Published online 3 October 2006
Published in Agron J 98:1575-1581 (2006)
DOI: 10.2134/agronj2006.0096
© 2006 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Production Papers
Iron Deficiency Chlorosis in Soybean
Soybean Seeding Rate and Companion Crop Effects
Seth L. Naeve*
Department of Agronomy and Plant Genetics, Univ. of Minnesota, 411 Borlaug Hall, 1991 Upper Buford Cir., St. Paul, MN 55108
* Corresponding author (naeve002{at}umn.edu)
Received for publication March 29, 2006.
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ABSTRACT
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Iron deficiency chlorosis (IDC) is an important production problem in soybean [Glycine max (L.) Merr.] grown in the upper Midwest. Although IDC-tolerant varieties are available to producers, even tolerant cultivars suffer from IDC-related stresses. Several cultural methods have been utilized in an attempt to reduce IDC losses. Interplant competition from increased soybean seeding rates or companion crops appears to reduce IDC symptoms. Two related studies were established to examine increased seeding rates and the planting of companion crops to reduce IDC of soybean in eight IDC-prone environments. In one study, soybean was planted at five seeding rates from 432 000 to 926 000 seeds ha–1. In a second study, a glyphosate-resistant soybean variety was planted as a sole crop or with companion crops of glyphosate-susceptible soybean or oat (Avena sativa L.). Seeding rate treatments affected IDC scores and yield, but these effects were confounded by environmental interactions. Overall, yield increases from higher seeding rates tended to be small and inconsistent, but among seven environments exhibiting mild to moderate IDC symptoms, increasing seeding rates from 432 000 to 926 000 seeds ha–1 increased yields by 281 kg ha–1 (16%). Blending of a glyphosate-susceptible with a glyphosate-resistant variety seemed to have little positive effect on yield of soybean (yield gains varied from –333 to + 566 kg ha–1). Likewise, using oat interseeded with soybean to reduce IDC symptoms and increase soybean yields is unlikely to be a profitable practice for soybean producers.
Abbreviations: AE, acid equivalent • Fe-EDDHa, Fe-ethylenediamine (o-hydroxyphenylacetic)acid • IDC, iron deficiency chlorosis • Susc., susceptible
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INTRODUCTION
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SOYBEAN is susceptible to IDC when grown on calcareous soils in the North Central region of the USA (Inskeep and Bloom, 1987; Franzen and Richardson, 2000). Hansen et al. (2004) estimated IDC's yearly economic impact on U.S. producers to be at least $120 million. Many management approaches have been used to reduce IDC. Foliar sprays of various Fe compounds have been evaluated but are only effective for mild chlorosis or for brief periods of time (Randall, 1981; Penas et al., 1990). Seed coating with Fe-EDDHA (Fe- ethylenediaminedi[o-hydroxyphenylacetic] acid) has also been successfully used and shown to increase seed yields and reduce IDC (Wiersma, 2005; Penas et al., 1990; Karkosh et al., 1988). When applied at very high rates (9–11 kg ha–1), seed coating with Fe-EDDHA has the potential to allow IDC-susceptible varieties to produce vegetative and grain yields similar to resistant varieties (Wiersma, 2005). However, these very high rates pose potential handling and planting problems and are unlikely to be economically viable solutions to IDC. In contrast, Goos and Johnson (2000) did not observe a yield increase with Fe-EDDHA seed treatment, and found that cultivar selection was the only practical and economically feasible approach to reducing IDC.
A second study by Goos and Johnson (2001) examined three cultivars planted in 78-cm rows at 370 000 and 740 000 seeds ha–1 at four locations within 1 yr. Seeding rate affected IDC scores at Stage V4 to V5 (Fehr and Caviness, 1977) and final soybean yields in all four environments. Doubling seeding rates reduced IDC scores by 0.3 to 0.4 units and increased yields roughly 250 kg ha–1 (9%). Penas et al. (1990) suggested that producers utilize high plant population (40 seed m–1 of row, regardless of row spacing), foliar sprays of FeSO4·7H2O + wetting agent, and resistant varieties to manage IDC.
