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Published in Agron J 98:295-301 (2006)
DOI: 10.2134/agronj2005.0198
© 2006 American Society of Agronomy
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Production Papers

Early-Maturing Soybean in a Wheat–Soybean Double-Crop System

Yield and Net Returns

S. Kyei-Boahen and L. Zhang*

Delta Research and Extension Center, Mississippi State Univ., 82 Stoneville Road, Stoneville, MS 38776

* Corresponding author (lzhang{at}drec.msstate.edu)

Received for publication June 30, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Double-cropping soybean [Glycine max (L.) Merr.] after harvesting wheat (Triticum aestivum L.) is an important production system in the mid-southern USA. Field studies were conducted at Stoneville, MS (33°26' N lat) to evaluate the seed yield and net returns from early soybean maturity groups grown in a double-crop system under limited irrigation and to compare the results with those from a full-season system. Soybean cultivars from maturity groups (MG) III, IV, and V were used. Double-cropped soybean yields ranged from 2055 to 3767 kg ha–1 and were 10 to 40% lower than their full-season counterparts. For both systems, the average yield and net returns from MG IV, in particular the late IV cultivars, were among the highest, whereas those from MG III and late V were generally the lowest. The net returns from the full-season soybean averaged $85 to 274 ha–1 higher than those from the double-cropped soybean; however, the net returns from the double-cropped wheat more than compensated for these differences. The 3-yr average wheat yield was 5170 kg ha–1 and accounted for more than 60% of the combined net returns from the double-crop system. These results indicate that yields and net returns from double-cropped MG III or IV soybean could be equal or greater than MG V cultivars, but the late MG IV provided the highest yield and economic return. The data indicated that wheat–soybean double-crop system using MG III or IV under limited irrigation was more profitable than the full-season soybean system.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOYBEAN double-cropped after winter wheat represents large hectarage in the mid-southern USA, including Mississippi (Hovermale et al., 1992), Arkansas (Arkansas Soybean Promotion Board, 2005; Caviness et al., 1986), Louisiana (Board and Hall, 1984), and Alabama (Touchton and Johnson, 1982). The current wheat-soybean double-crop system in the Midsouth involves planting wheat in mid- to late October or in early November. Late-maturing soybeans, such as maturity group (MG) VI or VII, are planted after harvesting wheat in mid-June or early July. When an MG VI soybean is planted in mid-June at Stoneville, MI (33°26' N lat), it usually matures in late October or early November. Field conditions at this period are often not conducive to soybean harvest or to preparing the same field for another wheat crop. Therefore, some farmers are not able to plant another wheat crop in some years when these late maturing soybeans are used. Wesley and Cooke (1988) indicated that wheat-soybean double cropping in the Mississippi Delta could be profitable when wheat price is above $0.10 kg–1, but it would not be sustainable after 20 November for most years due to the limited workday. Thus, adjustments have to be made to improve the sustainability, productivity, and profitability of this double-crop system.

Although the number of days from emergence to maturity for a specific soybean variety is predetermined by its gene combinations (Bernard, 1971; Buzzell and Bernard, 1975; McBlain and Bernard, 1987), the actual expression of this maturity under field conditions varies greatly depending on the geographic location and associated photoperiod (Cregan and Hartwig, 1984). In recent years, soybean farmers in the mid-southern USA have adopted earlier soybean maturity groups, such as MG IV, MG V, and, to a limited extent, MG III soybeans (Heitholt et al., 2005; Zhang, 2004). In Mississippi, no soybean variety with an MG value of 4.5 or lower was included in the state Official Variety Trial before 1998 (White et al., 1999). However, over 8% of the total varieties tested in the 2001 Official Variety Trial were MG 4.5 or less (White et al., 2002), indicating a gradual interest in early-maturing varieties. Studies conducted at Stoneville, MS in 1998 and 1999 indicated that earlier-maturing soybeans (e.g., MG III or early MG IV varieties) can be planted relatively late (i.e., after May) and still produce competitive yields (Zhang, 2004). The current production trend raises questions as to whether early-maturing varieties could be used in the double-cropping system without sacrificing yield and the best production practices that should be followed.

