Twenty-Year Biomass Yields of Eight SwitchgrassCultivars in Alabama

David Bransby & Ping Huang

# Springer Science+Business Media New York 2014

Abstract Long-term experiments with perennial cellulosicenergy crops are needed to determine stand longevity andyield trends over time, as affected by weather. Consequently,the objectives of this study were to determine the long-termbiomass yields of eight switchgrass (Panicum virgatum L.)cultivars in Alabama and to evaluate effects of weather vari-ables on annual yields. Two lowland cultivars (Alamo andKanlow) and six upland cultivars (Blackwell, Cave-in-Rock,Kansas-Native, Pathfinder, Summer, and Trailblazer) wereevaluated in a small-plot experiment in central Alabama.Plots were planted in 1989 and fertilized with 84 kg N ha−1

annually, split into two equal applications. The experimentreceived no P and K fertilizer, herbicides, or irrigation. Annualyields were determined by harvesting plots twice each year,from 1990 to 2009. Growing season (March 1 to October 31)precipitation ranged from 405 to 1,294 mm. All plotsremained productive during the experimental period.Average yields were generally higher than those reported fromother experiments and that of Alamo (23.5 Mg ha−1 year−1)was higher than yields of the other seven cultivars. Althoughyields of lowland cultivars varied over time, this variation wasnot related to age of stand or precipitation. In contrast, yieldsof all upland cultivars increased over the first 12 years, andyields of Blackwell, Cave-in-Rock, Kansas-Native, andSummer were positively related to precipitation. It is conclud-ed that under similar soil, environmental, and managementconditions, stands of switchgrass should be productive for20 years or more.

Keywords Switchgrass . Panicum virgatumL . Cultivars .

Biomass yield . Long-term . Precipitation

Introduction

The United States Department of Energy (DOE) recentlyconducted two studies to assess the potential of cellulosicbiomass crops for biofuel production on a national scale [1,2]. Yield of biomass per unit area and stand longevity wereimportant considerations in this assessment, because thesetraits have strong impacts on economic viability of biomasscrops, and the amount of land needed to produce a specifiedquantity of biomass [3]. Projected currently achievable yieldsreported in the DOE studies were developed from results offield experiments, mostly with small research plots, and expertopinion. For switchgrass in the southeastern USA, projectedannual yields ranged from 11.2 to 26.9 Mg ha−1 of drybiomass [1]. The referenced DOE studies also assumed thatstand life of herbaceous biomass crops was 10 years, and thatyields would be relatively stable for this length of time.However, availability of data to support these assumptions islimited. Fike et al. [4] reported 3 years of switchgrass yielddata, starting 7 years after planting, but a complete set of 10-year production records was not provided. The effect of pre-cipitation on switchgrass biomass yields across different siteswas determined byWullschleger et al. [5] from a large dataset.However, impacts of weather variables on yields of switch-grass grown at a single location have not been assessed, andsuch information would be more appropriate to determineweather-related risk for growers who are interested in produc-ing the crop on a commercial scale. Consequently, the objec-tives of this study were to determine the long-term biomassyields of eight switchgrass cultivars in Alabama and to eval-uate effects of weather variables on annual yields.

Materials and Methods

The experimental site was at the Auburn University E. V.Smith Research and Extension Center in south-central

D. Bransby (*) : P. HuangDepartment of Crop, Soil and Environmental Sciences,Auburn University, AL 36849, USAe-mail: bransdi@auburn.edu

Bioenerg. Res.DOI 10.1007/s12155-014-9448-2

Alabama (latitude 32° 30′ N, longitude 85° 40′ W) on aWickham sandy loam (fine-loam, mixed, semiactive,thermic Typic Hapludult) soil. Precipitation occurs throughoutthe year, averaging 1,335 mm on an annual basis. However,short summer droughts are common and are aggravated byhigh rates of evapotranspiration; average daily high tempera-ture is 32.8 °C in July and August.