In a survey by Hansen et al. (2003) of Minnesota growers, respondents identified their current management practices to prevent IDC. Producers used variety selection (70%), seeding management (42%, including seeding rate, row spacing, and planting date), and fertility management (11%, including foliar, seed, and soil treatments). Interestingly, seeding management appeared to be used in managing IDC by many Minnesota farmers despite the limited amount of research or research literature devoted to it. While producers believe that increased seeding rates can help alleviate IDC symptoms, there is a poor understanding of why it is advantageous and how managing soybean populations might improve growing conditions. Thus, IDC remains a significant problem and a challenge for producers, who have adopted a diverse set of practices to reduce IDC stress.
Under normal soybean production situations, increased seeding rates do not often translate to higher yields. Yield increases due to higher seeding rates beyond those required to establish a reasonable stand are usually small and inconsistent (Holshouser and Whittaker, 2002; Pedersen and Lauer, 2002; Lehman and Lambert, 1960). In wide rows, populations required to maximize yields are often very low. Devlin et al. (1995) found seeding rates of only 284 000 seeds ha–1 maximized soybean yields planted in 76-cm rows in Kansas.
A potential alternative to increasing soybean seeding rates would be the blending of a glyphosate {[N(phosphonomethyl)glycine]}-susceptible variety with a glyphosate-resistant one. The increased seeding rate and effective early season stand has the potential to increase the overall health of the glyphosate-resistant genotype during the early season when IDC stress is most apparent. Adding saved seed of a glyphosate-susceptible variety to a normal seeding rate of purchased glyphosate-resistant variety may be an economically viable alternative to increasing the seeding rate of the glyphosate-resistant variety alone.
While the mechanism behind IDC symptom mitigation by companion crops is largely unknown, the presence of a greater total plant biomass from increased populations leads to speculation that companion crops, such as interseeded oat, could be an alternative low-cost option to more expensive genetically modified soybean seed or seed blending. In fact, a large number of Minnesota soybean producers have experimented with planting soybeans into existing stands of small grains, most often oat (personal observation). Companion crops such as oat can be conveniently eliminated at the time of early season weed control when utilizing any of several common postemergence herbicide systems. Iron deficiency chlorosis is often most severe early in the growing season and gradually disappears in the midseason (Naeve and Rehm, 2006); therefore, requisite companion crop removal appears to coincide with a reduced need for the benefit from the companion crop. Mitigating early season stresses from IDC may position the soybean crop to recover more completely during the midseason when IDC symptoms abate.
Soybean is relatively tolerant of early season weed competition. Dalley et al. (2004) and Krausz et al. (2001) found yield losses in soybean planted in 76-cm rows only when competing weeds were greater than 30 cm in height. Vangessel et al. (2000) found no soybean yield loss from weed competition when weeds were removed at stage V4 (Fehr and Caviness, 1977). However, soybean is less tolerant of competition from weeds in more competitive growing conditions, such as higher weed densities and less than adequate rainfall (Dalley et al., 2004).
While variety selection is the primary management technique for managing IDC, many producers in Minnesota and the Dakotas are spending resources to further reduce IDC with increased soybean seeding rates and/or interseeded companion crops even in the absence of conclusive research evidence supporting these management practices. While Goos and Johnson (2001) did indicate that increased seeding rates could produce greater yields, seeding rate optima were not evaluated. The objectives of this study were to evaluate the effect of increased soybean seeding rates across a broad range of seeding rates and the effect of interseeding either glyphosate-susceptible soybean or oat on reducing IDC symptoms and increasing yield across eight IDC-prone environments.