This study was based on the hypothesis that a grower who plants wheat in early fall (i.e., late September or early October) would be able to harvest the wheat in late May or early June. Relatively early MG soybeans, such as early MG V or late MG IV or even earlier varieties, could be planted immediately after harvesting wheat. These soybeans could be harvested in mid- to late September or early October (Zhang et al., 2004) and could avoid some of the adverse growing and harvest conditions that occur late in the season. The objectives of this study were to (i) evaluate and compare the yields of early-maturing soybean cultivars planted in double-crop and full-season systems under limited irrigation, (ii) determine and compare the net returns from the double-crop and full-season systems, and (iii) determine the best soybean MG for a double-crop system.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The study was conducted at the Delta Research and Extension Center, Stoneville, MS from 2000 through 2004 on clay loam soil (fine, smectitic, thermic Chromic Hapluderts). The experimental design was a split-plot arrangement of treatments in a randomized complete block with four replications. The main plot factor was cropping system (full season and double-crop), and the subplot factor was maturity group. Winter wheat cultivar 84T06 was planted on 10 Oct. 2000, 18 Oct. 2001, and 15 Oct. 2003 on plots earmarked for the double-crop system. The wheat plots were fertilized approximately 1 wk after seedling emergence and in early March the following year using granular urea at 112 kg ha–1 for each application. The wheat was harvested on 20 May 2001, 4 June 2002, and 27 May 2004.

The wheat stubble and straw were burnt for subsequent planting of soybean on no-tilled seedbed. Several soybean cultivars from MG III, IV, and V were planted on 21 May 2001, 6 June 2002, and 28 May 2004 (Table 1) using a four-row planter and a seeding rate of 18 seeds m–1 of row to obtain approximately 385 000 plants ha–1. Row spacing was 0.51 m, and individual plots were 44 m long and 2.5 m wide. Cultivars in MG III and IV generally have indeterminate growth habit, whereas those in MG V have determinate growth habit. The same soybean cultivars used in the double-crop experiment were planted earlier on the other half of the experimental area on 18 Apr. 2001, 22 Apr. 2002, and 19 Apr. 2004 as full-season soybean. The seedbed preparation for the full-season soybean consisted of two passes with a disk harrow before seeding.


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  		Table 1. Soybean cultivars and maturity groups used at various planting dates for full-season and double-crop systems near Stoneville, MS, in 2001, 2002, and 2004.

 
Recommended pre-emergence and postemergence herbicides were applied to control weeds. The soybean plots were irrigated according to standard practice through an overhead lateral move system when necessary. The full-season soybean received 25 mm of irrigation water three times each year during June and July. The double-cropped soybean was also irrigated three times (25 mm of water) during July and August. The rainfall for each year was adequate for wheat production; therefore, no irrigation was applied.

Wheat yield for each year was determined by harvesting the whole plot using a field combine. A four-row plot combine equipped with plot weight and seed moisture measurement systems was used to harvest the full-season and double-crop soybeans. The wheat and soybean yields were adjusted to 130 g kg–1 moisture content.

Combined analysis of the soybean yield data was performed, and each cropping system was analyzed separately using mixed model procedures (SAS Institute, 2003). The appropriate error degrees of freedom were determined by the use of Satterthwaite's approximation. Within a cropping system, combined analysis of maturity groups across years was performed with maturity group and year of production, and their interaction considered fixed, whereas blocks nested within year was considered random. Least square means of the fixed effects were computed, and the PDIFF option of the LSMEANS statement was used to display the differences among least square means for comparison. This option uses Fisher's protected least significant difference, and comparison was conducted at P ≤ 0.05. Contrast statements were used to compare the full-season and double-cropped soybean yields for each MG.

