Wheat ( Triticumaestivum L . ) Cultivar Selection Affects Double-Crop and Relay-Intercrop Soybean ( Glycinemax L . ) Response on Claypan Soils

Field research (2003–2005) evaluated the effect of wheat row spacing (19 and 38 cm) and cultivar on double-cropped (DC) soybean response, 38-cm wheat on relay-intercrop (RI) response, and wheat cultivar selection on gross margins of these cropping systems. Narrow-row wheat increased grain yield 460 kg ha−1, light interception (LI) 7%, and leaf area index (LAI) 0.5 compared to wide rows, but did not affect DC soybean yield. High yielding wheat (P25R37) with greater LI and LAI produced lower (330 kg ha−1) soybean yields in an RI system than a low yielding cultivar (Ernie). Gross margins were $267 ha−1 greater when P25R37 was RI with H431 Intellicoat (ITC) soybean compared to Ernie. Gross margins were similar for monocrop H431 non-coated (NC) or ITC soybean, P25R37 in 19or 38-cm rows with DC H431 NC soybean, and P25R37 in 38-cm rows with RI H431 ITC soybean in the absence of an early fall frost.


Introduction
Claypan soils cover approximately 4 million hectares in the Midwestern U.S.These soils are poorly drained partially because of an argillic claypan layer 15 to 24 cm below the soil surface, and they are highly erodible due to slopes up to 20% and surface water runoff [1,2].Conservation tillage systems are recommended to reduce soil loss [3].Due to economic considerations and relatively strong corn and soybean prices, cropping systems have shifted from a rotation of corn (Zea mays L.) soybeans [Glycine max (L.) Merr.] and wheat (Triticum aestivum L.) to a rotation of corn and soybeans.Small grains such as wheat are important crops to increase water infiltration, organic matter, soil structure, and to reduce soil erosion from surface water runoff [4][5][6].No-till soybeans, double-cropped after wheat, had 75% of the runoff of monoculture, conventional tilled soybeans in upland silty soils [5].Some farmers have used double-cropped soybean production to increase wheat profitability [7][8][9].In the absence of government programs, a risk and return analysis in southeast Kansas favored double-cropped wheatsoybeans over monocropped wheat or soybeans [10].Relayintercrop production has been proposed in the Midwest to reduce production risks of double-cropping.Doublecropping soybeans after wheat is risky above 38 • latitude due not only to low rainfall and dry soils at planting, but also to a relatively short growing season that can be limited by early frost which reduces grain quality and yield [11,12].
Relay-intercrop production involves overlapping growth cycles of two or more crops.This production system is common with legumes seeded into small grains; however, the companion crops may compete for water, nutrients, and sunlight, which may slow development of either crop [13][14][15].Relay-intercropped soybean production involves seeding wheat in the fall and an intercrop seeding of soybeans into standing wheat.This cropping system has been proposed to reduce risk associated with double-crop soybean production, move double-crop production farther north, and increase farm profitability.Consistent relay-intercropped soybean production may qualify farmers for low-risk Federal Crop Insurance because soybeans are planted before the doublecrop soybean cut-off date.
One of the main opportunity costs farmers encounter with relay-intercropping is the risk of seeding wheat in 38cm wide rows and not being able to plant soybeans due to inclement weather conditions.Wheat is typically seeded in narrow rows (15 to 25 cm).Most wheat row spacing research has evaluated 10-to 30-cm wide rows in order to reduce weed competition [21][22][23][24].Beuerlein et al. [25] reported that high-yielding wheat planted in 38-cm wide rows had grain yields similar to a 19-cm row spacing.However, a single wheat cultivar in 41-cm rows yielded 91% of 21 cm rows in other research [13].Differences in light interception and competitiveness among wheat cultivars have been reported in narrow-row wheat [21,24].In 20-cm rows, there was 2% to 5% greater light interception than 80-cm wide row wheat [17], but the impact on early planted relayintercropped soybean cultivars has not been evaluated.High yielding monocropped soybean cultivars were high yielding in a relay-intercropping system since no cultivar × cropping system interaction was detected when soybeans were planted after wheat heading [19].Researchers have evaluated growth of late-planted, relay-intercropped soybean [20], but few studies have looked at wheat canopy development differences for early-planted, relay-intercropped soybean.Reinbott et al. [17] evaluated light interception of 20 and 80-cm row spacings, but no research has evaluated 38cm spacings, which are typical in today's crop production systems.
Since this previous relay-intercrop research was conducted, imidazolinone-tolerant wheat [26], glyphosateresistant soybean [27,28], polymer-coated soybean seed technology [29], and tractor guidance systems have been introduced while widespread use of planters on 38-cm widerow spacings is common.Polymer-coated seed technology regulates germination based on soil temperature and moisture content [29] which may allow earlier relay-intercropped soybean planting dates and reduce mechanical damage to wheat compared to later planting dates.Delayed soybean germination may also reduce interference between wheat and soybeans.However, no research has evaluated the impact of wheat cultivars on light interception and subsequent wheat yields, relay-intercropped soybean response, or the cost-effectiveness of these systems using currently available technology.The objectives of this research were to (1) evaluate the effect of wheat row spacing and cultivar on wheat and double-cropped soybean response, (2) evaluate the effect of wheat cultivar in a relay-intercropping system on wheat and soybean response, and (3) determine the effect of wheat cultivar selection on gross margins of relayintercropped and double-cropped soybeans compared to monocropped soybeans.organic matter.This study included fifteen treatments and was arranged as a randomized complete block design with three to four replications in plots 3 by 12.2 m.Individual treatments are explained in the sections below.

