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© 2003 Plant Management Network.
Accepted for publication 25 April 2003. Published 23 May 2003.

Producing Soybean and Wheat Cropping Systems in Rainfed Environments

Kent R. Keim, Lewis H. Edwards (Emeritus), J. Ron Sholar, Department of Plant and Soil Sciences, Oklahoma State University, Stillwater 74078

Corresponding author: Kent R. Keim. kkent@okstate.edu

Keim, K. R., Edwards, L. H., and Sholar, J. R. 2003. Producing soybean and wheat cropping systems in rainfed environments. Online. Crop Management doi:10.1094/CM-2003-0523-01-RS.

Abstract

Growers are interested in crops that can be used as alternatives to complement winter wheat grown as a monocrop. The objective of this study was to compare agronomic performance and net return of winter wheat and soybean cropping systems under rainfed environments. Cropping systems of monocrop winter wheat (Triticum aestivum L.), monocrop soybean [Glycine max (L.) Merr.], and 3-crop/2-year (early-season soybean, winter wheat, and doublecrop soybean in a biennial rotation) were evaluated at Lahoma, OK. Over three years, early-season and monocrop soybean yields averaged 35 and 31 bu/acre, respectively, but were not significantly different in any year. Doublecrop soybean averaged 13 bu/acre following wheat. Reduced yields for doublecrop soybean were attributed primarily to highly variable rainfall amount and distribution during growing seasons. Wheat yields were similar for monocrop and 3-crop/2-year systems, averaging 54 and 53 bu/acre, respectively. Crop production reflected the rainfall variability occurring in such a production environment. Enterprise budgeting was used to determine revenues, costs, and net returns for cropping systems. Annualized average net returns to land, labor, and management were $59/acre for monocrop soybean, $52/acre for monocrop winter wheat, and $77/acre for 3-crop/2-year. The results can be used as a tool to determine cropping systems to use in managing crop production.

Introduction

Grower interest has increased dramatically in crops serving as alternatives to complement winter wheat in continuous, winter wheat monocrop areas. Reasons may be agronomic, environmental, or economic (2,3,4,5,18). Soybean can be produced as an early-season or doublecrop with winter wheat in cropping systems, representing multiple cropping and increasing the number of crops harvested per unit area of land for a given time period.

In the middle South, the early-season soybean production design is to plant in April and harvest in August or September (11,12). Early-season soybean can be followed by planting winter wheat as a doublecrop (1,10). Alternatively, in winter wheat production regions with adequate frost-free periods and rainfall, such as the southern USA, double cropping soybean following wheat is possible (8,9,17,19). Limited summer rainfall is not conducive to successful doublecropping, particularly with respect to soybean following wheat. Limited summer rainfall (June to August) is common in monocrop winter wheat production areas.

Often, crop yields receive major consideration in evaluating crop production. Net returns, however, represent a primary determinant in value and utilization of cropping systems. Cropping systems need to be agronomically sound as well as profitable to be of use by growers.

The objective of this study was to compare yield and net returns in rainfed environments of winter wheat with that of the following two alternative cropping systems: (1) monocrop soybean, and (2) a 3-crop/2-year (early-season soybean, winter wheat, doublecrop soybean in a biennial rotation) cropping system.

Experiment, Results, and Findings

This study was conducted from 1997 through 2000 at the North Central Research Station at Lahoma, OK, under rainfed conditions. Cropping systems of monocrop winter wheat, monocrop soybean, and 3-crop/2-year were evaluated. The 3-crop/2-year cropping system consisted of early-season soybean planted in April and harvested in September; followed by winter wheat planted in October and harvested the following June; followed, in turn, by doublecrop soybean planted after wheat and harvested in November.

Wheat and soybean cultivars used were chosen from those currently available, which were considered to have the best yield potential and adaptation to the area. Targeted planting dates were directed at obtaining optimum seed yield levels. Cultivar Jagger (16) was used for all wheat plantings. Soybean cultivars used in the early-season were of a different maturity group (MG) than those used for monocrop and doublecrop. Cultivar Asgrow 4501 (MG IV) was planted for the early-season soybean in 1998 and 1999. Cultivar Asgrow 4602 (MG IV) was planted for the early-season soybean in 2000. Cultivar Asgrow 5601 (MG V) was planted for the monocrop and doublecrop soybean in 1998, whereas cultivar Asgrow 5602 (MG V) was planted for the monocrop and doublecrop soybean in 1999 and 2000. All soybean cultivars were glyphosate resistant.

