Agronomy Journal 94:509-517 (2002)
© 2002 American Society of Agronomy
PRODUCTION PAPER
Feasibility of Four-Year Crop Rotations in the Central High Plains
Alan J. Schlegel*,a,
Troy J. Dumlerb and
Curtis R. Thompsonb
a Southwest Res.-Ext. Cent., Kansas State Univ., Tribune, KS 67879
b Southwest Area Ext., Kansas State Univ., Garden City, KS 67846
* Corresponding author (aschlege{at}oznet.ksu.edu)
Received for publication April 26, 2001.
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ABSTRACT
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In the central Great Plains, there has been an increase in summer crops, such as grain sorghum [Sorghum bicolor (L.) Moench] or corn (Zea mays L.), grown in a wheat (Triticum aestivum L.)–summer crop–fallow rotation. This field study quantified the effects of increasing cropping intensity beyond a 3-yr rotation on soil water dynamics and wheat and grain sorghum production and profitability and determined the grain yields necessary to ensure greater profitability over 3-yr rotations. Cropping systems evaluated were wheat–wheat–sorghum–fallow (WWSF), wheat–sorghum–sorghum–fallow (WSSF), and continuous wheat (WW). Available soil water (ASW) at wheat planting was 82 mm greater following sorghum than following wheat; however, fallow efficiency decreased from 50% following wheat to 20% following sorghum. At sorghum planting, ASW was 22 to 32 mm greater following wheat than sorghum. Before sorghum, soil water accumulation during fallow was much greater following wheat than sorghum (168 vs. 60 mm). Fallow efficiency was also greater (34 vs. 23%). Wheat yields were 43% (1.2 Mg ha-1) greater following sorghum than wheat. Grain sorghum yields were 36% (1.5 Mg ha-1) greater following wheat than sorghum. Production costs were similar for all rotations. The WSSF rotation had the highest net returns, about $95 ha-1 compared with $92 ha-1 for a simulated WSF rotation, $76 ha-1 for WWSF, and $49 ha-1 for WW. In this study, grain yields required to make 4-yr rotations more profitable than a 3-yr WSF rotation were recrop sorghum yields of 3.5 to 4.0 Mg ha-1 and recrop wheat yields of 2.5 to 3.0 Mg ha-1.
Abbreviations: ASW, available soil water • NT, no-till • WCF, wheat–corn–fallow • WF, wheat–fallow • WSF, wheat–sorghum–fallow • WSSF, wheat–sorghum–sorghum–fallow • WUE, water use efficiency • WW, continuous wheat • WWSF, wheat–wheat–sorghum–fallow
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INTRODUCTION
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IN RECENT YEARS, cropping intensity in dryland systems has increased in the central Great Plains. The traditional wheat fallow (WF) system is being replaced with more intensive systems, such as wheat–summer crop–fallow. The primary summer crops are grain sorghum and corn. From 1990 to 1999, wheat grown on fallow land in western Kansas decreased about 25% from 1.42 to 1.07 million ha (Kansas Farm Facts 1991, 2000). During the same period, dryland grain sorghum increased from 200000 to 405000 ha and dryland corn from 10000 to 195000 ha. There was also an increase in the amount of continuous crop wheat, increasing from 149000 ha in 1990 to 179000 ha in 1999. The inherently low precipitation and high evaporation potential in the Great Plains limits dryland crop production. Adoption of conservation tillage practices that improve precipitation capture and reduce evaporation loss has allowed for more intensive cropping systems (Norwood et al., 1990; Norwood, 1992; Norwood 1994; Unger, 1984; Peterson et al., 1996; Peterson et al., 1999). Most producers are required to maintain residue cover to meet conservation compliance requirements of the 1996 Farm Bill.
Several studies have addressed the economics of intensive cropping systems. In a review of economic studies of dryland cropping systems in the Great Plains, Dhuyvetter et al. (1996) found that increasing cropping intensity and reducing tillage generally increased producer profitability. Dhuyvetter and Norwood (1994) and Williams (1988) found that WSF had higher returns than WF and that reduced tillage was more profitable than conventional tillage in western Kansas. Peterson et al. (1993) found that a wheat–corn–fallow (WCF) rotation was more profitable than a WF rotation in northeast Colorado. With improved grain yields, more crop selection, and existing regulatory requirements, the trend towards more intensive cropping systems using conservation tillage is expected to continue. The question arises, though, as to whether cropping systems more intensive than two crops in 3 yr will be economically feasible. Peterson et al. (1999) found that annualized grain production for 3- (WCF or WSF) and 4-yr [wheat–corn–millet–fallow or wheat–sorghum–sorghum–fallow (WSSF)] rotations was similar in eastern Colorado and doubled grain water use efficiency (WUE) compared with WF. They also found that opportunity cropping (growing a crop each year) maximized production but was less profitable than WCF. There has been little research on 4-yr crop rotations in the central High Plains.