In spring of 1989, two lowland cultivars of switchgrass(Alamo and Kanlow) and six upland cultivars (Blackwell,Cave-in-Rock, Kansas-Native, Pathfinder, Summer, andTrailblazer) were planted in a randomized complete blocksmall-plot experiment with four replicates. Plots that were1.5 m wide and 6.0 m long were planted with a seed drill ata seeding rate of 11.2 kg ha−1 with 0.2 m between rows.Nitrogen fertilizer was applied at a rate of 84 kg N ha−1 an-nually, split into two equal dressings, one in March andanother after the first harvest. No P and K fertilizer, irrigation,or herbicides were applied over the 20-year period of datacollection. Whole plots were harvested with a mower at aheight of 5 cm in July or August each year, and again inOctober. Biomass harvested from each plot was weighedimmediately after harvesting, following which subsampleswere obtained and weighed before and after drying in a forcedair oven for 72 h at 60 °C to determine dry matter content.

To compare annual biomass yield among cultivars, startingin the year after planting, data were analyzed by analysis ofvariance using the SAS v9.2 PROC MIXED procedure, withyear treated as a repeated measure variable. The heteroge-neous AR(1) was determined to be the appropriate covariancestructure. A critical P value of 0.05 was used as cutoff fortesting fixed effects, and determination of differences amongleast-squares means was based on an adjusted P value obtain-ed by using the option ADJUST = SIMULATE in theLSMEANS statement. To determine effects of age and weath-er variables on biomass yields of each cultivar, stepwiseregression analyses of biomass yield against age, growingseason (1 March to 31 October) precipitation, and date ofthe last frost were conducted with the SAS PROCAUTOREG procedure. Date of the last frost was included asa variable in this analysis because field observations indicatedthat new shoots of switchgrass which emerged after dormancyin winter were often killed by late frosts at the end of winter orin early spring, and it was hypothesized that such events mightaffect yield.

Results and Discussion

Growing season precipitation from 1989 to 2009 (Fig. 1) wasclose to the long-term average (824 mm), except for very highprecipitation in 1989 (1,294 mm, 57 % above average) and2009 (1,294 mm, 57 % above average), and very low precip-itation in 2000 (432 mm, 48 % below average) and 2007

(405 mm, 51 % below average). This low precipitation in2000 and 2007 represented droughts that rank among theworst on record and resulted in total failure of annual cropssuch as corn, cotton, and soybean if farmers did not haveaccess to irrigation [6]. The precipitation extremes that oc-curred during the course of this study made the data collectedparticularly suitable for regression analysis and determinationof biomass production responses to precipitation. Mean min-imum air temperature in mid-winter was −8.8 °C, and meanmaximum temperature in mid-summer was 37.5 °C.Temperatures over 40 °C were recorded in the summers of2000 and 2007, and a minimum temperature of −12 °C wasrecorded in the winter of 2002.

At the end of the 20-year experimental period, there was novisual evidence of disease, stand deterioration, or encroach-ment by weeds in any of the experimental plots. Analysis ofbiomass yield data indicated a cultivar × year interaction(P<0.01; Fig. 2). Notwithstanding this interaction, differencesin average biomass yields among cultivars over the 20-yearexperimental period are important, as are differences amongcultivars in the proportion of biomass produced each year inthe first and second harvests. Average yield for lowland typeswas higher than that of upland types, and within lowland typesyield for Alamo was higher than that of Kanlow (Table 1).Differences in yield among upland cultivars were relativelysmall. Long-term average yields recorded in this study were59 and 64 % higher than the average yields of 8.7 and12.9 Mg ha−1 for upland and lowland types, respectively, asreported from a large number of experiments across the USA[5]. The average annual yield of 23.5 Mg ha−1 recorded forAlamo translates into 470 Mg ha−1 over the 20-year period.These results suggest that the projected stand life of 10 yearsin the DOE studies [1, 2] is conservative, and projectedcurrently achievable yields of up to 26.9 Mg ha−1 are reason-able, especially if it is recognized that (a) cutting twice a yearin this study might have depressed yields of lowland cultivars,compared to cutting once a year [7], and (b) plot size rangingfrom several squaremeters to a commercial scale had no effecton estimates of switchgrass yield [5]. However, the latteranalysis might not be reliable due to inclusion of relativelyfew observations from commercial-scale fields. The propor-tion of annual yield obtained in the first harvest ranged from65.5 % for Alamo to 70.2 % for Kanlow (Table 1). Althoughdata on the time at which reproductive growth occurred fordifferent cultivars was not collected in this study, field obser-vations indicated that this varied considerably, and that earlierreproductive growth of some cultivars probably contributed toa greater proportion of annual yield being obtained in the firstharvest.