Crop production practices used for tillage, fertilization, and pesticides were considered as standard for the region. The entire plot area was planted to winter wheat 5 Nov 1997. Nitrogen was applied at the rate of 18 lb/acre, and phosphorus was applied at the rate of 46 lb/acre using diammonium phosphate as a topdress during February, 1998. For plots to be planted to early-season soybean in 1998, the winter wheat crop was destroyed by tillage. All subsequent fertilization was conducted according to soil test recommendations of Oklahoma State University Soil Fertility Laboratory, targeting wheat yield of 50 bu/acre. Soybean crops were not fertilized. Precipitation data was obtained from the Oklahoma Mesonet System (13).

Wheat, and early-season and monocrop soybean were planted using conventional tillage. Conventional tillage operations consisted of disking, chiseling, and field cultivating. Early-season and monocrop soybean were drilled in 8-inch rows. Doublecrop soybean was sown in 30-inch rows using a no-till row crop planter. Soybean seeding rates were projected at a final plant population of 140,000 viable seeds/acre. Glyphosate (Roundup Ultra®) was applied as a post-emergence herbicide at the rate of 28 fluid oz/acre. Wheat was planted in 8-inch rows using 76 lb/acre of seed. Bradyrhizobium japonicum inoculant was applied to soybean seed, since no known soybean crop had ever been grown on this soil. Soil was a Grant silt loam (fine-silty, mixed, superactive, thermic Udic Argiustoll).

Plot size was 20 × 40 feet. A 20-foot length was harvested from four bordered rows in early-season and monocrop soybean, and one bordered row in doublecrop soybean. Soybean plants were cut at the soil surface and seeds removed with a plot threshing machine. Wheat yield was measured by harvesting a 10-×-40-foot area with a commercial combine. All yields were weighed after threshing and drying to bring seed to uniform moisture content. Plot yield was expressed in units of bu/acre.

Net returns per hectare above specified costs were calculated as the difference between gross revenue and total specified costs. Net returns did not include charges for land, labor, and management. Gross value was calculated for each budget by multiplying actual commodity prices by experimental yields. Net market prices for wheat and soybean were determined by adjusting market price with the effective loan deficiency payment. Variable and fixed costs were calculated for budgets. The agronomic experiment specified all input requirements. Net returns were calculated on an annualized basis.

Experimental design was a randomized complete block (RCBD) with four replications. Study design enabled evaluating all possible crops in each calendar year. This was obvious for wheat and soybean grown in monocrop. For the 3-crop/2-year system, the study was initiated in 1997, and so beginning in 1998, both "years" of the system occurred. An additional treatment was added to each replication to accommodate offset years of the system. Having both years of the 3-crop/2-year system occur in the same calendar year eliminated confounding of any given year in the system with yearly environmental effects. Crop yield and net return data were analyzed using the Statistical Analysis System mixed model (2000 SAS Institute Inc., Cary, NC). Year and replication were considered random effects, and cropping system was considered a fixed effect.

Years included within this study were considered to be very different environments as represented by magnitudes of ranges for soybean and wheat yields (Table 1) and precipitation (Table 2). From a crop management perspective, crop production is most practically described on an annual basis. Thus, separately analyzing each year of the soybean and wheat yield data was warranted.

Table 1. Seed yields for cropping system study at Lahoma, OK.

†For each crop, soybean or wheat, values in a column followed by the same letter are not significantly different at the 0.05 probability level.

Table 2. Monthly and long-term average precipitation at Lahoma, OK.

†Major County

Early-season and monocrop soybean yields were not significantly different in any of the three years (Table 1). Yield of doublecrop soybean was significantly less than early-season soybean in all years, and monocrop soybean in two of the three years. These results agree with previously published results where yield from double crop soybean following wheat was less than monocrop soybean (19,20,21,22). Wesley et al. (20) reported yield of monocrop soybean exceeded yield from doublecrop soybean both in irrigated and nonirrigated environments, and attributed lower yields for doublecrop soybean to environmental factors, principally rainfall.

Extreme soil wetness due to excessive precipitation delayed the planting date for early-season soybean in 1999, and doublecrop soybean in 1998 (Table 3). In contrast, extremely dry conditions during June in 1998 prevented planting doublecrop soybean until 6 July, after adequate precipitation was received. In addition, in 1999, a wet weather pattern prevented wheat harvest until 28 June and delayed doublecrop soybean planting to 2 July. Only in 2000 was doublecrop soybean planted immediately following wheat harvest; however, no rainfall occurred in August or September in 2000 (Table 2). Thus, risks are associated with production of soybean as a doublecrop following wheat in semiarid regions with erratic rainfall patterns.

Table 3. Planting and harvesting dates for cropping system study at Lahoma, OK.

†Wheat in mono crop and 3-crop/2-year cropping systems planted and harvested on same date.

‡For each crop and year, upper number is planting date and lower number is harvest date.

Large ranges in planting dates have been reported for double crop soybean following wheat (3,20), due to inadequate moisture delaying or preventing planting of doublecrop soybean. Delayed planting dates for doublecrop soybean can reduce yield potential because the time frame before plant response to critical photoperiod inducing flowering is shortened.