Economics analyses were conducted to evaluate returns over variable and fixed production costs for the full-season and double-cropping systems using the Mississippi State Budget Generator (Laughlin and Spurlock, 2005). The variable production costs included costs of seeds, herbicides, fertilizer, irrigation, hired labor, fuel, and repair and maintenance of equipment. The fixed expenses included annual ownership costs of a tractor, equipment, and implements. All MG in each cropping system in a particular year were treated the same. Minor differences in production practices occurred across years, but production costs were generally similar. The specified expenses for each year totaled $164, $305, and $348 ha–1 for wheat, double-cropped soybean, and full-season soybean, respectively. The estimated cost of production did not include costs for land, management, and overhead. Seasonal average soybean prices received by Mississippi farmers from September of a particular year (2001, 2002, or 2004) to February of the following year (Mississippi Agricultural Statistics Service, 2005a) were used to calculate income from crop sale. The average soybean prices for 2001, 2002, and 2004 were $0.160, $0.201, and $0.214 kg–1, respectively. Income from wheat was calculated using $0.096, $0.106, and $0.136 kg–1 as average wheat prices for 2001, 2002, and 2004, respectively (Mississippi Agricultural Statistics Service, 2005b). Gross income was calculated as the product of yield and average seasonal crop price. Net returns above total specified expenses for the full-season and double-cropping systems were determined by subtracting the total specified expenses for each system from the gross income.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Rainfall from planting to maturity of wheat totaled 1080, 1234, and 840 mm in 2000/2001, 2001/2002, and 2003/2004, respectively, and was well distributed; there were no extended dry periods during this phase of the production cycle. In general, the amounts and distribution of rainfall during the soybean growing period for the three seasons from April to October were good (Fig. 1 ). Thus, the full-season and double-cropped soybeans received limited irrigation consisting of 25 mm of irrigation water three times during June and July (full-season) or during July and August (double-cropped). Rainfall during the last 10 d of June 2004 was high (295 mm) and resulted in flooding, which affected the growth of the double-cropped soybean. The maximum and minimum temperatures were near normal; however, the 2004 average temperatures for July and August were 1 to 2°C lower than those for 2001 and 2002 (Fig. 1).


Figure 1
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  		Fig. 1. Maximum and minimum air temperatures and rainfall for the 2001, 2002, and 2004 soybean growing seasons at Stoneville, MS (Source: National Weather Service, Cooperative Observer Network, Stoneville Experiment Station, Stoneville, MS).

 
Wheat yield ranged from 4838 to 5497 kg ha–1 (Table 2), with a 3-yr average of 5170 kg ha–1. This was much higher than the county's (Washington County) average yield of 3462 kg ha–1 for 2000 through 2004 (Mississippi Agricultural Statistics Service, 2005c) and those reported previously for the same location but a different site in Mississippi (Wesley and Cooke, 1988; Heatherly et al., 1996). Wesley and Cooke (1988) reported an average yield of 3233 kg ha–1, whereas Heatherly et al. (1996) indicated 2930 kg ha–1 for their 1988 experiment. The differences in yield are likely related to differences in varieties, soil type, and rainfall pattern between this study and earlier studies. The present study was conducted on clay loam soil, whereas the previous studies were done on clay soil.


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  		Table 2. Analysis of variance for soybean yields in wheat-soybean double-crop and full-season systems at Stoneville, MS in 2001, 2002, and 2004.

 
The wheat matured in late May and was harvested by the first week of June, which enabled the timely planting of the double-cropped soybean. The double-cropped soybean MG III matured before 20 September, the early maturing MG IV matured between 25 and 30 September, and the later-maturing MG IV matured by 6 October. The MG V cultivars generally did not reach maturity by 20 October and were affected particularly by cool night temperatures (Fig. 1) in addition to a steady decline in solar radiation. Thus, the pods were not completely filled by the beginning of November, and the maturity coincided with wet field conditions, making it difficult to harvest using a combine. In addition, late-season foliage-feeding insects, including stinkbug (Nezera viridula L.) and bean leaf beetle (Cerotoma trifurcata), caused considerable damage to the MG V cultivars. Earlier studies indicated that damaging populations of these insects occur during the later part of the growing season in the southern USA, with the early-maturing cultivars serving as host for these insects (Baur et al., 2000; Heatherly and Elmore, 2004). Therefore, yields for the MG V cultivars were not determined.

