed competition within the constraints of each individual WMS each year.
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In the irrigated experiments, water was applied by the furrow method through gated pipe whenever soil water potential at the 30-cm depth, as measured by tensiometers, decreased to between -50 and -70 kPa. The effect of irrigation on yield of soybean in the midsouthern USA is well documented (Heatherly, 1999b), but irrigation environment can also affect infestation levels of some weed species (Heatherly et al., 1994, 2001, 2002a). Amounts of irrigation water applied and irrigation starting and ending dates each year were 360 mm applied between 7 July and 24 August in 1999, 355 mm applied between 28 June and 23 August in 2000, and 160 mm applied between 19 June and 24 July in 2001. Applied water traversed the area in furrows created by the tractor wheels during seeding on this soft clay soil. Irrigation amounts were determined by the degree of cracking in this shrink–swell soil (cracks when dry, swells when wet) because water applied to it through surface irrigation flows downward to the depth of cracking and rises to the surface as the cracks fill (Mitchell and van Genuchten, 1993). Weather data presented in Table 2 were collected approximately 0.8 km from the experimental site by Delta Research and Extension Center personnel.
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A field combine modified for small plots was used to harvest the four center rows of each plot on 10 (nonirrigated) and 23 (irrigated) Sept. 1999, 15 (nonirrigated) and 19 (irrigated) Sept. 2000, and 10 (irrigated) Sept. 2001. The nonirrigated study was not harvested in 2001 due to extreme weed reinfestation resulting from incomplete soybean canopy closure and the above-normal rainfall (245 mm) that occurred from 11 August through 3 September. Soybean seed from all plots were cleaned by the harvesting machine; thus, correction for foreign matter content in seed of any treatment combination was not necessary in any year. Harvested seed were weighed and adjusted to 130 g moisture kg-1 seed.
Estimates of total expenses 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 for each treatment 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, sprayers, and the irrigation system were estimated by computing the annual capital recovery charge for each machine and applying its per-hectare rate to each field operation. Costs for machinery and operating expenses were based on prices paid by Mississippi farmers each year. Operating expenses included those for herbicides and adjuvants; seed; rollout vinyl pipe used in irrigation; labor; fuel, repair, and maintenance of machinery and irrigation systems; hauling harvested seed; and interest on operating capital. The price of seed for GR cultivars was $0.46 kg-1 more than that for non-GR cultivars in 1999 and $0.42 kg-1 more in 2000 and 2001; this extra cost was added to the weed management expense for GR cultivars. Weed management expenses after planting were calculated for each treatment and included charges for herbicides, surfactants, and application. All application charges included both operating expenses and ownership costs associated with tractors and sprayers. Irrigation expenses were based on a 65-ha furrow irrigation setup and included an annualized cost for the engine, well, pump, gearhead, generator, fuel tank and lines, and land leveling. The USDA loan rate of $0.196 kg-1 soybean for Mississippi was used to calculate income from each experimental unit each year. Net return above total specified expenses was determined for each experimental unit each year.
Analysis of variance [PROC MIXED (SAS Inst., 1996)] was used to evaluate the significance of treatment effects on seed yield and net return separately for nonirrigated and irrigated experiments. Analyses across years treated year as a fixed effect to determine interactions involving year. Analyses for individual years treated cultivar, WMS, and N level as fixed effects, and the replicate x cultivar error term was assigned as random in the PROC MIXED model. Mean separation was achieved with an LSD0.05.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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During this research, all years experienced undesirable weather at some time (Table 2). In 1999, average monthly maximum temperatures during April through June were near normal. High temperatures in conjunction with little rainfall in July and August resulted in severe stress for all cultivars in the nonirrigated environment. This stress was exacerbated by the relatively late planting date of 17 May in 1999 and the beginning bloom through full seed period occurring from late June through late August. In 2000, average monthly maximum temperatures from April through June were near normal while July and August temperatures were above normal. Rainfall in July and August of 2000 was only 16 mm. The beginning bloom through full seed period occurred from early June through mid-August. Again, these conditions resulted in severe stress for cultivars in the nonirrigated environment. In 2001, average monthly maximum temperatures were near normal in all months of the growing season. August 2001 rainfall was above normal and a record for the month. The beginning bloom through full seed period occurred from early May through late July. The low rainfall amounts in July and August of 1999 and 2000 resulted in greater irrigation amounts being applied in those years than in 2001.
