Switchgrass Biomass and Nitrogen Yield with Over-Seeded Cool-season Forages in the Southern Great Plains

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BioEnergy Research ISSN 1939-1234Volume 6Number 1 Bioenerg. Res. (2013) 6:44-52DOI 10.1007/s12155-012-9225-z

Switchgrass Biomass and Nitrogen Yieldwith Over-Seeded Cool-season Forages inthe Southern Great Plains

Twain J. Butler, James P. Muir,Chengjun Huo & John A. Guretzky

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Switchgrass Biomass and Nitrogen Yield with Over-SeededCool-season Forages in the Southern Great Plains

Twain J. Butler & James P. Muir & Chengjun Huo &

John A. Guretzky

Published online: 17 June 2012# Springer Science+Business Media, LLC 2012

Abstract In dry climates with long, hot summers and freez-ing winters, such as that of the southern Great Plains of NorthAmerica, switchgrass (Panicum virgatum L.) has proven po-tential as a cellulosic bioenergy feedstock. This trial looked atdry matter (DM) and N yield dynamics of switchgrass over-seeded with cool-season legumes and rye (Secale cereale L.),compared to switchgrass fertilized with 0, 56 and 112 kg Nha-1 yr-1 at an infertile and a fertile location. Optimal Nfertilizer rate on switchgrass was 56 kg N ha-1 at the infertilelocation. Legume yield was greater in the first season afterplanting, compared to subsequent years where annual legumeswere allowed to reseed and alfalfa (Medicago sativa L.) wasallowed to grow. This suggests that the reseeding model forannual legumes will not work in switchgrass swards grown forbiomass unless soil seed banks are built up for more than oneyear, and that overseeding with alfalfa may have to be repeat-ed in subsequent years to build up plant populations. Over-seeding rye and legumes generally did not suppress orenhance switchgrass biomass production compared to unfer-tilized switchgrass. However, cumulative spring and fall bio-mass yields were generally greater due to winter and spring

legume production, which could be beneficial for grazing orsoil conservation systems, but not necessarily for once-yearlylate autumn harvest biofuel production systems.

Keywords Switchgrass biomass yield . Nitrogen yield .

Cool-season legumes . Clover . Hairy vetch . Medics . Rye

AbbreviationsDM dry matterOM organic matter

Introduction

Extensive research in the southern Great Plains of NorthAmerica has identified switchgrass as a productive and widelyadapted cellulosic bioenergy feedstock [1]. In the dry, variablerainfall climates of this region, where temperatures often fallbelow 0 °C in the winter but regularly rise above 45 °C in thesummer, switchgrass, a perennial grass native to North Amer-ica, tolerates low soil fertility, responds to low levels of Nfertilizer, and persists under single harvest cultivation onceestablished [1–3]. Although precipitation amounts and distri-bution vary widely from year to year in this semiarid region,with consequent variable switchgrass DM yields [3, 4], thisdeep-rooted bunchgrass has the potential to provide cellulosicbioenergy feedstock in a region that is poorly adapted toannual dryland cropping systems.

Most of what we know about switchgrass production in thesouthern Great Plains comes from forage research. Nitrogen isessential for switchgrass persistence and improved yieldsunder cultivation in the southern Great Plains [3, 4]. Becauseof the high cost of N fertilizer, the practice of interseedinglegumes has been suggested as an alternative source of soil N

T. J. Butler : C. HuoThe Samuel Roberts Noble Foundation,2510 Sam Noble Parkway,Ardmore, OK 73401, USA

J. P. Muir (*)Texas AgriLife Research, Texas A&M System,1229 N. HWY 281,Stephenville, TX 76401, USAe-mail: [email protected]

J. A. GuretzkyDepartment of Agronomy and Horticulture,University of Nebraska-Lincoln,310 Keim Hall,Lincoln, NE 68583-0915, USA

Bioenerg. Res. (2013) 6:44–52DOI 10.1007/s12155-012-9225-z

Author's personal copy

[5] and organic matter, a technique with some success in thecentral and northern Great Plains of North America [6, 7]. Asimilar approach in the southern Great Plains has resulted inlimited or no improvement in the yield of switchgrass com-pared to switchgrass monoculture pastures [8, 9], despitereports of species mixtures improving productivity and year-round stability of forage and bioenergy systems [8, 10–13],persistence [14], and improvement in forage N concentrations[7, 15]. Improving forage productivity by interseeding peren-nial warm-season legumes with growth patterns that matchwarm-season switchgrass has also proven an elusive goal inthe southern Great Plains. Interspecific competition for limitedsoil moisture generally results in poor establishment, weakpersistence, and negligible yields of the legume [2, 16]. Wanget al. [17], in their review of switchgrass mixtures in NorthAmerica, confirmed in general that switchgrass and legumemixtures did not yieldmoreDM than switchgrassmonocultures.

