HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by MacKown, C. T. Articles by Rao, S. C. Search for Related Content PubMed Articles by MacKown, C. T. Articles by Rao, S. C. Agricola Articles by MacKown, C. T. Articles by Rao, S. C. Related Collections Crop Rotation Systems Other Forage Crops

FORAGE & GRAZINGLANDS

Agronomic Feasibility of a Continuous Double Crop of Winter Wheat and Soybean Forage in the Southern Great Plains

^a USDA-ARS Grazinglands Research Lab., 7207 W. Cheyenne St., El Reno, OK 73036
^b Texas Agricultural Experiment Station, 17360 Coit Rd., Dallas, TX 75252. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture or the Texas Agricultural Experiment Station

^* Corresponding author (Charles.MacKown{at}ars.usda.gov).

	ABSTRACT

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

 
In the southern Great Plains, dryland double-cropping soybean [Glycine max (L.) Merr.] after winter wheat (Triticum aestivum L.) could provide quality summer forage, partially offset mineral fertilizer N applied to winter wheat, and lessen soil erosion. Waiting for wheat grain to mature, however, delays soybean planting and subjects growth to dry and hot conditions. Planting soybean after a hay crop of wheat was investigated to determine the feasibility of the system as a source of livestock feeds and N uptake by both crops. Twelve treatment combinations of two wheat fertilizer N levels (0 and 112 kg N ha^–1) and six summer management treatments (fallow: conventional and no-till; soybean: grazed, cut for hay, green manure, and mulch) were arranged in strips across four replications. Soybean biomass ranged from 1.35 to 1.90 Mg ha^–1 when soybean grazing and harvest occurred at seed fill, and crude protein ranged from 129 to 220 g kg^–1 resulting in a 3-yr average N uptake of 44 kg ha^–1. Within each N fertilizer level, average wheat forage yields were not different, but yield increased 29% with N fertilizer and crude protein was inversely related to yield. Double-cropped soybean failed to offer any yield-enhancing N benefit to wheat or enhance soil N and C content after 3 yr, even when used as a green manure. Unless a producer is willing to accept the low productivity of soybean as a double crop with wheat, the feasibility of this dryland double-crop forage system is limited.

Abbreviations: CT, conventional tillage • MG, maturity group • NT, no tillage

	INTRODUCTION

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

 
WINTER WHEAT (Triticum aestivum L.) planted in the southern Great Plains is grown extensively for forage as well as a grain-only crop (Pinchack et al., 1996; Hossain et al., 2004). Most of the wheat used as forage is grazed from mid-November until early March as a dual-purpose crop (graze plus grain) or from mid-November until early May as full-season pasture. Shortly after heading around mid-April, some producers will cut wheat for a hay crop or silage. After wheat harvest, land is traditionally kept fallow during the summer, using tillage to reduce stubble and control weeds. Sources of summer pasture are mostly native or improved warm-season perennial grasses, although annual warm-season grass may also be used (Dalrymple, 1999). Early in the season, crude protein quality of these perennial grasses is quite good for stocker livestock production, but crude protein concentrations decline as the grasses mature (Perry and Baltensperger, 1979; Mitchell et al., 2001; Arthington and Brown, 2005). Planting a summer annual legume after wheat could provide additional high-quality forage after warm-season grasses begin to decline in quality, provide groundcover to lessen soil erosion, and perhaps provide an organic source of N to partially offset mineral fertilizer N required to produce a wheat crop.

Before 1941, soybean [Glycine max (L.) Merr.] was primarily grown in the USA as a hay crop. Recently there has been a resurging interest in the use of soybean as an annual forage crop in the USA with the release of several forage types (Devine et al., 1998a, 1998b; Devine and Hatley, 1998; Devine and McMurtrey, 2004). Forage evaluations of standard and forage soybean types in the Midwest (Sheaffer et al., 1992; Hintz et al., 1992; Readfern et al., 1999; Sheaffer et al., 2001) and southern Great Plains (Heitholt et al., 2004; Rao et al., 2005) are promising, and while double-cropping a winter cereal and forage soybean may be feasible for the Midwest (Sheaffer et al., 1992), dryland production of soybean after wheat grown for grain in Oklahoma is uncertain (Keim et al., 2003; Rao et al., 2005) given the variable spring and summer rainfall patterns of the southern Great Plains.