Materials and Methods
Seeding dates and fertilizer application rates are listed in Table 1.Fertilizer was applied according to University of Missouri soil test recommendations [30].Wheat was notill seeded into soybean stubble with a Great Plains Solid Stand 10 (Great Plains Manufacturing Inc., P.O.Box 218, Assaria, KS 67416) grain drill on 19-cm row spacings.Every other row was blocked to seed 38-cm wide rows at 124 kg ha −1 .Soybeans were no-till seeded in 38-cm wide rows at 494,000 seeds ha −1 with a John Deere 7200 (John Deere 7200, Deere and Co., 501 River Drive, Moline, IL 61265-1100) planter.Relay-intercropped soybeans were planted during favorable weather conditions prior to jointing (Zadoks 30) in wheat to avoid mechanical injury to wheat [31].Soybean plots were maintained weed-free with three applications of glyphosate (N-(phosphonomethyl)glycine) (Roundup WeatherMAX, Monsanto Co., 800 N. Lindbergh Blvd., St. Louis, MO 63167) at 0.84 kg ae ha −1 plus diammonium sulfate at 20 g L −1 and manual weed removal while no herbicides were required for wheat.
Soybean height and emergence was evaluated 30 d after planting relay-intercropped and monocropped soybeans.At this time, wheat shoots and soybean plants were counted in two, 30-by 76-cm quadrats in each plot, and the stage of soybean development [35] of each plant was determined.Prior to harvest, a 1.2-m section of soybean row was counted to determine plant population at harvest.Plots were harvested with a small-plot combine (Kincaid Equipment Manufacturing, P.O.Box 400, Haven, KS 47543) and moisture was adjusted to 130 g kg −1 prior to analysis.Wheat grain samples were collected and evaluated for test weight (GAC 2100, DICKEY-john Corporation, 5200 DICKEY-john Rd., Auburn, IL 62615).Data were subjected to ANOVA using the SAS general linear model procedure (PROC GLM) [36].