In summary, previous research indicates that mixtures, es-pecially legumes, will likely contribute to productive stands ofswitchgrass in the southern Great Plains if they do not competewith switchgrass during the time of its warm-season growth,and that they produce sufficient biomass themselves to make adifference to total annual yield because of additional springyield [2]. Annual, cool-season species currently used as foragesin the southern Great Plains may fit these criteria because theyare productive [18], and set seed and senesce in early spring asswitchgrass emerges from winter dormancy [8]. This contrastswith mixtures in the northern Great Plains where cool-seasonannual and perennial legume growth overlaps more withgrowth of switchgrass [6, 19]. To date, overseeding switchgrassin the southern Great Plains with cool-season annual legumeshas resulted in minimal legume yields, but has also establishedthat switchgrass production is not affected detrimentally by theoverseeded legumes [8, 11].

The objective of this study was to build upon previousresearch that shows low-input grass-legume systems are per-sistent and productive [10], and that switchgrass biomass yieldsmay benefit from overseeding of cool-season legumes [8]. Ourresearch differs from previous efforts in two ways: (1) weincluded rye along with various annual cool-season legumesin an effort to identify cool-season species that can be over-seeded into dormant switchgrass to improve year-long DM andN yields ha-1, and (2) we undertook this research at a site withlow soil fertility and at another with relatively high soil fertilityto determine how initial fertility interacts with switchgrass/cool-season species competition and N fertilizer application.

Methods and Materials

A randomized complete block design (RCBD) field experi-ment with four replications was conducted at a site in south-central Oklahoma and another in north-central Texas during

the 2007–2010 growing seasons. The Gene Autry location(34°17'N, 96°60'W, elevation 342 m) was on a Dale silt loam(fine-silty, mixed, superactive, thermic Pachic Haplustoll; pH6.8; 42 NO3-N, 246 P, 564 Kmg kg-1; 16.5 g OM kg-1). TheStephenville location (34°17'N, 96°12'W, elevation 370 m)was on a Windthorst fine sandy loam (fine, mixed thermicUdic paleustalfs; pH 7.0; 7 NO3-N, 9 P, 283 Kmg kg-1; 17.4 gOM kg-1). Treatments consisted of a factorial arrangement ofinorganic N fertilizer on monoculture stands of switchgrass,inorganic N fertilizer on cereal rye interseeded into switch-grass, and organic N from annual legumes interseeded intoswitchgrass. During each season (2007-08 and 2008-09), ce-real rye and six legumes were interseeded into switchgrass onapproximately 15 October with a Hege 1000 (Colwich, KS,USA) no-till drill at both locations. In May, four rates of Nammonium sulfate fertilizer (0, 56, 112, 168 kg N ha-1) werebroadcast to monoculture switchgrass plots, and in October tothe plots of cereal rye interseeded into switchgrass, while theplots of legumes interseeded into switchgrass did not receiveany inorganic N. Plot size was 4.7 x 11.8 m. The treatmentswere applied to established ‘Alamo’ lowland switchgrass atStephenville, TX (15 year-old stand) referred to as a lowfertility upland site, and to ‘EG1101’ switchgrass (a selectionout of Alamo) at Gene Autry, OK (2 year-old stand) referred toas a high fertility bottomland site. Based on soil test recom-mendations for the Stephenville site, pre-plant applications of52 kg ha-1 P and 56 kg ha-1 Kweremade prior to establishmenteach year. The Gene Autry site had previously been croppedwith wheat (Triticum aestivum L), and soil test results indicatedno P or K amendments were necessary. Legume entries includ-ed arrowleaf clover (Trifolium vesiculosum Savi ‘Apache’),crimson clover (T. incarnatum L. ‘Dixie’), hairy vetch (Viciavillosa L. ‘AU early cover’), alfalfa (‘Bulldog 505’), buttonmedic (M. orbicularis L. Bartal ‘Estes’), and rigid medic (M.rigiduloides E. Small PI227850), seeded at 9.0, 16.8, 22.4,16.8, 10.1, and 16.8 kg pure live seed ha-1, respectively. Cerealrye was seeded at 84 kg pure live seed ha-1 each season (2007-08 and 2008-09).

