Soil Science Society of America Journal 67:235-240 (2003)
© 2003 Soil Science Society of America
DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION
A Single Irrigation to Improve Early Maturing Soybean Yield and Quality
Daniel W. Sweeney*,a,
James H. Longa and
M. B. Kirkhamb
a Kansas State Univ., Southeast Agric. Res. Center, P.O. Box 316, Parsons, KS 67357
b Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506
* Corresponding author (dsweeney{at}oznet.ksu.edu)
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ABSTRACT
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When irrigation sources are limited, deficit irrigation at selected growth stages may help avoid crop stress at critical times. The objective of this study was to determine the effect of a single irrigation at different reproductive growth stages on yield and quality of early maturing (Maturity Group I) soybean [Glycine max (L.) Merr.] cultivars. The experiment was conducted from 1991 through 1994 on a Parsons silt loam (fine, mixed, thermic Mollic Albaqualf). The experimental design was a randomized complete block with a split-plot arrangement of treatments. Irrigation scheme (application of 2.5 or 5 cm at R4, R5, or R6) as the main plot was factorially arranged within each replicate, and cultivar (Hodgson 78 and Weber 84) was the subplot treatment. Also included was a randomized nonirrigated check whole plot planted with both cultivars. Yields from a single irrigation at R4, R5, or R6 were similar, and averaged approximately 20% more than yield with no irrigation (1.72 Mg ha-1). Irrigation at R4 increased seeds plant-1, whereas R5 and R6 irrigations increased weight per seed. Irrigation had minimal effect on seed protein and variable effect on oil content. Visual quality of harvested seed frequently scored poor and irrigation improved quality only in 1992 when maximum air temperatures were <35°C. A single irrigation at R5 in 1991 improved germination to nearly 60% compared with 30% with no irrigation. Germination averaged 81% in other years with no irrigation effects. These data show that a single irrigation at different reproductive growth stages can influence early maturing soybean yield and quality, but the improvements may be inadequate to justify the practice.
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INTRODUCTION
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PRODUCTION SYSTEMS utilizing early maturing soybean cultivars have several potential advantages for producers. Early maturing soybean can be more profitable than traditional soybean because economic risk is spread by crop diversification, higher harvest grain prices, work-load distribution, and timely fall field operations (Casey et al., 1998). Yields of early maturing soybean cultivars were equal or superior to yields of traditionally maturing soybean cultivars in the southern USA (Boote, 1981; Mayhew and Caviness, 1994; Bowers, 1995; Boquet, 1998) and in the mid-latitude regions of the USA (Kane and Grabau, 1992; Sweeney et al., 1995). Although yield of early maturing soybean cultivars is often acceptable, grain quality has been found lacking (Mayhew and Caviness, 1994) and this may diminish the economic advantage of early maturing soybean by diminishing the price in local markets (Casey et al., 1998).
Early maturing soybean cultivars may be planted early to try to avoid potential summer drought conditions during the grain development period. Drought stress during reproductive growth stages can reduce yields (Eck et al., 1987) and is important regardless of planting date (Heatherly, 1988). However, because rainfall distribution is often uneven in many humid and subhumid areas of the USA that lack large aquifer sources, early maturing soybean could benefit from supplemental irrigation. Limited irrigation can increase crop yield by avoiding moisture stress at critical times (Hiler and Howell, 1983). Thus, limited-amount irrigation in subhumid and humid regions could be scheduled to partially alleviate potential plant stress at critical growth stages, although it would likely be deficient to fully meet evapotranspiration needs. Rainfall would be relied upon to supply the remainder of the crop's water needs.
Delaying soil-water based irrigation until soybean is in reproductive stages of growth has resulted in equivalent or greater yields compared with a seasonal water balance approach (Ashley and Ethridge, 1978; Brady et al., 1974; Elmore et al., 1988; Heatherly, 1983; Klocke et al., 1989; Martin et al., 1979; Specht et al., 1989). This process may increase water use efficiency by eliminating irrigation during the vegetative stage when soil evaporation is high (Neyshabouri and Hatfield, 1986). However, identification of the critical reproductive stage for irrigation of early maturing soybean has been less clear. Elmore et al. (1988) found that delaying irrigation until R3-R4 would likely result in lower yields than commencing irrigation at R2 or a soil-moisture based irrigation schedule. For short season soybean in North Dakota, Stegman et al. (1990) concluded that drought stress in the R4 to R6 stages was most detrimental to yield. Korte et al. (1983a) found that a single irrigation during R3-R4 increased seeds per plant and irrigation at R5-R6 increased weight per seed. Because weight per seed may be reduced as a result of irrigation at R1-R2 (Korte et al., 1983a), irrigations during R3-R4 and R5-R6 result in greatest yield (Korte et al., 1983b).