Significant yield differences occurred among soybean cultivars in both cropping systems. Further evaluation of the data indicated that the yield differences between the early- and later-maturing cultivars within MG IV or V were significant. Hence, MG IV and V cultivars were classified as early- or later-maturing groups within each MG. For example, MG 4.0 to 4.4 was classified as early MG IV, and MG 4.5 to 4.9 was classified as late MG IV. All the MG III cultivars used in this study were later-maturing cultivars from that group. Statistical analysis combined across cropping systems and years indicated that the main effects of cropping system, year, MG, and interactions among these factors, with the exception of cropping system by MG, were statistically significant (Table 2). Further analysis for each cropping system across years also indicated year-by-MG interaction; therefore, data are presented separately for each year.

In two of the three years of this study, yields for the full-season MG III cultivars were not different from their late MG V counterparts (Table 3) and were generally the lowest. The full-season soybean yield ranged from 2192 kg ha–1 in 2004 to 4488 kg ha–1 in 2002, with early IV cultivars producing the highest yield and late V cultivars producing the lowest yield. Overall, the MG IV cultivars performed better in terms of yield than the MG III and V cultivars. This may be related to the indeterminate growth habit of the MG IV cultivars that extend the flowering and seed fill duration and perhaps the good growing conditions that permitted these cultivars to better express their genetic yield potential.


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  		Table 3. Wheat and soybean yields in wheat-soybean double-crop and full-season soybean systems near Stoneville, MS in 2001, 2002, and 2004.

 
In the double-crop system, soybean yields for the late MG IV cultivars was 26, 23, and 24% higher than those for MG III, early MG IV, and early MG V cultivars, respectively, in 2001 (Table 3). Yields for the early- and late-maturing MG IV cultivars were not different in 2002 or 2004. The 2002 yield data for the early and late MG IV cultivars were higher (417 kg ha–1) than that for MG III; however, yield for MG III was higher than that for the late IV but did not differ from the early MG IV in 2004. The small plant size and the relatively short flowering, pod-set, and seed-fill duration for the MG III cultivars accounted for the generally low yields in 2001 and 2002 compared with the MG IV cultivars. This was not the case in 2004; perhaps the short growing season favored the MG III cultivars relative to the late MG IV cultivars (Table 3). The results from this 3-yr study and those from Wesley and Cooke (1988) and Heatherly et al. (1996) indicate that MG V and VI cultivars may not have a yield advantage over MG IV and III cultivars when planted in a double-crop system and could be a risk if planted after early June in a season with terminal wet conditions. Yields at the same location (Stoneville) in Mississippi and under similar production practices for double-cropped MG VI averaged 2331 kg ha–1 (Wesley and Cooke, 1988), whereas yields for MG V averaged 2641 kg ha–1 (Heatherly et al., 1996). In the present study, the use of early-maturing wheat coupled with relatively early planting of the wheat allowed early wheat harvest. This enabled the double-cropped soybeans to be planted earlier than the conventional double-crop system and likely explains why the yields of MG III and IV cultivars in Table 3 are comparable to those reported previously for MG V and VI. In 2001, when planting occurred on 21 May, the early MG V cultivars were harvested, but seed fill for the late MG V cultivars was not fully complete by the first week of November. In 2002 and 2004, planting occurred relatively late (on 6 June and 28 May, respectively); hence, all the MG V cultivars could not be harvested due to wet field conditions during the later part of these growing seasons. In addition, there was a high incidence of green stems, which made the stems harder to cut during harvesting. The cool temperatures and the wet conditions with associated high humidity, which typically occur after mid-October in the Mississippi Delta region, delay the maturation and drying of late-maturing soybean cultivars and often result in low yield and poor seed quality. Thus, the implication of using late-maturing cultivars in a double-crop system is the delayed planting of winter wheat during fall and consequently delayed harvest of the wheat and subsequent planting of soybean the following growing season.