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MATERIALS AND METHODS
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Field studies were conducted in western Minnesota at three locations in 2002 (Environments 1, 7, 10) and 2003 (Environments 2, 8, and 11) and in two locations in 2004 (Environments 3 and 9). Studies were located within Wilken, Grant, Swift, and Chippewa counties. All research locations described here were common with eight locations described in Naeve and Rehm (2006), and are identified here by the same nomenclature. Location selection, soil sampling and analysis, and all cultural practices are as described in Naeve and Rehm (2006), except where noted. Soybean staging follows Fehr and Caviness (1977).
Two separate but adjacent studies were established at each location. One study evaluated the influence of seeding rate on IDC, while the other study examined the effect of two different companion crops (oat and glyphosate-susceptible soybean) in the presence of a single seeding rate of soybean. The primary cultivar was the glyphosate-resistant variety, Asgrow AG0801, which has been extensively grown in the region due to its superior IDC resistance and excellent yield potential in challenging environments (Orf et al., 2002). This variety was chosen so that seeding rate and companion crop effects could be examined under normal production practices, where producers actively choose the highest-yielding and most IDC-tolerant variety available, when needed. Planting, harvest, and grain analysis were conducted as in Naeve and Rehm (2006).
Seeding Rate Study
The variety, AG0801, was planted at five seeding rates (432 000, 556 000, 679 000, 803 000, and 926 000 live seeds ha–1) in 76-cm rows. The practice of local producers is approximately 400 000 to 500 000 seeds ha–1. Six replications were used in a randomized incomplete block design (Cochran and Cox, 1992), where blocks consisted of three plots each. Planting dates were within 16 to 28 May each year. Visual IDC scores were made where 1 = green and 5 = severe chlorosis with some necrosis (Cianzio et al., 1979). In 2002 IDC scores were recorded on 21 June, 3 July, 12 July, 24 July, and 3 August (IDC Scores 1, 2, 3, 4, and 5, respectively). In 2003 scores were recorded on 1 July, 16 July, and 29 July (IDC Scores 2, 3, and 4, respectively). In 2004 scores were recorded on 1 July, and 20 July (IDC Scores 2 and 4, respectively). Soybean population density after emergence was recorded at V2.
Companion Crop Study
To evaluate potentially more economically feasible alternatives to the high seeding rates, AG0801 (432 000 seeds ha–1) was planted with an additional 124 000 or 247 000 seeds ha–1 (approximately 19 and 37 kg ha–1, respectively) of the glyphosate-susceptible variety Pioneer 9017, or with an additional 18.7 or 37.3 kg ha–1 of oat (approximately 670 000 and 1 300 000 seeds ha–1, respectively).
Oat is the most common crop used by Minnesota producers for IDC alleviation, possibly due to its low cost, availability, and/or ease of control with glyphosate. Preliminary research results (Naeve, unpublished data, 2001) demonstrated that corn (Zea mays L.) functioned exceptionally well as a companion crop with soybean for reducing symptoms of IDC; however, this plant was too competitive and caused significant yield reductions. Alternatively, annual ryegrass (Lolium multiflorum Lam.) was found to be noncompetitive, but did not alleviate IDC symptoms. Oat appears to provide the best balance between IDC symptom reduction and competitiveness with the soybean crop. The oat used in this study was a recurrent selection population, selected for earliness (2 wk earlier than conventional varieties) and short stature (60–70 cm when cultivated under normal oat production practices), to potentially decrease the intercrop competitiveness of oat on soybean.
The companion crops (glyphosate-susceptible soybean and oat) were eliminated with glyphosate at 0.84 kg a.e. (acid equivalent) ha–1 when the oat was approximately 40 cm tall and the soybean crop was at Stage V4 to V5. Applications were made between 23 and 30 June of each year. This timing was thought to provide the best balance between reducing early season IDC stress and potential yield loss. Both processes seem to occur through intercrop competition; thus, with late application of glyphosate, gains from IDC reduction would likely be offset by yield losses from water, nutrient and light competition. Visual IDC scores were recorded as described above. Score 1 (2002 only) was recorded before glyphosate application. All other scores were recorded after removal of the companion crop. Ten days after glyphosate application, soybean populations were recorded again to determine whether the glyphosate-susceptible soybean crop was entirely removed.