In the subsequent seasons (2008-09 and 2009-10) follow-ing establishment years (2007, 2008), the annual legumeswere allowed to reseed and alfalfa to regrow. Initial establish-ment and reseeding success was assessed by measuring le-gume and rye yield in the spring and switchgrass DM yields inthe autumn. Legume DM yields were estimated by harvestingtwo 0.25 m-2 quadrats from each plot when each legumereached 50 % bloom, and the remaining biomass was notharvested to maximize N contribution from the legumes tothe switchgrass. In 2008, plots at both locations were har-vested approximately on 7 Apr, 16 Apr, 1 May, 16 May, 18May, and 1 June for rigid medic, crimson clover, buttonmedic, arrowleaf clover, hairy vetch, and alfalfa, respectively.In 2009, legumes in both the seeded and reseeding plots atboth locations were harvested approximately on 10 Apr, 15

Bioenerg. Res. (2013) 6:44–52 45


Apr, 1 May, 7 May, 15 May, and 20 May for rigid medic,crimson clover, button medic, arrowleaf clover, hairy vetch,and alfalfa, respectively. In 2010, reseeding legumes at bothlocations were harvested approximately on 7 Apr, 19 Apr, 21Apr, 13 May, 15 May, and 25 May for rigid medic, crimsonclover, button medic, arrowleaf clover, hairy vetch, and alfal-fa, respectively. The cereal rye plots were harvested on ap-proximately 2 April each year (2008 and 2009) at bothlocations, and the entire biomass was removed with a Hege(Colwich, KS) plot harvester. Switchgrass was harvested eachyear at both locations on approximately 7 October using aHege plot harvester. In all harvests, forage samples were driedin a forced draft oven set at 60 °C to a constant weight for 3 to4 days and weighed; mass was determined and reported onDM basis. Samples were dried and ground through a Wileymill (Thomas–Wiley laboratory Mill, Thomas Scientific,Swedeboro, NJ, USA) to pass a 1-mm screen and analyzedfor N concentrations using near-infrared reflectance spectros-copy (NIRS) analysis. Nitrogen yields were calculated bymultiplying DM yields by the relevant N concentration.

After the growing season in January 2009, approximately12 soil cores were taken from a 15-cm depth from eachexperimental planting (2007-09 and 2008-09). A soil depthof 15 cm was selected because it is the most common recom-mendation, and because the detection of differences in soil Nand OM from deeper in the profile is unlikely, especially inplots where legumes decomposed on soil surface. This tookplace one season after growth for the 2008 planting and twoseasons after the 2007 planting of legumes. Soil samples wereanalyzed for pH, OM, NO,3-N, P, and K (water soluble) usinga private laboratory (Ward Lab, Kearney, NE, USA).

The 30-year average rainfall from October to Septemberfor Gene Autry, OK was 973 mm (Fig. 1); the percentage ofthat long-term average that fell during the same period was61 % in 2007-2008, 61 % in 2008-2009, and 76 % in 2009-2010. Thirty-year average rainfall from October to Septemberfor Stephenville, TX was 776 mm (Fig. 2); the percentage ofthat long-term average that fell during the same period was81 % in 2007-2008, 87 % in 2008-2009, and 118 % in 2009-2010. During their establishment, the plots received 2.5 cmsupplemental irrigation to promote germination immediatelyafter cool-season annual cover crops were seeded in Octoberat both locations.

Data were separated into sets from the establishmentseason of the cover crop (2007-08 and 2008-09) and fromsubsequent years in which the legumes were allowed toreseed (2008-09 and 2009-10). Statistical analyses for es-tablishment seasons and reseeding seasons were performedusing Proc Mixed of SAS [20]. Establishment year andreplication were treated as random effects, while locationand treatments were considered fixed effects. The piecewisedifferential (PDIFF) function of the least squares means(LSMEANS) procedure was used to compare means for

DM yield and nutritive values. Significance was determinedat P≤0.05.

