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Genetic gain for new soybean [Glycine max (L.) Merr.] cultivars is substantial, and research suggests this is partially explained by greater tolerance to increased plant densities. We conducted an experiment to determine if greater tolerance to increased plant density and more efficient use of cumulative intercepted photosynthetically active radiation (ClPAR) partially explained the yield difference of two old soybean cultivars and two new cultivars. Soybean cultivars were grown in 38-cm spaced rows at final plant densities of 4.9, 14.8, 24.7, 34.6, and 44.5 plants [m.sup.-2] for 2 yr in Ames, IA. New cultivars averaged 914 kg [ha.sup.-1] and 4.9 g [plant.sup.-1] greater yield compared with old cultivars averaged across years and plant densities. Yield plateaus occurred at plant densities of 23.0 plants [m.sup.-2] for new cultivars and 19.9 plants [m.sup.-2] for old cultivars. Each cultivar group showed a similar yield decline per plant to increased plant density. New and old cultivars showed a similar response to increasing values of ClPAR, attaining a 95% relative yield plateau at 466 MJ [m.sup.-2]. Soybean yield response to increased plant density and ClPAR has not changed over time, and genetic gain from new cultivars is associated with more efficient use of light. doi: 10.2135/cropsci2009.02.0063

Soybean [Glycine max (L.) Merr.] oil with oleic acid (18:1) content >500 g [kg.sup.-1] is desirable for a broader role in food and industrial uses, Seed oil in commercially grown soybean genotypes averages about 230 g [kg.sup.-1] oleic acid. Some maturity group (MG) II to V plant introductions (PIs) have elevated oleic concentrations of 300 to 500 g [kg.sup.-1]. Temperature of the growing environment during the reproductive growth stage affects oleic concentration in soybean oil. The objective of this study was to evaluate stability of oleic acid content among 15 PIs with elevated 18:1 and three checks grown in 16 environments. Oleic acid concentration of the high 18:1 checks N98-4445A and M23 ranged from 383 to 694 g [kg.sup.-1] and 428 to 572 g [kg.sup.-1] with averages of 575 g [kg.sup.-1] and 508 g [kg.sup.-1], respectively. The PIs with the highest 18:1 were MG II to III, with most lines averaging >400 g [kg.sup.-1] oleic acid concentration over 16 environments. Generally, PIs in MG II to III were less stable across environments than those in MG V. However, MG III PI 379559D ranged from 381 to 513 g [kg.sup.-1] with an average 439 g [kg.sup.-1] oleic acid concentration and was the most stable in oleic acid content of the 15 PIs studied. PI379559D was more stable than N98-4445A or M23. PI417360 and PI506852 averaged highest in 18:1 (>330 g [kg.sup.-1]) among MG V PIs studied. Combining genes from these PIs and other sources with elevated 18:1 may be useful in developing higher oleic acid soybean genotypes.

The development of high-oleate soybean [Glycine max (L.) Merr.] germplasm, currently in demand due to its nutritional value and oxidative stability, may be hampered by the instability of fatty acid composition across environments, which is largely attributed to temperature changes and its effects on fatty acid biosynthesis during the period of oil deposition. In the present study, we evaluated three soybean populations that segregated for oleate content and maturity at multiple environments in North Carolina. Oleate content was positively correlated with daily temperatures, averaged over the seed-filling stage, for the experimental lines of the late-maturing population but was negatively correlated for the lines of the early-maturing populations. Oil content was positively correlated with the average daily temperature during seed filling in all three populations, regardless of their early- or late-maturity profile. Negative correlations between oleate content and temperature during the period of oil deposition have not been previously reported. Moreover, in contrast to previous reports, maximum, minimum, and average daily temperatures were highly correlated during seed filling in all environments of this study. We conclude that cautious interpretation of these findings is necessary due to the correlation between temperature and photoperiod during the period of oil deposition.

Isoflavones are naturally occurring compounds found in soybean [Glycine max (L.) Merr.]. Soybean isoflavone, as a quantitative trait, is subject to significant genotype x environment interaction, which makes breeding for this trait difficult. Thirty [F.sub.4:7] soybean lines, derived from crosses of 'RCAT Angora' x CK-01 and 'Heinong 35' x RCAT Angora were classified within each population as high, intermediate, or low isoflavone. The lines, parents, and two maturity checks were grown in four locations in 2005 and six locations in 2006 across Ontario and Quebec, Canada. Isoflavone content of the mature seed was determined by near-infrared reflectance. The effects of genotype, environment, and the genotype x environment (G x E) interaction were significant. Consistently performing genotypes from the two populations were identified by several stability parameters. Genotype-genotype x environment (GGE) biplot demonstrated an ability to provide information on both the genotypes and the environments in which they were evaluated. The identification of genotypes with consistent placement in either the high- and low-isoflavone classes suggested that breeding for relative isoflavone content in soybean is possible, although breeding for absolute stability remains a challenge, given the large environmental influence on soybean isoflavone levels.

There is uncertainty about manure P availability for crops and need for P fertilization when manure is applied. This study evaluated effects of liquid swine (Sus scrofa domesticus) manure and fertilizer P on early plant growth and P uptake, grain yield, and grain P removal for corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. Sixteen trials were conducted in Iowa from 2000 to 2002. Replicated treatments were the combinations of three manure rates (0, 29, and 57 kg total P [ha.sup.-1] on average across sites) and four P fertilizer rates (0 to 30 kg P [ha.sup.-1]). Initial soil Bray-[P.sub.1] (15-cm depth) was 9 to 89 mg P [kg.sup.-1]. Fertilizer rates applied across all plots were 168 kg N [ha.sup.-1] for corn and 56 to 120 kg K [ha.sup.-1] for both crops according to soil-test K. Measurements were early plant dry weight (DW) and P uptake (V5-V6 stage), grain yield, and grain P removal. Increases in plant DW, P uptake, and grain P removal were unrelated to soil-test P (STP), and were more fiequent to manure than to fertilizer. Yield increases (P [less than or equal to] 0.05) were observed at six sites with Bray-[P.sub.1] [less than or equal to] 20 mg P [kg.sup.-1], with response only to manure at one site and to both manure and fertilizer at five sites but to fertilizer only when manure was not applied. The study provided no evidence of lower crop-P availability from liquid swine manure than fertilizer P for corn or soybean in these Iowa fields. Abbreviations: DW, dry weight; OM, organic matter; STE soil-test P.

