Sharkey Clay Research Articles

Sorption of Bentazon and Degradation Products in Two Mississippi Soils

Bentazon degradation in soil typically proceeds with development of bound residue. Low sorption of bentazon suggests that this residue consists of degradation products; however, there is little data on the sorption behavior of these products. This study was undertaken to determine the sorption of bentazon and the degradation products 2-amino-N-isopropyl benzamide, 2-aminobenzoic acid, andN-methyl bentazon in Dundee silt loam and Sharkey clay, two common agricultural soils of the Mississippi Delta. Greater sorption of bentazon and degradation products in the Sharkey soil was related to finer texture and higher organic C content. Isotherms were nonlinear, with sorption increasing in the order bentazon ≪ 2-amino-N-isopropyl benzamide <N-methyl bentazon. In general, methanol extraction indicated reversible sorption of these compounds. Therefore, it is unlikely that sorption of either 2-amino-N-isopropyl benzamide orN-methyl bentazon contributes to bound residue. In contrast, essentially all of the14C sorbed from radiolabeled 2-aminobenzoic acid solution remained bound after methanol extraction. However, due to degradation of 2-aminobenzoic acid, it was not possible to conclude the extent to which this compound contributed to bound14C. Sorption of14C from 2-aminobenzoic acid exceeded that ofN-methyl bentazon, and at low initial concentrations (≤ 20 μM) was nearly 1000-fold greater than bentazon sorption.

Yield and Germinability of Seed from Irrigated and Nonirrigated Early‐ and Late‐Planted MG IV and V Soybean

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−1) and least in 1993 (1432 vs 1295 kg ha−1). In 1992, the 4195 kg ha−1 average yield with irrigation from the 15 April planting exceeded the 2979 kg ha−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.

Leaching of Nitrate, Atrazine, and Metribuzin from Sugarcane in Southern Louisiana

AbstractWe have studied the leaching losses of NO3, atrazine [6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine], and metribuzin [4‐amino‐6‐(1,1‐dimethylethyl)‐3‐(methylthio)‐1,2,4‐triazine‐5(4H)‐one] applied to sugarcane (Saccharum officinarum L.) planted in Mississippi River alluvial soil in southern Louisiana. Nitrogen (122 kg/ha) and atrazine (2.24 kg/ha) were applied in June, and atrazine (2.24 kg/ha) and metribuzin (1.12 kg/ha) were applied in December; losses through a Sharkey clay (very fine, montmorillonitic, nonacid, thermic Vertic Haplaquepts) into subsurface drains (5.5‐ and 10.9‐m spacing) were measured for about 100 d in both seasons. Five days after application NO3‐N appeared in its highest concentrations (5–11 mg/L) in the drain water; after this first event, concentrations remained below 10 mg/L throughout the summer season. After application in the summer atrazine appeared in the subsurface drains at its highest seasonal concentrations (114–144 µg/L) on the day of application; after 4 to 7 wk these concentrations remained below 3.0 µg/L. Total losses in the summer amounted to 3 to 8% of the NO3 application and 0.6 to 1.2% of the atrazine application. Almost 50% of the NO3 leaching into the subsurface drains occurred after Day 76, whereas 82% of the atrazine leached into the drains by Day 8. After the winter application, high concentrations of atrazine (67–81 µg/L) and metribuzin (52–94 µg/L) were measured within 8 d. Similarly, large concentrations of atrazine occurred in the drain water throughout the winter season. The much higher concentrations of atrazine during the winter study, compared with the summer, coincided with soil surface concentrations that were 3 to 10 times those of the summer. Total losses during the winter were 0.4 to 2.0% (atrazine) and 0.4 to 1.7% (metribuzin) of the applications. Evidence for preferential flow into the drains of the NO3 and the herbicides is presented.

Comparison of Single and Multiple Applications of Propanil and Residual Herbicides in Dry-Seeded Rice (Oryza sativa)

Experiments were conducted on Sharkey clay and Gigger silt loam soils in northeast Louisiana from 1990 through 1993 to compare season-long barnyardgrass control, rice yield, and estimated economic returns with single and multiple applications of propanil alone or with molinate or thiobencarb. Multiple herbicide applications were more effective than single applications in controlling barnyardgrass and provided the greatest rice yields and economic returns. Applying residual herbicides with propanil improved barnyardgrass control in five of six experiments and increased rice yield and economic returns in three of six experiments.