The objectives of this research were to compare and quantify soil water storage, crop water use, crop production, and profitability of 4-yr and continuous cropping systems and to estimate the minimum grain yields required for 4-yr rotations to be more profitable than 3-yr rotations.
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MATERIALS AND METHODS
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The research was conducted in west-central Kansas at the Southwest Research-Extension Center near Tribune from 1996 through 2000. The soil was a Richfield silt loam (fine, smectitic, mesic Aridic Argiustoll) with a pH of 7.4 and organic matter content of 15 g kg-1. Average climatic data are 405 mm of annual precipitation (62% from May to August), 11°C mean temperature, and 1800 mm of open pan evaporation (April through September). The predominant cropping systems in the region are WF and wheat–summer crop–fallow. The cropping systems evaluated were wheat–wheat–sorghum–fallow (WWSF), WSSF, and continuous wheat (WW). All sorghum and WW were grown using no-till (NT) practices. For wheat following sorghum, reduced tillage was used for 1997 and 1998 and NT for 1999 and 2000. The reduced tillage systems utilized a combination of herbicides and tillage (sweep plow with three 1.5-m V-blades) for weed control during fallow, whereas NT relied solely on herbicides {glyphosate [N-(phosphonomethyl)glycine] and 2,4-D (2,4-dichlorophenoxyacetic acid)}. For in-crop weed control, atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) (0.84 kg ha-1) and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] (2.24 kg ha-1) were applied pre-emerge for sorghum, and metsulfuron-methyl {methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)-amino]carbonyl]amino]sulfonyl]benzoate} (7 mg ha-1), dicamba (3,6-dichloro-2-methoxybenzoic acid) (70 mg ha-1), and 2,4-D (70 mg ha-1) were applied spring topdress for wheat. Plot size was 12.2 by 36.5 m. The experimental design was a randomized complete block with four replications. All phases of each rotation were present each year.
Winter wheat was planted in September at a rate of 56 kg ha-1. In 1997–1998, ‘Tam 107’ was planted with a John Deere 9400 hoe drill with 30-cm row spacing. In 1999–2000, ‘Tam 110’ was planted with a John Deere 750 single-disc drill with 19-cm row spacing. In 1996, grain sorghum (‘Pioneer 8875’) was seeded at 3.4 kg ha-1 in 38-cm rows with a John Deere 750 drill in early June. For other years, grain sorghum (‘Pioneer 8771’ in 1997 and 1998 and ‘Pioneer 8699’ in 1999) was planted at 80000 seeds ha-1 in 76-cm rows with a John Deere 7300 planter in late May or early June. Starter fertilizer [monoammonium phosphate (NH4H2PO4), 11–52–0] was band-applied with the wheat seed each year at a rate to supply 10 and 46 kg ha-1 N and P2O5, respectively. Ammonium polyphosphate (10–34–0) was band-applied near the sorghum seed at planting in 1999 to supply 9 and 31 kg ha-1 N and P2O5, respectively. Fluid fertilizer (urea ammonium nitrate, 28% N) was broadcast-applied in early spring for both sorghum and growing wheat at 67 and 90 kg ha-1 in 1996–1997 and 1998–2000, respectively.
Soil water content was measured gravimetrically at planting and after harvest of each crop to a depth of 1.8 m in 0.3-m increments. Soil water is reported as available soil water (ASW), i.e., soil water content minus wilting-point moisture content (-1.5 MPa matric potential). Crop water use was calculated as soil water depletion (soil water at planting less soil water at harvest) plus growing season precipitation (precipitation received between planting and harvest soil water determinations). Water use efficiency was calculated as grain yield (kg ha-1) divided by crop water use (mm). Fallow accumulation of soil water was calculated as soil water at planting less soil water at harvest of previous crop. Fallow efficiency was calculated as fallow accumulation divided by precipitation received during fallow (time period between soil water determinations of harvest of previous crop and planting of current crop).