According to Keim et al. (2003), unfavorable environmental conditions immediately after the harvest of wheat grain is a critical factor in the production risk of a dryland continuous wheat-soybean double-crop system in Oklahoma. Changing a double-crop system of wheat grown for grain followed by soybean to one that harvests wheat as a hay crop or full-season pasture would allow soybean planting up to 6 wk earlier and leave more soil water (Brown, 1971; Musick et al., 1994) for the soybean crop. Consequently, we investigated the performance of soybean and its impact on productivity of a winter wheat hay crop grown with and without mineral N fertilizer using conventional tillage (CT) and no-tillage (NT) practices. Our research objectives were to assess the feasibility of this dryland double-crop system as a source of livestock feeds by measuring the quantity and quality of soybean forage and to compare the effects of using soybean for grazing, as a hay crop, or as a cover crop on the production and N accumulation of a winter wheat hay crop.

	MATERIALS AND METHODS

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

 
Location and Previous Cropping
Experimental plots were located at the Grazinglands Research Laboratory near El Reno, OK (35.56932 lat;-98.04455 long), on a Norge silt loam (fine-silty, mixed, active, thermic, Udic Paleustoll). The site (1.2 ha) was a winter wheat field that was harvested for grain in early June 1999. Wheat stubble was disked after grain harvest, and disked twice more before planting at the end of June with pigeon pea [Cajanus cajan (L.) Millsp.] intended for a grazing trial, but Holstein steers (Bos taurus L.) preferentially grazed the abundant pigweed (Amaranthus spp.) and crabgrass (Digitaria spp.), leaving the pigeon pea mostly ungrazed. In mid-October 1999, pigeon pea was swathed, baled, and removed from the site before disking. Nitrogen fertilizer treatment plots of 0 and 112 kg N ha^–1 (N0 and N1, respectively) were created by incorporating granular urea before planting winter wheat (cv. Jagger; 112 kg seed ha^–1). Each N treatment plot was 14.6 m wide by 79.1 m long, arranged in a randomized complete block design and replicated four times with 9.8 m alleys between blocks. Alleys were planted with winter wheat but not fertilized with N. Wheat grain was harvested in early June 2000, wheat stubble burned, and pigeon pea NT planted. Because of poor pigeon pea emergence and intense weed pressure, the site was disked in early August. The site was disked and dragged with a harrow, fertilizer N applied to the same plots as before, dragged with a harrow to incorporate the fertilizer, and planted with Jagger winter wheat as before. In early May 2001, winter wheat at anthesis was cut for hay. Hay yield (dry wt.) from the N0 treatment was 7.09 Mg ha^–1, which was significantly less (P < 0.05) than the 7.91 Mg ha^–1 from the N1 treatment.