Recognizing that switchgrass often does not reach fullproduction until its third growing season [6], biomass yieldsof Alamo and Kanlow in the first year after planting (1990)were remarkably high (Fig. 2). This might have been due to

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the high precipitation which occurred in the year of planting(Fig. 1), resulting in more rapid stand development than usualfor these cultivars. Yield of Alamo and Kanlow varied in asomewhat cyclic pattern over time, but based on regressionanalyses, this variation was not related to precipitation or ageof stand (Table 2). In contrast, average annual yield of uplandcultivars increased by 11.4 Mg ha−1 over the 20-year experi-mental period, from of 9.4Mg ha−1 in 1990 to 20.8Mg ha−1 in2009. This resulted in large differences between upland andlowland types in the first year being transformed to relativelysmall differences by the end of the experimental period(Table 3). Trends evident in Fig. 2 suggest that yield of uplandcultivars was close to a maximum after 12 years (by 2001),following which no further increase was evident. This obser-vation was supported by additional regression analyses for the1990–2001 period, which indicated strong effects of stand ageon biomass yield for upland varieties (Table 4), but no effectfor the 2001–2009 period (regression data not shown). The

increase in yield among upland cultivars over the first 12 yearsof the experiment ranged from 0.36 Mg ha−1 year−1 forTrailblazer to 1.10 Mg ha−1 year−1 for Kansas-Native, withan average of 0.75 Mg ha−1 year−1 for all cultivars (Fig. 2,Tables 3 and 4).

The date of the last frost had no effect on yield, but annualbiomass yields of Blackwell, Cave-in-Rock, Kansas-Native,and Summer were positively related to precipitation: on av-erage, yield of these cultivars decreased by 123 kg ha−1 foreach 25-mm decrease in precipitation (Table 2). These resultsdiffer from those of Wullschleger et al. [5] who found noeffect of growing season precipitation above 600 mm onbiomass yield of switchgrass. However, data from that studywere from multiple sites and were confounded with soil typeand cultivar, whereas our data were from a single site and didnot involve any confounding. Low precipitation in 1992appeared to have marked effects on yield of upland cultivars,but no effect on lowland cultivar yields (Figs. 1 and 2).

Fig. 1 Annual growing season(1 March to 31 October)precipitation from 1989 to 2009and long-term averageprecipitation (837 mm) indicatedas a horizontal line

Fig. 2 Annual biomass yield ofeight switchgrass cultivars from1990 to 2009

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Results in the drought of 2007 were similar, except that yieldof Kanlow was also depressed. However, yields were notdepressed in the severe drought of 2000, possibly due tomore even distribution of the limited precipitation that didoccur during that year (data not shown). Even though yieldsof some upland varieties were reduced in drought years, theeffect of drought was still relatively small. For example, useof the prediction equation in Table 2 for Cave-in-Rock indi-cated that for a 10-year-old stand and a 50 % reduction inprecipitation, from the average of 837 to 418 mm, projectedyield decreased by only 15 %, from 15.1 to 12.9 Mg ha−1. Incomparison, projected yield decreases for cotton, maize, andsoybean were 42, 49, and 50 %, respectively, based onregression analyses of yield from these crops against precip-itation for the same location and period [8]. Therefore, theprojected decrease in switchgrass yields was substantially lessthan that of annual crops over the same range of precipitation.This relatively high tolerance of drought by switchgrass isprobably related to existence of a large, permanent rootsystem [9].