Wheat Row Spacing and Double-Crop Soybeans.
The experiment included a factorial arrangement of wheat row spacing (19 cm and 38 cm) and cultivar ["Pioneer 25R37" (Pioneer Hi-Bred International, Inc., PO Box 1000, Johnston IA 50131-0184), "AgriPro 502CL" (AgriPro Seeds, Inc., P.O.Box 30, Berthoud, CO 80513) and "Ernie"] which was followed by double-crop H431 (Hubner Seed Co., Inc., West Lebanon, IN 47991) noncoated (NC) soybeans.Doublecropped soybeans were planted in 38-cm rows the same day wheat was harvested.Pioneer 25R37 was selected as a high yielding cultivar in the region, and Ernie was a common public soft red winter wheat cultivar [37].Since AgriPro 502CL was a new imidazolinone-tolerant cultivar with limited seed supply at the initiation of this experiment, we evaluated light interception only in the P25R37 and Ernie cultivars.Incident and diffused photosynthetically active radiation (PAR) measurements were utilized as a nondestructive method to measure soybean leaf area index (LAI) and evaluate differences in crop canopy development [38,39].Five PAR measurements were recorded at ground level with a one-m SunScan Canopy Analysis System (Dynamax, Inc., 10808 Fallstone Road Suite #350, Houston, TX 77099) near solar noon positioned diagonal to three 38 cm rows and five 19 cm rows on 11 June each year.A simultaneous measurement of incident PAR, time, and zenith angle was recorded to calculate percent light interception (LI) and LAI.
All data were combined over years since there was no 3-way interaction (year × wheat row spacing × wheat cultivar).In the absence of an interaction between these factors, we present main effects for wheat cultivar and row spacing.All means were separated using Fisher's Protected LSD (P = .1).

Gross Margins of Monocropped, Relay-Intercropped, and
Double-Cropped Soybeans.Monocropped soybean cultivars (H431 ITC, H431 NC, and DK 38-52) were included along with the previously described factorial treatments to evaluate the cost-effectiveness of wheat cultivar selection on relay-intercrop and double-crop systems compared with monocrop soybean [10].In keeping with previous research [10,12,14], we calculated gross margins for the cropping systems using the product of yield and average receipts for wheat ($0.21/kg) and soybean ($0.32/kg) over the past three years [33,34] minus input costs (Table 2) [32].A single factor ANOVA revealed a significant treatment interaction with years.Wheat yield, soybean yield, receipts, expenses, and gross margin data were subjected to an F Max test for homogeneity at P = .01[40] and combined over years because variances were homogenous.Means were separated using Fisher's Protected LSD at P = .01.

Wheat Row Spacing and Double-Cropped Soybeans.
Ernie and AgriPro 502CL shoot density was 22% to 26% greater in narrow-(19 cm) than a wide-row (38 cm) spacing in late-May (Table 3).This did not cause an interaction in LI or LAI between row spacing and cultivar (Table 4).Narrowrow (19 cm) wheat LI and LAI was 7% and 20% greater, respectively, than wide (38 cm) rows.Subsequently, grain yields increased 470 kg ha −1 in narrow-compared to widerow spacing.This was unlike more recent research in Ohio where wide-and narrow-row wheat yields were similar [25].P25R37 intercepted 4% more light and had a greater LAI than Ernie when wheat was in the milk (Zadoks 75) to dough (Zadoks 85) stage of development [31].This was 43 to 48 d after relay-intercropped soybeans were planted  [32] and returns [33,34]   in this experiment.Differences in light interception among wheat cultivars may affect the ability of wheat to compete with weeds [21,24].Canopy closure in narrow-row wheat may help suppress weeds more than wide-row spacing.Narrow-row wheat spacing has reduced yield of early planted soybeans in other research [17].This was especially true when wheat interfered with soybeans from mid-June until wheat harvest, a critical time in relay-intercropping when wheat and soybeans compete for water, nutrients, and light.As a result of an open canopy in 38-cm wide-row wheat, there may be greater weed densities or more weed species present than in 19-cm row widths.We observed no difference in weed species diversity or density between row spacings.However, in relay-intercropping, this eventuality could be important because, as wheat-row spacing increases, winter annual weeds may reduce wheat grain yields [22,23] and subsequently reduce soybean yields due to additional interference with the soybean plant.Imidazolinone-tolerant wheat cultivars, such as AgriPro 502CL, may allow residual acetolactase synthase (ALS) herbicide applications in wheat [29] that would be beneficial to relay-intercropped soybeans.Wheat cultivar grain yields were ranked P25R37 > Ernie > AgriPro 502CL when averaged over row spacing (Table 4).Wheat grain moisture at harvest was slightly wetter in narrow rows (0.8%) for P25R37 than in wide rows (Table 3), which  could result from increased air circulation through the widerow canopy.Ernie and AgriPro 502CL test weight was 22 to 33 kg m −3 greater in narrow rows compared to wide rows, but test weights did not differ when P25R37 was seeded in a wide-or narrow-row arrangement.This was similar to research in Illinois where test weights were greater in narrow-(20 cm) than wide-row (40 to 80 cm) spacings [13].
Soybean plant population at harvest was lower when Ernie was seeded in 38-cm rows, possibly because the amount of residue in the field affected available soil moisture for soybean plants [41]; but there was no effect of wheat row spacing on double-crop soybean grain yield.Similarly, wheat residue management had no effect on double-crop soybean yield in eastern Arkansas [41].