Summer Management Treatments
Using the existing plots, management treatment plots were randomly assigned as strip plots across the two N treatments. Six management treatments included both CT and NT practices within each replicate of the N strips. The treatments were summer fallow CT, summer fallow NT, soybean grazed NT, soybean hay NT, soybean cover crop CT (green manure), and soybean cover crop NT (mulch). Plot size was 14.6 x 7.9 m except for grazed NT plot size, which was 14.6 x 39.6 m. Three successive double crops of soybean followed by winter wheat were evaluated in 2001/2002 (Year 1), 2002/2003 (Year 2), and 2003/2004 (Year 3). Each year after harvest of the Jagger winter wheat hay crops (during third week of April just after anthesis), all plots were sprayed with glyphosate [N-(phosphonomethyl)glycine; 1.5–3.1 kg a.i. ha^–1, depending on year] for weed control before planting glyphosate-resistant soybean (late Maturity Group [MG] IV in Year 1 [Garst 4888RR] and Year 2 [Garst 472RR]; Garst, late MG III in Year 3 [Midland 9G380RR/STS]). Soybean seeds (73 kg ha^–1) treated with Bradyrhizobium japonicum inoculant were planted in 40-cm rows with a NT drill during the middle of May. The first of at least two disk operations of the summer fallow CT plots occurred immediately after soybean was planted. Summer annual weeds in the NT treatments were controlled with one or more applications of glyphosate (1.5–2.5 kg a.i. ha^–1, depending on year). Soybean aboveground biomass measurements were made just before grazing commenced for a 2- to 3-wk period beginning mid-July to mid-August. Plots intended for hay harvest and plants in unfenced alley areas were cut with a small plot forage harvester (Hege 212, Wintersteiger, Inc., Salt Lake City, UT) and the biomass removed. Soybean plants in the green manure and mulch plots were cut with a sickle bar mower (Troy-Bilt model 34063, MTD Consumer Group, Inc., Cleveland, OH), and the biomass in the green manure plots was incorporated into the soil with disking. After termination of grazing, the remaining soybean crop (mainly stems) was cut with a flail mower (John Deere 370, John Deere, Moline, IL). Early to mid-September all plots were sprayed with glyphosate for weed control, and the CT plots were disked before planting Jagger winter wheat seed (112 kg ha^–1) in 20-cm rows with a NT drill between late September and early October. Mid-November each year the N1 plots received broadcast applications of granular urea at 112 kg N ha^–1.

Forage Measurements
Measurements of aboveground biomass of soybean plants were obtained either by hand harvesting or use of a small plot forage harvester. Two sets of 20 randomly selected plants in each graze NT, green manure, and mulch plot were clipped to a stubble height of 5 cm, oven-dried to constant weight in a forced-air oven at 65°C, weighed, and then ground in a cyclone mill for N analysis. Soybean plant density in each of these plots based on the number of plants in two randomly selected rows, each 1.5 m long, was used to convert the weight of clipped plants into biomass yields. A small plot forage harvester was used to obtain soybean forage biomass measurements for the hay NT plots. Two passes through each plot with the harvester removed 16.4 m² each pass and left a stubble height of 5 cm. For each harvester pass, the harvester weighed the cut soybean hay and a composite subsample was collected and the fresh weight measured in the field before processing for dry weight and N analysis as described above for the hand-harvested soybean plants. Moisture content of the subsample was used to calculate the dry weight of forage obtained with the small plot harvester. The small plot forage harvester was used to obtain wheat hay biomass as described for the soybean hay harvests. Soybean and wheat oven-dried samples were measured for total N using an automated flash-combustion analyzer (LECO CHN1000; Leco Corp., St. Joseph, MI). Both of the two subsamples from each plot were analyzed for total N. Forage crude protein values were calculated by multiplying total N concentration by 6.25.

Soil Measurements
A hand-held soil probe was used to collect soil samples (2.5-cm-diameter cores) for the 0- to 15-cm and the 15- to 30-cm depths. About 25 to 30 cores were taken from each plot and combined by depth within a plot and air-dried for analysis. The Soil, Water, and Forage Analytical Laboratory at Oklahoma State University, Stillwater, analyzed samples collected mid-July 1999. The 0- to 30-cm depth contained about 35 ± 4 kg nitrate N ha^–1 and had nondeficient P and K soil test indexes of 214 ± 41 and 1310 ± 100, respectively (means ± SE of the four replicated blocks). Soil samples were collected as before in mid-July 2004 and air-dried, crushed to pass a 2-mm screen, then extracted with 1 N KCl and analyzed by flow injection (FIAstar 5010 Analyzer, Foss North America, Inc., Eden Prairie, MN) for ammonium N (AN 50/84; Tecator 1984) and nitrate N (AN 62/83; Tecator 1983). Soil total N and C in air-dried samples were measured using an automated combustion analyzer (VarioMax, Elementar Americas, Inc., Mt. Laurel, NJ). The July 1999 samples from the 0- to 15- and 15- to 30-cm depths contained 0.79 ± 0.03 and 0.68 ± 0.02 g total N kg^–1 soil, respectively, and 8.32 ± 0.33 and 7.21 ± 0.22 g total C kg^–1 soil, respectively (means ± SE of the four replicated blocks).