Visual IDC scores, seed yield and quality values from eight environments were used in all statistical analyses. All analysis of variance (ANOVA) were preformed with the GLM procedure using SAS v 9.1 (SAS Institute, Cary, NC). Treatments and environments were compared with Duncan's multiple range tests. Sources of variation and means were declared significant at P < 0.05.
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RESULTS
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Research Environments
Although all research locations were selected based on past history of IDC, not all environments exhibited strong IDC symptoms. Soil analysis indicated medium to high levels of available P and K except for the soils associated with Environment 7 where P was low (Naeve and Rehm, 2006). Additionally, Environment 7 was entirely nonchlorotic. The corresponding soil had relatively high levels of available Fe (DTPA extractable, 13.4 mg kg–1), likely mitigating any IDC symptoms.
Rainfall patterns were quite variable across environments with summer rainfall totals ranging from 248 mm in Environment 2 to 492 mm in Environment 9 (Fig. 1
). Moreover, the distribution of this rainfall was quite different across environments. In 2002 (Environments 1, 7, and 10), the greatest rainfall tended to occur during the midseason, whereas in 2004, (Environments 3 and 9) most of the rainfall occurred during the early and late season periods. Temperatures were similarly impacted by year. Mid-season temperatures were about 4°C cooler in 2004 than in 2002 and 2003.
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Fig. 1. Rainfall and average ambient temperature profiles for seven IDC environments from 15 May to 30 September. Rainfall distribution is shown as totals for early, mid-, and late-season precipitation by site. These seasonal totals represent 15 May through 30 June, 1 July through 14 August, and 15 August through 30 September.
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Seeding Rate Study
Seeding rates of 432 000, 556 000, 679 000, 803 000, and 926 000 seeds ha–1 resulted in early season (V2) stands of 408 000, 501 000, 629 000, 727 000, and 841 000 plants ha–1, respectively, when averaged across all environments. Despite these very high seeding rates in 76-cm rows, little evidence of reduced emergence or increased seedling death due to interplant competition was documented. Emergence averaged a 90 to 94% for treatments across all environments.
Seeding rate had a significant affect on IDC Scores 2, 3, and 4 when examined across all environments (Table 1). However, environments did not respond to increased seeding rates similarly, as is illustrated by significant environment x seeding rate interactions for IDC scores. Table 2 shows IDC scores for early, middle, and late July (Scores 2, 3, and 4) for each environment. Four of eight environments showed significant treatment effects on Score 2. Locations with less severe IDC symptoms tended to have nonsignificant treatment effects; however, a trend toward reduced chlorosis in higher seeding rates existed across most environments. In 2002, scores were recorded on five dates from late June through early August. Lower seeding rates tended to have higher IDC scores (more chlorosis) at each date. However, at only one of the two environments (Environment 1) were significant treatment effects on IDC score apparent (IDC Scores 2 and 3).
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Table 1. Population study: Significance (P > F) of environment, soybean seeding rate, and their interaction on three IDC scores, seed yield, seed protein, oil, and sum of protein + oil concentrations across eight environments.
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Table 2. Effect of seeding rate on IDC Scores, seed yield, seed protein, oil, and seed protein + oil concentration in eight IDC-prone environments.
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Seeding rate had a significant treatment effect on seed yield, and seed protein and oil concentration across all environments (Table 1); however a significant environment x seeding rate interaction for yield and protein was also identified. Oil concentration was affected by seeding rate without a corresponding environmental interaction. Among the seven chlorotic environments with minor chlorosis symptoms, increasing seeding rates from 432 000 to 926 000 seeds ha–1 resulted in an average yield increase of 281 kg ha–1, or 16%.
Because Environment 7 was nonchlorotic (Table 2), data from this environment were removed for re-analysis. The resulting ANOVA (data not shown) was nearly identical to the full analysis, except that the environment by seeding rate interaction on seed yield was no longer significant (P > F = 0.48), indicating that among environments that were at least mildly chlorotic, seeding rate significantly and positively affected yield.