Determining the critical timing for irrigation of crops in humid and subhumid areas may allow for judicious use of limited water supplies. The objective of this research was to determine the effect of a single irrigation of 2.5 or 5 cm applied at R4, R5, or R6 on yield, yield components, and seed quality of early maturing soybean.
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MATERIALS AND METHODS
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The experiment was conducted from 1991 through 1994 near Parsons, KS at the Southeast Agricultural Research Center of Kansas State University. The topsoil was a Parsons silt loam of approximately 30 cm, overlying a "claypan" B horizon. The topsoil had an available water holding capacity of approximately 5 cm and the subsoil has a low percolation rate of <0.15 cm h-1 (Owens et al., 1990). Selected background soil chemical analyses in the 0- to 15-cm depth were 6.5 pH (1:1 soil/water), 29 mg kg-1 P (Bray-1), and 90 mg kg-1 K (1 M NH4C2H3O2 extract) analyzed by North Central Region recommended procedures (Dahnke, 1980).
The experimental design was a randomized complete block with a split-plot arrangement of treatments in three replications. In each replication, whole plots included a 3 x 2 factorial arrangement of irrigation timing and amount, plus a nonirrigated plot. The factorial irrigation schemes were 2.5 or 5 cm applied at R4, R5, or R6. Designations of R4 (full pod), R5 (beginning seed), and R6 (full seed) followed descriptions by Fehr and Caviness (1977). Irrigation dates are shown in Table 1 and irrigation was applied regardless of rainfall or soil moisture conditions. Two Maturity Group I soybean cultivars, ‘Hodgson 78’ and ‘Weber 84’, comprised the subplots.
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Table 1. Irrigation dates for soybean at growth stages R4, R5, and R6 and soil moisture in the 0- to 15-cm depth prior to irrigation events in 1991 through 1994 at Parsons, KS.
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Individual subplot size was 4.6 by 9.1 m with whole plots being 9.1 by 9.1 m for each irrigation scheme. Whole plots were separated on all sides by 9.1 by 9.1 m border areas to avoid irrigation drift or runoff. Irrigation was by a solid-set, aluminum pipe overhead sprinkler system. Part-circle impact sprinklers were located on each of the four corners of the irrigated whole plots, and cumulatively delivered approximately 2.5 cm h-1. For plots receiving 5 cm total, 2.5 cm of irrigation was applied on consecutive days to allow for infiltration and minimize runoff. All plots were disked in early spring and were later tilled with a spring-tooth field cultivator prior to planting. Annually, 13 kg P ha-1 and 25 kg K ha-1 were applied as a commercial, prilled mixture of ammonium phosphate and potassium chloride prior to cultivation. Herbicides used on all plots each year were 1.7 kg a.i. ha-1 metalochlor (C14H20ClNO2) and 0.11 kg a.i. ha-1 imazaquin (C17H17N3O3) applied preplant and incorporated by field cultivation. Soybean was drill-seeded at 490 000 seeds ha-1 in 18-cm wide rows on 10 May 1991, 13 May 1992, 26 May 1993, and 10 May 1994.
Plant population was counted in each plot during early vegetative growth each year. The seven center rows of each plot were harvested with a plot combine. Grain was weighed and yield was adjusted to 130 g kg-1 moisture content. To determine mean weight per seed, duplicate random samples of 100 seeds were taken from the harvested seed, weighed, and adjusted to 130 g kg-1 moisture content. Seeds per plant were calculated from yield, weight per seed, and population data for each plot. Grain samples were sent to the Iowa State University Grain Quality Laboratory for determination of protein and oil content by near-infrared spectrophotometer analysis. Grain quality, based on each entire harvest subsample, was estimated visually based on the degree of wrinkling, defective seed coats, greenishness, and moldy or rotten seeds using the scale of 1 = very good, 2 = good, 3 = fair, 4 = poor, and 5 = very poor. Samples were sent to the Kansas State Seed Testing Laboratory for standard germination tests by Association of Official Seed Analysts (AOSA) (1988) procedures.
Rain and air temperature data were recorded at a National Weather Service Cooperative Observer Network Site (Station ID #14-6242-09) located on the Research Center and that is <0.5 km from the study site. This weather station has been active since 1948 and follows National Weather Service observational standards.
