Establishing and Managing Switchgrass as an Energy Crop

David J. Parrish, Professor, and John H. Fike, Associate Professor, Department of Crop and Soil Environmental Sciences, Virginia Tech, Blacksburg 24061; David I. Bransby, Professor, Department of Agronomy and Soils, Auburn University, Auburn, AL 36849; and Roger Samson, Executive Director, REAP-Canada, Box 125 Centennial Centre CCB13, Ste. Anne de Bellevue, QC H9X 3V9

Abstract

When it was first adopted as a crop, switchgrass was evaluated and improved for forage uses; but it has more recently been extensively studied as an energy crop, where its biomass might be used as feedstock for bioenergy. Of the two morphological forms or cytotypes of switchgrass, the lowland cultivars tend to produce more biomass; but upland cultivars are generally of more northern origin and more cold tolerant and therefore are usually preferred in the North. Attention to weed control, planting date, planting depth, and seed dormancy can greatly increase establishment success with this species. Stands of switchgrass should be harvested no more than twice per year, and one cutting often provides as much biomass as two. Harvesting after aboveground biomass has senesced can aid persistence, facilitate harvest operations, conserve N, and improve feedstock quality; but other harvest patterns may provide a better fit in some situations. If its biology is properly taken into consideration, switchgrass can offer great potential as an energy crop.

Introduction

Agriculture captures sunlight and uses that energy to build simple matter into complex, useful products. Thus, forage plants distributed across a landscape act as solar collectors and produce biomass that can be converted on-the-hoof into food and other products. Managers of forages and grazing lands, then, will be able to relate readily to the notion of energy crops – plants grown to capture the sun’s energy in biomass, which can then be processed into bioenergy forms. Bioenergy includes biofuels (liquids and gases) and biopower, a term being used to describe heat or electricity produced from burning biomass. This review focuses on switchgrass (Panicum virgatum L.) and how the species might best be managed if used for bioenergy.

Why might we want to grow crops to produce energy? (i) If it has not already, demand for the Earth’s hydrocarbon fuel resources will inevitably outstrip the supply. (ii) Fossil fuel consumption increases atmospheric CO₂, which promises major ecological consequences. Clearly, we need renewable, less-polluting alternatives to hydrocarbon fuels.

Renewable, less-polluting alternatives to hydrocarbon fuels include energy produced from biomass (12). If the cellulose and hemicellulose that plants make are depolymerized into simple sugars, they can be fermented into ethanol. Alternatively, some technologies can take raw lignocellulosic biomass and convert it industrially into biogas or petroleum-like products. The lignocellulosic designation (often shortened to just cellulosic) recognizes that lignin, along with holocellulose (cellulose plus hemicellulose), is a major biomass component – and that lignin may present particular challenges in conversion processes (12).

After corn (Zea mays), the herbaceous species that has gained the most attention as an energy crop in the USA is switchgrass (23,28,33). This native prairie grass (hereafter noted as SG) gained biofuel prominence after the US Department of Energy (DOE) studied it for a decade and a half, in a time when little other research was being done on herbaceous energy crops. The DOE decision to focus on SG came after screening numerous non-woody species. For those initial studies, SG was included as a benchmark species at the suggestion of Dale Wolf, a forage agronomist at Virginia Tech. During the 5-year herbaceous energy-crop screening phase, SG proved to be the most productive candidate overall (33). It was therefore adopted as a "model species" for another 10 years of agronomic, breeding, and biotechnology studies (23,33). Beginning in 2002, DOE funding for herbaceous energy crops was curtailed (23). More recently, the USDA has begun to take over the reins for such work (32), although DOE remains involved in research related to biomass conversion technologies and carbon sequestration.

Switchgrass, a perennial, C4/warm-season species, occurs naturally over most of the eastern two-thirds of the USA, in Central America, and in southern Canada (17). It is a very diverse species, with striking differences between plants. This diversity, which presumably reflects evolution and adaptation to new environments as the species spread across the continent (7,8,28), provides a range of valuable traits for breeding programs (40).

Switchgrass has two distinct forms, or "cytotypes" (6,7,8), or "morphological types" (10,11): upland and lowland. Upland SG types are generally shorter (≤ 8 ft tall) and less coarse than lowland types. Lowland cultivars may grow to ≥ 9 ft in favorable environments. Both types are deeply rooted (> 6 ft in favorable soils) and have short rhizomes. The upland types tend to have more vigorous rhizomes. Consequently, the lowland types can seem to have a bunchgrass habit, while the upland types tend to be more sod forming. Lowland forms appear more plastic in their morphology, producing larger plants if stands become thin or are planted in wide rows (10,21). On the other hand, lowland types appear to be more sensitive to moisture stress than upland types (10).