Statistical Analyses
Experimental subunits (N fertilizer treatments and summer management treatments) were arranged in strips across each replication. Both subunits were randomized independently with each replication. Because the arrangement of plots remained the same across three successive double crops, the data were analyzed using a mixed linear model with repeated measures (PROC MIXED) (Littell et al., 1996; SAS Institute, 2005). Replicates and interaction terms with replicate were considered random terms, and all other terms (year, N fertilizer treatment, soybean management treatment) were considered fixed for the ANOVA of soybean and wheat traits. The ANOVA of soil traits was conducted by year using PROC MIXED. Replicates and interaction terms with replicate were considered random terms, and N fertilizer and soybean management treatment factors were considered fixed for the ANOVA of soil traits. Pairwise comparisons of least-square means obtained with the PDIF option were sorted with the pdmix800 macro developed by Saxton (Saxton, 1998).

	RESULTS AND DISCUSSION

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

 
Precipitation for each of the 3 yr of the study was less than or nearly equal that of the 1994 through 2005 average of 814 mm yr^–1 recorded at a Mesonet station within 3 km of the research plots. Yearly precipitation deficits from the 12-yr average were 207 mm in 2001, 23 mm in 2002 and 339 mm in 2003. There were marked differences in the monthly precipitation distribution patterns from the 12-yr average (data not shown), and there were substantial precipitation deficits for the interval between harvest of wheat as a hay crop and the beginning of grazing and cutting the soybean crop: 67 mm (–21%) for 114 d in 2001, 183 mm (–51%) for 123 d in 2002, and 123 mm (–44%) for 81 d in 2003. Compared to the 12-yr daily average, total precipitation between planting and harvest of wheat as a hay crop was 114 mm less (–31%) in 2001/2002 (Year 1), 2 mm more (+ 0.6%) in 2002/2003 (Year 2), and 71 mm less (–20%) in 2003/2004 (Year 3).

Summer management treatment effects were observed on soybean biomass, soybean and wheat protein concentration, and N accumulated by wheat forage (Table 1). Fertilizer N impacted all three wheat forage traits but only affected soybean protein concentration. We anticipated that the level of applied N to wheat would affect how the soybean forage traits responded to cultural practice, but the N application x summer management treatment interaction was not significant. The N application x summer management treatment did affect wheat biomass production, but not the other two wheat forage traits. Nearly all year x summer treatment interactions and year x N rate interactions were significant for both soybean and wheat forage traits. Forage biomass, crude protein, and above-ground N accumulation for the three-way interaction of winter wheat N application x summer management treatment x year were not significant for either soybean or winter wheat crops.

View larger version (166K): [in this window] [in a new window]   Figure 1. Appearance of soybean plants after termination of grazing by cattle. Grazing of plants in the left photograph began after growth stage R5; soybean pods are absent from plants when grazing began before growth stage R3 (beginning pod), as illustrated in the right photograph from a different experiment.

Winter Wheat Forage Responses
Biomass
An understanding of the effects (favorable and unfavorable) of soybean cropping on the productivity of winter wheat grown for forage is also needed to assess the feasibility of this double-crop system. Forage yields of winter wheat among summer management treatments were not consistent across the three successive years of the double-crop sequence. The significant summer management treatment x year interaction (Table 1) was due to low yield of wheat forage for the fallow NT and soybean mulch treatments within Year 1, the low yield of wheat forage for the soybean green manure treatment within Year 2, and a uniform forage biomass among the management treatments in Year 3 (Table 2). Wheat forage yields were affected more by year than treatment and ranged from 4.46 to 9.25 Mg ha^–1. Winter wheat forage yield was 5.44 Mg ha^–1 in Year 1 of the three successive years and corresponded to the greatest preceding soybean crop yield and the least amount of precipitation (114 mm less than 12-yr average) received between planting and the harvest of the wheat crop. These conditions probably reduced subsoil moisture and increased water deficit stress to the wheat crop. The best forage yield was not obtained in Year 2, when precipitation was normal, but was greatest in Year 3 (8.50 Mg ha^–1), with a 71-mm deficit from the 12-yr average. This may have been partly due to the lowest preceding soybean crop yield and a favorable distribution of precipitation and average daily temperature in Year 3 that would promote late winter and early spring growth. Between January 1 and forage harvest on 21 April, Year 3 had 84 mm more precipitation and 15% greater growing-degree-days (base 0°C) than Year 2 (data not shown).