Companion Crop Study
All treatments were planted with 432 000 seeds ha–1 of the glyphosate-resistant variety AG0801. Treatments 432 000 + 124 000 susc. (susceptible) and 432 000 + 247 000 susc. were planted with an additional 123 500 and 247 000 seeds ha–1 of the glyphosate-susc. variety 9017, respectively. Initial stand counts for these three treatments resulted in soybean populations of 404 000, 489 000, and 539 000, plants ha–1. The treatments 423 000 + 19 kg oat and 423 000 + 37 kg oat resulted in early season stand counts of 426 000 and 364 000 plants ha–1, respectively, indicating that the higher seeding rate of oat reduced soybean emergence by about 16%. Populations of treatments containing glyphosate-susc. soybean seed were reduced to 450 000 and 449 000 plants ha–1 after glyphosate application. While not all susc. soybean plants were removed by the glyphosate application, it did reduce populations of these treatments to similar levels. Soybean populations in the control and oat treatments did not differ between early and late stand counts.
Companion crop treatments did not significantly affect IDC Scores 2 or 4 when analyzed across all environments (Table 3). Table 4 identifies several instances when individual environments showed significant treatment effects within scores; however, clear trends between treatments and IDC scores could not be detected. This is illustrated by the significant treatment by environment interaction on IDC Scores 2 and 3 shown in Table 3.
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Table 3. Competition study: Significance (P > F) of environment, companion crops and their interaction on three IDC scores, seed yield, seed protein, oil, and sum of protein + oil concentrations across eight environments.
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Table 4. Effect of companion crops on IDC Scores, seed yield, seed protein, oil, and seed protein + oil concentration in eight IDC-prone environments.
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Figure 2
shows IDC scores over time for the two chlorotic environments studied in 2002. When analyzed by location, significant treatment effects were noted in IDC Scores 1 through 4 in Environment 10 and in IDC Score 3 in Environment 1. The oat treatments tended to be greener than the control treatment at time of Score 1, but these treatments became more chlorotic after the oat was removed by glyphosate. Any additional greening effect of the oat companion crop gained by this early season intercrop competition was lost soon after the removal of the oat.
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Fig. 2. Visual iron deficiency chlorosis (IDC) scores of soybean planted at 432 000 seeds ha–1 and with the addition of glyphosate-susc. soybean or oat. Glyphosate-susc. soybean was added at the rates of 124 000 or 247 000 seeds ha–1 (+124 K susc. and +247 K susc., respectively). Oat was added at rates of 19 and 37 kg ha–1 (+19 kg oat and +37 kg oat, respectively). Figures represent studies at two chlorotic environments in 2002 (Environments 1 and 10). Iron deficiency chlorosis scores were recorded on five dates. n = 6 ± SE.
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Treatments significantly affected soybean yield, but strong treatment by environment interactions can also be seen (Table 3). The control treatment yielded significantly more than the companion crop treatments in Environments 3 and 10 (Table 4). Treatments with glyphosate-susc. soybean as a companion crop tended to produce slightly higher yields in environments with more severe IDC (Environments 1 and 9), but these environments did not produce significant treatment effects per se. Seed protein, oil, and protein plus oil concentrations were not affected by treatment when all environments were combined for analysis (Table 3). Protein and oil were affected by treatment only in Environment 3. Here, greater protein levels were found in the glyphosate-susc. soybean companion crop treatments, and greater oil concentrations were found in the oat companion treatments.