The ecology of SG is tied to grazing animals and fire. The phenology and physiology of the species are well suited to competing for resources in tall-grass prairie ecosystems. This knowledge can be used to advantage in the establishment and management of the species as an energy crop (28). Mycorrhizal relationships are very important also, with fungal symbionts presumably providing advantages both in mineral and water absorption (5).

Cultivar/Ecotype Selection

Natural selection has produced distinct populations of SG that are adapted to the eastern two-thirds of the USA and portions of southern Canada (3,6,7,8). Some of those native populations have been collected, increased, and released as named cultivars. In other cases, SG cultivars have resulted from the blending and/or breeding of various accessions.

In addition to being grouped based on morphological type, i.e., upland or lowland, cultivars or accessions can be described based on their area of origin (usually "southern" or "northern" USA). Thus, a cultivar or accession might be designated SL (southern lowland), SU (southern upland), NL (northern lowland), or NU (northern upland) (8,10,11). These four categories are sometimes called germplasm groups (8). As a generalization, there are relatively few NL and SU and an abundance of NU (8).

Growers might tend to choose a SG cultivar derived from local accessions, since those plants would presumably be best adapted to the locale; but this logic is only partially valid for biomass production (3). Locally adapted plants may be better able to tolerate the climatic conditions of the region, but they may not be the most productive (8). Furthermore, SG cultivars that have been selected for their good forage qualities might be well suited for biogas production; but they would likely be problematic – from a chemical constituency standpoint – as a feedstock for making ethanol or other liquid fuels (40).

Quality both in forages and in biomass feedstocks is related to how readily they can be processed – either in an animal’s gut or in a conversion facility. For most bioenergy conversion processes, SG grown to full maturity creates a fibrous biomass that provides good feedstock quality. One major exception is that high lignin content is an anti-quality factor in ethanol production (12). Breeders are working to develop SG lines that will be preferred for biomass purposes (40).

As a rule-of-thumb, one should plant a cultivar originating from south of the location where it will be grown. This general approach takes advantage of two facts: (i) the production of flowers and seedheads in SG is triggered by photoperiod and is delayed in more northern, longer-day areas; and (ii) the conversion from vegetative to reproductive growth in SG effectively marks the end of above-ground biomass accumulation for the season. Hence, SG cultivars that flower later than locally originating types may remain vegetative for longer and produce more stem and leaf, i.e., more biomass.

But southern provenance carries with it a liability. Such cultivars may lack the hardiness to survive in more northern locations. In fact, the work of Casler and colleagues (3,6,7,8) has shown clearly that some otherwise very productive cultivars of southern origin are susceptible to winter injury, or may be lost altogether, when planted too far north. Moser and Vogel (25) suggested cultivars should not be planted more than 300 mi north of their area of origin. Casler et al. (7) have suggested cultivars should not be used more than one hardiness zone north (or south) of their origin.

Southern lowland types currently are the cultivars of choice from the Deep South through the transition zone in the USA (10,11). Originating from Texas, Alamo is particularly prone to stand reductions or losses when planted in northern locations; and its tendency to "green up" earlier in the spring than some other cultivars also exposes it to injury from late-spring frosts. Northern-derived upland types are more likely to be hardy enough to survive winters in more northern locations. When selecting cultivars for a new area of production, farmers might be well advised to plant several cultivars and monitor their persistence under the management system to be imposed.

Establishment

Switchgrass can be grown on sites that would not be favorable for many other crops, including land that would be too erodible for corn. Plantings tend to do better on soils of finer texture (but not heaviest clays) because of their greater water-holding capacity, but SG has a rather remarkable adaptability to many soil types.

Switchgrass has a perhaps undeserved reputation for being difficult or slow to establish (9,28,34). Climatic factors can hinder establishment, and inattention to agronomic details can result in poor performance; but difficulties encountered in SG establishment often spring from the innate dormancy of SG seeds (28). Freshly-harvested SG seeds often germinate at 5% or less if planted into a warm soil (35). Over time, this dormancy can disappear as the seeds "after-ripen" (35,36). Accordingly, older seeds (≥ 1 year old) are often more desirable. Seed tag information is unhelpful with regards to germinability; the percent germination reported on the tag is determined by conducting a standard test (2) in which seeds are exposed to a 14-day wet "prechill," which stratifies seeds and breaks dormancy. It is sometimes suggested that SG be planted in early spring to allow the seeds to stratify in situ, a strategy that allows the seeds to eventually germinate; but this method can lead to problems with weeds that germinate before or with the SG. Some seed producers are now bulk-stratifying SG seeds, which can greatly reduce dormancy.