Genotypic differences for canopy wilting have been reported for soybean [Glycine max (L.) Merr.], but no wilting data have been published, and mechanisms for differences remain unresolved. In field studies in 2002 and 2003, differences for wilting among 19 genotypes were consistent across years. Plant introductions (PI 416937 and PI 471938) were among the slowest wilting genotypes, and breeding lines (93705-34 and 93705-95) were among the fastest wilting. Row spacing (18 vs. 80 cm wide) did not affect wilting, indicating that lateral rooting did not contribute to genotypic differences. In a separate field study, volumetric soil water content at 15- and 50-cm depths was generally greater for slow-wilting PI 416937 than for fast-wilting 93705-95. Wilting for both genotypes responded similarly to soil water content ([r.sup.2] = 0.63-0.74). In a growth chamber study, transpiration declined similarly for fast- and slow-wilting genotypes in response to soil water deficit. Wilting response to soil water was the same for slow-wilting P1416937 and fast-wilting genotypes 93705-34 and A5959. Slow-wilting 93705-36 began wilting at a lower soil water content than did PI416937, 93705-34, and A5959, indicating that more than one mechanism may be responsible for slow wilting.

Early-maturing soybean [Glycine max (L.) Merr.] cultivars require less irrigation than full-season cultivars and may mature before drought periods most often occur in the midsouthern United States. These cultivars require high plant-population densities for radiation interception and acceptable yields, which increase costs. We hypothesized that seven-leaflet genotypes would have greater leaf area per plant, resulting in more radiation interception and higher yield than near-isogenic three-leaflet genotypes at similar populations. Near-isogenic lines from maturity groups 00 to 1.8 were seeded at rates from 4 to 80 [m.sup.-2]. The fraction of photosynthetically active radiation (PAR) intercepted by plots was measured using digital imagery and used to estimate cumulative intercepted PAR (CIPAR). Although seven-leaflet isolines had greater leaf area per leaf than three-leaflet isolines, leaf area per plant was similar between three- and seven-leaflet isolines because the three-leaflet isolines had a slightly greater number of mainstem leaves than seven-leaflet isolines. Generally, seven-leaflet isolines had 10 to 21% greater CIPAR at populations [less than or equal to] 40 [m.sup.-2] compared to three-leaflet isolines. At populations [less than or equal to] 20 [m.sup.-2], seven-leaflet isolines generally had higher yields than three-leaflet isolines, but yields at these low populations were inherently low and agronomically unacceptable.

Soybean [Glycine max (L.) Merrill] seed with low raffinose, stachyose, and phytin is desired for feeding nonruminant animals to improve feed efficiency, increase mineral uptake, and reduce flatulence, but may have reduced agronomic quality. Composition of soluble carbohydrates in seed parts and sensitivity of mature seed to imbibitional chilling were determined for low raffinose and stachyose (LRS) seed; low raffinose, stachyose, and phytin (LRSP1, LRSP2) seed; and normal raffinose, stachyose, and phytin (CHECK) seed. Cotyledons and axes of seed from the three modified lines had low raffinose, stachyose, and verbascose compared to the CHECK. Cotyledons from LRS seed had significantly higher concentrations of galactinol and the di- and tri-[alpha]-galactoside derivatives of myo-inositol, D-pinitol, and D-chiro-inositol than cotyledons from LRSP1 and LRSP2 seed. Seed coats of all four lines were similar in soluble carbohydrate composition indicating the modifications were expressed in embryo tissues. Mature seed of LRS and CHECK was tolerant to imbibitional chilling, but LRSP1 and LRSP2 seed were sensitive to imbibitional chilling. The higher accumulation of cyclitol [alpha]-galactosides in embryos (cotyledons) of LRS seed (90% cotyledons, 2% axis, 8% seed coat) may have contributed to imbibitional chilling tolerance.

Soybean [Glycine max (L.) Merr.] seed produced in high-temperature, high-humidity production environments is prone to have substandard germination. Hardseededness, wrinkled seed coats, and infection by Phomopsis Iongicolla Hobbs are all known to affect soybean seed germinability. Ancestors of modern U.S. soybean cultivars may lack the necessary variability to impart high germinability to new cultivars without the introgression of new genetic diversity. The purpose of this research was to identify soybean germplasm possessing positive traits affecting high seed germinability for seed produced under high-temperature environments, such as in the early soybean production system (ESPS) of the midsouthern United States. Seed was produced in the ESPS for 486 U.S. plant introductions (PI), 25 ancestral lines of U.S. cultivars, and four U.S. cultivars at Stoneville, MS, in 2002 and 2003. Standard and accelerated-aging germination percentages, hardseededness, seed coat wrinkling, and incidence of P. longicolla were estimated for each line. Based on the 2002 and 2003 field data, 42 PIs were selected for further testing, along with three cultivars, in two greenhouse temperature regimes (36[degrees] and 42[degrees]C) at Stoneville, MS, in 2005. Standard field germinations for the 25 ancestral lines ranged from 26 to 82%. Sixty-three accessions were identified as having a mean standard field germination of [greater than or equal to] 90%,