Wheat Cultivar and
Relay-Intercropped Soybean.The choice of soybean cultivar did not affect wheat shoot density; however, wheat variety selection affected shoot density since P25R37 had a greater shoot density than Ernie where H431 ITC was planted (Table 5).In the relayintercrop system, environmental differences among years affected wheat test weight, moisture, and yield between cultivars.P25R37 had higher test weights, moisture, and yield in 2003 and 2005 when compared to Ernie, but test weights and yields were similar in 2004 when there was more rainfall in May (Table 6) and lower overall yields.However, relay-intercropped soybean yields were 330 kg ha −1 greater when seeded into Ernie than P25R37 (Table 5).This indicated that a high-yielding wheat cultivar such as P25R37 may interfere with soybean and reduce relay-intercropped soybean yield more than a low-yielding wheat cultivar (Ernie).This was probably related to a greater LI and LAI of P25R37 when compared to Ernie in the absence of soybean (Table 4).Soybean cultivar response to wheat cultivars was similar, regardless of which high-yielding (P25R37) or low-yielding (Ernie) cultivar we evaluated; therefore, data were pooled over wheat cultivar (Table 7).Emergence of H431 ITC was delayed 10 to 14 d (visual observation) and was at least one vegetative stage of development [35] later when compared to noncoated H431 or DK 38-52 at this point in time (data not presented).Noncoated H431 were at stage V1 to V2 while coated (H431 ITC) soybeans were either still emerging or at the cotyledon stage of development (VC) 30 d after planting (DAP) soybean.Soybean plant population of DK 38-52 or H431 NC was 55% to 56% greater than H431 ITC 30 DAP (Table 7).Similarly, DK 38-52 and H431 NC plants were 4 to 9 cm taller than H431 ITC when relayintercropped into P25R37 or Ernie 30 DAP.H431 ITC plant heights were taller where P25R37 was planted due to greater elongation between the internodes and narrower stems than Ernie (visual observation).Early in the season, soybeans commonly grow rapidly as indicated by taller plants, when weeds [42] or noncoated, relay-intercropped soybeans are present [20].However, all relay-intercropped plants were shorter than monocropped soybeans at harvest (visual observation), in keeping with other studies [12,[18][19][20].Soybean plant population at harvest remained 13% to 18% greater in noncoated (H431 NC and DK 38-52) cultivars than H431 ITC, but grain yield was ranked H431 ITC > H431 NC > DK 38-52.Relay-intercropped H431 ITC grain yield was 280 to 590 kg ha −1 greater than noncoated soybean.

