Switchgrass Establishment Research Articles

Assessing winter cereals as cover crops for weed control in reduced-tillage switchgrass establishment

Weed pressure is a major challenge in switchgrass (Panicum virgatum L.) establishment. The objective of this study was to assess whether cereal cover crops alone can control weeds and improve switchgrass stand establishment in reduced-tillage system or whether herbicide treatments should be applied. Field experiments were conducted at the University of Massachusetts Agricultural Experiment Station in Deerfield in 2010–2011 and 2012–2013 growing seasons. A split plot design with three replications was used. The main plots consisted of three cover crop treatments [fallow, oat (Avena sativa L.) and rye (Secale cereale L.)]. The sub-plots were herbicide treatments that consisted of no herbicide or post-emergence application of atrazine (A) (1.1kga.i.ha−1) and quinclorac (Q) (0.37kga.i.ha−1). Herbicides were applied when switchgrass was at 4-leaf stage (6 weeks after planting). Switchgrass tiller density was 100% improved with application of A+Q. Rye provided more effective weed control than oat; however, tiller density of switchgrass was also lower in rye treatment compared with oat and fallow. Switchgrass biomass yield was significantly lower in nontreated herbicide plots (0.22Mgha−1) than plots receiving A+Q (0.41Mgha−1). Our findings suggested that supplemental herbicide application is needed with using cover crops to improve switchgrass establishment.

Switchgrass Establishment Influenced by Cover Crop, Tillage Systems, and Weed Control

Successful establishment of switchgrass (Panicum virgatum L.) is often challenging. The objective of this study was to improve switchgrass stand establishment through integrated management practices that included cover crops, tillage systems and herbicide application. The study was conducted at the University of Massachusetts Agricultural Experiment Station in Deerfield during the growing seasons of 2012 and 2013. A split split-plot design with three replications was used in both years. The main plots consisted of three cover crop treatments (fallow, oat [Avena sativa L.], and rye [Secale cereale L.]). The sub-plots were two seeding methods (no-till drill, and cultipacker seeder [Brillion]). The sub sub-plots were herbicide treatments that consisted (1) pre-emergence (PRE) application of atrazine (A) and quinclorac (Q) and (2) a Broad Spectrum application of PRE A + Q that was supplemented with post-emergence (POST) application of 2,4-D and dicamba. The no-till seeding method in both years resulted in higher stand density and biomass. Weed control was improved with the Broad Spectrum herbicide (0.90 Mg ha−1) compared with A + Q treatment (1.3 Mg ha−1). No-till seeding produced considerably higher tiller numbers (190 tiller m−2) than that of cultipacker seeder which in turn resulted in significant weed suppression. In general, when planted after rye, switchgrass produced fewer tillers than after oat or fallow. No-till seeding into oat mulch with the application of Broad Spectrum herbicides resulted in highest switchgrass yield (2,098 kg ha−1) in the establishment year.

Breeding bird community response to establishing intercropped switchgrass in intensively-managed pine stands

Intercropping switchgrass (Panicum virgatum L.) between tree rows within young pine (Pinus spp.) plantations is a potential method to generate lignocellulosic biofuel feedstocks within intensively managed forests. Intensively managed pine supports a diverse avian assemblage potentially affected by establishment and maintenance of an annual biomass feedstock via changes in plant communities, dead wood resources, and habitat structure. We sought to understand how establishing switchgrass on an operational scale affects bird communities within intercropped plantations as compared to typical intensively managed loblolly pine (Pinus taeda L.) forest. We conducted breeding bird point counts using distance sampling for three years (2011–2013) following establishment of intercropped switchgrass stands (6 replicates), traditionally-managed pine plantations, and switchgrass-only plots (0.1 km2 minimum) in Kemper Co., MS. We detected 59 breeding bird species from 11,195 detections. Neotropical migrants and forest-edge associated species were less abundant in intercropped plots than controls the first two years after establishment and more abundant in year three. Short distance migrants and residents were scarce in intercropped and control plots initially, and did not differ between these two treatments in any year. Species associated with pine-grass habitat structure were less abundant initially in intercropped plots, but converged with pine controls in subsequent years. Switchgrass monocultures provided minimal resources for birds. If songbird conservation is a management priority, managers should consider potential reductions of some breeding birds for one to two years following intercropping. It is unclear how these relationships may change outside the breeding season and as stands age.