Weed Management Expense and Weed Control
Weed management costs for GR cultivars were always less with POST (only glyphosate used) than with PRE + POST (nonglyphosate herbicides used) (Table 1). For non-GR cultivars, PRE + POST was cheaper than POST in 1999 and 2001 while costs of the two were similar in 2000. Costs for PRE + POST applied to GR cultivars were greater than for PRE + POST applied to non-GR cultivars because of the greater cost for seed of GR cultivars. Costs for POST applied to GR cultivars were less than for POST applied to non-GR cultivars. This cheaper weed management with postemergent glyphosate compared with non-glyphosate postemergent herbicides over the course of this study agrees with results of Nelson and Renner (1999) and Heatherly et al. (2002a)(2002b). Over the 3 yr of this study, POST for GR cultivars cost the least, and PRE + POST for GR cultivars cost the most.
All WMSs provided excellent weed control at the end of the weed control period (immediately before irrigation initiation). In the nonirrigated environment, control of predominant weed species {barnyardgrass [Echinochloa crus-galli (L.) Beauv.], browntop millet [Brachiaria ramosa (L.) Stapf], hyssop spurge (Euphorbia hyssopifolia L.), johnsongrass [Sorghum halepense (L.) Pers.], pitted morningglory (Ipomoea lacunosa L.), and prickly sida (Sida spinosa L.)} at harvest ranged from 93 to 100% in 1999 and 2000, with no significant differences between years (Table 3). Control of these species averaged across years was 94% regardless of cultivar, WMS, and N level, with one exception: Small but significant differences in johnsongrass control among cultivars occurred, but control was at least 89% in all cultivars. The POST WMS was as effective in controlling weeds as the PRE + POST WMS. Application of early-season N (35 kg ha-1) had no effect on weed control in the nonirrigated environment.
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Soybean Seed Yield and Net Return
Nonirrigated
All yields were extremely low (average 730 and 1185 kg ha-1 in 1999 and 2000, respectively) as a result of drought stress each year (Table 4), and all net returns were negative (Table 5). Soybean seed yield was not significantly affected by N level or WMS in either year (Table 4). In both years, a GR cultivar produced the greatest yield. Use of 35 kg N ha-1 resulted in smaller average net returns in both years as a result of the additional cost with no concomitant increase in yield (Table 5). The cultivar x WMS interaction was significant for net return in both years. A GR cultivar produced a greater net return than the non-GR cultivar within each WMS. Within cultivar, the PRE + POST WMS resulted in more net return for the non-GR cultivar each year while the opposite was true for the GR cultivars. Again, all yields were low and all net returns negative. It is obvious from these results that any surmised yield reduction from use of glyphosate on GR soybean in the field will not occur when drought stress is severe.
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Use of 35 kg N ha-1 resulted in smaller average net returns in all years as a result of the additional cost, with no significant yield increase sufficient to offset cost of N (Table 5). The cultivar x WMS interaction was significant for net return in 1999 and 2000. Use of the non-GR cultivar resulted in greater net returns when PRE + POST was used than did use of GR cultivars. When POST was used, use of a GR cultivar resulted in net return that was similar to or greater than that from the non-GR cultivar. Within cultivar, the PRE + POST WMS resulted in greater net return from the non-GR cultivar each year while the opposite was true for the GR cultivars. In 2001, use of the non-GR cultivar resulted in a greater net return than that from use of AG 4702. The WMS x N level interaction was significant for net return in 2001. When the PRE + POST WMS was used, N level had no significant effect; when POST was used, using 35 kg ha-1 N resulted in smaller net return. Net return from use of POST was greater only when the zero N level was used.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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In the nonirrigated environment, all net returns were negative because of extremely low yields resulting from extreme drought stress, and use of early-season N reduced net return even more. In the irrigated environment, non-GR cultivars generally produced greater yield and net return than GR cultivars. Net returns from non-GR cultivars were greater with PRE + POST than with POST WMS in 2 of the 3 yr while net returns from GR cultivars were greater with POST than with PRE + POST in all years. Neither glyphosate nor early-season N significantly affected yield of GR cultivars in the POST WMS that received glyphosate compared with nonglyphosate postemergent herbicides in both irrigated and nonirrigated environments. This contrasts with results from a 1-yr field study conducted by King et al. (2001), who inferred from this study that glyphosate tends to decrease seed yields of GR cultivars grown with limited soil water.
In the present study, early-season N application to soybean did not benefit yield of either GR or non-GR cultivars and resulted in smaller net returns. These results also indicate that using PRE + POST compared to POST-only weed management with GR cultivars will result in smaller net returns because of the increased cost incurred from using pre-emergent herbicides in conjunction with greater cost for seed of GR cultivars.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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), barnyardgrass (ECHCG), johnsongrass (SORHA), pitted morningglory (IPOLA), prickly sida (SIDSP), and hyssop spurge (EPHHS) at time of soybean harvest in glyphosate-resistant (GR) and non-GR soybean cultivars grown under two weed management systems (WMS
) with and without early-season N (0 and 35 kg N ha-1) in nonirrigated and irrigated environments at Stoneville, MS, 1999–2001
.