Effect of Irrigation and Weed Control Treatment on Yield and Net Return from Soybean (Glycine max)

Field studies were conducted from 1987 through 1990 on Sharkey clay to determine effect of weed control treatment (WTRT) on yield and net return from soybean grown with no irrigation (NI) or irrigation (I) during reproductive development. Weed control treatments were: 1) PRE glyphosate plus metribuzin plus metolachlor or pendimethalin, followed by POST bentazon plus acifluorfen or 2,4-DB plus linuron (high WTRT); 2) PRE glyphosate at a rate to kill existing weeds at planting (medium WTRT); or 3) PRE paraquat at a rate to desiccate but not necessarily kill existing weeds at planting (low WTRT). POST cultivation was used in all treatments. Weed ground cover in the high, medium, and low WTRT's was composed of annual grass and moderately competitive broadleaf species, and averaged 7, 25, and 49%, respectively, at harvest in 1990. Yield declined with decreasing weed control in the I treatment after the first year, but average net returns from the high and medium WTRT's were not different, and both exceeded average returns from the low WTRT. Yields from the high and medium WTRT's in the NI treatment were not different, and both exceeded yield from the low WTRT. Average net returns from the medium WTRT were greater than those from the high and low WTRT's in all years. Yield increase from irrigation and difference in average net returns between I and NI declined with decreasing weed control.

Timing of Furrow Irrigation Termination for Determinate Soybean on Clay Soil

AbstractIrrigation commencing at or near beginning bloom (R1) and continuing during reproductive development of determinate soybean [Glycine max (L.) Merr.] grown in the southern USA in dry years increases seed yield. The number of irrigations between the first and last irrigation event can influence the direct costs of irrigation; however, the optimum time to stop irrigation in relation to crop maturity has not been identified. ‘Leflore’ (1987–1990) and ‘A5980’ (1988–1990) determinate cultivars were grown on Sharkey clay (very‐fine, montmorillonitic, nonacid, thermic Vertic Haplaquept) at Stoneville, MS, to determine the effect of timing of last furrow irrigation on seed yield, yield components, yield increase, and net returns. All irrigation treatments within a cultivar were started at R1 (beginning bloom) or later. One treatment was dropped from the sequence with each successive furrow irrigation event through R6 (full seed). Irrigation applied through mid‐R5 (seed‐filling period) during a dry year produced the greatest seed yield increase of 1855 kg ha−1, which was 610 kg ha−1 greater than the yield of the treatment terminated at R4 (full pod), and equal to the yield of the treatment terminated at R6. Yield increases due to irrigation during wet years were smaller or nonexistent. In all cases, increased number of seeds was the sole contributor to increased yield. In the dry years, irrigation terminated at either mid‐ to late‐R2 or mid‐R5 produced the greatest yield increase per unit of irrigation water, but irrigation continued into the seedfill period and then terminated produced the highest net return. These results indicate that furrow irrigation of soybean grown on a cracking clay soil in dry years should be continued through mid‐R5 to achieve maximum seed yield and net returns.

Rice (Oryza sativa) Weed Control with Soil Applications of Quinclorac

Field studies were conducted from 1988 to 1990 on a Sharkey clay to evaluate residual weed control in rice with quinclorac applied PPI, PRE to dry soil, and PRE to moist soil. Quinclorac applied at 0.4 or 0.6 kg ai ha−1PPI or PRE to dry or moist soil controlled more than 80% of barnyardgrass, pitted morningglory, and hemp sesbania without rice injury. Quinclorac applied at 0.3 kg ha−1controlled these three weed species substantially but inconsistently. No rice injury was observed from any quinclorac treatment. Except for one of three years when irrigation was delayed for 7 d after PRE application to dry soil, application timing did not consistently affect weed control or rice yield.

Drought Stress and Irrigation Effects on Germination of Harvested Soybean Seed

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 < 80%) seed from all cultivars in all experiments. Irrigation initiated during flowering and continued into the seed‐fill period significantly increased standard germination of seed from all experiments. Irrigation during the full reproductive period was required to realize both maximum seed yield and maximum germination of harvested seed. The results show that drought stress can result in reduced germination of harvested soybean seed, and that irrigation can be used to overcome this effect.