Evidence for enhanced N availability during switchgrass establishment and seeding year production following inoculation with rhizosphere endophytes

Improvement in sustainable production of switchgrass (SG, Panicum virgatum L), as a purpose-grown biomass feedstock crop, could be realized through investigation of plant–microbe interactions associated with plant growth promoting rhizobacteria (PGPR), capable of biological nitrogen fixation (BNF). The objective of this study is to increase establishment year production of SG biofuels by inoculation with a mixed PGPR inoculum. We isolated pure strains of N2-fixing, and other PGPR, from SG rhizomes. The bacteria were identified as Paenibacillus polymyxa, an N2-fixing bacterium, and other PGPR capable of solubilizing phosphate and/or producing auxins. Field trials utilizing these strains in a mixed PGPR inoculum showed that inoculated plants contained more N in tillers during anthesis but not at senescence, suggesting that more N could be cycled to belowground roots and rhizomes for winter storage. The amount of N removal in biomass and recovery of fertilizer N were also greater for inoculated than uninoculated plants. PGPR inoculation also resulted in positive N balances, suggesting improved access to N from non-fertilizer N sources, possibly through BNF and improved soil N uptake. Overall, inoculation of SG with PGPR enhanced N acquisition and could be an effective strategy to increase the establishment year production of this crop.

Maximizing Land Use during Switchgrass Establishment in the North Central United States

Switchgrass (Panicum virgatum L.) grown under three differing establishment methods was evaluated for yield, quality, and potential ethanol production across seven environments in Wisconsin and Michigan. The three establishment methods included: (i) June seeding into fallow ground; (ii) June seeding following winter rye (Secale cereale L.) forage; and (iii) August seeding following winter wheat (Triticum aestivum L.). Planting switchgrass in June was successful following the rye forage crop, but planting in August after wheat resulted in complete stand loss. While harvested biomass in a two‐cut system was equal to or greater than that realized in a one‐cut system (6.9 vs. 6.8 Mg ha−1), biomass quality for ethanol production was highest following a killing frost (226 vs. 224 g ethanol kg−1 biomass). The higher overall biomass production in the two‐cut system generally compensated for this difference however, with ethanol yields similar between the two systems (1950 vs. 1970 L ethanol ha−1 for the two‐ and one‐cut system, respectively). In addition to ethanol production, we found that forage nutritive value in the first cut of the two‐cut system was of sufficient quality to satisfy the dietary needs of several classes of livestock. Harvesting established switchgrass for hay with an early season cutting in a two‐cut system provides producers with an alternative forage source while not affecting total seasonal biomass yield if harvested at the appropriate growth stage and cutting height to leave sufficient photosynthetic material for regrowth.

Seed Safening from Herbicidal Injury in Switchgrass Establishment

ABSTRACTWeed control during the establishment year of switchgrass (Panicum virgatum L.) is critical for a successful stand. Laboratory and field studies were conducted for 2 yr to evaluate the effectiveness of seed safeners for protecting switchgrass seed from the preemergent herbicide metolachlor. Initial evaluations of six seed safeners determined that fluxofenim was the only successful protectant tested for switchgrass seedling germination. Safener rates 0, 25, 50, and 100% of the recommended rate (40 mg a.i. 100 g per seed), method of application (coating vs. controlled hydration), ecotype (lowland vs. upland), dry matter yield, and number of crowns per row were analyzed in replicated field trials conducted in Starkville, MS, Ames, IA, Blacksburg, VA, and Brookings, SD, in a randomized complete block design. Two establishment years were planted in single‐row (6.1 m) plots spaced 50.8 cm apart. Subsequent biomass yield was harvested at the end of each growing season. Exceptional stands were achieved with all rates of safener in both years due to the reduction or elimination of annual grassy weeds and no crop injury was experienced at any test location. The controlled hydration (combination of 25, 50, or 100% fluxofenim) method resulted in significantly greater yields and crown counts than the coating technique at most locations for both years.