Double-cropped soybean yields for all 3 yr were generally lower than those for the full-season yields, with average yield differences ranging from 258 to 988 kg ha–1 in 2004 to 1452 to 1694 kg ha–1 in 2002 (Table 3). Several studies have shown that soybean grown in full-season cropping system has a significant yield advantage over soybean planted in a double-crop system (LeMahieu and Brinkman, 1990; Wesley, 1999; Ashlock et al., 2000; Pfeiffer, 2000). The yield reduction in the present study (10–40%) is comparable to the 16 to 33% reduction reported by Pfeiffer (2000) in experiments conducted in Kentucky. Wesley et al. (1988) also reported yield reduction when yields from irrigated and nonirrigated full-season soybean MG V were compared with irrigated and nonirrigated counterparts in a double-crop system. In our study and in the study by Wesley et al. (1988), which involved irrigation, yield losses were not eliminated by water application, indicating that other factors may be involved (Egli and Bruening, 2000). Many studies have indicated that reduced daylength decreases the duration between seedling emergence and beginning seed development stage (R5), leading to inadequate vegetative growth for optimal yield of late-planted or double-cropped soybean (Boerma and Ashley, 1982; Board and Settimi, 1986; Ball et al., 2000). In a simulation model, Egli and Bruening (1992) also demonstrated that in the absence of water stress, a lower level of insolation during the reproductive growth stage was the major cause of yield losses from delayed plantings. They found that low temperatures contributed to yield reduction only for cultivars that matured after late October. Similar trends occurred in the present study, indicating that the double-cropped plants were shorter and smaller in size and that the trends were more pronounced in MG III. Flowering, pod set, and seed-fill were all shortened, resulting in yield reduction.

The higher yield differences among the full-season and double-crop soybeans in 2002 could be due primarily to the right amounts and distribution of rainfall, during April and May in particular, resulting in soil moisture levels that were probably more favorable for vigorous crop growth compared with levels from 2001 and 2004. Total rainfall for April and May in 2002 were 83 and 72 mm, respectively, whereas corresponding values of 101 and 129 mm in 2001 and 105 and 184 mm in 2004 occurred (Fig. 1), resulting in occasional flood conditions. The high rainfall during the last half of June 2004 (307 mm) caused flooding conditions and may partially explain the relatively low soybean yields in both cropping systems in 2004. Contrast between MG III yields for the two cropping systems was not significant in 2004 and could be attributed to a more pronounced effect of the flood on the full-season cultivars (Table 3). The ability of soybean plant to recover from a particular stress declines as it advances in development from flowering to the pod-filling stage (Ritchie et al., 1997). The full-season MG III cultivars were at full pod stage when the flood occurred and hence were less able to recover compared with the double-cropped MG III cultivars, which were at V3 as defined by Fehr and Caviness (1977).

The average specified production expenses for the full-season soybean ($348 ha–1) was higher than the double-cropped soybean ($305) due to additional production practices, such as tillage and herbicide application, that occurred in the full-season environment. Moreover, the limited irrigation schedule for the double-cropped soybean as a result of the relatively good rainfall pattern helped minimize the production costs of this system. The production costs for wheat was $164 ha–1 and, like the production costs for soybean, did not vary across years.

