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Biomass Yield and Stand Characteristics of Switchgrass in South Central U.S. Environments

^a USDA-ARS-AFSRC, 1224 Airport Rd., Beaver, WV 25813
^b Texas Agric. Res. Station, 1229 North U.S. Hwy. 281, Stephenville, TX 76401
^c USDA-ARS-GRL, 7207 W. Cheyenne Rd., El Reno, OK, 73036
^d Texas Agric. Res. Station, 17360 Colt Rd., Dallas, TX 75252
^e Dep. of Soil and Crop Sci., Texas A&M University, College Station, TX 77843
^f Texas Agric. Res. Station, 3507 HWY 59 E, Beeville, TX 78102

^* Corresponding author (kim.cassida{at}ars.usda.gov)

	ABSTRACT

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

 
Optimizing feedstock production from switchgrass (Panicum virgatum L.) requires careful matching of genotype to environment, especially for southern U.S. regions. Nine genotypes from four combinations of ecotype and morphological type were harvested once yearly in autumn for 3 or 4 yr at five locations across Texas, Arkansas, and Louisiana that varied in latitude and precipitation. Genotypes were evaluated for dry matter yield (DMY), plant density, tiller density, lodging, and rust (caused by Puccinia spp.) infection. Genotype x environment (GxE) interactions were identified for most traits. Biomass yield of all genotypes tended to increase with latitude, but lowland morphological types may have been more sensitive than upland morphological types to differences in moisture availability. Yield (5.82 vs. 14.97 Mg ha^–1, respectively) and persistence (final stand density, 3.99 vs. 5.96 plants m^–2) were lower for upland than for lowland genotypes, particularly at higher rainfall and more southern sites. Lowland genotypes were often able to compensate for stand thinning by increasing individual plant size, but upland genotypes were not. Lodging and rust scores were higher for upland than for lowland genotypes. Yield (13.65 vs. 9.75 Mg ha^–1) and final plant density (5.58 vs. 4.95 plants m^–2) were higher for southern than northern ecotypes. The southern-lowland combination exhibited the best yield and persistence over the study region, and genotypes within this group exhibited variability in yield among sites. Therefore, development of switchgrass cultivars for biomass production in the southern USA should focus on the southern-lowland genotypes.

Abbreviations: DMY, dry matter yield • GxE, genotype x environment interaction • L, lowland morphological type • N, northern ecotype • S, southern ecotype • U, upland morphological type

	INTRODUCTION

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

 
SWITCHGRASS has attracted considerable interest as a biofuel crop for cofiring in southern coal plants (Sanderson et al., 1996). Yields of most currently available cultivars are lower in this region than in the Great Plains, midwestern, or eastern regions of the USA (Bransby et al., 1989; Sanderson et al., 1996, 1999b; Lemus et al., 2002). The south central region of the USA is characterized by mild to moderate winters, hot and often dry summers, low native soil fertility, and rainfall patterns that vary from dry and bimodal in the western end of the region to evenly wet and humid in the east. None of these conditions of themselves suggest potential cultivation problems with switchgrass, a plant that is widely adapted across the USA (Moser and Vogel, 1995) with specific genotypes evolved to fit a variety of local conditions.

Ecotype and morphological type differences among switchgrass genotypes play a large role in adaptation to specific environments. Photoperiod and precipitation and humidity are reported as the most important environmental factors influencing adaptation to a region (Moser and Vogel, 1995). Switchgrass is a short-day plant and ecotypes are differentiated primarily by response to photoperiod (Moser and Vogel, 1995). Northern ecotypes flower earlier, are shorter, yield less, and have a longer winter dormant period with better winter survival than southern ecotypes when grown at the same latitude. Precipitation of the region of origin is a factor because ecotypes from the relatively dry Great Plains often have good drought tolerance, but poor tolerance to foliar pathogens when grown in higher rainfall environments. Switchgrass morphology is typified as lowland or upland (Moser and Vogel, 1995). Compared with upland types, lowland types are taller, coarser, more rust resistant, grow faster, have a stronger bunch-grass growth habit, and are found naturally on floodplains and areas with temporary poor drainage.