The center of each plot was combine-harvested in late June or early July for wheat and in October for grain sorghum. Harvest width was 1.9 m for wheat and 1.5 m for sorghum. Grain yields were adjusted to 125 g kg-1 moisture. Yield components (spikes m-2, spikes head-1, and 1000-kernel weight) were determined from 1998 to 2000. Aboveground biomass samples were taken at harvest, dried, and weighed. Wheat straw and sorghum stover were calculated as aboveground biomass minus grain yield. Surface residue cover was determined after planting by the line transect method.
Analysis of variance was performed to evaluate treatment effects on dependent variables using the GLM routine of SAS (SAS Inst., 1996). Single degree-of-freedom contrasts were used to determine differences among crop rotations.
An economic analysis compared the relative costs and returns for each system. Costs for tillage, herbicide applications, planting, and harvest were based on average custom rates for western Kansas (Kansas Custom Rates, 2000). Seed and herbicide expenses were based on local costs. Fertilizer N cost was $0.44 kg-1, and fertilizer P was $0.55 kg-1 P2O5. Grain prices used in the budget were the average prices at harvest from 1996 to 2000 in western Kansas. Gross income was calculated each year by multiplying crop yield by harvest grain prices. Government program payments under the 1996 Farm Bill and land costs were not included because they have no effect on the relative profitability of the various systems. However, participation in the farm program was assumed because minimum grain prices were set equal to local loan levels when cash prices were less than loan rate. Wheat–sorghum–fallow plots were not established in the experiment. However, for sake of comparison with WWSF, WSSF, and WW, net returns for WSF were calculated. For this calculation, we assumed that the wheat yields in WSF would be the same as the yield of the first wheat crop in WWSF and the sorghum yields the same as the first sorghum crop in WSSF. It is possible that actual yields could be different in WSF than our calculated yields, either higher or lower, due to changes in disease infestation or water storage dynamics, but for the sake of the economic comparison, we used the calculated WSF yields.
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RESULTS AND DISCUSSION
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Precipitation
Water is the most limiting factor for production of dryland crops in the central Great Plains, so precipitation during fallow and the growing season greatly influences grain yield. Growing season precipitation for wheat was below normal in 2 of the 4 yr while for sorghum, it was above normal each year (Table 1). In 1996, growing season precipitation for sorghum was more than twice the 30-yr average. Long-term growing season precipitation averages 40 mm more for wheat than for sorghum. However, during this study, growing season precipitation was 68 mm more for sorghum than for wheat. Fallow precipitation varied considerably from year to year but was above the 30-yr average for all crops except 1996 sorghum. In contrast to growing season precipitation, fallow precipitation was relatively greater before wheat than sorghum. Averaged across years, fallow precipitation was above normal by 80% (102 mm) for wheat following wheat and 32% (118 mm) for wheat following sorghum compared with 6% (11 mm) above normal for sorghum following sorghum and 37% (121 mm) for sorghum following wheat.
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Table 1. Precipitation during fallow periods and growing season near Tribune, KS, and the long-term means for each period.
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Soil Water
Available soil water at wheat planting was greater for wheat following fallowed sorghum stubble than wheat following wheat in 3 of the 4 yr, with an average increase of 82 mm (Table 2). In earlier research at this location, ASW at wheat planting was 56 mm greater with WSF than WW with conventional tillage (Norwood et al., 1990) and 114 mm greater with NT (Schlegel et al., 1999). Similarly, Jones and Popham (1997) reported that ASW at wheat planting in Texas was 49 mm greater with WSF than with WW. With systems having the same length of fallow, there were no differences in ASW at wheat planting. The amount of ASW at wheat planting was greater following fallowed sorghum stubble than following wheat throughout the soil profile but especially below 60 cm (Fig. 1)
. The longer fallow period for wheat following sorghum allowed water movement deeper into the profile while water accumulation during the shorter fallow period following wheat was concentrated nearer the surface. At harvest, profile ASW was similar for all cropping systems at about 40 to 60 mm.
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Table 2. Profile available soil water (ASW) at wheat planting and harvest as affected by cropping system, 1997–2000.