Temporal and spatial availability of K can influence soybean [Glycine max (L.) Merr.] productivity. This study quantified the impact of initial soil K concentrations, soil-water content, and soybean K uptake on soil K pools (water-extractable solution-phase K [[K.sub.sol]], 1 mol [L.sup.-1] N[H.sub.4]OAc-extractable K, referred to as exchangeable K ([K.sub.exch]), and 5-min sodium tetraphenylboron [NaTPB] extractable K [[K.sub.TPB]]) and compared predictions of plant K availability using the NaTPB vs. N[H.sub.4]OAc tests. Soil and soybean samples were collected five times between the VE and R6 development stages in 2003 and 2004 from fallow and cropped notill Toronto--Millbrook silt loam Alfisols at the Throckmorton Purdue Agricultural Center. Gravimetric soil water content was measured weekly during the growing season. Low-, medium-, and high-K fertility plots were replicated four times. Initial [K.sub.exch] levels ranged from 60 to 290 and from 50 to 90 mg [kg.sup.-1] at the 0- to 10- and 10- to 20-cm depths, respectively. Medium- and high-fertility soils had the highest grain yields. The 0- to 5-cm soil layer had the highest [K.sub.exch] levels, water availability, and soybean K uptake. Compared with [K.sub.exch], [K.sub.sol] levels were less stratified and the surface layer was less dynamic. The NaTPB extraction was a better predictor of soybean K uptake in 1 yr, but across both years, N[H.sub.4]OAc was superior. Potassium measurements in the 0- to 10-cm soil layer provided slightly better estimates of plant K uptake than those in the 0- to 20-cm layer. Greater water availability and K uptake in the 0- to 5-cm soil layer suggest that under evenly distributed, intermittent rainfall conditions in somewhat poorly drained soils, vertical soil K stratification might not be a concern for no-till soybean production. Abbreviations: ET, evapotranspiration; [K.sub.exch], exchangeable potassium; [K.sub.fix], fixed potassium; [K.sub.sol], solution-phase potassium; [K.sub.TPB], 5-min sodium tetraphenylboron extractable potassium; NaTPB, sodium tetraphenylboron.

Agricultural practices affect the production and emission of C[O.sub.2] and [N.sub.2]O from soil. The purpose of this 2-yr field study was to determine the effects of tillage (conventionally tilled [CT] and no-till [NT]) and fertilizer source (composted cattle manure and inorganic N-P-K fertilizer) on the C[O.sub.2] and [N.sub.2]O content in soil profiles under corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. The mean C[O.sub.2] and [N.sub.2]O gas contents (i.e., mass of gas per unit soil volume) in the soil profile were determined periodically during two field seasons by sampling the soil atmosphere using plastic tubes installed at three depths (10, 20, and 30 cm) within the crop row. The soil C[O.sub.2] content was greater in CT than NT soil and in manure-amended than inorganically fertilized plots during 1 yr of the study. The soil [N.sub.2]O content was not affected by tillage practices or fertilizer sources. A significant autocorrelation between sampling dates in both years suggested that the C[O.sub.2] and [N.sub.2]0 contents in the soil profile were not erratic or random, but temporally dependent on site-specific factors. The peak C[O.sub.2] and [N.sub.2]0 levels were measured within 50 d after seeding, probably because soil moisture conditions slowed diffusive gas flux but were favorable for microbial activity. Fluctuations in soil C[O.sub.2] and [N.sub.2]O contents were not related to the seasonal variation in soil temperature. At most sampling dates, there was a significant (P < 0.05) positive correlation between the C[O.sub.2] and [N.sub.2]O content in the soil profile, suggesting similarity in the rate of gas accumulation and diffusive flux for C[O.sub.2] and [N.sub.2]O in soils. The C[O.sub.2] and [N.sub.2]O content in the soil profile appeared to be controlled more by soil moisture than soil temperature or agricultural practices. Abbreviations: CT, conventionally tilled; DAS, days after seeding; NT, no-till; WFPS, water-filled pore space.

Growing legumes in the southern Great Plains (SGP) during fallow periods between wheat (Triticum aestivum L.) crops can protect soil, add N to soil, and supply high-quality summer forage. This study determined the water used, water use efficiency (WUE), and soil water depletion of forage (n = 3) and grain (n = 1) soybean [Glycine max (L.) Merr.] cultivars on silt loam soils following winter wheat harvest during the 2003 to 2005 summer growing seasons. Interactions (P < 0.01) in soil water content occurred between sampling date, cultivars, and years. More soil water was recorded under soybean in 2003 than 2004 or 2005, but was lower than in fallow plots. Year effects (P < 0.05) were recorded for standing crop, water used, and WUE. While cultivar effects were not significant P > 0.10). All cultivars used 2.0 to 2.5 times more water in 2004 and 2005 than 2003 (driest year). Forage production in 2003 (3034 kg [ha.sup.-1]) was 55% lower than in 2004 and 2005. Higher WUE (kg [ha.sup.-1] [mm.sup.-1]) was recorded in 2004 (16.0) than 2003 (13.2) or 2005 (10.9). Incorporating soybean into fallow periods following wheat generally limits the amount of soil water available for fall production of wheat forage in the SGP.

Field experiments were conducted over two years at two locations in Illinois to investigate the response of soybean [Glycine max (L.) Merr.] to simulated hail injury applied at the cotyledon (VC) stage. Removal of one or both cotyledons from all plants or of both cotyledons from alternating plants did not reduce mature plant height or yield in any environment. Cutting the stem off just below the unifoliolate node, alone or in conjunction with the removal of a single cotyledon, reduced yield by 11 to 21% in three of the four environments. It appears that soybean cotyledons are of little importance once the unifoliolate leaves have expanded.