All soybean data were analyzed across years using the Proc Mixed procedure of the Statistical Analysis System (Littell et al., 1996). Because of year by treatment interactions, seed quality data were reanalyzed within years. Treatment means were compared using Fisher's protected LSD. Contrasts were made to compare the average treatment response of no irrigation with treatments irrigated at R4, R5, and R6.
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RESULTS AND DISCUSSION
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Climatic Conditions
Conditions in 1991 were hot and often dry (Fig. 1)
. During the 6-wk period of July and early August, more than half the days had maximum air temperatures that were 
35°C. In 1992, there were no days with maximum temperatures 35°C and total rainfall for June-August exceeded 50 cm. In 1993, the first half of the June-August period was cool and wet, but conditions were drier and hotter later. In 1994, less rainfall and more hot days were recorded during late June and early July than from mid-July to 31 August. Typical climatic conditions for half-month periods from June through August are that average rainfall ranges from 4.1 to 5.8 cm and average maximum air temperature ranges from 29 to 33°C (30-yr data not shown).
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Fig. 1. Rainfall (bars) and number of days that maximum air temperature was 35°C (squares) during the months of soybean reproductive growth from 1991 through 1994 at Parsons, KS.
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Yield and Yield Components
Average soybean yield for the 4-yr period from 1991 through 1994 was increased by approximately 20% by a single irrigation at the R4, R5, or R6 growth stages as compared with no irrigation (Table 2). However, there was no significant yield differences among irrigation timing treatments. Stress effects on short season soybean during R4 to R6 may be most detrimental to yield (Stegman et al., 1990), but if the frequency and distribution of rainfall is adequate, yield increases from irrigation may be small (Boquet, 1998). In our study, yield was not affected by an interaction between year and irrigation treatments, but the relative increase from irrigation varied with year.
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Table 2. Average yield and yield components of early maturing soybean from 1991 through 1994 as affected by growth stage irrigation, irrigation amount, and cultivar.
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Irrigations at R5 and R6 produced heavier seed than those produced from no irrigation or irrigation at R4 (Table 2). Apparently, partially reducing drought stress during seed fill resulted in larger seeds. In contrast, irrigation at R4 resulted in a greater number of seeds per plant than did no irrigation, whereas single irrigations at R5 or R6 did not. This agrees with results of Kadhem et al. (1985) for traditionally maturing soybean. They found an increase in weight per seed from single irrigations near R6 compared with irrigations before or near R4, whereas seeds plant-1 were greater with single irrigations before or at R4 than later in seed fill.
Yield increases from irrigation during full reproductive development have been attributed mainly to increased number of seeds (Heatherly and Spurlock, 1993), and irrigation during pod elongation can reduce ovule abortion within developing pods (Korte et al., 1983a). Therefore, in a limited irrigation scheme for early maturing soybean, irrigation at R4 may increase seed number and provide a greater yield potential if rain occurs during seed fill. In our study, increasing the irrigation amount from 2.5 to 5 cm increased yield by 6%, apparently because of a minimal increase in weight per seed (Table 2). Weber 84 yielded 10% more than Hodgson 78 because of a greater increase in seed number that offset the larger seed produced by Hodgson 78.
Harvest Seed Quality and Germination
In contrast to yield, quality components generally were affected by year x treatment interactions. This suggests that, even though year main effects may be significant, yearly environmental conditions influence the effect of irrigation on quality parameters. Quality parameters of harvested seed were reanalyzed within years and are shown in Table 3.
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Table 3. Analysis of variance for the effect of growth stage irrigation, irrigation amount, and cultivar on seed protein and oil content, visual quality, and germination rate of early maturing soybean.