For switchgrass cultivated as biofuel in California, invasiveness limited by several steps

The expected production of biomass-derived liquid fuels in the United States may require cultivation of millions of acres of bioenergy crops, including perennial grasses such as switchgrass. Switchgrass is not native to California and possesses many qualities in common with other perennial grasses that are invasive. To evaluate the potential invasiveness of switchgrass in California, we conducted risk analysis and climate-matching models as well as greenhouse and field evaluations of switchgrass, looking at its environmental tolerance and competitive ability against resident riparian vegetation. We concluded that dryland regions of California are not suitable to vigorous establishment and invasion of switchgrass. However, riparian areas appear to be far more likely to support switchgrass populations. With effective mitigation practices in place throughout the development, growth, harvest, transport and storage processes, it should be possible to minimize or eliminate the movement of seeds and vegetative propagules to sensitive habitats. Consequently, we believe that switchgrass is unlikely to become a significant problem in California, even with widescale production.

Effect of Various Herbicides on Warm‐season Grass Weeds and Switchgrass Establishment

ABSTRACTSwitchgrass (Panicum virgatum L.) is typically slow and difficult to establish. Field experiments evaluated switchgrass establishment and the response of warm‐season grass weeds (large crabgrass [Digitaria sanguinalis (L.) Scop.], broadleaf signalgrass [Urochloa platyphylla (Munro ex C. Wright) R.D. Webster], Johnsongrass [Sorghum halepense (L.) Pers.], and Texas panicum [Urochloa texana (Buckley) R.D. Webster]) to a preemergence (cloransulam) or postemergence (foramsulfuron, pendimethalin, quinclorac, foramsulfuron + pendimethalin + quinclorac, nicosulfuron, nicosulfuron + pendimethalin + quinclorac, nicosulfuron + quinclorac + atrazine, and monosodium methanearsonate [MSMA]) herbicides. Experimental design was a split‐split‐plot design with presence/absence of preemergence herbicide cloransulam as main plot, weed species as subplot treatment, and postemergence herbicides as sub‐subplot treatments. Herbicides foramsulfuron + pendimethalin + quinclorac was applied at the 1‐ to 2‐ and 2‐ to 3‐leaf stage, whereas nicosulfuron was applied at 2‐ to 3‐leaf stage and nicosulfuron combinations and MSMA were applied at 3‐ to 4‐leaf stage. Using cloransulam as a preemergence herbicide resulted in lower switchgrass seedling numbers and generally did not improve establishment. Interseeding Johnsongrass allowed for the greatest number of switchgrass seedlings and was the least competitive weed, whereas interseeding large crabgrass resulted in the least amount of switchgrass and proved to be the most competitive weed. Foramsulfuron + pendimethalin + quinclorac resulted in effective weed control (>70%) for all four species evaluated and improved switchgrass establishment (13–26%) compared to the untreated control (0–3%); however, greater improvement in stand establishment is needed.