A primary focus for soybean [Glycine max (L.) Merr.] breeders recently has been the development of cultivars with improved oil qualities such as reduced palmitic acid (16:0) and linolenic acid (18:3). A backcross breeding program was used to develop five low 16:0, two low 16:0 + 18:3, and one low 18:3 modified fatty acid breeding lines (MFALs). Research objectives were (i) to determine planting date effects on fatty acid content in the eight MFALs and (ii) to compare the MFALs to parental cultivars for seed composition and agronomic traits. The eight MFALs and four control cultivars were evaluated at two planting dates at Clemson, SC, in 2001, 2003, and 2004. Planting dates were chosen to simulate full season and double crop planting dates for South Carolina soybean production. Agronomic traits including seed yield, plant height, lodging, maturity date, seed size, and seed quality were measured, and seeds were analyzed for protein, oil, and fatty acid levels. Planting date had a significant effect on all agronomic variables, as well as on protein, oil, and palmitic and linolenic acid. There was a decrease in palmitic acid at the late planting date, while the early planting date resulted in a decrease in linolenic acid levels. The effect of genotype was significant for all agronomic and seed composition variables measured when averaged across planting dates. It appears that planting date may be manipulated to reduce palmitic or linolenic acid of MFALs, although the extent of the reduction varies with genotype.

Soybean [Glycine max (L.) Merr.] reproduction is sensitive to temperatures > 35[degrees]C. Two studies were conducted to determine temperature effects on soybean pollen germination (PG) and to detect genotypic differences. Pollen collected from 44 genotypes (Maturity Groups III to VI) grown outdoors was subjected to in vitro temperatures from 15 to 50[degrees]C at 5[degrees]C intervals. Genotypes differed significantly for in vitro PG percentage (mean of 81%) and tube length (mean of 437 [micro]m). Mean cardinal temperatures ([T.sub.min], [T.sub.opt], and [T.sub.max]) were 13.2, 30.2, and 47.2[degrees]C for PG and 12.1, 36.1, and 47.0[degrees]C for pollen tube growth. Genotypes differed for leaf cell membrane thermostability (CMTS), but CMTS did not correlate with pollen parameters. Cumulative temperature response index, CTRI (unitless), of each genotype calculated as the sum of eight individual stress responses (ISRs) derived from maximum PG, maximum pollen tube length (PTL), and the maximum ([T.sub.max]), minimum ([T.sub.min]), and optimum ([T.sub.opt]) temperatures for PG and for PTLs was used to group genotypes for temperature tolerance. Heat-tolerant genotype (DG 5630RR) was less sensitive to high temperature (38/30[degrees]C) compared with heat-intermediate (PI 471938) and heat-sensitive (Stalwart III) genotypes that had deformed pollen, with reduced apertures and collumellae heads. Hence, pollen can be used as a screening tool for heat tolerance. Most sensitive to temperature was D88-5320 with a CTRI of 6.8, while AG 4403RR was most tolerant with a CTRI of 7.5. Elevated [C[O.sub.2]] did not modify reproductive parameters or CTRI. The study also revealed that heat tolerance of vegetative tissue had little or no relationship with the heat tolerance of reproductive tissue. Maturity groups lacked a specific trend for tolerance to high temperature. The identified high temperature-tulerant genotypes and temperature-dependent pollen response functions might be useful in soybean breeding and modeling programs, respectively.

Three physiological traits that may affect performance of soybean [Glycine max (L.) Merr.] when soil water availability is limiting are (i) water use efficiency (WUE), (ii) regulation of whole plant water use in response to soil water content, and (iii) leaf epidermal conductance ([g.sub.e]) when stomata are closed. Six soybean plant introductions (PIs), eight breeding lines derived from them, and nine cultivars were compared for variability in these three traits during vegetative growth in two greenhouse studies. In the first experiment, whole plant water use, normalized both to plant size and evaporative demand (the normalized transpiration ratio, NTR), was monitored during a 10-d cycle of gradually increasing drought stress and then for an additional 2 d following rewatering. The critical soil water content at which each plant began to reduce its water use (FFS[W.sub.c]), was determined. The WUE was estimated as the ratio of total plant dry weight to total water used. In the second experiment, [g.sub.e] was determined for these same 23 genotypes by measuring leaf water vapor exchange after a 36-h dark adaptation. Substantial variation was found among genotypes for WUE, FFS[W.sub.c], [g.sub.e], and also the extent to which NTR recovered on rewatering. Generally, adapted cultivars had greater WUE and lower [g.sub.e] than did PIs. However, PI 471938 and its progeny N98-7264 were clear exceptions to this trend. An unexpected finding was that WUE was significantly negatively correlated with [g.sub.e] across genotypes.

There is no information on accumulation of dehydrin proteins during seed development and maturation of soybean [Glycine mar (L.) Merr.] in response to drought stress. Our objective was to study accumulation of dehydrin-like proteins in developing soybean seeds in response to drought stress. A greenhouse experiment and a field experiment were conducted. In the greenhouse experiment, three treatments were imposed on soybean plants after beginning of linear seed filling ([R.sub.5]): well-watered (WW), gradual stress (GS) imposed before severe stress, and sudden severe stress (SS). In the field treatments were irrigation (I) and nonirrigation (NI) (rainfed) conditions imposed from [R.sub.5] to [R.sub.8] (mature seeds). Greenhouse results indicated dehydrin-like proteins (28 and 32 kDa) were detected 18 d after [R.sub.5] ([R.sub.5,8]) in developing seeds from drought-stressed plants but not in seeds from the well-watered plants. In the mature seeds, dehydrin-like proteins (28, 32, and 34 kDa) were detected in seeds from drought-stressed plants as well as the well-watered plants. In the field, dehydrin-like proteins accumulated similarly under irrigation and nonirrigation conditions, with the first detection for dehydrins (28 and 32 kDa) at 22 d after [R.sub.5] ([R.sub.6]). Accumulation of dehydrin-like proteins was maximalin seeds harvested at 43 d after [R.sub.5] (seed physiological maturity).