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Protein concentration was not affected in any year by a single irrigation at R4, R5, or R6 compared with no irrigation (Table 3). The differences in 1992 and 1993 in seed protein content of both cultivars irrigated at different growth stages were 
4% (data not shown). Foroud et al. (1993) found similar small or no differences in protein content between irrigation treatments depending on year. Overall oil content was low in 1991 (Fig. 2) , possibly because of stressful conditions from low rainfall and high temperatures (Fig. 1). However, in that year a single irrigation at R5 resulted in significantly greater oil content (Table 3) than did no irrigation or irrigation at R4 or R6. The average maximum air temperature for the week surrounding the R5 growth stage in 1991 was 37°C and mean temperature ([max. + min.]/2) was 30°C (Table 4). Kane et al. (1997c) reported strong correlation between increased seed oil content and higher mean seed-fill temperatures for early maturing soybean. In their study, mean temperatures were <25°C (Kane et al., 1997b). Piper and Boote (1999), using a large USA Soybean Uniform Test database, found that oil concentration responded quadratically to increasing mean temperature with a maximum at about 28°C. Although their data set did not go above 28.7°C, extrapolation would suggest that oil concentration in soybean seed would diminish at mean air temperatures >28°C. Perhaps in our study in 1991, high temperatures resulted in depressed oil concentrations, but a single irrigation at R5 may have partially compensated for the lack of rainfall (Table 4) and allowed the soybean to increase oil content during seed-fill under high temperatures. Oil content was unaffected by irrigation during 1992 or 1994 (Table 3) and these years appeared to be the least stressful during the study (Fig. 1 and Table 4). In 1993, oil content was greater when a single irrigation was applied at R4 or R6 as compared with no irrigation or irrigation at R5. This contrasted with results from 1991, especially because maximum air temperature in 1993 at R4 and R6 was lower than at R5 (Table 4). Kane et al. (1997c) concluded that environmental effects on harvested seed protein and oil were generally relatively small. This also appeared to be true for our data.
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Fig. 2. Soybean seed oil content as affected by no irrigation or a single irrigation at the R4, R5, or R6 growth stages in 1991 and 1993.
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Table 4. Average air temperatures and total rainfall for the week surrounding R4, R5, and R6 soybean reproductive growth stages in 1991 through 1994 at Parsons, KS.
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In general, visual grain quality was fair to poor for early maturing soybean (Fig. 3)
. Only in 1992, when temperature and moisture stress appeared less than other years (Fig. 1), was visual quality improved by any single irrigation as compared with no irrigation (Table 3). An irrigation at R5 improved seed quality rating by one unit compared with no irrigation (Fig. 3). The potential for low yields with high day temperatures during seed fill of soybean (Gibson and Mullen, 1996; Kane et al., 1997a) may have been reflected in visual quality in our study, as evidenced by poor scores during 1991 and 1993 (Fig. 3).
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Fig. 3. Visual soybean seed quality as affected by no irrigation or a single irrigation at the R4, R5, or R6 growth stage from 1991 through 1994. Seed quality was estimated visually based on the amount and degree of wrinkling, defective seed coats, greenishness, and moldy or rotten seeds using the scale of 1 = very good, 2 = good, 3 = fair, 4 = poor, and 5 = very poor.
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Germination of harvested seed was affected by irrigation in 1991 (Table 3). Overall germination in 1991 was <60%, but a single irrigation at R5 or R6 resulted in greater germination rate than with no irrigation or an irrigation applied at R4 (Fig. 4)
. The response was greater for Weber 84 than Hodgson 78 (interaction data not shown). Although overall germination rates varied with year (data not shown), the average germination of seed from 1992 through 1994 was unaffected by irrigation (Fig. 4). Drought stress at R5 and R6 has been found to reduce germination of traditional maturing soybean in Iowa (Smiciklas et al., 1992). However, germination of early maturing soybean in other locations was found to be low and may be related to disease pressure (Mayhew and Caviness, 1994; TeKrony et al., 1996).
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Fig. 4. Soybean seed germination as affected by no irrigation or a single irrigation at the R4, R5, or R6 growth stages in 1991, and the average for 1992 through 1994.
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SUMMARY
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Single irrigations applied at different reproductive growth stages can increase yield and quality of early maturing soybean above that from no irrigation. However, yield increases for a single irrigation, averaged over a 4-yr period, were only 20% over nonirrigated soybean for this location and likely may be less when environmental conditions are favorable for high yields. Irrigation at R4, R5, or R6 produced similar yields but in different ways. Irrigations at R4 increased seeds plant-1, whereas R5 and R6 irrigations increased weight per seed. Irrigation had minimal effect on seed protein and effects on oil content were variable in the stressful years or nonsignificant in years that were more environmentally favorable. Visual quality of harvested seed was frequently poor and irrigation improved quality only in 1992, a year when maximum air temperatures were <35°C. A single irrigation at R5 improved germination when environmental stress was greatest (1991), but had no effect in other years. Thus, a single irrigation at different reproductive growth stages can influence early maturing soybean yield and quality, but the improvements may not be economically viable.
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ACKNOWLEDGMENTS
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The authors thank David Kerley, Robert Black, and the late Phillip Markley for their field assistance.
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NOTES
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Kansas Agric. Exp. Stn. Contribution no 02-166-J.
Received for publication November 2, 2001.
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ABSTRACT
INTRODUCTION