The response of winter wheat forage biomass to summer management was not consistent across N0 (0 kg N ha^–1) and N1 (112 kg N ha^–1) fertilizer levels. The interaction of these effects was significant because the relative biomass differences of each treatment for the two levels of N applied to winter wheat were not constant (Table 1 and Table 3). For example, the relative difference in biomass of wheat forage was 18% for summer fallow treatments compared to 46% for the soybean mulch treatment. Within each N fertilizer level, differences in winter wheat forage yields among treatments were not significant, but as expected the overall average of the N1 treatment produced 29% more biomass than the 6.0 Mg ha^–1 forage yield of the N0 treatment (Table 3). The limited return of N from the soybean forage was insufficient to match the winter wheat biomass response achieved by applying 112 kg N ha^–1 of fertilizer. Compared to the summer fallow treatments the presence of soybean as a green manure crop at even the N0 fertilizer level did not provide a significant N benefit to the following wheat crop. After 3 yr of growing soybean in the same plots the average winter wheat biomass of the soybean green manure and mulch treatments at N0 (7.58 Mg ha^–1) was nearly identical to the 7.57 Mg ha^–1 average of the other summer management treatments at N0 and 24% less than the N1 fertilizer level averaged across all treatments (data not shown).

Crude Protein
During the 3 yr, forage crude protein concentrations shortly after anthesis varied nearly 3.7-fold among the individual plots and ranged from 56 to 209 g kg^–1 with an overall average of 126 g kg^–1. About 22% of the wheat forage samples mostly from N0 plots (data not shown) and Year 3 (Table 4) had a crude protein concentration less than 105 g kg^–1, a forage crude protein level required for growing 225-kg medium-frame calves at an average daily gain of about 0.7 kg d^–1 (National Research Council, 1984). While there was a significant summer management treatment x year interaction (Table 1 and Table 2), differences in forage crude protein among summer management treatments in Year 1, but not Year 2 or Year 3, account for the significant interaction. When forage crude protein values were averaged across N fertilizer levels and management treatments, the values were significantly different and inversely related to forage biomass values (Table 2). As expected, forage crude protein levels of winter wheat fertilized with 112 kg N ha^–1 were greater than those for winter wheat grown without fertilizer N (Table 3). Inclusion of soybean as a double crop did not benefit the crude protein concentration of the following winter wheat forage crop.

Soil Nitrogen and Carbon
Except for one case, soil mineral N (NH₄⁺ + NO₃^–), total N, and total C measured following the last wheat harvest of the three sequential years of double-cropping soybean and winter wheat forage were not significantly different due to amount of N applied to the wheat crop, summer management treatment, or the interaction of these effects (Table 5). The one unexpected exception was less total C measured in the 15- to 30-cm soil depth of the soybean crop cut for hay compared to the soybean manure and mulch treatments (Table 6). We believe contributions of soybean biomass were insufficient and the amount of time for NT to have a positive effect on soil organic matter levels was inadequate to observe any substantial change in the soil concentration of total N and total C. The Norge silt loam soil bulk density was 1.44 g cm^–3 for the 0- to 15-cm depth (data not shown) and based on July 1999 soil analyses would contain about 1.71 Mg ha^–1 total N and 18.0 Mg ha^–1 total C. An upper limit for the contribution of aboveground soybean N to the soil N pool would be 133 kg ha^–1 after 3 yr, which is less than 8% of the soil total N pool in the upper 15 cm of soil. This upper limit is for the soybean manure treatment and assumes soybean N was derived entirely from N₂ fixation and after 3 yr all incorporated N remained in the soil. Similarly for the soybean manure treatment, the contribution of aboveground biomass C after 3 yr was 1.96 Mg ha^–1, or less than 11% of the total C pool in the upper 15 cm of soil, and does not account for substantial losses of C due to mineralization of the soil-incorporated aboveground biomass. The input of aboveground soybean N and C would be substantially less for the other soybean treatments. Extending the seasonal growth period could substantially increase late-maturity soybean cultivars' contribution to aboveground N and C inputs to the soils in the southern Great Plains. The extended season would still provide an opportunity to plant wheat, provided precipitation is sufficient to support additional soybean productivity (Rao et al., 2005).