Influence of Growth Stage on Rice (Oryza sativa) Tolerance to Triclopyr

Field studies were conducted from 1985 to 1989 on a Sharkey clay to examine injury to a semi-dwarf rice cultivar, ‘Lemont’, from triclopyr or triclopyr plus propanil. Triclopyr applied in the booting stage reduced yield two of three years, with the observed yield reduction possibly caused by epinasty of the rice flag leaf. Triclopyr application to three- to four-leaf rice caused hyponasty. Triclopyr did not reduce plant height, seed weight, germination, or total milling yield. Triclopyr plus propanil caused more leaf burn that triclopyr alone, but yields were not reduced compared with the untreated control. This research indicated that triclopyr and triclopyr plus propanil can be used in rice production with the semi-dwarf cultivar, Lemont, with the potential to minimize drift to non-target crops due to the greater flexibility in application timing compared with 2,4-D application.

Water management and location effects on root and shoot growth of irrigated lowland rice

Abstract In the United States rice (Oryza sativa L.) is entirely flood irrigated. Delaying the application of the floodwater beyond the recommended time is practiced by some Arkansas producers and may (with supplemental rainfall) potentially reduce the water inputs required to produce a crop. Water stress, however, may affect root growth characteristics which could impact fertility management decisions relative to continuous flood culture. Therefore a field study was initiated to identify the responses of root and shoot growth and yield to water management and location. Two water management treatments were imposed on the ‘Tebonnet’ cultivar, i) normal water management where a flood was applied at the four‐ to five‐leaf stage (continuous flood), and ii) flood delayed (but with supplemental irrigation) until 1.25 cm internode elongation. Studies were located on two soils in eastern Arkansas, a Sharkey clay and Crowley silt loam in 1988 and 1989. The floodwater was removed from plots approximately 10 d befor...

Evaluation of manganese inorganic fertilizer sources and Mehlich‐3 extractant for prediction of manganese deficiency in soybeans on a Sharkey silty clay

Abstract A greenhouse experiment was conducted on two Sharkey silty clay (very fine, montmorillonitic, nonacid, thermic, Vertic Haplaquept) soils (SharkeyA and‐B) to compare MnSO4 and two Mn‐oxysulfatc sources (oxysulfate‐A and‐B) and to evaluate the Mehlich‐3 extractant. Soils were collected from a soybean [Glycine max (L.) Merr.] field with (Sharkey‐A) and without (Sharkey‐B) a history of Mn deficiency symptoms. Treatments consisted of two lime treatments, O and 2000 mg kg‐1, and three Mn rates, 0, 20, and 40 mg kg‐1. Each source was broadcast in granular form. Manganese sulfate was also applied in solution. Soybean plants were grown for 40 days. Dry weight, whole‐plant Mn concentration, and total Mn uptake were measured. Extractable soil Mn was determined using the Mehlich‐3 extractant. Dry weight was increased by applied Mn only on the Sharkey‐A soil, especially for the limed treatment. The Mehlich‐3 extractant delineated between the responsive (2.3 mg Mn kg‐1) and non‐responsive (6.0 mg Mn kg‐1) Shar...

Soil Water Potential: Effects on Soybean Looper Feedin on Soybean Leaves

Defoliation by soybean looper, Pseudoplusia includens (Walker), often is uniformly high over portions of soybean [Glycine max (L.) Merr.l fields but is uniformly low, or progresses more slowly, over other portions of the same fields. Through the use of insect bioassays the effect of soil water potential (SWP) and two soil types were investigated to determine if they are factors associated with observed soybean looper defoliation patterns. Tests were conducted using excised leaves from greenhouse‐grown plants and laboratory‐reared insects. In Test I, a significant (P ≤ 0.01) reduction in 10‐d larval weights and an increase in larval development periods was caused by plants grown at reduced SWP for 15 d before bioassay initiation. No differences in 10‐d larval weights or development periods associated with Dubbs silt loam (fine‐Silty, mixed, thermic Typic Hapludalf) or Sharkey clay (very‐fine, montmorillonitic, nonacid, thermic Vertic Haplaquept) soils occurred in Test I. In Test II, a large, significant reduction occurred in 10‐d larval weights and an increase occurred in development periods that was associated with plants grown at reduced SWP for 27 d before bioassay initiation. A small, significant decrease in 10‐d larval weights and an increase in larval development periods under reduced SWP was associated with Sharkey clay in Test II. In Test III, a small, significant reduction in 10‐d larval weights and an increase in development periods was associated with plants grown for 24 d at moderately reduced SWP. The effects observed in these tests are great enough to be taken into consideration when conducting host‐plant resistance research and when making insect control decisions.