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DISCUSSION
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The seeding rate study described here supports but does not confirm the findings (Goos and Johnson, 2001) that seeding rate can influence yield as well as IDC scores. While they found significant yield increases to seeding rates from 370 000 to 740 000 seeds ha–1 in four of four environments in 1 yr, we found significant and positive treatment effects from seeding rates of 432 000 to 926 000 in only one of eight environments across 3 yr. One nonchlorotic environment showed very large yield decreases with increased seeding rate. While this phenomenon is rare with soybean, it may be due to the low soil P, and/or due to the rainfall pattern in this environment, where more than 200 mm of rainfall likely promoted excess vegetative growth in July and early August. The 76-cm-row system used in this study likely exacerbated both potential yield-enhancing and yield-depressing effects of these very high seeding rates. When the nonchlorotic Environment 7 was removed from the analysis, the treatment by environment interaction was no longer significant. This indicates that increased seeding rates can affect soybean yield in chlorotic environments. But, this may not be the case when considering all IDC-prone environments. Within chlorotic environments, yield increased by an average of just 281 kg ha–1, indicating that yield responses to increases seeding rate in chlorotic environments are relatively small and inconsistent. It is notable that this yield increase was similar to the 250 kg ha–1 increase reported by Goos and Johnson (2001). It is possible that yield enhancement from seeding rates above 432 000 seeds ha–1 may be much smaller than the yield losses sustained from reductions in seeding rate below 432 000 seeds ha–1.
Blending glyphosate-susc. soybean seed with glyphosate-resistant soybean was evaluated on calcareous soils as a potential means to reduce input costs over increased seeding rates of pure stands of glyphosate-resistant genotypes. If proven to be successful, producers could blend glyphosate-susc. soybean seed that has been produced on their own farm with purchased glyphosate-resistant seed at very little additional cost. Unfortunately, in only three environments examined (1, 8, and 9) did this practice appear to be successful. These environments also showed increased yields in the seeding rate study. While, together, these results imply that early season stands may be important for mitigating IDC stress related yield loss in some environments, it is clear that this effect is neither large nor universal.
While many producers plant soybean crops into existing stands of small grains to reduce stresses from IDC, the potential of short-statured oat planted directly in the row with soybean appeared to offer the greatest opportunity to reduce IDC symptoms and potentially increase yields, while minimizing the negative interplant competition effects on soybean yield. However, this study found little evidence to support the interseeding of oat with soybean to reduce yield losses from IDC. It is possible that companion crops may be acting to remove some unknown IDC-promoting soil chemical factor, such as NO3 (Wallace, 1988). If this is the case, it is possible that planting of the soybean crop into an existing stand of small grains may produce more positive results than were noted here. We feel that studies examining timing effects of oat companion crops for IDC symptom mitigation are warranted. Overall, these studies indicate that increased seeding rates may increase yields of soybean crops with visible IDC symptoms, but they do not support the practice of interseeding of companion crops as a method to increase soybean yields under IDC conditions.
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ACKNOWLEDGMENTS
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This project was made possible only though the financial generosity of the Minnesota Soybean Research & Promotion Council. I thank Tracy O'Neill and Sheri Huerd for their expert technical assistance, and Richard Vanden Heuvel for his critical review.
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NOTES
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Research supported in part by the Minnesota Soybean Research and Promotion Council.
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REFERENCES
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- Cianzio, S.R., W.R. Fehr, and I.C. Anderson. 1979. Genotypic evaluation of iron deficiency chlorosis in soybeans by visual scores and chlorophyll concentration. Crop Sci. 19:644–646.[Abstract/Free Full Text]
- Cochran, W.G., and G.M. Cox. 1992. Experimental designs. Wiley, New York.
- Dalley, C.D., J.J. Kells, and K.A. Renner. 2004. Effect of glyphosate application timing and row spacing on corn (Zea mays) and soybean (Glycine max) yields. Weed Technol. 18:165–176.
- Devlin, D.L., D.L. Fjell, J.P. Shroyer, W.B. Gordon, B.H. Marsh, L.D. Maddux, V.L. Martin, and S.R. Duncan. 1995. Row spacing and seeding rates for soybean in low and high yielding environments. J. Prod. Agric. 8:215–222.
- Fehr, W.R., and C.E. Caviness. 1977. Stages of soybean development. Spec. Rep. 80. Iowa Agric. Home Econ. Exp. Stn., Iowa State Univ., Ames.