Results and Discussion

Two years after the experiment started, soil pH, OM, N, P, andK concentrations did not differ among treatments (overseededwith legumes or with rye and N fertilizer, or no overseedingand N fertilizer) at the Stephenville location, which averaged6.6 pH; 2.0 g OM kg-1, 1.0 g NO3-N kg-1, 3.6 g P kg-1, 204.1 gK kg-1 or at the Gene Autry location, which averaged 6.8 pH;1.6 g OMkg-1, 3.7 g NO3-N kg-1, 49.7 g P kg-1, 224.1 gK kg-1 .Due to limited reseeding of legumes (2008-09 and 2009-10),soil samples were not taken in the final year since it is unlikelythat there would be any differences. Soil analyses indicated thatsoil OM and N did not increase with overseeded cool-seasonforage. Soil OM at both locations and N at Gene Autry de-creased relative to initial concentrations; it was likely incorpo-rated into the biomass or volatilized. One of the reasons whytreatment differences were not apparent may be due to the shortinterval between cool-season crop senescence and switchgrassgrowth. Changes in soil characteristics as a result of croppingsystems often require several years to produce measureablechanges [21] and N can be the slowest mineral to release fromlegumes [22]. In the presence of switchgrass, any mineralcontribution of the cool-season crop to the soil would morelikely be incorporated into switchgrass growth, rather thanaccumulated in the soil. Switchgrass is considered to be adap-ted and productive onmarginal soils with lower fertility, similarto the Stephenville site, while high fertility sites similar to GeneAutry are typically best suited to row crops like wheat andsorghum (Sorghum bicolor L.) in this region.

Dry matter accumulation

First year overseeded (2007-08 and 2008-09)

Location interactions were significant; therefore means arereported by location. The application of 56 kg N fertilizerha-1 increased switchgrass yields 67 % at Stephenville, buttripling that rate to 168 kg only increased yields an addi-tional 18 % (Table 1). Muir et al. [3], in their study ofswitchgrass on the southern Great Plains, reported similarswitchgrass yield levels. They also found that the greatestresponse to N fertilizer was detected at the lower fertilizerrates, and reported that this response was most discernablein years and locations with greater rainfall. Overseeding rye,crimson clover and rigid medic without the addition of Ndid not increase the total yield compared to switchgrasswithout an overseeded cool-season crop and no N fertilizerat Stephenville. This result was similar to those reported byBow et al. [8] for cool-season annual legumes in north-

46 Bioenerg. Res. (2013) 6:44–52


central Texas. Likewise, overseeding with N-fertilized rye (56to 168 kg N yr-1) did not increase total yields compared toswitchgrass with no cool-season legume and at comparable Nfertilizer levels. Overseeding with rye at equivalent N fertilizerrates resulted in DM yields during the cool season (rye) ratherthan during the warm season (switchgrass). As a result, ascereal rye yields increased, switchgrass yields decreased pro-portionately. Annual legume yields during the seeding year inthis study were greater than those reported by Bow et al. [8],

who overseeded arrowleaf clover, button medic, and commonvetch (V. sativa L.) onto established switchgrass; legumeyields, however, were considerably less than those reportedby Muir and Bow [18] for the same species when planted ontilled soil. In the current study, overseeding hairy vetch, buttonmedic, arrowleaf clover and alfalfa, by contrast, increasedtotal yields by 55, 47, 54, and 103 % respectively, comparedto no-fertilizer switchgrass grown without an overseededcool-season species. Overseeding switchgrass with hairy

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Bioenerg. Res. (2013) 6:44–52 47


vetch, button medic, and arrowleaf clover produced the sametotal yields as fertilizing switchgrass with 56 kg N ha-1 , whileoverseeding with alfalfa was equivalent to fertilizing switch-grass with 168 kg N ha-1. Bow et al. [8] reported up to 69 %increases in total yields after overseeding cool-season annuallegumes into switchgrass under similar conditions during ayear with 821 mm, but no benefit to overseeding during a yearwith less rainfall.

Compared to Stephenville, switchgrass DM yields weregreater at Gene Autry in plots without added N fertilizer,ranging from 12.5 to 17.8 Mg ha-1 yr-1 (Tables 1 and 2), fargreater than those recorded at other southern Great Plainslocations [Stephenville in the present study, 4, 8]. These yieldswere likely a reflection of fertile soil (2.0 vs. 1.6 g OMkg-1, 42vs. 7 mg NO3-N kg-1 soil, 246 vs. 9 mg P kg-1, and 564 vs.283 mg K kg-1) because the application of N fertilizer in thefirst year did not increase yields relative to no-N fertilizer plotswhether rye was overseeded or not. The same pattern heldwith overseeding rye where no differences in yield weremeasured among rye-fertilized and control plots. Becauserainfall at Gene Autry was less than at Stephenville duringthese years, it is safe to conclude that superior soil fertility atGene Autry compared to Stephenville at the beginning of thestudy, namely 500, 2600, and 99 % greater concentrations ofsoil NO3-N, P and K, respectively, was the principal differ-ence. When switchgrass was not fertilized but overseededwith rye, button medic, arrowleaf clover, crimson clover,and rigid medic, cumulative year-total yields were greater thanswitchgrass by itself by 33, 41, 43, 50, and 32%, respectively.Unlike at Stephenville, however, overseeding switchgrasswith hairy vetch and alfalfa did not improve yields comparedto switchgrass without N fertilizer. These data indicate that

hairy vetch and alfalfa required greater soil moisture than wasavailable at Gene Autry to maximize yields, while the otherlegumes were more drought tolerant — although they likelydid not reach maximum production capacity either. Annuallegume yields at Gene Autry, when compared to sites withsimilar climate, were much greater than those reported else-where for overseeded switchgrass [8], and even for thosereported in ploughed soils [18]; greater by 132 % in the caseof arrowleaf clover.