A potential method to incorporate quantity/intensity into routine soil test interpretations

Abstract Exchangeable potassium (K), extracted with 1M ammonium acetate, widely used as the measure for plant available K does not estimate the K supplying capacity of the soil. This research was undertaken to quantity the K supplying capacities of some Missouri soils and evaluate a modified K Quantity‐Intensity (Q/I) approach for adaptation into routine soil analysis. The K supplying capacities as analyzed by the modified Q/I and plant removal were measured on the 0–20 cm and 20–40 cm depths of Broseley loamy fine sand (loamy, mixed, thermic Arenic Hapludalf), Mexico silt loam, (fine, montmorillonitic, mesic Udollic Ochraqualf), Waldron clay loam [fine, montmorillonitic (calcareous) mesic Arenic Fluvaquemt] and the 0–20 cm of Sharkey clay (very fine, montmorillonitic, nonacid, thermic Vertic Haplaquet). The dominant clays in the clay fractions were identified by X‐ray diffraction to be composed of montmorillonite and illite. The high linear coefficient of correlation (R2 = 0.92∗∗) between the potassium b...

Soybean Cultivars' Response to Flood Irrigation of Clay Soil

AbstractSurface irrigation is widely used to increase seed yield of soybean [Glycine max (L.) Merr.] grown on clayey soils of the southern Mississippi River alluvial plain. A field study was conducted on Sharkey clay (Vertic Haplaquept, very‐fine, montmorillonitic, thermic) in 3 yr to measure the effect of flood irrigation of varying duration on yield and yield components of soybean. Treatments were (Treatment 0) nonirrigated; (Treatment I) 24 h to flood (soil submerged 24 h after onset of irrigation) and then drained; (Treatment 2) 24 h to flood plus 24‐h moving water flood, then drained; (Treatment 3) 24 h to flood plus 48‐h moving water flood, then drained, (Treatment 4) soil submerged with static water for 24 h, then drained; and (Treatment 5) soil submerged within 2 h of onset of irrigation, and then drained. All irrigation treatments commenced at first bloom and repeated whenever soil water potential of a treatment dropped below −70 kPa at 30cm depth. ‘Sharkey’ soybean was more responsive than was ‘Centennial’, but all irrigation treatments resulted in significant yield increases of both cultivars in years when rainfall was deficient during reproductive development. Treatment 3 flooding resulted in significantly lower seed yield than did Treatment 1 flooding, while Treatments 1 and 2 floodings achieved similar yield increases. In a rainy year (1988), Treatment 3 average yield was lower than yields from all other treatments. This indicates that even relatively short periods of flooding can be detrimental to soybean if complemented with untimely rainfall. Standing water for 24 h (Treatment 4) produced yield increases that equalled or exceeded those of all the moving water treatments. These results showed that flood inundation of less than 48 h will result in the consistently largest significant yield increase for soybean, and that soybean genotypes differ in their response to flood irrigation.

Soybean and Soybean Cyst Nematode Response to Soil Water Content in Loam and Clay Soils

Soybean cyst nematode (Heterodera glycines Ichinohe) (SCN) a major pest of soybean [Glycine max (L.) Merr.] in the southeastern USA. The objective of this study was to determine the effect of wet and dry silt loam and clay soils on SCN development and corresponding soybean yield response in a greenhouse study. ‘Tracy‐M’ soybean was grown in pots that contained either Dubbs silt loam(fine‐silty, mixed, thermic Typic Hapludalf or Sharkey clay (very fine, montmorillonitic, nonacid, thermic Vertic Haplaquept) surface soils that were watered to maintain adequate soil water potential (SWP) (−30 kPa SWP, wet treatment), or had water added at onehalf the rate of the adequate treatment (dry treatment). Nematode treatments were either infested or noninfested. Experiments were conducted in 1987, 1988, and 1989, and new soil was infested each year. Number of cysts increased significantly with time in the wet silt loam treatment, stayed the same as the initial infestation in the dry silt loam treatment, and declined significantly in both the wet and dry clay treatments in all years of the study. Infestation by SCN caused a reduction in seed yields in both soils, but the decrease was less in the clay soil. Soil water content in each soil interacted with level of SCN infestation to influence soybean yield. The interaction was not dependent on similar SCN infestation levels between soils, or similar differences in SCN infestation between wet and dry soil treatments within soil. Yield declines associated with SCN infestation were highly correlated with number of seed. These results indicate that SCN will not maintain populations in fine‐textured clay soil under greenhouse conditions, nor result in the magnitude of yield decline that often is measured in coarse‐textured soil.