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Fig. 1. Distribution of available soil water (ASW) within the soil profile at winter wheat planting, 1997–2000, Tribune, KS. Capital letter denotes specific crop in the cropping system. WSSF, wheat–sorghum–sorghum–fallow; WWSF, wheat–wheat–sorghum–fallow; WW, continuous wheat.
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The amount of soil water accumulation during fallow before wheat varied widely from year to year (Table 3). More than 200 mm of water was stored from sorghum harvest to wheat planting in 2000 compared with a loss of soil water in 1999 for the same fallow period. In 2 of the 4 yr, more water was stored during the 3-mo fallow following wheat harvest than in the longer fallow (11 mo) of wheat following sorghum. Smika and Wicks (1968) suggested that a shorter fallow period is one approach for increasing water storage efficiency. Averaged across years, there were no differences in fallow soil water accumulation before wheat among any of the cropping systems. This seems to conflict with one of the main purposes of fallow, that of storing soil water. However, this is only a reflection of the dynamics of soil water storage. The soil's ability to store water is inversely related to water content; that is, a relatively dry soil can store more water (and more efficiently) than a relatively wet soil. In the years where fallow accumulation was low for wheat following sorghum, as in 1999 (-59 and -1 mm accumulation), the soil was relatively wet (274–322 mm ASW) at sorghum harvest 2 yr prior in 1997 (Table 4). In contrast, fallow accumulation before 2000 wheat (following sorghum) was more than 200 mm when the soil at previous harvest (1998 sorghum) was relatively dry (only about 80 mm ASW). The efficiency of storing water during fallow is notoriously low, generally about 40% or less in the Great Plains (Peterson et al., 1996). In this study, fallow efficiency of wheat following sorghum (11 mo) ranged from 39% to net losses, with an average of 20%. Similarly, fallow efficiency before wheat in WSF rotations was 17% in Texas (Jones and Popham, 1997) and 23% in southeastern Colorado (McGee et al., 1997). With fallow efficiency of 20%, this means that 80% of the precipitation from sorghum harvest to wheat planting was lost, illustrating the inefficiency of fallow in capturing precipitation. By shortening the fallow period to 3 mo (wheat following wheat), fallow efficiency was increased to about 50%, but this still represents a loss of half of the fallow precipitation.
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Table 3. Soil water accumulation and fallow efficiency before wheat planting as affected by cropping system, 1997–2000.
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Table 4. Profile available soil water (ASW) at grain sorghum planting and harvest as affected by cropping system, 1996–1999.
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At sorghum planting, average ASW was 22 to 32 mm greater following wheat than following sorghum (Table 4). Farahani et al. (1998) found similar results in southeast Colorado with 25 mm greater ASW at sorghum planting following wheat than sorghum in a WSSF rotation. This is expected because of the longer fallow period with sorghum following wheat compared with sorghum following sorghum (11 vs. 7 mo). Norwood et al. (1990) reported ASW at sorghum planting in southwest Kansas was 71 mm greater in WSF than continuous sorghum with conventional tillage. In contrast, Jones and Popham (1997) found that ASW at sorghum planting was similar (only a 9-mm difference) for WSF and continuous sorghum averaged across a 10-yr period in Texas. Differences in ASW at sorghum planting were more pronounced in the top 60 cm of soil rather than deeper in the profile as with wheat (Fig. 2)
. As noted earlier, ASW at sorghum harvest varied greatly from year to year, with about 80 mm in 1998 to more than 300 mm for recrop sorghum in 1997. The wet soil at 1997 sorghum harvest was due to precipitation in the fall after the sorghum had matured but before soil water determinations.
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Fig. 2. Distribution of available soil water (ASW) within the soil profile at grain sorghum planting, 1996–1999, Tribune, KS. Capital letter denotes the specific crop in the cropping system. WSSF, wheat–sorghum–sorghum–fallow; WWSF, wheat–wheat–sorghum–fallow.
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Water accumulation in the fallow period before sorghum following wheat ranged from about 250 mm in 1998 and 1999 to a net loss in 1996 (wwSf; capital letter denotes the specific crop in the cropping system) and averaged about 165 mm (Table 5). There were no differences in fallow accumulation for either 4-yr rotation for sorghum following wheat. For the recrop sorghum, fallow accumulation averaged about 35% of that of the longer fallow period (60 compared with 168 mm). In contrast, Farahani et al. (1998) found that fallow accumulation was only 20 mm less for the second sorghum crop compared with the first in a WSSF rotation in southeast Colorado (80 vs. 100 mm). Usually fallow efficiency is greater with shorter fallow periods, but during this study, fallow efficiency was less for sorghum following sorghum than following wheat. Jones and Popham (1997) reported fallow efficiency before NT sorghum was 32% for continuous sorghum compared with 21% in a WSF rotation in Texas. However, in this study, the relatively wet soil at sorghum harvest in 1997 and 1998 and low winter precipitation before1996 sorghum were not conducive for soil water accumulation.