Self-reseeded and regrowth years (2008-09 and 2009-10)

Rye did not reseed the first year following planting so it wasnot included in DM regrowth analyses (Table 2). Becauseyears (2008-09 and 2009-10) were considered random effects,data were averaged across those two seasons at each location.There were location by treatment interactions; thereforemeans are reported by locations. The application of 56 kg Nha-1 at Stephenville resulted in an average of 52 % greaterswitchgrass DM yield; 112 kg N ha-1 resulted in an additional21 % yield and 168 kg N ha-1 a further 20 %. At Stephenville,switchgrass plots with overseeded alfalfa had the greatestregrowth total, 76 % greater than 0 kg N ha-1 switchgrasswithout any overseeding. Despite alfalfa yielding <1 Mgha-1 yr-1, switchgrass in plots with alfalfa yielded an additional2.3 Mg ha-1 yr-1 over plots with no legume or N fertilizer andthe equivalent yield of switchgrass fertilized with 56 kg Nha-1 yr-1. Alfalfa and switchgrass combined yields were theequivalent of switchgrass fertilized with 112 kg N ha-1 yr-1.Although not to the same degree (1.4 Mg ha-1 yr-1), switch-grass plots overseeded with arrowleaf clover likewise yieldedmore than control plots when legume and grass were

Table 1 Dry matter (DM) yieldof switchgrass fertilized with Nfertilizer or overseeded withlegumes or cereal rye averagedacross the seasons (2007-08 and2008-09) that the cool-seasoncrop was seeded

Stephenville, TX was consideredto be relatively low fertility site,while Gene Autry was consideredto be a relatively high fertility site

Means followed by same letterdo not differ at P<0.05

Stephenville TX Gene Autry OK

Cover crop Switchgrass Total Cover crop Switchgrass Total

kg DM ha-1

Rye+0 N 131 d 3341 de 3429 f 4247 abc 14034 a 16727 ab

Rye+56 N 1322 bcd 4034 cd 4916 cde 5972 a 13231 a 17212 a

Rye+112 N 2734 ab 4206 cd 6028 abc 5285 ab 14096 a 17620 a

Rye+168 N 3034 a 4571 bc 6595 a 5972 a 14153 a 18135 a

0 N - 3261 de 3261 f - 12533 a 12594 b

56 N - 5435 ab 5435 abc - 14544 a 14544 ab

112 N - 5467 ab 5467 abc - 15667 a 15667 ab

168 N - 6431 a 6431 ab - 14115 a 14115 ab

Hairy vetch 1430 bcd 3629 cde 5060 cde 3662 bc 11832 a 15527 ab

Button medic 2093 abc 2709 e 4804 cde 2660 cd 15107 a 17783 a

Arrowleaf clover 2152 abc 2866 e 5019 cde 5634 a 12360 a 18011 a

Crimson clover 1155 bcd 2746 e 3902 ef 2975 bcd 15860 a 18851 a

Alfalfa 2531 ab 4088 cd 6620 a 3191 bcd 13326 a 16517 ab

Rigid medic 438 d 3325 de 3764 ef 1304 d 15295 a 16656 ab

48 Bioenerg. Res. (2013) 6:44–52


combined, comparable to pure switchgrass plots fertilizedwith 56 kg N ha-1 yr-1.

At the Gene Autry location, neither N fertilizer application,legume regrowth, nor reseeding increased yields vis-à-visunfertilized switchgrass plots without legumes (Table 2). Thiswas true even for plots overseeded with alfalfa, despite itscontribution of over 1.5 Mg DM ha-1 yr-1. The combination ofaverage switchgrass yields of nearly 18 Mg ha-1 yr-1 (compa-rable to regional switchgrass yields when fertilized at over200 kg N ha-1 yr-1 and with greater precipitation rates [3]), andno response to N fertilizer, indicated that soil N was stillabundant at this location during all three years of the trial.