- Franzen, D.W., and J.L. Richardson. 2000. Soil factors affecting iron chlorosis of soybean in the Red River Valley of North Dakota and Minnesota. J. Plant Nutr. 23:67–78.[ISI]
- Goos, R.J., and B.E. Johnson. 2000. A comparison of three methods for reducing iron deficiency chlorosis in soybean. Agron. J. 92:1135–1139.[Abstract/Free Full Text]
- Goos, R.J., and B.E. Johnson. 2001. Seed treatment, seeding rate, and cultivar effects on iron deficiency chlorosis of soybean. J. Plant Nutr. 24:1255–1268.[ISI]
- Hansen, N.C., V.D. Jolley, S.L. Naeve, and R.J. Goos. 2004. Iron deficiency of soybean in the North Central U.S. and associated soil properties. Soil Sci. Plant Nutr. 50:983–987.
- Hansen, N.C., M.A. Schmitt, J.E. Anderson, and J.S. Strock. 2003. Iron deficiency of soybean in the upper Midwest and associated soil properties. Agron. J. 95:1595–1601.[Abstract/Free Full Text]
- Holshouser, D.L., and J.P. Whittaker. 2002. Plant population and row-spacing effects on early soybean production systems in the Mid-Atlantic USA. Agron. J. 94:603–611.[Abstract/Free Full Text]
- Inskeep, W.P., and P.R. Bloom. 1987. Soil chemical factors associated with soybean chlorosis in calciaquolls of western Minnesota. Agron. J. 79:779–786.[Abstract/Free Full Text]
- Karkosh, A.E., S.K. Walker, and J.J. Simons. 1988. Seed treatments for control of ion deficiency chlorosis of soybean. Crop Sci. 28:369–370.[Abstract/Free Full Text]
- Krausz, R.F., B.G. Young, G. Kapusta, and J.L. Matthews. 2001. Influence of weed competition and herbicides on glyphosate-resistant soybean (Glycine max). Weed Technol. 15:530–534.
- Lehman, W.F., and J.W. Lambert. 1960. Effects of spacing of soybean plants between and within rows on yield and its components. Agron. J. 52:84–86.[Abstract/Free Full Text]
- Naeve, S.L., and G.W. Rehm. 2006. Genotype by environment interactions within iron deficiency chlorosis tolerant soybean genotypes. Agron. J. 98:808–814.[Abstract/Free Full Text]
- Orf, J.H., L.L. Hardman, P.J. Schaus, and A. Killam. 2002. Soybeans, p. 63–80. In Minnesota varietal trials results. MP106-2002. Minnesota Agric. Exp. Stn., St. Paul.
- Pedersen, P., and J.G. Lauer. 2002. Influence of rotation sequence on the optimum corn and soybean plant population. Agron. J. 94:968–974.[Abstract/Free Full Text]
- Penas, E.J., R.A. Wiese, R.W. Elmore, G.W. Hergert, and R.S. Moomaw. 1990. Soybean chlorosis studies on high pH bottomland soils. Univ. Nebraska Inst. Agric. Nat. Res. Bull. 312.
- Randall, G.W. 1981. Correcting iron chlorosis in soybeans. Soils Fact Sheet 27 (revised). Minnesota Agric. Ext. Serv., St. Paul.
- Vangessel, M.J., A.O. Ayeni, and B.A. Majek. 2000. Optimum glyphosate timing with or without residual herbicides in glyphosate-resistant soybean (Glycine max) under full-season conventional tillage. Weed Technol. 14:140–149.
- Wallace, A. 1988. Iron chlorosis induced by detopping soybeans. J. Plant Nutr. 11:793–796.
- Wiersma, J.V. 2005. High rates of Fe-EDDHA and seed iron concentration suggest partial solutions to iron deficiency in soybean. Agron. J. 97:924–934.[Abstract/Free Full Text]
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ABSTRACT
INTRODUCTION