The Dehydration Responsive Element-Binding protein/C-repeat Binding Factor (DREB/CBF) plays important roles in regulating physiological processes and downstream gene expression for drought stress in Arabidopsis thaliana, rice, and wheat. 'Essex' soybean [Glycine max (L.) Merr.] was identified as tolerant and a wild soybean PI 407155 (Glycine soja Sieb. & Zucc.) as more tolerant to dehydration stress in a greenhouse screen. In this study, we cloned and characterized the Glycine DREB1 (GlyDREB1) from the dehydration-induced PI 407155 and Essex using a semi-quantitative RT-PCR, rapid amplification of eDNA ends (RACE), and northern blot. We also investigated the physiological characteristics including biomass accumulation, moisture content, and electrolyte leakage for both soybean genotypes under dehydration stress. Analysis of homologous GlyDREB1 genes from PI 407155 and Essex indicated differences in both the timing and strength of expression under dehydration stress. The GlyDREB1 in PI 407155 was rapidly induced in 1 h of dehydration shock and the transcription level was much higher than that in Essex. The root system of PI 407155 maintained higher moisture content and biomass accumulation than that of Essex on the 15 d without irrigation. The percentage electrolyte leakage in Essex was almost twice that in PI 407155 under 5 d no irrigation treatment. Both molecular and physiological studies indicated that PI 407155 had higher level of tolerance to dehydration stress than Essex, therefore providing a new potential soybean genome for developing dehydration stress cultivars.

Argentina is the largest soybean [Glycine max (L.) Merrill] meal and oil exporter in the world, with crops covering a 23[degrees] to 39[degrees] S latitude range, allowing the presence of genotypes from different maturity groups (MG). Multi-environment yield trials (MET) for commercial cuitivars are conducted each year across the crop area. The aim of this paper was to evaluate the consistency of MG effects and its interaction with environments (E), first to investigate if different mega-environments (ME) for oil, protein, and oil + protein exist in Argentina, and second to identify superior MG regarding these traits. We analyzed a 3-yr series of oil and protein data from MET involving six MG and more than 14 E per year. Statistical analysis was based on ANOVA and graphical displays from E-centered biplots to explore MG-related effects and to identify ME. No ME were identified for oil content because of MG II, III, and IV showed higher content than other groups in every E. Two or three ME (depending on the growing season) were identified for protein and oil + protein contents; in one of them MG VI cultivars had the highest value of these compounds whereas in the other set of E, higher yielding cultivars were from MG II-III. The oil variations among E depended mainly on MG effects suggesting broad adaptations of short MG, whereas MG x E interaction effects for protein and oil + protein were higher than for oil, and enough to create opportunities for handling environment-specific adaptations.

The effects of alfalfa, red clover, and perennial ryegrass as cover crops on soybean cyst nematode (SCN) and soybean and corn yields were evaluated in Waseca, Lamberton, and Rosemount, MN. The cover crops were interseeded in soybean at 0 or 2 wks after planting soybean in 2002 and killed with herbicide before planting corn in 2003. As expected, SCN-susceptible soybean supported higher SCN population density than SCN-resistant soybean. Reduction of SCN population density by red clover (up to 40%) and alfalfa (up to 55%) was observed in some sampling occasions at Lamberton and Rosemount, probably due to reduced soybean growth, but the effect was inconsistent. No significant reduction of SCN population by the two crops was detected at Waseca. While perennial ryegrass did not affect SCN population density in most cases, up to 46% higher egg population densities were observed in the perennial ryegrass treatment as compared to the control at Waseca. SCN-resistant soybean produced higher yield than susceptible soybean at all sites. While alfalfa reduced soybean yield at Lamberton (up to 50%) and Rosemount (up to 11%), red clover and perennial ryegrass reduced soybean yield only at Lamberton (up to 38%) and Waseca (up to 34%), respectively. No difference in corn yield was observed at Waseca. At Lamberton, alfalfa and red dover planted at the time of planting soybean reduced corn yield in the following year 17 and 13%, respectively, and perennial ryegrass planted 2 wks after planting soybean reduced corn yield 13%. At Rosemount, significant reduction of corn yield was observed with red clover (15-21%) interseeded in SCN-snsceptible soybean and with alfalfa (12%) and red clover (12%) interseeded in SCN-resistant soybean at the time of planting soybean. The results suggest that an even later planting date of cover crops in soybean may reduce yield loss due to competition and make these cover crops more appropriate for use in the soybean-corn rotation in Minnesota.

Burning of wheat (Triticum aestivum L.) residue followed by plowing is a common management practice in wheat--soybean [Glycine Max (L.) Merrill] production systems in the mid-southern USA. However, this residue management practice is not environmentally friendly and may not be sustainable. The objectives of this study were to (i) evaluate the effects of N fertilization of wheat, residue burning, and tillage on soil surface carbon dioxide (C[O.sub.2]) flux in a wheat--soybean double-crop production system, and (ii) evaluate the role of soil temperature and soil moisture in controlling C[O.sub.2] flux in a relatively warm, subhumid environment. Soil surface C[O.sub.2] flux was measured nine times between June 2002 and October 2003 during the soybean growing season under all combinations of conventional--(CT) and no-tillage (NT) at high and low N fertilization levels with and without residue burning at two locations in the Mississippi River delta region of eastern Arkansas. Soil surface C[O.sub.2] flux was 37.6% higher (P < 0.01) from CT than from NT and 6.1% higher (P < 0.05) from the low than the high N rate treatment at one location, but not at the other. Burning did not affect soil surface C[O.sub.2] flux except for a significant burning x N rate (P = 0.016) and burning X time interaction (P = 0.032) at one location, but not at the other. Both soil temperature and moisture parameters were significantly negatively correlated with temperature-normalized soil surface C[O.sub.2] flux, but soil temperature, particularly at the 10-cm depth, explained more of the variation than did soil moisture parameters. The results of this study indicate that tillage and N fertilization of prior wheat, but not residue burning, affect the loss of C as C[O.sub.2] from the soil and that there are additional soil and/or environmental factors, other than near-surface soil moisture and temperature fluctuations, perhaps microbial biomass, that contribute to controlling soil surface C[O.sub.2] flux in wheat-soybean production systems in the subhumid region of southern USA.