The difficulty in growing switchgrass in southern regions may be related to adaptation of ecotypes and morphological types to particular regions. Improved genotypes of switchgrass may be developed from a single collection or incorporate traits from several ecotypes or morphological types in an effort to meet agronomic needs for uses such as animal feed, conservation plantings, or biofuel feedstock production. As a result, varieties may exhibit a great degree of GxE variation, particularly for dry matter yield (DMY) (Hopkins et al., 1995a, 1995b; Vogel and Jung, 2001; Casler and Boe, 2003). Managing this through proper matching of variety to environment and intended use may be critical to successful utilization. Most released varieties are of the upland morphological type (Moser and Vogel, 1995) and the northern ecotype (USDA, 1995). However, lowland types often have higher yields in a given environment (Lemus et al., 2002; Sanderson et al., 1996, 1999b; Christian et al., 2002), and Vogel and Jung (2001) proposed that southern ecotypes had higher potential biomass productivity, albeit with a higher risk of winterkill. Casler et al. (2004) reported that showed that yield potential of lowland genotypes decreased and that of upland genotypes increased from 36 to 46° latitude, and that morphological type had greater impact on potential yield than ecotype. The winterkill problem has probably discouraged widespread use of southern germplasm in development of switchgrass cultivars, but winterkill is unlikely to be a critical factor for switchgrass intended for use south of 36° latitude. The consistent success of the southern lowland cultivar Alamo in southern U.S. trials (Sanderson et al., 1996; 1999b) underscores the possibility that development of cultivars specifically for the south may alleviate switchgrass productivity problems in this region.

We compared switchgrass genotypes that differed in ecotype and morphological type in a 4-yr trial conducted at five locations of the south central USA. Our objective was to determine how genotypes responded to environments. Locations were selected to differ in latitude and the amount and distribution of precipitation, and genotypes represented four combinations of ecotype and morphological type. Dry matter yield, persistence, and stand characteristics are reported herein, with biomass chemical composition and yield of fuel components reported elsewhere.

	MATERIALS AND METHODS

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

 
Switchgrass germplasm was evaluated at five locations with differing latitudes and annual rainfall: College Station, Dallas, and Stephenville, TX; Hope, AR; and Clinton, LA (Table 1). Soil types were: College Station—Weswood silty clay loam (fine silty, mixed thermic Fluventic Ustochrept); Dallas—Houston black clay (fine, montmorillonitic thermic Udic Pellusterts); Stephenville—Windthorst fine sandy loam (fine, mixed thermic Udic Paleustalfs); Arkansas—Bowie fine sandy loam (fine-loamy, siliceous, thermic Fragic Paleudult); and Louisiana—Dexter silt loam (fine-silty, mixed thermic Ultic Hapludalf). Switchgrass genotypes were classified as upland or lowland by morphological type and as northern or southern according to latitude of origin within the south central region. They included the following: Alamo—a southern lowland variety from south Texas; Caddo, a northern upland variety from Stillwater, OK; SL931, SL932, SL941—southern lowland synthetic lines from central and southern Texas; NL931, NL942—northern lowland synthetic lines from Oklahoma and southern Kansas; NU942—a northern upland synthetic line from Oklahoma and southern Kansas; and SU942—a southern upland synthetic line from central and southern Texas. All synthetic lines were developed at Oklahoma State University by C. Taliaferro.

All stands were fertilized once per year within 4 to 6 wk of initiation of spring growth. Phosphorus and potassium were applied annually according to local soil recommendations in Arkansas and Louisiana but were not required at the other sites. Nitrogen was applied at 150 kg ha^–1 to all stands except at Arkansas, where 168 kg ha^–1 was applied. The center two (6-row plots) or three (7-row plots) rows of each plot were harvested once per year at a 10-cm stubble height. A single annual harvest was targeted to occur in late summer or autumn when the standing crop stopped initiating new tillers and leaves were no longer green. In Louisiana, upland entries were not harvested in 2001 because stands had declined to negligible yields, and yields were recorded as zero for these entries. Harvested material was weighed, dry matter content determined from subsamples collected from each plot, and DMY calculated. Precipitation and air temperature data were summarized for each location (Table 1).