For both cropping systems, the net returns from MG IV cultivars (in particular the late MG IV cultivars) were among the highest, whereas those from MG III and late V were generally the lowest (Table 4). Net returns from the full-season early MG V cultivars were among the highest in 2001 and 2004 but were too risky to be used in a double-cropped system due to unfavorable harvest conditions that occur during the maturity period of this group. The net returns from the full-season soybean reported in the present study are comparable with the values for irrigated MG III and IV cultivars planted during mid April–early May in Arkansas (Popp et al., 2004). In the study by Popp et al. (2004), net returns above direct costs for these cultivars in 2001 ranged from 236 to $384 ha–1, whereas our study indicated net returns to be 272 to $319 ha–1 for the same year. Our data for 2004 were consistent with these values, but the relatively high yield from the 2002 full-season soybean resulted in high net returns that ranged from 498 to $554 ha–1. Wesley and Cooke (1988) also reported net returns of $262 ha–1 for irrigated Centennial, an MG VI soybean cultivar, in a full-season system. This net return is comparable to the 3-yr average net return ($253 ha–1) from the full-season late MG V cultivars in the present study but $72 ha–1 less than the average net return reported for the early MG V in Table 4.


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  		Table 4. Net returns above specified production costs{dagger} for wheat–soybean double-crop and full-season soybean at Stoneville, MS in 2001, 2002, and 2004.

 
The average net returns above specified expenses from the full-season soybeans were relatively higher than the double-cropped soybeans (Table 4). The differences were $97, $274, and $85 ha–1 in 2001, 2002, and 2004, respectively. The difference in average net returns for the 2002 study was threefold higher than the other years because of the relatively large yield differences between the full-season and double-cropped soybeans for that year. The combined net returns from the double-cropping system (wheat plus soybean) were higher than the net returns from the full-season system by a range of 134 to $278 ha–1 in 2001, 86 to $135 ha–1 in 2002, and 415 to $571 ha–1 in 2004. The data indicate that an average of 60% of the returns from the double-cropping system was derived from the wheat in 2001 and 2002, whereas over 75% of the 2004 returns was attributable to the wheat. The high wheat yield (5497 kg ha–1) coupled with the relatively high wheat price of $0.136 kg–1 accounted for the high total returns for the double-cropping system in 2004. In the report by Wesley and Cooke (1988), the double-cropping system had a total income advantage of $114 ha–1 over the full-season system, with the wheat accounting for 51% of the returns. In similar studies on no-till soil using a late MG V soybean cultivar, Heatherly et al. (1996) reported combined net returns of $645, $99, and $72 for 1988, 1989, and 1990, respectively, from a wheat-soybean double-cropped system. According to the authors, the wheat contributed 41% of the combined returns reported for 1988 when wheat and soybean produced the highest yields. In the present study, the average wheat yield was 5170 kg ha–1, compared with 3233 kg ha–1 (Wesley and Cooke, 1988) and 2930 kg ha–1 (Heatherly et al., 1996), hence the differences in contribution to total net returns.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Planting soybean MG III or IV cultivars after harvesting wheat reduces the risk of low yields or crop failure, which often occurs in a conventional double-crop system involving MG V and VI cultivars. These early-maturing cultivars, when planted in the last week of May or in the first week of June, were harvested by the first week of October; however, although only the early MG V cultivars were harvested in 2001, all the MG V cultivars could not be harvested in 2002 and 2004 due to late maturity, which coincided with unfavorable weather conditions. Seed yield and net returns for the MG IV, in particular the late MG IV cultivars, were highest in the double-crop and full-season systems. The yield of an individual MG was higher in the full-season system than that in the double-cropped system, but the wheat yield made the double-crop system more profitable than the full-season system. The wheat contributed an average of 60 to 75% of the combined net returns from the double-crop system. For the double-crop system to be sustainable, an early-maturing wheat cultivar must be planted early in the fall to ensure early wheat harvest for the timely planting of soybean. Because of the short growing season, late-maturing cultivars, including MG V, may not reach physiological maturity in a season with terminal cool and wet conditions. The study indicates that the yields for the late MG IV cultivars were consistently high across years and cropping systems; thus, adoption of later-maturing cultivars of soybean MG IV may improve yields of double-cropped soybeans and may reduce the risk of total crop failure. In addition, the timely harvest of late MG IV cultivars may allow field preparation for another winter wheat crop to be planted on the same land during fall.


   ACKNOWLEDGMENTS
 
This study was supported by the Mississippi Soybean Promotion Board. The authors thank Mr. Bart Freeland for providing the weather data.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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