Effects of Row Spacing, Seeding Rate, and Planting Date on Establishment of Switchgrass

Switchgrass (Panicum virgatum L.) has been identified as a potential biomass crop for marginal cropland, but establishment can be problematic. In Exp. 1, treatments (seeding rate and row spacing) were arranged as a randomized complete block in a split‐plot design with four replicates in which the main plot treatment was row spacing (17.8, 35.6, 71.1, or 88.9 cm) and the subplot treatment was seeding rate (1.12, 2.24, 3.36, or 4.48 kg pure live seed [PLS] ha−1). In Exp. 2, treatments (seeding rate and planting date) were arranged as a randomized complete block in a split‐split‐plot design with four replicates in which the main plot treatment was planting date (September, February, or May), the subplot treatment was switchgrass ecotype (upland ‘Cave‐in‐Rock’ or lowland ‘EG1101’), and the sub‐subplot was seeding rate (2.24, 4.48, 6.72, 8.96, or 11.20 kg PLS ha−1). In Exp. 1, row spacing did not affect seedling density, but switchgrass at the widest row spacing (88.9 cm) produced the least dry matter (DM) yield during the establishment year. However, row spacing had no effect on DM yield after the establishment year. In Exp. 2, increasing seeding rate increased seedling density but had no effect on DM yield. In addition, seedling density was greatest for both ecotypes planted in September; however, February‐ and May‐planted EG1101 produced the greatest DM yield. While upland switchgrass may be successfully planted in autumn in the southern Great Plains, lowland switchgrass should be planted only in the spring because of seedlings’ susceptibility to freezing.

Switchgrass establishment and seeding year production can be improved by inoculation with rhizosphere endophytes

Nitrogen-fixing bacteria (NFB) and other plant growth promoting rhizobacteria (PGPR) have been used to increase growth and yield of corn and sugarcane, two graminaceous crops utilized as bioenergy feedstocks. Switchgrass (SG, Panicum virgatum L.), a native perennial grass of North America, is also a desirable bioenergy crop because it can be grown on marginal lands and thus does not necessarily displace food production. Yet no investigations have been conducted on the use of NFB and/or PGPR as inoculants to increase SG growth and productivity. Studies reporting high biomass yields from SG with minimal to no fertilizer nitrogen (N) input prompted us to hypothesize that SG is obtaining part of its N from an alternative source, perhaps through biological nitrogen fixation from root/rhizome associated PGPR. We isolated bacteria from SG rhizomes that had not received fertilizer N input for over ten years and which increased plant growth in the absence of the fertilizer N under growth chamber conditions. The bacteria were identified as a strain of Paenibacillus polymyxa, a N2-fixing bacterium, and several other PGPR capable of solubilizing phosphate and/or producing auxins. Our objective was to test this PGPR inoculum under field conditions with low-N inputs. Here, we present the findings regarding SG productivity in the establishment year. Switchgrass seeds inoculated with the PGPR culture had greater tiller density, and taller tillers resulting in 40% higher yield than uninoculated plants. PGPR have the potential to be effectively utilized as inoculants to enhance SG yields in a low-N input production system.

Evaluation of intercropped switchgrass establishment under a range of experimental site preparation treatments in a forested setting on the Lower Coastal Plain of North Carolina, U.S.A.

There is growing interest in using switchgrass (Panicum virgatum L.) as a biofuel crop and for its potential to sequester carbon. However, there are limited data on the establishment success of this species when grown as a forest intercrop in coastal plain settings of the U.S. Southeast. Therefore, we studied establishment success of switchgrass within experimental intercropped plots and in pure switchgrass plots in an intensively managed loblolly pine (Pinus taeda) plantation in eastern North Carolina. Pine trees were planted in the winter of 2008, and switchgrass was planted in the summer of 2009. Establishment success of switchgrass was measured over the growing season from May to October 2010, and quantified in terms of percent cover, height (cm), tiller density (number of tillers m−2), leaf area index and biomass (Mg ha−1). At the end of the growing season, pure switchgrass plots were taller than the intercropped treatments (114 ± 2 cm versus 98 ± 1 cm, respectively), but no significant treatment effects were evident in the other variables measured. Switchgrass biomass across all treatments increased from 2.65 ± 0.81 Mg ha−1 in 2009 to 4.14 ± 0.45 Mg ha−1 in 2010. There was no significant effect of distance from the pine row on any switchgrass growth parameters. However, we anticipate a shading effect over time that may limit switchgrass growth as the pines approach stand closure.