Selection of row spacing was determined by two factors. First, an assumption was that growers would be adapting equipment used for wheat to use with soybean. Therefore, a drill was used to plant early-season and monocrop soybean. For both those treatments, the seedbed was conventionally tilled. Such tillage was compatible with the drill used. Second, no-till was used for doublecrop soybean, because any tillage was considered to be unacceptable with respect to the associated time lag in planting and soil moisture loss. Since the drill was not capable of planting in no-till conditions an available no-till planter was used. Comparison of yield for doublecrop soybean versus early-season and monocrop was confounded by row spacing, which was 8-inch for early-season and monocrop soybean, and 30-inch for doublecrop soybean. Appropriate row spacing is, however, dependent on yield-limiting conditions, especially water availability (7).

Wheat yields for any year were not significantly different between the two cropping systems, monocrop versus 3-crop/2-year (Table 1). Thus, these results indicate that wheat may be successfully produced with equivalent yield levels in either a monocrop or 3-crop/2-year cropping system. For 2000, wheat yields were 20% lower than either of the other two years of study. Wheat in both systems was planted and harvested the same day for a given cropping season (Table 3). The wheat plots received the same series of cultural practices.

In our study, wheat was planted in a conventionally tilled seedbed. In other regions, traditional tillage operations of chisel plowing, disking, and harrowing have been used to prepare a seedbed for wheat. Traditional seedbed preparation for wheat was used regardless of doublecrop soybean being planted no-till or following conventional tillage (14,15). We observed no change in yield of wheat could be attributed to including wheat in a rotation with soybean. In contrast, Heatherly et al. (9) reported low and declining wheat yields in a three-year study of continuous wheat-soybean doublecropping using no-till wheat planting. Higher wheat yields have been reported for monocrop compared to wheat-soybean doublecropping (8). However, expenses associated with maintaining and preserving cropland between successive monocrop wheat crops were greater than the value of additional yield.

A critical factor of wheat-soybean cropping systems is timeliness of operations. With continuous wheat-soybean doublecrop, time available between harvesting one crop and planting the subsequent crop is minimal. The 3-crop/2-year system relaxes the time constraints associated with a continuous wheat-soybean doublecrop system. Should environmental conditions be unfavorable for planting doublecrop soybean immediately following wheat harvest, soybean planting can be delayed until conditions are more favorable. This occurred in two of the three years of study. Delaying double crop soybean planting in the 3-crop/2-year system would have no adverse effect on subsequent wheat planting date as in continuous wheat-soybean doublecropping. With the 3-crop/2-year system, no crop is in the field between the time doublecrop soybean is harvested in November and early-season soybean is planted in April of the following year. This time frame provides an opportunity for tillage or chemical fallow, or a combination of the two. Early-season soybean in the 3-crop/2-year system is planted as soon as is possible after 1 April, thus allowing timely harvest and yield potential (Table 3), while enabling optimum planting time of mid-October for highest, expected wheat yield (6).

Wheat net returns appeared to be more stable than soybean net returns across years, both as a monocrop or a component crop in the 3-crop/2-year system (Table 4). Range of values for net returns for monocrop soybean were erratic across years, resulting from highly variable yields. Net returns for early-season soybean in the 3-crop/2-year system reflect impact of variable soybean yields in response to environmental conditions. For two of the three years, doublecrop soybean resulted in negative net returns.

Table 4. Net returns to land, labor, and management for cropping systems at Lahoma, OK.

†No significant differences observed among cropping systems within or averaged over years at the 0.05 probability level.

‡Dollars per acre on an annualized basis.

This study demonstrated the 3-crop/2-year cropping system was agonomically feasible in a rainfed environment historically producing monocrop wheat. As a crop, wheat produced relatively stable yields, both in monocrop and a wheat-soybean biennial rotation. Soybean can be grown in cropping systems such as those used in this study. Early-season and monocrop cropping systems were not significantly different in terms of soybean yield. Soybean yields were highly dependent upon amount and distribution of precipitation, particularly when planted as a doublecrop following winter wheat. Net returns are an important criterion for selecting feasible or optimal cropping systems. Based upon this study net returns of the two alternative cropping systems were not statistically superior to monocrop wheat. Comparison of yield and value of crops needs to be considered in choosing among cropping systems. Findings from this study will provide a tool to assess and determine management of cropping systems.

Acknowledgements

The authors are grateful to Carla Goad and Jon Biermacher for assistance with statistical and net return analyses, and to Ray Sidwell and staff of the North Central Research Station at Lahoma, OK, for technical assistance and support. Published with the approval of the Director of the Oklahoma Agricultural Experiment Station, Oklahoma State University, Stillwater, OK 74078. This work was supported in part by the Oklahoma Agricultural Experiment Station and the Oklahoma Soybean Board.

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