Additional measurements were obtained in some site–years. At harvest, tillers were counted from three plants per plot at Dallas and Louisiana in 1998 to 2000 and at Stephenville in 1998 and 2001. At Arkansas, Dallas, and Louisiana, stand density at harvest was determined every year by counting the number of crowns in each harvest strip. At College Station and Stephenville, stand density at harvest was determined only in the final year of the trial. Lodging as a percentage of plot area was estimated visually at Arkansas in 1999 and 2000, at Dallas in 2000, at Louisiana and Stephenville in 2001, and at College Station in 1999 and 2001. Leaf rust (caused by Puccinia spp.) scores were estimated visually as percentage of leaf area affected at Arkansas in 2000.

Within sites, experimental design was a randomized complete block with four replications. All statistical analyses were conducted by SAS (SAS Inst., 2001). The trial was analyzed as a multi-site, multi-year trial with appropriate error terms to test year and site effects in the analysis of variance. Sites were selected to cover a specific range of environments so were analyzed as a fixed variable. Year was analyzed as a fixed variable because the perennial nature of the crop caused performance in any given year to be related to the previous year's survival. When year x genotype interactions were significant within sites, means were compared within years. Means for locations and years were separated by Fisher's protected LSD (P < 0.05). Comparisons among genotype groups were made by orthogonal contrasts as follows: (i) upland vs. lowland morphological type (UM vs. LM) and (ii) northern vs. southern ecotype (NE vs. SE). Individual genotypes and years were also compared by Fisher's protected LSD 0.05. A correlation analysis was performed within morphological type groups to evaluate relationships between DMY and water availability throughout the growing season, among lodging scores, stand DMY, and density; and among stand tillering characteristics. All mention of statistical significance refers to P < 0.05 unless otherwise specified.

	RESULTS AND DISCUSSION

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

 
Dry Matter Yield
There was a year x genotype interaction at every site except Arkansas (Table 2), but consistent effects of ecotype and morphological type (Fig. 1) allowed for general comparison. Genotypes in the lowland morphological type group had greater DMY than those in the upland group in every site–year (average over all site–years, 14.97 vs. 5.82 Mg ha^–1, respectively). Genotypes in the southern ecotype group had greater DMY than those in the northern group in all but two site–years (average over all site–years, 13.65 vs. 9.75 Mg ha^–1, respectively). Interaction was most likely attributable to variation in the magnitude of differences among years and to a general trend at most sites toward stable or increasing DMY over time for lowland genotypes and decreasing DMY over time for upland genotypes.

View larger version (37K): [in this window] [in a new window]   Fig. 1. Dry matter yield of northern lowland (NL), southern lowland (SL), northern upland (NU), and southern upland (SU) groups of switchgrass entries harvested once yearly at Clinton, LA, Hope, AR, and Dallas, Stephenville and College Station, TX. Within years, numbers followed by *, **, *** indicate differences at the 0.05, 0.01, and 0.001 levels, respectively, for orthogonal contrasts of (1) upland vs. lowland and (2) northern vs. southern.

View larger version (26K): [in this window] [in a new window]   Fig. 2. Average annual dry matter yield and stand density at final harvest for lowland versus upland morphological types and northern and southern ecotypes of switchgrass grown at five sites (A–Hope, AR; C–College Station, TX; D–Dallas, TX; L–Clinton, LA; and S–Stephenville, TX) varying in latitude.