Switchgrass establishment as affected by seeding depth and soil type

Switchgrass (Panicum virgatum L.) has been identified as a potential bioenergy crop for the North Central Region of the USA. One of the limitations to successful production of switchgrass is poor seedling establishment affected by the interaction between seeding depth and soil physical characteristics. The objective of this study was to determine the effect of seeding depth and soil types on switchgrass seedling establishment in a greenhouse and field experiment. The greenhouse experiment included seven seeding depths (0, 13-, 19-, 25-, 38-, 51-, and 64-mm deep), three soil types, and one cultivar, Dacotah. Soil types for the greenhouse experiment were brought in from three North Dakota State University research sites located at Fargo (silty-clay, 6.1% organic matter (OM)), Prosper (fine-silty, 3.3% OM), and Carrington (coarse loamy, 2.5% OM). The field experiment was conducted at Fargo and Prosper, ND, in 2010, and included four seeding depths (13-, 19-, 30-, and 38-mm deep) and two cultivars Dacotah and Forestburg. Results from the greenhouse experiment indicated pure live seed emergence (PLSE) differed among seeding depths and soil textures. Only the surface seeding (0-mm depth) had a significantly lower PLSE. Seeding depths from 13- to 64-mm depth were not significantly different and fluctuated between 64 and 74% PLSE. The silty-clay soil at Fargo had significantly higher PLSE (74%) across all seeding depths than the fine-silty soil at Prosper, and the coarse-loamy soil at Carrington (60% PLSE). In the field experiment, the Forestburg cultivar had very poor emergence at all seeding depths and environments. The PLSE for ‘Forestburg’ was 10%, from the 13-mm depth averaged across both locations. The PLSE for Dacotah across both locations was 80, 42, 18, and 5% for the 13-, 19-, 30-, and 38-mm depths, respectively. Under controlled conditions (greenhouse experiment), seeding depth was not a major factor affecting seedling establishment except for the surface seeding where the lack of seed and soil contact and surface dryness reduced emergence. Pure live seed emergence and emergence index were greater in the silty-clay soil at Fargo across all seeding depths, presumably due to a better seed–soil contact. Under field conditions, the 13-mm depth had significantly higher PLSE indicating other factors are interacting with the seeding depth. As a general recommendation to growers switchgrass should be seeded no deeper than 13-mm depth. Testing the germination of the seed lot, without stratification, before planting is recommended to better estimate field establishment success.

Effects of Seeding Date and Weed Control on Switchgrass Establishment

We tested the effects of seeding date and weed control during switchgrass establishment in a field experiment that was conducted in central Pennsylvania in 2007 and repeated in 2008. Switchgrass was no-till seeded in early May, late May, and mid-June, and three postemergence weed management treatments were evaluated, including Mow (only a single mowing), Broadleaf (2,4-D + dicamba), and Broad Spectrum (2,4-D + dicamba + atrazine + quinclorac). Switchgrass density increased at later seeding dates, except in 2008, when the middle seeding date had the lowest density. In both years, weed biomass in late summer was lowest in the last seeding date of the Broad Spectrum treatment. In contrast, switchgrass biomass in late summer was greatest in the first seeding date of the Broad Spectrum treatment in both years. In the year after establishment (production year), plots were split to test the effects of supplemental weed control, composed of metsulfuron + 2,4-D applied in May, on total aboveground yield. Supplemental control in the production year increased total aboveground yield in the Mow treatment only, indicating that effective weed control during the establishment year might reduce the need for weed control in the following year. Although maximum aboveground yield was achieved when switchgrass was seeded in May and herbicides were used, results from our experiment suggest that seeding switchgrass at a relatively high seeding rate in June in our study region and mowing annual weeds to reduce competition and prevent seed production could be an effective strategy if minimizing herbicide use is a priority.