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Table 5. Soil water accumulation and fallow efficiency before grain sorghum planting as affected by cropping system, 1996–1999.
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Grain Yield
Wheat yields were greater following sorghum than following wheat in 3 of the 4 yr (Table 6). Averaged across years, the yield increase from the longer fallow period was 43% (1.2 Mg ha-1). This corresponds to earlier research at this location where wheat yields in WSF were 1.1 Mg ha-1 greater than in WW under conventional tillage (Norwood et al., 1990) and 1.4 Mg ha-1 greater than in WW under NT (Schlegel et al., 1999). Grain yield of continuous and recrop wheat were similar to the county average yield (2.7 Mg ha-1) of wheat following fallow (Kansas Farm Facts, 1997, 1998, 1999, and 2000). Evaluation of yield components showed that the number of spikes per meter and kernels per spike were greater for wheat following sorghum than following wheat (Table 7). Kernel weight was not affected by cropping system. Wheat yields and yield components were similar for WW and wWsf. This indicates that the lower yield of wheat following wheat compared with following sorghum occurs in the first year of WW and does not change with additional wheat crops. Earlier work found that wheat yields in WW tended to decrease across time relative to WSF because of increased infestations of winter annual grasses (Schlegel et al., 1999).
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Table 7. Yield components of winter wheat (1998–2000) and grain sorghum (1998–1999) as affected by cropping system averaged across years.
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Grain sorghum yield was greater following wheat than following sorghum in all years (Table 8) and was greater than the county average yield of 4.6 Mg ha-1 (Kansas Farm Facts, 1997, 1998, 1999, and 2000). The yield reduction for the second sorghum crop (wsSf) compared with sorghum following wheat ranged from 15 to 48% (1.1–2.6 Mg ha-1), with an average of 30% (1.7 Mg ha-1). Norwood et al. (1990) reported sorghum yields were 1.1 Mg ha-1 less with continuous sorghum than with WSF in southwest Kansas while Jones and Popham (1997) found that yields in Texas were only 0.6 Mg ha-1 less with continuous sorghum than with WSF. The number of kernels per meter was greater for sorghum following wheat, but other yield components were similar for all systems. Plant population was about 9% greater for sorghum following wheat (4.5 plants m-2) than following sorghum (4.1 plants m-2). There were no differences in grain yield for sorghum following wheat in any year, but yields averaged across years were 0.55 Mg ha-1 greater with wSsf than with wwSf.
Crop Water Use and Water Use Efficiency
Water use by wheat was greater for wheat following sorghum than following wheat (Table 9). The magnitude of the increase (90 mm) corresponded to the greater (82 mm) amount of ASW at planting (Table 2). Water use efficiency was also greater (22%) for wheat following sorghum than following wheat. Jones and Popham (1997) reported greater WUE for wheat in WSF or WF than in WW while Norwood (1992) reported no difference in WUE for WSF, WF, or WW.
Water use by sorghum was similar in 2 yr in all systems; however, averaged across years, water use was greater following wheat than sorghum (Table 10). There were no differences in water use for wSsf and wwSf. In comparing crops, water use was about 90 mm less for sorghum (following wheat) than for wheat (following sorghum). Similar to wheat, WUE by sorghum was greater with a longer fallow period. Crop WUE was greater for grain sorghum than for wheat. For instance, WUE was 13 kg ha-1 mm-1 for sorghum (following wheat) compared with 8 kg ha-1 mm-1 for wheat (following sorghum). Similarly, Khan (1996) found that WUE of wheat was about 53% that of sorghum. The inclusion of a crop with greater WUE, such as sorghum, in a rotation with wheat increases the WUE of the entire cropping system. The cropping system WUE increased with increased frequency of sorghum in the rotation from 6.5 kg ha-1 mm-1 for WW to 9.0 and 10.6 kg ha-1 mm-1 for WWSF and WSSF, respectively. Inclusion of other summer crops, such as corn, in intensified cropping systems may further increase WUE. Norwood and Currie (1997) found 14% greater WUE for corn than sorghum under NT management in a wheat–summer crop–fallow system in southwest Kansas.