Conclusions

Annual precipitation during the 20-year period spanned by ourresearch varied substantially, from serious drought conditionsto years in which precipitation was well above the long-termaverage. This 889-mm range in growing season precipitationoffered an ideal opportunity for determining weather-relatedrisks associated with production of switchgrass. Results sug-gest that under similar soil, environmental, and managementconditions in the Southeast, stands of switchgrass should beproductive for over 20 years. Alamo provided the highestaverage long-term biomass yields. Yields of all cultivars wereconsiderably higher than the average yields reported from alarge number of experiments across the USA and comparablewith the upper limits of projected currently achievable yieldsreported by the DOE. Annual yields of lowland cultivarsvaried over time, but this variation was not related to standage or precipitation. Yields of upland cultivars increasedsharply with age; that for Cave-in-Rock and Kansas-Nativemore than doubled within the first 12 years of the study. Forsome upland cultivars, lower yields were associated with a

Table 1 Average annual biomass yield from 1990 to 2009 and proportionof yield in the first harvest (expressed as a percent of the total annualyield) for eight switchgrass cultivars

Type Cultivar Yield (Mg ha−1 year−1) First harvest (%)

Lowland Alamo 23.5a 65.5c

Kanlow 18.9b 70.2a

Upland Blackwell 11.8d 67.3abc

Cave-in-Rock 15.4c 66.0bc

Kansas-Native 14.8 cd 66.5abc

Pathfinder 13.6 cd 69.8ab

Summer 15.2 cd 66.3abc

Trailblazer 12.3 cd 69.8ab

Means within each column with different letters differ (P<0.05)

Table 2 Regression estimates and statistics for annual biomass yield as afunction of age (years) and growing season rainfall (mm) for eightswitchgrass cultivars from 1990 to 2009

Type Cultivar Intercept Ageestimate

Rainfallestimate

Regress r2 Totalmodel r2

Lowland Alamo 22.2** −0.1084 0.0040 0.051 0.651

Kanlow 19.7** 0.0283 −0.0003 0.001 0.391

Upland Blackwell 4.9** 0.3778** 0.0030* 0.178 0.610

Cave-in-Rock 6.3** 0.4286** 0.0058** 0.255 0.630

Kansas-Native 5.8* 0.5474** 0.0034 0.174 0.666

Pathfinder 5.9* 0.3589* 0.0049* 0.141 0.464

Summer 7.2** 0.3836** 0.0046* 0.165 0.543

Trailblazer 6.7** 0.2637** 0.0031* 0.330 0.573

*P<0.05; **P<0.01

Table 3 Biomass yields (Mg ha−1) of eight switchgrass cultivars in 1990,2001, and 2009

Type Cultivar Year

1990 2001 2009

Lowland Alamo 34.6a 30.7a 25.1a

Kanlow 23.2b 24.3b 22.5ab

Upland Blackwell 6.8c 16.4d 17.6b

Cave-in-Rock 10.4c 22.4b 21.5ab

Kansas-Native 8.7c 21.7bc 20.4ab

Pathfinder 10.2c 17.5 cd 18.7b

Summer 10.6c 21.0bc 21.4ab

Trailblazer 9.5c 15.9d 18.3b

Means within each column with different letters differ (P<0.05)

Table 4 Regression estimates and statistics for yield (1990–2001) as afunction of age (years) and growing season rainfall (mm) for eightswitchgrass cultivars

Type Cultivar Intercept Ageestimate

Rainfallestimate

Regress r2 Totalmodel r2

Lowland Alamo 27.6** −0.3342 −0.0020 0.012 0.709

Kanlow 15.1** 0.5089 0.0014 0.025 0.515

Upland Blackwell −0.4 0.7457** 0.0075** 0.486 0.691

Cave-in-Rock 0.0 1.0634** 0.0090** 0.502 0.785

Kansas-Native 0.5 1.1403** 0.0058 0.542 0.794

Pathfinder 1.9 0.4953* 0.0092 0.185 0.343

Summer 0.8 0.8114** 0.0101** 0.386 0.683

Trailblazer 4.5 0.3563* 0.0055 0.177 0.338

*P<0.05; **P<0.01

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decrease in growing season precipitation. However, based onthe relative decrease in yield over the same range of precipi-tation, results show that switchgrass is considerably moretolerant to drought than most annual crops.

Acknowledgments The authors express their appreciation to Mr.Stevan Nightengale and Ms. Susan Sladden who both conducted fieldwork and data collection in this project for over 20 years.

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