Gross Margins of Monocropped, Relay-Intercropped, and
Double-Cropped Soybeans.P25R37 and Ernie grain yield was reduced 490 and 770 kg ha −1 , respectively, when H431 NC soybean were relay-intercropped into 38-cm wide-row wheat compared to DC 38-cm wide-row wheat (Table 8).P25R37 wheat grain yield was greater than Ernie in both double-cropped and relay-intercropped production systems.Soybean grain yield was similar among monocropped H431 ITC, H431 NC and DK 38-52 cultivars.Relay-intercropped H431 ITC soybean yield was 64% of monocropped coated or noncoated H431.Soybean yields were 19% to 32% greater when H431 ITC was relay-intercropped into 38-cm wheat than double-cropped H431 NC soybeans following 19-or 38-cm wheat row spacings.However, no significant increase in soybean yield resulted when relay-intercropped H431 NC was compared to double-crop H431 NC.
Although the greatest gross receipts and margins were from P25R37 with double-cropped H431 NC soybeans, gross margins were statistically similar for monocropped H431 NC or ITC soybeans, P25R37 in 19-or 38-cm rows with double-crop H431 NC soybean, and P25R37 in 38-cm rows with relay-intercropped H431 ITC.Early frost did not affect double-cropped soybean yields in this study.Doublecropping soybean with wheat was as profitable or more profitable than monocropping soybean in the Eastern Great Plains and other regions of the U.S. [7][8][9].Gross receipts of P25R37 with relay-intercropped H431 ITC were similar to P25R37 with double-cropped H431 NC soybean, but expenses were $29 ha −1 greater with the relay-intercropping system.The development of relay-intercropped soybeans was similar to monocropped soybeans (visual observation), which indicated there was no risk of early fall frost reducing grain yield or quality which is commonly associated with double-cropped soybeans.

Conclusions
Narrow-row (19 cm) wheat increased grain yield 460 kg ha −1 , light interception 7%, and leaf area index 0.5 when compared to wide rows, while wheat-row spacing did not affect double-cropped soybean yield.A high yielding wheat cultivar (P25R37) with greater LI and LAI had lower (330 kg ha −1 ) soybean yields in a relay-intercropping system than a lowyielding cultivar (Ernie).However, gross margins were $267 ha −1 greater when P25R37, compared to Ernie, was relay-intercropped with H431 ITC.Gross margins were similar for monocropped H431 NC or ITC soybeans, P25R37 in 19-or 38-cm rows with double-cropped H431 NC soybean, and P25R37 in 38-cm rows with relay-intercropped H431 ITC.In an attempt to reduce risk associated with double-cropped soybean production due to an early fall frost and dry conditions at planting, a relay-intercropping system using coated soybeans was as cost-effective as monocropped and double-cropped soybeans, even in the absence of a yield limiting early frost in the three years of this research.Farmers considering relay-intercropped soybean production in order to keep wheat in their crop rotation should utilize highyielding wheat cultivars and coated-soybeans to maximize gross margins.Early-planted, relay-intercropped soybean into 38-cm wide-row wheat production must consider not only the soybean cultivar and seed coat technology, but also a high yielding wheat cultivar.

Table 1 :
Fertilizer rates, application dates, and seeding dates for wheat and soybean cropping systems from 2003 to 2005.

Table 2 :
Custom crop-production expenses used to calculate gross margins.
bFertilizer cost was an average of local distributors in the fall, 2009.c Soybean seed cost was the actual cost at the time of this research.Relative seed cost differences between coated and noncoated soybean were assumed to be similar.

Table 3 :
The effects of wheat row spacing and cultivar on wheat shoot density, wheat grain moisture, wheat grain test weight, double-crop (DC) soybean population, and yield.

Table 4 :
Wheat light interception, leaf area index, and grain yield for row spacing and cultivar main effects.Data were averaged over years.
a Treatment was not evaluated.

Table 5 :
Wheat cultivar shoot density, test weight, moisture, and soybean yield main effects for cultivar in a relay-intercropping system planted in 38-cm wide rows in2003, 2004, and 2005.All data except shoot density were averaged over relay-intercropped soybean cultivars (H431 NC, H432 ITC, and DK 38-52).

Table 6 :
Monthly and average rainfall during the soybean growing season from 2003 to 2005.