Understanding soil organic N (ON) pool enrichment may help explain why rates of N fertilization required to attain maximum corn (Zea mays L.) yields are usually lower for corn following soybean [Glycine max (L.) Merr.] than for corn following corn. Our objectives were to quantify the ON pools within a 16-ha Iowa field and to correlate those results with corn yield. Spring and fall measurements of ON content (0-15 cm soil) as amino acids (AAs), amino sugars (ASs), and N[H.sub.4.sup.+] were made using samples collected between 1997 and 1999 from 10 soil map units. The chemical extraction method determined an average 87% of the total N content (n = 10 soils) as identified ON but gave reduced ON recovery from depression soils that experienced periods of water ponding. The total AA concentrations measured in May were positively correlated ([r.sup.2] = 0.84, P < 0.01) with corn yield during a dry year (1997) and 7 out of 10 soils provided near maximum yields. A wetter 1999 boosted overall corn yields 6.6% but resulted in a poorer relationship between May AA concentrations and corn yield. Microbial N compounds measured (May 1997) as glucosamine, galactosamine, and ornithine were also positively correlated with corn yield ([r.sup.2] = 0.84, p < 0.01; [r.sup.2] = 0.94, P < 0.001; [r.sup.2] = 0.93, P < 0.001, respectively). The ON concentration decreased during corn production from May to September 1997 an average of 367 kg N [ha.sup.-1] but increased following soybean growth in 1998 by 320 kg N [ha.sup.-1]. The chemical extraction methodology identified soils that may not require the full amount of N fertilizer currently being applied, thus decreasing the potential for N loss to surface and ground water resources without decreasing opportunities to achieve optimum yield.

Nitrogen supplied by [N.sub.2] fixation to soybean [Glycine may (L.) Merr.] may not be sufficient to maximize yield. Field studies were conducted in 2002, 2003, and 2004 on Sharkey clay soil (very-fine, smectitic, thermic Chromic Epiaquert) at Stoneville, MS (33°26' N lat). The objective was to determine the effect of high rates of N applied as a replacement for [N.sub.2] fixation in nonirrigated and irrigated environments. Eight cultivars ranging from Maturity Group II to IV were planted on 17 Apr. 2002, 2 Apr. 2003, and 25 Mar. 2004. Not all cultivars were evaluated in all 3 yr. Glyphosate herbicide was used in all 3 yr and a non-glyphosate herbicide treatment was applied in 2002. Cultivars grown in 2003 were also evaluated under an application of 21.3 kg [ha.sup.-1] of Mn. All cultivar, herbicide, and Mn treatments were evaluated in irrigated and nonirrigated environments with fertilizer N (PlusN treatment) or without fertilizer N (ZcroN treatment). In the PlusN treatment, granular N[H.sub.4]N[O.sub.3] was surface applied at soybean emergence at rates of 290 kg [ha.sup.-1] in 2002, 310 kg [ha.sup.-1] in 2003, and 360 kg [ha.sup.-1] in 2004. When analyzed over all management practices (years, cultivars, herbicide, and Mn treatments), the PlusN treatment resulted in significantly decreased ureide concentration (57.2 and 53.5% reduction) and significantly increased biomass accumulation (14.1 and 16.7%), N accumulation (12.8 and 28.1%), and seed yield (7.7 and 15.5%) for the irrigated and nonirrigated environments, respectively. The majority of the yield increase in each environment resulted from increased number of seed (9.5% irrigated and 16.2% nonirrigated). These results confirm the sensitivity of [N.sub.2] fixation to drought and indicate that Ns fixation may limit yield of soybean grown in both irrigated and nonirrigated environments of the midsouthern USA, and that [N.sub.2] fixation deficiencies occur before the beginning of processes that determine number of seed., WATER DEFICIT STRESS (drought) is difficult to define and quantify because the magnitude of its effect depends on numerous crop (species, cultivar, phenology, etc.) and environmental factors (intensity, duration, evaporative [...]

There is considerable uncertainty relative to the emissions of [N.sub.2]O from legume crops. A study was initiated to quantify [N.sub.2]O fluxes from soils cropped to alfalfa (Medicago sativa L.) and soybean (Glycine max L.), and to improve our understanding of soil and climatic factors controlling [N.sub.2]O emissions from these crops. Measurements were made on three soils cropped to alfalfa, soybean, or timothy (Phleumpratense L.), a perennial grass used as a control. In situ soil-surface [N.sub.2]O emissions ([F.sub.N2O]) were measured 47 times during the 2001 and 2002 growing seasons. Soil water, N[H.sub.4]-N, N[O.sub.3]-N, and [N.sub.2]O contents, and soil temperature were also determined to explain the variation in gas fluxes. Emissions of [N.sub.2]O were small under the grass where very low soil mineral N content probably limited denitrification and [N.sub.2]O production. Soil mineral N contents under legumes were lip to 10 times greater than under timothy. However, soil mineral N contents and [F.sub.N2O] were not closely related, thus suggesting that the soil mineral N pool alone was a poor indicator of the intensity of [N.sub.2]O production processes. Higher [F.sub.N2O] were measured under legume than under timothy in only 6 out of 10 field comparisons (site-years). Moreover, the emissions associated with alfalfa (0.67-1.45 kg N [ha.sup.-1]) and soybean (0.46-3.08 kg N [ha.sup.-1]) production were smaller than those predicted using the emission coefficient proposed for the national inventory of greenhouse gases (alfalfa = 1.60-5.21 kg N [ha.sup.-1] ; soybean = 2.76-4.97 kg N [ha.sup.-1]). We conclude that the use of the current emission coefficient may overestimate the [N.sub.2]O emissions associated with soybean and alfalfa production in eastern Canada.