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Table 10. Crop water use and water use efficiency (WUE) of grain sorghum as affected by cropping system, 1996–1999.
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Straw and Stover Production
Wheat straw at harvest ranged from 6.3 to 8.2 Mg ha-1, with no significant differences among systems (Table 11). The ratio of straw or stover to yield ranged from 2.02 to 2.90 kg kg-1, considerably greater than the standard value of 1.7 kg kg-1 used by the Natural Resources Conservation Service (USDA-NRCS, 2000). The ratio of straw or stover to yield was about 40% greater for wheat following wheat than following sorghum. Similar to wheat, sorghum stover production was similar for all systems. Also similar to wheat, the ratio of straw or stover to yield for sorghum of 1.46 to 1.71 kg kg-1 was greater than the commonly used value of 1 kg kg-1 (USDA-NRCS, 2000). Wheat provided more residue cover for the subsequent crop at planting than did sorghum. For example, residue cover at wheat planting following sorghum averaged 18% compared with more than 50% following wheat. This is a reflection of the shorter fallow period for wheat following wheat than sorghum (3 vs. 11 mo) and the fact that more sorghum stover is required than wheat straw to provide the same amount of residue cover on the soil surface. For instance, 30% residue cover requires about 600 kg ha-1 of wheat straw compared with 1100 kg ha-1 of sorghum stover (USDA-NRCS, 2000). Because residue cover at wheat planting following sorghum is relatively low even with NT management, it is important in tilled systems to minimize tillage in the fallow period after sorghum and before wheat to maintain as much residue cover as possible for reducing wind erosion potential. From a cropping system perspective, the low residue cover conditions at wheat planting following sorghum occur less frequently in the longer rotations (1 of 4 yr with either WWSF or WSSF compared with 1 in 3 yr with WSF), providing a potential erosion control benefit to the longer rotations.
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Table 11. Wheat straw and sorghum stover production and surface residue cover at planting of winter wheat (1998–2000) and grain sorghum (1998–1999) as affected by cropping system.
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Production Costs and Economic Returns
Most production costs were similar for WW and both 4-yr rotations (WWSF and WSSF). Differences in costs between wheat following sorghum in WWSF and WSSF rotations and recrop wheat in WWSF and WW rotations can primarily be attributed to higher pre-crop weed control costs for wheat following sorghum (Table 12). However, there was also a decrease in tillage and harvest costs for recrop wheat in the WWSF and WW rotations. Total cost differences were about $35 ha-1. Even greater differences exist between sorghum following wheat in WWSF and WSSF rotations and recrop sorghum (WSSF), with pre-crop herbicide costing $62 ha-1 more for sorghum after wheat than recrop sorghum (Table 13). The reductions in weed control costs for recrop wheat and sorghum offset some of the revenue loss associated with the lower yields for recrop wheat and sorghum.
Economic returns for WW and the 4-yr rotations were largely dependent on yields. An economic analysis, based on representative production costs, average Southwest Kansas crop prices in the harvest month, and observed yields, indicated that the WSSF rotation had the highest average returns. Returns for WSSF were about $3 tillable ha-1 (total of crop and fallow land) higher than a simulated WSF rotation, about $19 tillable ha-1 higher than the WWSF rotation, and about $46 tillable ha-1 higher than the WW rotation (Fig. 3)
. Comparing the 4-yr rotation in the first 2 yr of the study, the WWSF rotation had the highest returns, but in Years 3 and 4, returns were about $49 to $56 ha-1 higher from WSSF than from WWSF. This occurred because recrop wheat yields in the WWSF rotation averaged about half the yield of wheat following sorghum. Isolating the returns of each crop in the WWSF rotation, wheat following sorghum had the highest average return at about $31 tillable ha-1. Sorghum had the second highest return at $29 tillable ha-1 while recrop wheat had the lowest return at about $15 tillable ha-1. In the WSSF rotation, sorghum following wheat had the highest average returns at about $38 tillable ha-1, followed by returns of $36 tillable ha-1 for wheat, and $21 tillable ha-1 for recrop sorghum. Returns for WW averaged $49 tillable ha-1.