View larger version (21K): [in this window] [in a new window]   Fig. 3. Cumulative precipitation over the growing season (April to harvest) during harvest years in Clinton, LA, Hope, AR, and Dallas, Stephenville and College Station, TX. Values reported for April are cumulative for January through April. Normal cumulative rainfall at harvest for the five sites is 131.0, 110.8, 80.6, 61.3, and 84.2 cm, respectively.

Individual genotype DMY averaged across years is shown in Table 2. Alamo was used as the standard of comparison for the experimental southern lowland genotypes. No experimental genotype was consistently superior to Alamo across sites, but Alamo was first-ranked only in Dallas. Genotype SL931 was first-ranked for DMY in Arkansas and College Station and second-ranked in Louisiana, but only yielded statistically more than Alamo in Arkansas. Genotype SL941 was first-ranked in Louisiana but not statistically different from Alamo. Among the southern lowland group, both Alamo and SL941 displayed great variability in ranking among sites, ranging from first-ranked to last-ranked. Alamo yields in this trial were comparable to those reported for single harvest biomass yields in Texas (Sanderson et al., 1999a, 1999b; Muir et al., 2001). At Louisiana and Stephenville, average Alamo yields were similar to the 12.1 Mg ha^–1 reported in Iowa (Lemus et al., 2002), but Alamo DMY was higher than in Iowa at our remaining sites. In contrast, our Caddo yields were low at all locations relative to the 7.8 Mg ha^–1reported in Iowa (Lemus et al., 2002). In agreement with our data, Muir et al. (2001) reported reduced first-year switchgrass yields in Stephenville compared with subsequent years. Gradual improvements in stand productivity underscore the importance of collecting enough years of data when evaluating slow-developing perennial crops.

Stand Characteristics
Stand density declined from the initial population for all genotype groups except lowland morphology in Louisiana and Stephenville (Table 3). Final stand density was higher for lowland than for upland morphological type at Louisiana (6.45 vs. 0 plants m^–2, respectively), Stephenville (6.32 vs. 5.93 plants m^–2, respectively), Dallas (5.66 vs. 4.51 plants m^–2, respectively), and Arkansas (5.74 vs. 4.88 plants m^–2, respectively). Final stand density was higher for southern than for northern ecotypes at Dallas (5.44 vs. 5.07 plants m^–2, respectively) and Stephenville (6.40 vs. 5.93 plants m^–2, respectively). Final stand density (Fig. 2) was not related to latitude (P > 0.05) for any genotype group, in contrast to results of Casler et al. (2004) who found that lowland survival decreased while upland genotype survival increased with latitude.

View this table: [in this window] [in a new window]   Table 3. Switchgrass plant density after three harvest years in Hope, AR, and Dallas, TX, and after four harvest years at Stephenville and College Station, TX.

View larger version (23K): [in this window] [in a new window]   Fig. 4. Stand density of northern lowland (NL), southern lowland (SL), northern upland (NU), and southern upland (SU) groups of switchgrass entries over 3 yr in Hope, AR, Clinton, LA, and Dallas, TX. The dotted line represents stand density of 6.45 plants m–2 at establishment. In Clinton, the NL and SL lines are superimposed. Within years, numbers followed by *, **, *** indicate differences at the 0.05, 0.01, and 0.001 levels, respectively, for orthogonal contrasts of (1) upland vs. lowland and (2) northern vs. southern.

View larger version (30K): [in this window] [in a new window]   Fig. 5. Numbers of tillers per plant at harvest for northern lowland (NL), southern lowland (SL), northern upland (NU), and southern upland (SU) groups of switchgrass entries in Dallas, TX. Within years, numbers followed by *, **, *** indicate differences at the 0.05, 0.01, and 0.001 levels, respectively, for orthogonal contrasts of (1) upland vs. lowland and (2) northern vs. southern.