Management practices, such as no-tillage (NT) and high-intensity cropping sequences, have the potential to enhance C and N sequestration in agricultural soils. The objectives of this study were to investigate the impacts of conventional-tillage (CT), NT, and multiple cropping sequences on soil organic carbon (SOC) and nitrogen (SON) sequestration and on distribution within aggregate-size fractions in a southcentral Texas soil after 20 yr of treatment imposition. No-tillage management increased soil aggregation compared with CT, with the bulk of SOC and SON storage present in larger aggregate-size fractions (>2 mm, 250 [micro] to 2 mm) at both soil depths. Multiple cropping systems, such as a grain sorghum [Sorghum bicolor (L.) Moench]/ wheat (Triticum acstivum L.)/soybean [Glycine max (L.) Merr] (SWS) rotation and a wheat/soybean (WS) doublecrop had the highest SOC and SON storage, while the continuous monoculture soybean treatment had the lowest storage. Soil organic C and SON storage were significantly greater under NT than CT for all cropping sequences at 0 to 5 cm and for SWS and WS at 5 to 15 an. At the 0- to 5-cm depth, NT increased SOC storage by 64% and SON storage by 76% compared with CT. However, at 5 to 15 cm, NT only increased SOC storage by 28% and SON storage by 40%. The use of NT showed a greater impact for increasing SON storage than for SOC storage, suggesting that N cycling is an important factor related to soil C sequestration potential.

Given the potential for high amounts of variability in yield-affecting soil factors, some soil property or properties may serve as a basis for site-specific soil management. The objectives of this study were to determine the variability of selected soil properties and to determine the relationships between these soil properties and soybean [Glycine max (L.) Merr.] yield. Soil samples were collected from the center point of 0.5-ha grids in three fields and were analyzed for soil-test-extractable Ca, Mg, K, P, pH in water, and texture. Relative elevation, slope, aspect, and soybean yield were also determined at these points. Coefficient of variation, Pearson correlation coefficients, and principal component (PC) analysis coupled with stepwise regression were used to analyze the data. Two of the three fields had medium to high P and K values but low yields while the third field had low P and K values but relatively higher yields, suggesting factors other than P and K levels were affecting yield. Soil variability, with the exception of pH, was highest in the North field. Potassium in this field exhibited evidence of high amounts of small-scale spatial or temporal variability. Across ali three fields, pH had the lowest amount of variability while variability in soil fertility varied from year to year and field to field. Fertility parameters had to be considered with other soil factors to determine their relationship to yield. Topography-yield relationships varied from field to field. Areas with higher clay content in all three fields had higher yield, suggesting clay could be used as a basis for site-specific soil management.

Environmental stress during reproductive growth of soybean [Glycine max (L.) Merr.] may reduce seed germination and vigor. Most studies of environmental influence on soybean seed germination and vigor have concentrated on the effects of drought, maximum day temperature, or average daily temperature during seed fill. This study was conducted to determine the effects of different combinations of day temperatures and night temperatures during reproductive growth on soybean seed quality. Day/night temperatures of 30/20, 30/30, 35/20, and 35/30 [degrees] C were imposed at flowering and pod set (R1-R5), seed fill and maturation (R5-R8), and during the entire reproductive growth period (R1-R8). High day and night temperature reduced soybean seed germination and seedling vigor. The extent of reductions in germination and vigor by high temperature was influenced by duration of temperature exposure and phase of reproductive growth. Temperatures during flowering and pod set exerted little influence on soybean seed quality. A combination of 35 [degrees] C day temperature and 30 [degrees] C night temperature during seed fill and maturation or 35 [degrees] C day temperature from flowering to maturity decreased germination. Seedling vigor was reduced by 35/20, 30/30, and 35/30 [degrees] C during seed fill and maturation and the entire reproductive period. Increase in night temperature from 20 to 30 [degrees] C decreased seedling vigor at both 30 and 35 [degrees] C day temperatures. These results suggest that locational and seasonal differences in soybean seed quality may be induced by high temperatures during reproductive growth and indicate that night temperatures along with day temperatures are important in determining soybean seed quality.

Unbiased prediction of plant growth stages is essential for accurate simulation of stage-specific responses to environmental factors. The phenology model in SOYGRO V5.42 was compared with the phenology model in CROPGRO V3.0 for prediction of flowering and maturity date. Data came from 17 sources in North America and covered a wide range of maturity groups. An additional large-scale data set from the U.S. Soybean Uniform Tests was used to evaluate predictions of maturity date. Parameters of the phenology models were estimated with an optimization procedure in which the downhill simplex method determined the direction of the search. While the optimization procedure was valuable to estimate the parameters, additional criteria were required to obtain realistic values. Based on the root mean square error (RMSE) criterion between predicted and observed dates, SOYGRO and CROPGRO predicted flowering equally well. Development rate after flowering was underpredicted by SOYGRO in cool environments so that in some years, maturity was predicted very late. CROPGRO has a separate temperature function after beginning seedfill, which decreased the RMSE for prediction of maturity date compared with SOYGRO, especially for early maturity cultivars. Allowing the critical short day length to increase after flowering date in the CROPGRO model consistently decreased the RMSE for prediction of beginning seed fill and maturity. CROPGRO was superior to SOYGRO for prediction of maturity date.