Nitrogen concentration and accumulation

First year overseeded

Switchgrass N concentrations at Stephenville were low(Table 3), but similar to those reported by Bow et al. [8] andGuretzky et al. [4] in the southern Great Plains. Neither theapplication of N fertilizer to pure switchgrass plots nor theoverseeding of legumes increased switchgrass N concentra-tions at Stephenville. The only case in which switchgrass Nconcentration was greater than the control plots at Gene Autrywas for switchgrass plots overseeded with rye and fertilizedwith 112 kg N ha-1 yr-1, but as at Stephenville, N fertilizer hadno effect on pure switchgrass stand N concentrations. Theapplication of N fertilizer likewise did not affect N concentra-tion in rye at either location. At Stephenville, hairy vetch hadgreater N concentrations than any other legume, and was 32%greater than arrowleaf clover, which had the lowest concen-tration. At Gene Autry, hairy vetch had 49 % greater Nconcentrations than arrowleaf clover.

Applying 56 kg N ha-1 yr-1 at Stephenville increased Nyields 79% in pure switchgrass compared to no-fertilizer plots

the year of seeding, but additional fertilizer produced noadditional increase (Table 3). Nitrogen yields increased 20-fold as a result of 112 kg N ha-1 yr-1 fertilizer application onrye and 91 % in switchgrass in rye plots compared to the no-fertilizer plots. This culminated in a 244 % increase in total Nyield from switchgrass overseeded with rye and fertilized with112 kg N. Overseeding rye onto switchgrass fertilized with112 kg N ha-1 in February increased cumulative year-total Nyield by187 %, compared to switchgrass by itself at the samefertilizer rate applied in May.

At Stephenville, N yield differed among legume entries.Alfalfa yielded more N than any other legume the first yearafter seeding, followed by buttonmedic and hairy vetch whichdid not differ (Table 3). Rigid medic was the only entry thatdid not improve switchgrass N yields vis-à-vis the unfertilizedswitchgrass-only plots. Switchgrass overseeded with hairyvetch yielded 128 % more N than the no-fertilizer, no-overseed switchgrass. Switchgrass overseeded with alfalfayielded 66 % more N than switchgrass fertilized with168 kg N ha-1. Switchgrass overseeded with hairy vetch,button medic, or alfalfa had N yields greater than those ofswitchgrass fertilized with 168 kg N ha-1, 370 % greater in thecase of alfalfa.

At Gene Autry, overseeding rye onto switchgrass did notincrease subsequent switchgrass N yield compared to theswitchgrass-only control plot, and the application of N madeno difference in N yield in either pure switchgrass or plotsoverseeded with rye (Table 3). When N yields were totaled forrye and switchgrass, all plots overseeded with rye yielded atleast 78 % greater N than pure switchgrass plots, regardless offertilizer application. Switchgrass grown in plots oversownwith arrowleaf clover, crimson clover and alfalfa had greaterN yields than control plots that received 56 kg N ha-1 yr-1. As aresult, overseeding with rye and legumes increased cumulativeyear-long (overseeded component and switchgrass) N yields

Table 2 Dry matter (DM) yieldsof self-reseeding annual legumesoverseeded in switchgrass aver-aged across the self-reseedingseasons of 2008-09 and2009-10

Stephenville, TX was consideredto be a relatively low fertility site,while Gene Autry was consideredto be a relatively high fertility site


Stephenville Gene Autry


kg DM ha-1

0 N - 4282 d 4282 ef - 17889 a 17886 ab

56 N - 6517 c 6517 bc - 18234 a 18231 ab

112 N - 7868 b 7868 b - 17725 a 17725 ab

168 N - 9454 a 9454 a - 17386 a 17386 ab

Hairy vetch 902 a 4754 d 5656 cde 699 b 15609 a 16308 ab

Button medic 132 b 4500 d 4633 def 0 b 15676 a 15676 b

Arrowleaf clover 1196 a 4519 d 5716 cd 605 b 17388 a 18001 ab

Crimson clover 1131 a 4380 d 5513 cde 64 b 16790 a 16855 ab

Alfalfa 969 a 6586 c 7556 b 1574 a 17381 a 18956 a

Rigid medic 0 b 3773 d 3773 f 7 b 18107 a 18114 ab

LSD 665 1112 1398 868 2887 2860

Bioenerg. Res. (2013) 6:44–52 49


compared to switchgrass-only control plots, by 207 kg ha-1 inthe case of arrowleaf clover. Plots overseeded with arrowleafclover yielded 64 % greater N ha-1 yr-1 than those overseededwith rigid medic and 34 % greater N ha-1 yr-1 than thoseoverseeded with rye when no N fertilizer was applied.