Growth promotion and colonization of switchgrass (Panicum virgatum) cv. Alamo by bacterial endophyte Burkholderia phytofirmans strain PsJN

BackgroundSwitchgrass is one of the most promising bioenergy crop candidates for the US. It gives relatively high biomass yield and can grow on marginal lands. However, its yields vary from year to year and from location to location. Thus it is imperative to develop a low input and sustainable switchgrass feedstock production system. One of the most feasible ways to increase biomass yields is to harness benefits of microbial endophytes.ResultsWe demonstrate that one of the most studied plant growth promoting bacterial endophytes, Burkholderia phytofirmans strain PsJN, is able to colonize and significantly promote growth of switchgrass cv. Alamo under in vitro, growth chamber, and greenhouse conditions. In several in vitro experiments, the average fresh weight of PsJN-inoculated plants was approximately 50% higher than non-inoculated plants. When one-month-old seedlings were grown in a growth chamber for 30 days, the PsJN-inoculated Alamo plants had significantly higher shoot and root biomass compared to controls. Biomass yield (dry weight) averaged from five experiments was 54.1% higher in the inoculated treatment compared to non-inoculated control. Similar results were obtained in greenhouse experiments with transplants grown in 4-gallon pots for two months. The inoculated plants exhibited more early tillers and persistent growth vigor with 48.6% higher biomass than controls. We also found that PsJN could significantly promote growth of switchgrass cv. Alamo under sub-optimal conditions. However, PsJN-mediated growth promotion in switchgrass is genotype specific.ConclusionsOur results show B. phytofirmans strain PsJN significantly promotes growth of switchgrass cv. Alamo under different conditions, especially in the early growth stages leading to enhanced production of tillers. This phenomenon may benefit switchgrass establishment in the first year. Moreover, PsJN significantly stimulated growth of switchgrass cv. Alamo under sub-optimal conditions, indicating that the use of the beneficial bacterial endophytes may boost switchgrass growth on marginal lands and significantly contribute to the development of a low input and sustainable feedstock production system.

Assessing habitat susceptibility and resistance to invasion by the bioenergy crops switchgrass and Miscanthus × giganteus in California

One potential externality of the bioeconomy is the unintentional introduction and widespread dissemination of bioenergy crops that become invasive species. As one component of parameterizing risk assessment of invasiveness, we evaluated the colonization, survival, and establishment potential of switchgrass (Panicum virgatum) and giant miscanthus (Miscanthus × giganteus) in a riparian and dryland habitat in central California, where both species are non-native, under varying levels of soil moisture availability and competition over two years. Survival (<11%) and performance (≤3 tillers) of transplants of both species were poor in the dry upland environment regardless of competitive regime. In contrast, the lowland riparian habitat supported switchgrass (34–61% survival) and giant miscanthus (5–19% survival) transplants, with survival varying based on soil moisture availability and competitive environment. After two years, switchgrass grown without competition in the first year in the lowland habitat produced about six-fold more tillers that were twice as tall and yielded eight times more aboveground biomass than switchgrass growing in an intact resident plant community. In a second experiment, introduced switchgrass seed and giant miscanthus rhizome survival was extremely poor (<20%) in the upland habitat. Our results indicate that dryland regions of California have very high resistance to either switchgrass or giant miscanthus. However, riparian areas, particularly disturbed low competition areas, are capable of supporting the establishment of switchgrass. Though limited in habitat representation, this is the first study to evaluate experimental introductions of bioenergy crops into potentially susceptible habitat.