No single method or approach for establishing SG will work in all situations. However, attention to several key factors can increase the likelihood of success (28,41):

• Plant after the soil is well warmed (> 70°F); this runs contrary to standard practice in many places, but plantings as late as midsummer can be successful if moisture is favorable.

• Use seeds that are highly germinable, i.e., not dormant.

— Seeding rates of 2 lb/acre could theoretically be adequate for highly germinable seedlots.

— Higher planting rates (≤ 10 lb/acre) are usually used to compensate for dormant seeds and to increase the chances for successful-looking (less weedy) stands in the year of establishment.

• Plant to ½- to ¼-inch deep (can be ¾ inch for coarser soils); incorporation and good seed-to-soil contact are crucial; no-till or conventional methods can be used, but use a planter that places seeds accurately and pack or firm the soil well after planting.

• Provide no fertilization at planting to minimize weed competition.

• Control weeds.

Weeds can be a major impediment to SG establishment (28). Good weed control at the time of planting is crucial. In an optimal scenario (non-dormant seeds planted into a well-warmed soil), SG seedlings can out-compete many weeds that might co-emerge. Perennials that escape pre-planting control can be problematic, but broadleaf species can often be defeated with post-emergence herbicides.

Planting failures due to weed infestations may be more apparent than real. Some experienced producers tolerate heavy weed infestations in the first year, perhaps only mowing at a height slightly above the SG seedlings to minimize the effects of overtopping. In subsequent years, the growth habit and phenology of the now-established SG plants often help them to out-compete many weeds. A good rule of thumb appears to be that, if there are ≥ 2 plants/ft² at the end of the establishment year, the planting can succeed (34,41). Mature, fully productive stands often have < 1 plant/ft² (10). Initially weedy SG stands can become solid stands with good management.

The conventional wisdom is to avoid N fertilization during the first year to minimize weed pressure. Where weeds are not a problem and a site has low to moderate soil fertility, SG seedlings might benefit from a more aggressive fertilization program. In the greenhouse and a soil-less medium, SG seedlings produced more biomass with increasing increments of N up to 250 lb/acre (20). We certainly do not recommend such rates, and we suspect mycorrhizal and other symbiotic associations often reduce the need for fertilizer supplements in a field setting (5).

Fertilization of Established Stands

A range of responses to macronutrients, especially N, has been reported, with needs differing widely depending on management (13,14,18,20,21,22,26,38). Plantings are not always responsive to applied N, and several workers have reported significantly more N being removed in harvested SG biomass than was applied as fertilizer (28). Excessive N fertilization may promote lodging and reduce stand density in single-harvest biofuel systems (13), and N-use efficiency can be quite low in soils capable of mineralizing significant N (20).

Managing SG for bioenergy can greatly reduce N requirements compared to managing for hay. As would be expected, making multiple harvests within a year requires more N fertilization due to greater N removal (38). In contrast, harvesting biomass only once and at the end of the season removes far less N [e.g., (14,20,22)]. Nitrogen levels in SG biomass may be ≥ 2% during the growing season, but N can drop to ~0.5% in senescent or dead biomass after the plant has "recycled" N into crowns and roots (22). Thus, a single, post-senescence harvest will remove only ~10 lb of N per ton biomass. The lower levels of N in a feedstock can also be a positive quality factor for some conversion technologies (12).

With a growing understanding of the N economy of SG and the technology of conversion, consensus recommendations for fertilizing SG as an energy crop may be beginning to emerge. The limited response of SG to an increasingly expensive input – fertilizer N – suggests N applications above 50 lb of N per acre per year may provide little or no economic return (22,26,28). In some areas of the eastern USA, as much as half of the N that might be needed falls out of the sky as NOx-enriched precipitation (12). As a general guideline, SG bioenergy growers might apply 10 lb of N per acre per ton biomass harvested. In some situations, as little as 5 lb of N per acre per ton may be adequate to sustain maximum once-per-season, post-senescence-harvest yields.

Responses to P and K are variable and limited. Maintaining soil P and K at moderate levels likely will support high productivity in single-harvest systems. Replenishment needs will be lower if post-senescence harvests are delayed, allowing for nutrient leaching.

Weeds, Insects, and Diseases

The true extent of weed, insect, and disease problems in established SG stands is unknown. Limited research has been conducted in any of these areas. Most information is anecdotal. It is also unclear how pervasive these problems may become as SG acreage increases.