Self-reseeded and regrowth years

At Stephenville, self-reseeded populations of button medicand regrowth of alfalfa had 68 and 61 % greater N concentra-tion, respectively, than arrowleaf clover. Arrowleaf and crim-son clover had the lowest N concentration of all legumes(Table 4). At Gene Autry, hairy vetch and alfalfa had 51 and32 % greater N concentrations, respectively, than arrowleafclover. At Stephenville, switchgrass grown following crimson

clover or with alfalfa had 23 % greater N concentrations thanswitchgrass grown in control plots without N fertilizer. Nodifferences in switchgrass N concentration were measured atGene Autry or among switchgrass control plots with any Nfertilizer levels at Stephenville.

Except for rigid medic that failed to reseed and buttonmedic that reseeded poorly, there were no differences in Nyield among regrowth or reseeded legumes at Stephenvillewhere yields among the remaining four entries averaged29 kg N ha-1 yr-1 (Table 4). At Stephenville, switchgrassgrown with alfalfa yielded the equivalent N as switchgrassthat received 168 kg N ha-1 as fertilizer. Combined switch-grass and crimson clover or alfalfa N yields at Stephenvillewere greater than those for switchgrass alone with 168 kg Nha-1 fertilizer. Combined switchgrass and hairy vetch or

Table 3 Nitrogen (N) concen-tration and accumulation ofswitchgrass fertilized with Nfertilizer or overseeded withlegumes or cereal rye averagedacross the 2007-08 and2008-09 establishmentseasons

Stephenville, TX was consideredto be relatively low fertility site,while Gene Autry was consideredto be a relatively high fertility site




N concentration

g N kg-1 DM

Rye+0 N 13.9 d 6.7 ab - 14.2 c 10.2 ab -

Rye+56 N 13.8 d 7.2 a - 14.1 c 9.8 ab -

Rye+112 N 15.4 d 7.4 a - 15.7 c 11.2 a -

Rye+168 N 15.8 d 5.4 ab - 16.2 c 10.7 ab -

0 N - 4.2 b - - 8.2 b -

56 N - 4.6 b - - 8.0 b -

112 N - 5.4 ab - - 8.3 b -

168 N - 4.2 b - - 9.0 ab -

Hairy vetch 35.4 a 6.4 ab - 38.6 a 8.0 b -

Button medic 29.4 bc 6.4 ab - 37.0 a 8.2 b -

Arrowleaf clover 26.9 c 5.6 ab - 25.9 b 9.1 ab -

Crimson clover 31.4 b 5.9 ab - 32.0 ab 8.6 ab -

Alfalfa 32.3 b 6.7 ab - 29.0 b 10.2 ab -

Rigid medic 31.2 b 5.1 ab - 37.0 a 8.5 ab -

Nitrogen accumulation

kg N ha-1 yr-1

Rye+0 N 2 e 23 de 25 de 61 c 171 a 232 b

Rye+56 N 18 d 35 b 43 cd 84 b 168 a 252 ab

Rye+112 N 42 bc 44 a 86 b 83 bc 197 a 280 a

Rye+168 N 48 b 36 ab 84 b 97 b 194 a 291 a

0 N - 14 f 14 e - 103 c 103 d

56 N - 25 d 25 de - 117 c 117 d

112 N - 30 c 30 de - 130 bc 130 d

168 N - 27 cd 27 de - 127 bc 127 d

Hairy vetch 51 b 32 bc 83 b 141 a 124 bc 265 ab

Button medic 62 ab 31 bc 93 b 98 b 145 b 243 ab

Arrowleaf clover 28 cd 28 cd 56 c 146 a 164 ab 310 a

Crimson clover 36 c 23 de 59 c 95 b 163 ab 258 ab

Alfalfa 82 a 45 a 127 a 92 b 169 a 261 ab

Rigid medic 14 de 19 ef 33 d 48 c 141 b 189 c

50 Bioenerg. Res. (2013) 6:44–52


arrowleaf clover N yields were greater than those for switch-grass alone with 112 kg N ha-1 fertilizer.

At Gene Autry, regrowth of alfalfa yielded greater N thanall other reseeded legumes with the exception of hairy vetch(Table 4). At this same location, none of the fertilizer oroverseeded treatments resulted in switchgrass N yields greaterthan the control, again indicating that the 42 mg NO3 kg

-1 soilat this location were sufficient to mask any N-fertilizer bene-fits. Even when regrowth or reseeded legume and switchgrassN yields were combined for the whole year, none of thetreatments were superior to the control.