	CONCLUSIONS

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

	ACKNOWLEDGMENTS

 
The authors are grateful for the technical support of Jeff Weik, Delmar Shantz, Kory Bollinger, and Niel Fobes for their help with field and laboratory work that made this research possible.

	NOTES

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

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication October 25, 2006.

	REFERENCES

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

 

• Arthington, J.D., and W.F. Brown. 2005. Estimation of feeding value of four tropical forage species at two stages of maturity. J. Anim. Sci. 83:1726–1731.[Abstract/Free Full Text]
• Bowen, W.T., J.Q. Quintana, J. Pereira, D.R. Bouldin, W.S. Reid, and D.J. Lathwell. 1988. Screening legume manures as nitrogen sources to succeeding non-legume crops: I. The fallow soil method. Plant Soil 111:75–80.[CrossRef][Web of Science]
• Brown, P.L. 1971. Water use and soil water depletion by dryland winter wheat as affected by nitrogen fertilization. Agron. J. 63:43–46.[Abstract/Free Full Text]
• Dalrymple, R.L. 1999. Crabgrass: A synopsis. p. 1–5. In Crabgrass for forage: Management from the 1990's. NF-FO-99-18. The Noble Foundation, Ardmore, OK.
• Devine, T.E., and E.O. Hatley. 1998. Registration of ‘Donegal’ forage soybean. Crop Sci. 38:1719–1720.[Free Full Text]
• Devine, T.E., E.O. Hatley, and D.E. Starner. 1998a. Registration of ‘Derry’ forage soybean. Crop Sci. 38:1719.
• Devine, T.E., E.O. Hatley, and D.E. Starner. 1998b. Registration of ‘Tyrone’ forage soybean. Crop Sci. 38:1720.
• Devine, T.E., and J.E. McMurtrey, III. 2004. Registration of ‘Tara’ soybean. Crop Sci. 44:1020–1021.[Free Full Text]
• Fehr, W.R., and C.E. Caviness. 1977. Stages of soybean developments. Spec. Rpt. 80. Iowa State University, Ames.
• Heitholt, J.J., D. Kee, J.B. Farr, J.C. Read, S. Metz, and C.T. MacKown. 2004. Forage from soybean provides an alternative to its poor grain yield in the southern Great Plains. Crop Manage. 6 April 2004. Available at www.plantmanagementnetwork.org/pub/cm/research/2004/soybean/ (verified 3 May 2007).
• Hintz, R.W., and K.A. Albrecht. 1994. Dry matter partitioning and forage nutritive value of soybean plant components. Agron. J. 86:59–62.[Abstract/Free Full Text]
• Hintz, R.W., K.A. Albrecht, and E.S. Oplinger. 1992. Yield and quality of soybean forage as affected by cultivar and management practices. Agron. J. 84:795–798.[Abstract/Free Full Text]
• Hossain, I., F.M. Epplin, G.W. Horn, and E.G. Krenzer, Jr. 2004. Wheat production practices used by Oklahoma grain and livestock producers. Available at http://osuextra.okstate.edu/pdfs/B-818.pdf (verified 3 May 2007). Oklahoma Agric. Exp. Stn. B-818. Oklahoma State University, Stillwater.
• Keim, K.R., L.H. Edwards, and J.R. Sholar. 2003. Producing soybean and wheat cropping systems in rainfed environments. Crop Manage. Available at www.plantmanagementnetwork.org/pub/cm/research/2003/rainfed/ (verified 2 July 2007).
• Littell, R.C., G.A. Milliken, W.W. Stroup, and R.D. Wolfinger. 1996. SAS system for mixed models. SAS Inst., Cary, NC.