Drought stress in the midsouthern USA reduces yield of soybean [Glycine max (L.) Merr.] planted in May and June and sometimes affects germination of harvested seed. Irrigation often increases yield, and may also increase seed germinability. Field experiments were conducted on Sharkey clay (Vertic Haplaquept, very-fine, montmorillonitic, termic) in 1992, 1993, and 1994 at Stoneville, MS. The objective was to determine the effects of earlier-than-normal (April) and normal (May) planting dates on yield and germinability of harvested seed of Maturity Group (MG) IV and V soybean cultivars grown in nonirrigated (NI) and furrow-irrigated (I) environs. In 1992, germination of seed of all cultivars harvested from the NI (14-73%) and I (36-77%) 15 April plantings was lower than that of seed harvested from the 27 May plantings. In 1993 and 1994, germination of all harvested seed averaged more than 90%. Without irrigation, seed yield from the April plantings exceeded that from the May plantings every year. The difference was greatest in 1992 (3170 vs. 2190 kg [ha.sup.-1]) and least in 1993 (1432 vs 1295 kg [ha.sup.-1]). In 1992, the 4195 kg [ha.sup.-1] average yield with irrigation from the 15 April planting exceeded the 2979 kg [ha.sup.-1] average yield from the 27 May planting. In 1994, yield of all irrigated cultivars in the 21 April planting equalled or exceeded the yield of the same cultivars planted on 13 May. Yields of MG V cultivars were equal to or greater than those of MG IV cultivars in both I and NI plantings in all years. These results demonstrate that April vs. normal May plantings of both MG IV and V cultivars can increase yields when grown with and without irrigation in the midsouthern USA.

Species composition and population densities of weed communities of arable land reflect agronomic practices. The trend toward reducing tillage in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production changes the environment where weeds are managed, survive, and reproduce. The shift from tillage systems that include extensive annual soil disturbance to systems that minimize soil disturbance will cause major changes in weed population dynamics. These changes often reduce the effectiveness of weed control practices. Reduced herbicide efficacy has slowed adoption of conservation tillage because many conservation tillage systems rely heavily on herbicides for weed management. Poor understanding of weed population dynamics and lack of suitable control alternatives often result in increased herbicide use in conservation tillage systems. While results have varied among experiments, some general trends in weed population dynamics have arisen as tillage is reduced. These include increased populations of perennial, summer annual grass, biennial, and winter annual species. Densities of large-seeded dicot species often decrease. The ecological and management aspects of these changes are varied and complex. Effective, economical, and environmentally sound weed management in conservation tillage systems will require integration of new information with established principles of weed management. New management systems and control technologies are needed to develop integrated weed management systems for the altered ecosystems created by conservation tillage production systems.

Drought stress consistently reduces soybean (Glycine max (L.) Merr.) yield, while irrigation increases yields above nonirrigated levels in the southern USA in most years. Little information is available regarding drought stress and irrigation effects on quality of harvested soybean seed in this region, however. Field experiments using Maturity Group (MG) IV, V, and VI soybean cultivars were conducted in 1990 and 1991 at Stoneville, MS, to evaluate effects of drought and irrigation during reproductive development on subsequent germination of harvested seed. Treatments included a nonirrigated (NI) control and furrow irrigation (I) applied at various times during the reproductive period (R1-R6) in four different experiments. All experiments were grown on Sharkey clay (very-fine, montmorillonitic, nonacid, thermic Vertic Haplaquept), a major soil in the alluvial plain of the lower Mississippi River Valley. Irrigation significantly increased seed yield and number of seed from all cultivars in all experiments. Effect of irrigation on weight per seed was none or small relative to effects on seed yield and number of seed. Drought stress resulted in significantly inferior quality (standard germination

A study was conducted to compare nutrient losses in surface runoff over a range of watershed conditions and tillage methods. Soil and nutrient loss occur when soybean is cultivated on slopes. Six small watersheds, where corn and soybean are grown in rotation, were studied for nutrient concentrations and transport in surface runoff for six years in east-central Ohio. During the growing season of corn, a greater NO3-N concentration in the runoff was observed.

The conservation tillage methods and herbicides used to control weed reduced soil losses on sloping soybean fields. Runoff and erosion on six small watersheds where corn and soybean were farmed in rotation for six years were determined to assess soil loss in these new settings. The production is confined to flat lands, when traditional tillage and mechanical cultivation are used to control weed.

Two infectious diseases that cause most yield losses in soybean [Glycine max (L.) Merr.] are soybean cyst nematode (SCN), caused by Heterodera glycines Ichinohe, and sudden death syndrome (SDS), caused by Fusarium solani (Mart.) Sacc. f. sp. glycines (Fsg). Because SCN and SDS have a synergistic effect on yield when they occur jointly in the field, breeders are attempting to develop varieties with dual resistance to these two diseases. Using independent culling as a selection strategy, we screened a set of 31 new soybean plant introductions (PI) that were field evaluated in 1995 at two locations in Southern Illinois. We identified 11 elite PIs that were resistant to SCN race 3, had yellow seed coat, a relatively good field response to SDS, and a moderate seed yield. These superior genotypes can be used as potential parents in soybean breeding programs.

A study evidence for the involvement of a new genetic locus that controls soybean cultivar specificity is demonstrated based on the finding from the symbiosis between Sinorhizobium fredii USDA257 and soybean [Glycine max (L.) Merr. Compared to the formerly characterized soybean cultivar specificity locus nolBTUVW, inactivation of the gcv locus had no discernible effect on the secretion of nodulation outer proteins of USDA257.