Conclusions

A comparison of yields between Stephenville and Gene Autrybrings into question the general conclusion reported by otherstudies in the southern Great Plains that rainfall was the mostimportant limiting factor in switchgrass overseeding trials.Rainfall at Stephenville was 19 % greater (120 mm yr-1), onaverage, than at Gene Autry, yet yields of both the cool-seasonspecies and switchgrass were far greater at Gene Autry. Supe-rior soil fertility at Gene Autry, 42 vs. 7 g NO3-N, 246 vs. 9 g P,

and 564 vs. 283 gK kg-1 soil, compared to Stephenville, wasthe likely explanation. The lack of biomass yield response to Nfertilizer application at Gene Autry, when such applicationshad a definite effect at Stephenville, supported this conclusion.Optimal N fertilizer rate on switchgrass, when N was limitingat Stephenville, was 56 kg N ha-1.

Legume yield was greater in the first season after plantingcompared to subsequent years that were allowed to reseed or,in the case of alfalfa, regrow. This suggests that the reseedingmodel for cool-season annual legumes will not always work inswitchgrass swards grown for biomass in the southern GreatPlains. It is unclear as to why legumes did not regenerate whenallowed to reseed. It was also observed that arrowleaf cloverfailed to regenerate in other forage systems during this time-frame, when normally it is considered an excellent reseeder. Inthe case of arrowleaf clover, proper environmental conditions(cool-cloudy weather with high humidity) are needed forsuccessful regeneration. Although rainfall occurred, distribu-tion was erratic and the typical cool temperatures with cloudcover and high humidity were not experienced during theseyears of evaluation. Lack of soil disturbance in perennialwarm-season grass systems grown for biomass (i.e. no hoofaction at soil surface) may be a factor and merits further

Table 4 Nitrogen (N) concen-tration and accumulation ofswitchgrass fertilized with Nfertilizer or self-reseeded withlegumes averaged across theself-reseeding seasons of2008-09 and 2009-10

Stephenville, TX was consideredto be a relatively low fertility site,while Gene Autry was consideredto be a relatively high fertility site




N concentration

g N kg-1 DM

0 N - 3.5 a - - 4.8 b -

56 N - 3.0 a - - 4.3 b -

112 N - 3.2 a - - 5.8 a -

168 N - 3.4 a - - 5.6 a -

Hairy vetch 29.1 b 3.7 a - 39.2 a 5.6 a -

Button medic 36.0 a 3.8 a - - 4.5 b -

Arrowleaf clover 21.4 c 3.5 a - 25.9 d 4.5 b -

Crimson clover 26.4 b 4.3 a - 33.0 c 4.3 b -

Alfalfa 34.4 a 4.3 a - 34.4 bc 4.5 b -

Rigid medic - 3.6 a - 36.0 b 4.3 b -

Nitrogen accumulation

kg N ha-1 yr-1

0 N - 15 e 15 e - 86 ab 86 ab

56 N - 20 c 20 cde - 79 ab 79 ab

112 N - 25 b 25 cde - 102 a 102 ab

168 N - 32 a 32 cd - 97 a 97 ab

Hairy vetch 26 a 18 cde 44 b 27 ab 87 ab 114 ab

Button medic 5 b 17 cde 22 cde - 70 b 70 b

Arrowleaf clover 26 a 16 de 42 bc 16 b 78 ab 94 ab

Crimson clover 30 a 19 cd 49 ab 2 b 73 b 75 b

Alfalfa 33 a 28 b 61 a 54 a 78 ab 132 a

Rigid medic - 14 e 14 e 1 b 82 ab 83 ab

Bioenerg. Res. (2013) 6:44–52 51


investigation. Additional research will show whether severalyears of overseeding may be needed to build up soil seedbanks sufficiently to initiate and sustain adequate self-reseeding. Likewise, seeding additional alfalfa in subsequentyears may increase legume plant populations.

Overseeding rye and legumes generally did not suppressor enhance switchgrass production compared to the unfer-tilized check, regardless of soil fertility or rainfall. Over-seeding cool-season forages or winter biomass feedstockonto dormant switchgrass is therefore a viable option inclimates such as those of the southern Great Plains, wherewinters are mild and the cool-season annuals germinate andset seed earlier than in cooler climates such as the northernGreat Plains. However, cumulative cool-season species andswitchgrass yields were generally greater than switchgrass byitself, due to the contribution of winter production. This addi-tional production could be beneficial for soil conservation andmultiple-use grazing and bioenergy systems, but not neces-sarily for biofuel systems where production cost considera-tions favor single, late-autumn harvests.

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Documents

Switchgrass Biomass and Nitrogen Yield with Over-Seeded Cool-season Forages in the Southern Great Plains