• Mitchell, R., J. Fritz, K. Moore, L. Moser, K. Vogel, D. Redfearn, and D. Wester. 2001. Predicting forage quality in switchgrass and big bluestem. Agron. J. 93:118–124.[Abstract/Free Full Text]
• Musick, J.T., O.R. Jones, B.A. Stewart, and D.A. Dusek. 1994. Water-yield relationships for irrigated and dryland wheat in the U.S. southern plains. Agron. J. 86:980–986.[Abstract/Free Full Text]
• National Research Council. 1984. Nutrient requirements of beef cattle. 6th ed. Natl. Acad. Press, Washington, DC.
• Perry, L.J., Jr., and D.D. Baltensperger. 1979. Leaf and stem yields and forage quality of three N-fertilized warm season grasses. Agron. J. 71:355–358.[Abstract/Free Full Text]
• Pinchack, W.E., W.D. Worall, S.P. Caldwell, L.J. Hunt, N.J. Worall, and M. Conoly. 1996. Interrelationships of forage and steer growth dynamics on wheat pasture. J. Range Manage. 49:126–130.
• Rao, S.C., H.S. Mayeux, and B.K. Northup. 2005. Performance of forage soybean in the southern Great Plains. Crop Sci. 45:1973–1977.[Abstract/Free Full Text]
• Readfern, D.D., D.R. Buxton, and T.E. Devine. 1999. Sorghum intercropping effects on yield, morphology, and quality of forage soybean. Crop Sci. 39:1380–1384.[Abstract/Free Full Text]
• Salvator, K., and W.E. Sabbe. 1995. Evaluation of the fertilizer value and nutrient release from corn and soybean residues under laboratory and greenhouse conditions. Commun. Soil Sci. Plant Anal. 26:469–484.[Web of Science]
• SAS Institute. 2005. SAS/STAT user's guide, release 9.1.3, Windows version 5.1.2600. SAS Inst., Cary, NC.
• Saxton, A.M. 1998. A macro for converting mean separation output to letter groupings in PROC MIXED. p. 1243–1246. In Proc. 23rd SAS Users Group Intl. SAS Inst., Cary, NC.
• Sheaffer, C.C., G.C. Marten, R.M. Jordan, and E.A. Ristau. 1992. Sheep performance during grazing in a double cropping system. J. Prod. Agric. 5:33–37.
• Sheaffer, C.C., J.H. Orf, T.E. Devine, and J.G. Jewett. 2001. Yield and quality of forage soybean. Agron. J. 93:99–106.[Abstract/Free Full Text]
• Tecator. 1983. Application note (AN 62/83): Determination of the sum of nitrate and nitrite in water by flow injection analysis. Foss North Am., Inc., Eden Prairie, MN.
• Tecator. 1984. Application note (AN 50/84): Determination of ammonia nitrogen by flow injection analysis and gas diffusion. Foss North Am., Inc., Eden Prairie, MN.
• Thönnissen, C., D.J. Mimore, J.K. Ladha, R.J. Holmer, and U. Schmidhlater. 2000a. Tomato crop response to short-duration legume green manures in tropical vegetable systems. Agron. J. 92:245–253.[Abstract/Free Full Text]
• Thönnissen, C., D.J. Mimore, J.K. Ladha, D.C. Olk, and U. Schmidhlater. 2000b. Legume decomposition and nitrogen release when applied as green manures to tropical vegetable production systems. Agron. J. 92:253–260.[Abstract/Free Full Text]
• Zeiher, C., D.B. Egli, J.E. Leggett, and D.A. Reicosky. 1982. Cultivar differences in N redistribution in soybeans. Agron. J. 74:375–379.[Abstract/Free Full Text]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by MacKown, C. T. Articles by Rao, S. C. Search for Related Content PubMed Articles by MacKown, C. T. Articles by Rao, S. C. Agricola Articles by MacKown, C. T. Articles by Rao, S. C. Related Collections Crop Rotation Systems Other Forage Crops