Effectiveness of Sulfosulfuron and Quinclorac for Weed Control during Switchgrass Establishment

We conducted a field experiment in 2007 and 2008 and repeated it in a separate field in 2008 and 2009 to test the effectiveness of two herbicides applied at two different times on weed control during switchgrass establishment. At 4 and 6 wk after switchgrass planting (WAP), sulfosulfuron was applied at 0.04 and 0.07 kg ai ha−1with nonionic surfactant and quinclorac was applied at 0.28, 0.42, and 0.56 kg ai ha−1with methylated seed oil. Herbicides applied at 4 WAP tended to be more effective than at 6 WAP. Sulfosulfuron provided greater control of smooth pigweed; however, quinclorac provided greater control of yellow foxtail, a grass weed that has traditionally been difficult to control with herbicides in switchgrass. Average yellow foxtail control was 73, 87, and 93% for quinclorac at 0.28, 0.42, and 0.56 kg ha−1, respectively, compared to 62 and 60% for sulfosulfuron at 0.04 and 0.07 kg ha−1, respectively. Switchgrass injury (chlorosis and height reduction relative to the untreated control) was observed, but most symptoms were not detectable by 8 wk after treatment (WAT) for most treatments. Plots that received quinclorac at 0.56 kg ha−1at 6 WAP tended to have relatively low weed biomass and high total aboveground yield in the establishment year and relatively high total aboveground yield in the year after establishment.

Effectiveness of weed management methods in establishment of switchgrass and a native species mixture for biofuels in Wisconsin

Abstract Establishment of native grassland species can be impaired by weed populations, presenting considerable economic risk for growers of perennial bioenergy crops. We established switchgrass (Panicum virgatum) in southwestern Wisconsin using four weed management strategies, and established a mixture of diverse grassland species to evaluate productivity of diverse mixtures relative to monocultures. Weed management for switchgrass included pre-emergent glyphosate (1.12 kg ae/ha; GLY), pre-emergent glyphosate (1.12 kg ae/ha) + post-emergent 2,4-D (1.06 kg ae/ha; GLY + 2,4-D), pre-emergent glyphosate (0.14 kg ae/ha) and imazapic (0.07 kg ae/ha; S-IMAZ + GLY), and oats planted as a companion crop + pre-emergent glyphosate (1.12 kg ae/ha; GLY + OATS). The diverse species mixture was established with pre-emergent glyphosate (0.14 kg ae/ha) and imazapic (0.07 kg ae/ha; D-IMAZ + GLY). We estimated plant species composition in 2008 and 2009, and biomass yield in autumn of each year. Imazapic decreased grass weed abundance and 2,4-D decreased broadleaf weed abundance in switchgrass in 2008, whereas there were no differences in weed abundances in 2009. Abundance of grass weeds was negatively correlated with biomass yield, and S-IMAZ + GLY produced greater yield in 2009 (7.31 ± 0.75 Mg ha−1) than GLY and GLY + OATS. Of the four switchgrass establishment treatments we tested, only S-IMAZ + GLY yielded significantly greater biomass than the diverse species mixture. Total costs of establishment were greatest for D-IMAZ + GLY and equivalent among the four switchgrass treatments. These results contribute to current understanding of the agronomic potential and economic risks of native grassland species for bioenergy crops in the first two years of establishment.

Field-scale soil property changes under switchgrass managed for bioenergy

The capacity of perennial grasses to affect change in soil properties is well documented but information on switchgrass (Panicum virgatum L.) managed for bioenergy is limited. An on-farm study (10 fields) in North Dakota, South Dakota, and Nebraska was sampled before switchgrass establishment and after 5 years to determine changes in soil bulk density (SBD), pH, soil phosphorus (P), and equivalent mass soil organic carbon (SOC). Changes in SBD were largely constrained to near-surface depths (0–0.05 m). SBD increased (0–0.05 m) at the Nebraska locations (mean=0.16 Mg m−3), while most South Dakota and North Dakota locations showed declines in SBD (mean=−0.18 Mg m−3; range=−0.42–0.07 Mg m−3). Soil pH change was significant at five of the 10 locations at near surface depths (0–0.05 m), but absolute changes were modest (range=−0.67–0.44 pH units). Available P declined at all sites where it was measured (North Dakota and South Dakota locations). When summed across the surface 0.3 m depth, annual decreases in available P averaged 1.5 kg P ha−1 yr−1 (range=0.5–2.8 kg P ha−1 yr−1). Averaged across locations, equivalent mass SOC increased by 0.5 and 2.4 Mg C ha−1 yr−1 for the 2500 and 10 000 Mg ha−1 soil masses, respectively. Results from this study underscore the contribution of switchgrass to affect soil property changes, though considerable variation in soil properties exists within and across locations.