Lodging of switchgrass was reported at all sites except Stephenville (the site with the least rainfall) in some or all years (Table 4). Lodging was most severe at Arkansas, where high winds and heavy rain occurred by late May–early June each year. Lodging was greater for upland than for lowland genotypes in five of the six site–years. The exception was College Station in 1999, where the reverse occurred. Southern ecotype entries lodged more than northern entries in three of six site–years within the lowland morphological group, but ecotype did not affect lodging scores within the upland group. Within morphological type groups, lodging score at harvest was not correlated with DMY or plant density at harvest. For three site–years (Dallas 1999, Arkansas 1999 and 2000) where data for both lodging at harvest and plant density data the following year were collected, lodging was correlated with stand loss in the following year (calculated as plant density at harvest minus plant density in the following spring) among upland genotypes (r = 0.60, P = 0.09), but not among lowland genotypes. From this data it is impossible to conclude whether upland plants died because they lodged, or lodged because they died. Across all sites, larger tillers were less likely to lodge (tiller size and lodging score, r = –0.95, P < 0.01 and r = –0.59, P < 0.05, for upland and lowland genotypes, respectively).

View this table: [in this window] [in a new window]   Table 4. Lodging score at harvest of northern lowland (NL), southern lowland (SL), northern upland (NU), and southern upland (SU) groups of switchgrass genotypes at Hope, AR, Clinton, LA, College Station, Texas, and Dallas, TX.

Diseases of roots and shoots occurred. Rust outbreaks were seen in Arkansas in all years, but were not observed at other locations. In the most severe outbreak (2000), upland entries had higher peak rust scores than lowland entries (28.3 vs. 21.0% of leaf affected, respectively, P < 0.05), and there were no differences in rust scores between ecotypes. However, rust scores for genotypes within morphological type groups were not correlated with yield, stand density at harvest, or stand density the following spring, indicating that rust had no lasting impact on stand performance. In contrast, Hopkins et al. (1995b) found weak positive correlations between rust scores and yield and reported no differences in rust incidence between upland and lowland genotypes in the Midwest. Plant-parasitic nematodes from nine genera were identified in soil under the switchgrass plants at Louisiana, College Station, Arkansas, and Stephenville following harvest in 2001 (Cassida et al., 2002). In that report, lesion nematodes (Pratylenchus spp.) were associated with poor persistence of upland genotypes at the Arkansas and Louisiana sites, but the specific effect of parasitic nematodes on switchgrass remains unknown.

	CONCLUSIONS

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

 
On average, genotypes in the lowland morphological type group yielded approximately three times more biomass and had greater persistence than upland genotypes. Differences in DMY and persistence between lowland and upland morphological types tended to be greater at higher rainfall sites. Lowland genotypes showed a strong ability to compensate for stand thinning over time by increasing individual plant size, tillers per plant, or tiller size, while upland genotypes were not able to fully compensate for thinning in these environments. In addition, upland genotypes were more likely to lodge and had higher rust scores than lowland genotypes. On average, southern ecotypes yielded 40% more biomass than northern ecotypes, but there was little difference in persistence between ecotypes. Across sites, genotype SL931 was the most consistent of the experimental lines at equaling or bettering DMY of Alamo. All genotypes tended to yield more biomass as latitude increased, and lowland genotypes were more sensitive to differences in moisture availability than upland genotypes. We conclude that switchgrass feedstock production for the south central region of the USA should focus on southern lowland genotypes, and that upland genotypes have limited usefulness for biofuel production in the region because of low yields and poor persistence. Southern lowland genotypes exhibited small differences in yield across sites that suggest selection for particular environments may be beneficial.

	NOTES

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

 
This research was sponsored by the U.S. Department of Energy's Biomass Program through contract 19XSY091C with Oak Ridge National Laboratory (ORNL). ORNL is managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725.

Received for publication December 30, 2003.

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Related articles in Crop Science Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (7) Citing Articles via Google Scholar Google Scholar Articles by Cassida, K. A. Articles by Ocumpaugh, W. R. Search for Related Content PubMed Articles by Cassida, K. A. Articles by Ocumpaugh, W. R. Agricola Articles by Cassida, K. A. Articles by Ocumpaugh, W. R. Related Collections Forage Management Biofuels