Weed control and soybean response to preplant tillage and planting time

Clay soils in the southeastern U.S.A. Typically are tilled for seedbed preparation and weed control, prior to seeding soybean ( Glycine max (L.).Merr.). However, during extended wet periods when clayey soils cannot be tilled, this practice interferes with timely planting of soybean. Consequently, a 3-year study (1985–1987) investigated the role of tillage and seeding date on irrigated soybean production on Sharkey clay (Vertic Haplaquept) near Stoneville, Mississippi. Plantings were made each year, in early May and late may or early June, in seedbeds subjected to the following treatments: (1) autumn tillage and winter fallow; (2) autumn tillage and winter wheat ( Triticum aestivum L.) cover; (3) late winter or early spring tillage; (4) prepared seedbed; or (5) no tillage between harvest and planting. A disk-harrow and spring-tooth harrow were used for all tillage operations. Weeds were effectively controlled during each growing season in all treatments by selected preplant, preemergence, and postemergence herbicides, plus postemergence cultivation. Seedbed tillage had no consistent effect on soybean yield, but in two of the three years, early plantings yielded significantly more seed than late plantings. The increase from early planting was 862 kg ha −1 (3598 vs. 2736 kg ha −1) in 1986, and 381 kg ha −1 (2899 vs. 2518 kg ha −1) in 1987. These results indicate that seedbed tillage of clayey soil should be avoided if it interferes with timely planting of soybean.

Plant Population Density and Row Spacing Effects on Soybean at Post‐Optimal Planting Dates

AbstractInformation on cultural practices for determinate soybean, Glycine max (L.) Merr., at post‐optimal planting dates is needed in the southern USA where planting is often delayed by weather or doublecropping. The objective of this study was to examine the effects of plant population density (PPD) on determinate soybean at two postoptimal planting dates in relation to interrow spacing, plant architecture, and yield components. A 3‐yr. soybean field study on Sharkey clay (very fine montmorillonitic, thermic, nonacid, vertic Haplaquept) was planted at post‐optimal dates of 20 June and 3 July. ‘Davis’ (MG VI) and ‘Braxton’ (MG VII) soybean were planted in 0.5 and 1.0 m row spacing (RS) at PPD at 6.4,13, 26, 38, 51 plants m−2 under irrigated and nonirrigated conditions. Yields were higher at the 0.5 m RS than at the 1.0 m RS, and were higher at the June date than at the July date. Highest yields at the June date (2.99 and 1.97 Mg ha−1) were with PPD of 38 and 13 plants m−2 at the 0.5 and 1.0 m RS, respectively. Yields were highest at the July date (2.45 and 1.53 Mg ha−1) with PPD of 51 and 26 plants m−2 at the 0.5 and 1.0 m RS, respectively. Increase in PPD up to 38 plants m−2 increased main stem length and decreased all other plant components. Decrease in main stem yield was proportionately smaller than the increase in PPD resulting in a net yield gain from higher PPD. Decrease in branch stem yield was proportionately greater than the increase in PPD resulting in a net yield loss from PPD increase. For determinate soybean planted at a 0.5 m RS an increase in PPD above that needed at optimal dates was necessary to obtain highest yields at post‐optimal planting dates. Soybean planted at the 1.0 m RS had much lower yield and was much less responsive to increase in PPD.

Sprinkler Irrigation Effects on Determinate Soybean Yield and Lodging on a Clay Soil