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Fig. 3. Net returns for various crop rotations with differing lengths of fallow, Tribune, KS. WWSF, wheat–wheat–sorghum–fallow; WSSF, wheat–sorghum–sorghum–fallow; WSF, wheat–sorghum–fallow; WW, continuous wheat.
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The second part of the economic analysis determined the yield level of recrop wheat and sorghum necessary for the WWSF and WSSF rotations to be more profitable than a 3-yr WSF rotation. To make this determination, wheat yields in a WSF rotation were varied from 1 to 5 Mg ha-1 while sorghum yields and corresponding returns were held constant at the average of the wSsf crop over the 4 yr of the study. Assuming that yields in the first wheat crop of the WWSF rotation would be identical to the WSF wheat yields, Fig. 4
shows the recrop wheat yields in the WWSF rotation necessary to equal returns of the WSF rotation. For example, with the price of wheat at $0.121 kg-1, a recrop wheat yield of 2.83 Mg ha-1 would be needed in a WWSF rotation to equal WSF returns if wheat yielded 3 Mg ha-1 in a WSF rotation. The higher yields in our example are necessary in the recrop wheat because land is essentially being taken away from more profitable sorghum in a WSF rotation to plant second-year wheat in the WWSF rotation.
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Fig. 4. Required recrop wheat yields in a wheat–wheat–sorghum–fallow (WWSF) rotation, at several wheat prices, such that returns equal those of wheat–sorghum–fallow (WSF), based on analyses at Tribune, KS.
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Figure 5
shows a similar analysis for recrop sorghum yields necessary in a WSSF rotation to equal WSF returns. In contrast to recrop wheat, there is more tolerance for reduction in yield of recrop sorghum. For instance, if the WSF sorghum yielded 5 Mg ha-1 at a price of $0.087 kg-1, the recrop sorghum yield needed to equal the return of WSF would be 3.72 Mg ha-1. This occurs because in this study, sorghum was more profitable than wheat.
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Fig. 5. Required recrop grain sorghum yields in a wheat–wheat–sorghum–fallow (WSSF) rotation, at several sorghum prices, such that returns equal those of wheat–sorghum–fallow (WSF), based on analyses at Tribune, KS.
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SUMMARY
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Precipitation was above normal for the study period, which resulted in above-average grain yields. Grain yields averaged 4.0 Mg ha-1 for wheat and 5.4 Mg ha-1 for grain sorghum when grown after 11 mo of fallow. Yields of the second wheat or grain sorghum crop in 4-yr rotations were reduced about 30% compared with the first crops. Wheat yields were the same for the second wheat crop in a WWSF rotation as for WW. At wheat planting, ASW was about 80 mm greater with wheat following sorghum than with continuous or recrop wheat. However, fallow accumulation before wheat was the same for all systems regardless of fallow length. In a WSSF rotation, fallow accumulation was 120 mm greater for the first compared with the second sorghum crop but resulted in only a 20 mm increase in ASW at planting. In 2 yr, precipitation late in the sorghum growing season created relatively high ASW at sorghum harvest, which affected fallow accumulation and ASW at planting of the subsequent crops. Crop water use efficiency was 22% lower for recrop or continous wheat and 11% lower for recrop sorghum. While water use was 22% greater for wheat (following sorghum) than sorghum (following wheat), WUE was 62% greater for sorghum than wheat. Averaged across the entire rotation, WUE increased with increasing amounts of sorghum in the cropping system. The amount of wheat straw and grain sorghum stover at harvest was 20 to 70% greater than the values used by the Natural Resources Conservation Service. Surface residue cover at planting was about 50 to 70% for all crops, providing excellent protection against wind erosion, except for wheat following sorghum where residue cover was 
20%. Production costs were nearly identical for WW and the 4-yr rotations. Economic returns for all rotations were largely dependent on yields. The WSSF rotation had the highest net returns of about $95 ha-1 compared with $92 for a simulated WSF rotation, $76 for WWSF, and $49 for WW. This study found that 4-yr rotations are feasible as indicated by similar returns for WSSF and WSF rotations; however, there is little expectation for producers to intensify their cropping systems without an incentive of greater profitability.
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NOTES
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Contrib. 01-414J from the Kansas Agric. Exp. Stn. Mention of a trade name is for identification only and does not imply endorsement or preference to other products not mentioned.
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ABSTRACT
INTRODUCTION