Modification of native grasses for biofuel production may increase virus susceptibility

Bioenergy production is driving modifications to native plant species for use as novel biofuel crops. Key aims are to increase crop growth rates and to enhance conversion efficiency by reducing biomass recalcitrance to digestion. However, selection for these biofuel-valuable traits has potential to compromise plant defenses and alter interactions with pests and pathogens. Insect-vectored plant viruses are of particular concern because perennial crops have potential to serve as virus reservoirs that influence regional disease dynamics. In this study, we examined relationships between growth rates and biomass recalcitrance in five switchgrass (Panicum virgatum) populations, ranging from near-wildtype to highly selected cultivars, in a common garden trial. We measured biomass accumulation rates and assayed foliage for acid detergent lignin, neutral detergent fiber, in vitro neutral detergent fiber digestibility and in vitro true dry matter digestibility. We then evaluated relationships between these traits and susceptibility to a widely distributed group of aphid-transmitted Poaceae viruses (Luteoviridae: Barley and cereal yellow dwarf viruses, B/CYDVs). Virus infection rates and prevalence were assayed with RT-PCR in the common garden, in greenhouse inoculation trials, and in previously established switchgrass stands across a 300-km transect in Michigan, USA. Aphid host preferences were quantified in a series of arena host choice tests with field-grown foliage. Contrary to expectations, biomass accumulation rates and foliar digestibility were not strongly linked in switchgrass populations we examined, and largely represented two different trait axes. Natural B/CYDV prevalence in established switchgrass stands ranged from 0% to 28%. In experiments, susceptibility varied notably among switchgrass populations and was more strongly predicted by potential biomass accumulation rates than by foliar digestibility; highly selected, productive cultivars were most virus-susceptible and most preferred by aphids. Evaluation and mitigation of virus susceptibility of new biofuel crops is recommended to avert possible unintended consequences of biofuel production on regional pathogen dynamics.

Carbon Sequestration under Irrigated Switchgrass (Panicum virgatumL.) Production

Perennial herbaceous crops such as switchgrass are important sources of cellulosic biomass for the developing bioenergy industry. Assessments of how much C will be lost or sequestered into soil and the turnover rates of that C are needed to assist producers and policymakers in determining the long‐term sustainability of biomass production. We used the natural13C abundance of soils to calculate the quantity and turnover of C4−C inputs in irrigated fields cropped to switchgrass monocultures. Soil profile root biomass produced after three seasons averaged 3.9 Mg C ha−1m−1Five years of cropping showed a 1200 kg ha−1increase in soil organic C (SOC) in the 0‐ to 15‐cm depth increment, with no change below 15 cm. The surface 15 cm of soil cropped to ‘Kanlow’ and ‘Shawnee’ had a δ13C enrichment of 3‰ above the native uncultivated soil, with 3.6‰ for ‘Cave in Rock,’ with an average 2‰ enrichment compared with the soil collected before switchgrass establishment. Enrichment in the 30‐ to 60‐ and 60‐ to 90‐cm depths averaged 1.7 and 0.9‰, respectively. The amount of soil profile C4–C determined by δ13C analysis showed a greater C input than determined by the difference in total C mass between the uncultivated native and cropped soils. The average accrual rate of C4–SOC was estimated at 1.0 Mg ha−1yr−1Estimates of the mean residence time of the C3−C under the irrigated C4monocultures of switchgrass were &gt;60 yr in the 0‐ to15‐cm and 30 to 55 yr in the 15‐ to 30‐cm depth increments. On average, 24% of SOC in the 0‐ to 15‐cm depth was derived from C4cropping.