AbstractDrought stress can reduce the yield of soybean [Glycine max (L.) Merr.] grown in the middle and lower Mississippi River Valley. A 3‐yr field study on Sharkey clay (very‐fine, montmorillonitic, nonacid, thermic Vertic Haplaquept) determined the effects of four soil water levels and two row spacings on the growth and yield of cultivars Forrest (MG V), Centennial (MG VI), and Braxton (MG VII). Irrigation was applied in the four treatments when (i) soil water matrix potential (Ψs) at both the 15 and 30 cm depths reached −70 J kg−1 (I1), (ii) Ψs at all depths between 15 and 76 cm reached −70 J kg−1 (I2), (iii) the same as in I1, but only after the R1‐R2 growth stage, and (iv) there was no irrigation (NI). Soybean was planted at interrow spacings (RS) of 0.5 and 1.0 m. Irrigation significantly increased yield of each cultivar at each RS in each year, but there were no yield differences among I1, I2, and I3 regimes at either RS. Average NI yields were 2.86,3.20 and 2.97 Mg ha−1 in 1982, 1983, and 1984, respectively. Yield increase from irrigation was 10,8, and 16% in 1982, 1983, and 1984, respectively. Irrigated yields of Forrest, Centennial, and Braxton were 3.47, 3.17, and 3.40 Mg ha−1, respectively, but water use efficiency was greatest with Forrest. Irrigation prior to R2 increased lodging up to 100%, and up to 50% when irrigated after R2. Severe lodging may have reduced irrigated yields. Relative yield increase from irrigation was limited by adequate rainfall and soil water storage, which resulted in high NI yields. The most efficient water regime was one based on an early maturing cultivar and γs measured at a depth of 76 cm.

Flood Duration Effects on Soybean Growth and Yield

AbstractFlooding is often a problem in areas of the southern USA with high rainfall and impermeable soils. A field study was conducted on two poorly drained, slowly permeable soils, a Sharkey clay (veryfine, montmorillonitic, nonacid, thermic Vertic Haplaquept) and a Crowley silt loam (fine, montmorillonitic, thermic Typic Albaqualf), to determine the influence of prolonged flooding on the growth and seed yield of eight determinate soybean [Glycine max. (L.) Merr.] cultivars. Soybean was continuously flooded 3 cm above the soil surface at either the V4 or R2 growth stage for 2, 4, 7, or 14 d. Soil water pressures were monitored at four depths in the Sharkey profile. Flood duration effects on the soybean plant were manifested in yellowing and abscission of leaves at the lower nodes, stunting, and reduced dry weight and seed yield. Canopy height and dry weight decreased linearly with duration of the flood at both growth stages. The growth rates were 25 to 35% less when soybean was flooded at R2 than at V4. The additional 4 wk of vegetative growth after flooding allowed the soybean flooded at V4 to recover to a greater extent than the cultivars flooded at R2. A linear relation was found between seed yield and monthly average crop growth rates for the 4 wk following flooding. A linear decrease in seed yield with flood duration was also found. On the Sharkey clay, rates of yield reduction were 157 and 124 kg ha−1 d−1 of the flood for the soybean flooded at R2 and V4, respectively. On the Crowley silt loam, yield reduction rates were 101 and 53 kg ha−1 d−1 of the flood duration for the soybean flooded at R2 and V4, respectively. Crop susceptibility factors (CS) were determined by dividing the decline in yield by the unstressed control, where the 2‐d flooded soybean cultivars were considered the controls. Values of CS ranged from 0.0 to 0.6 and were linearly related to flood duration after 2 d. The slopes of the lines were 1.5 times greater for the flood applied at R2 than at V4, and 2.4 times greater with soybean grown on the Sharkey clay than on the Crowley silt loam. The determinate soybean cultivars were more susceptible to prolonged flooding during early reproductive growth than early vegetative growth and when grown on the clayey soil than on the silt loam.

Planting System and Weed Control Effects on Soybean Grown on Clay Soil

AbstractDryland soybean [Glycine max (L.) Merr.] production on clay soils in the Mississippi River alluvial flood plain (Delta) is economically marginal, and production inputs must be minimized to achieve economic feasibility. One scheme for reduction of inputs is stale seedbed planting. Three chemical weed control options (none, preemergence only, and post‐emergence only) were compared in both stale and tilled seedbeds on a Sharkey clay (very‐fine, montmorillonitic, thermic Vertic Haplaquept) in 1981 and 1982. Four soybean cultivars representing Maturity Groups V, VI, and VII were used. Yields from the two seedbed treatments with preemergence weed control were equal, but seed yield from the tilled seedbed treatment with no herbicide averaged 225 kg ha−1 greater than the stale seedbed treatment with comparable weed control. Percent weed cover was lowest both years in the tilled seedbed with postemergence weed control. Weed species present in the greatest abundance were prickly sida (Sida spinosa L.), pitted morningglory (Ipomoea lacunosa L.), and slender aster (Aster exilis Ell.). An unfilled seedbed is a viable choice with effective weed management to enable producers to minimize tillage inputs and still maintain yield on clay soils.