After establishment, weeds would not be expected to become a problem in well-managed SG stands because of the dense canopy and resultant light competition. Even poor stands at establishment can become robust in their second or third year (34). We are beginning to hear southern reports of invasions of SG stands by winter-annuals in stands harvested to ground level at the end of the season. In such cases, it may be desirable to leave more stubble or to harvest in time for some regrowth, which would help to smother, or shade, invading weeds.

Insects, too, are likely to be of limited significance post-establishment, although this may change as acreages increase. Grasshoppers are known to feed on SG, and a stem-boring moth has been reported in South Dakota (27). The occurrence of several species of nematodes has been correlated with reduced yields and persistence of SG stands in Texas (9).

Rusts, spot blotch, viruses, and smuts on SG have all been reported; and disease pressures on SG are likely to be greater in humid environments. Heritable resistance to rust is reported (16). In Texas, upland cultivars appear more susceptible to rusts than do lowland types (10). Spot blotch was considered the most important disease in SG in Pennsylvania, and limited P nutrition was thought to be a factor in low resistance (42). Smuts may be the biggest cause for concern in large-scale SG-for-biomass scenarios. Reports from Iowa indicate that yields and stands can decline significantly with smut infestations (15).

Yields and Harvest Considerations

Biomass for energy (from any species) is a low-value commodity that initially will probably not attract a price of more than ~$55/ton delivered (4). Therefore, producing high yields with low inputs is even more important than with higher-value crops. Bransby et al. (4) estimated the break-even yield for SG at $55/ton was about 4.5 ton/acre; yields above this level would be needed for the crop to be profitable. Yields reported by McLaughlin and Kszos (23) for the top two cultivars in 13 trials across the USA ranged from 4.2 to 10.2 ton/acre/year, and averaged 6.5 ton/acre/year. However, these yields were recorded on small research plots and could be 20% or more above what might be expected in commercial operations. On the other hand, as cultivars specifically developed for biomass/biofuel purposes become available, we may expect to see average yields increase (24,40). Furthermore, studies of spatial variation in SG biomass yield revealed a range of 1.3 to > 9 ton/acre within very short distances in a typical field (39), suggesting management may be able to coax higher yields also.

Yield potential of SG generally increases with decreasing latitude (with the exception of the Florida peninsula) because of the associated increase in the length of the growing season and better adaptation of the more productive cultivars (7). As a generalization, there is also an increasing yield gradient from the Rocky Mountains to the eastern USA, probably correlated with quantity and reliability of rainfall (28).

With one rather significant exception (38), harvesting once a year typically provides similar yields to two cuts per year, especially for lowland types (24,28). Two upland cultivars, Cave-in-Rock and Shelter, have shown a slight seasonal yield advantage with two cuttings in some locations (13,14). While two harvests may sometimes provide higher yields, the increase in yield will generally not be large enough to justify the cost of the second harvest or the additional N required. In some cases, cutting twice or more per year lowers yields and reduces stand persistence compared to a single harvest [e.g., (13,18)].

Harvesting right after seedhead production can provide maximum biomass yields within a growing season; but continuously harvesting at the annual peak of biomass accumulation can reduce long-term yields and stand persistence (18,19,28). A single harvest in late fall – after plants have "recycled" key materials to belowground parts (28) – may provide highest long-term, sustainable biomass yields and best feedstock quality (22,26). However, weather, soil conditions, weed control, and other considerations may favor different harvest patterns in some locales.

In the South, where a harvest in late fall can lead to invasion of winter annuals, an early-fall harvest may be preferred. Such management can provide high yields plus modest autumnal regrowth, which may reduce invasion by winter weeds, protect next season’s early shoots from late-spring frosts, and provide habitat for wildlife. However, this harvest schedule will call for more N (than if harvested later), since early-fall harvests remove higher-N vegetation.

In some places, it may be feasible to leave SG standing and harvest it well into the winter (28) or to mow it in the fall and bale it in the spring. These approaches would spread out the harvest period and optimize equipment usage. They also allow for leaching of some nutrients from the dead biomass, which potentially improves feedstock value and reduces fertilizer inputs (1). Employing such strategies is very dependent on local climatic conditions. The delayed harvesting of SG appears most suited to northern areas, as the colder temperatures plus low N content of the biomass reduce potential for decomposition. The potential for wildfires can be high with these strategies, especially if the biomass is left standing over winter.

A cutting height of 6 to 10 inches is appropriate for mid-summer or early-fall harvests, leaving sufficient axillary buds for regrowth (18). If SG is harvested after full senescence, cutting height can potentially be set as low as feasible. Where winter hardiness is a concern, however, a 4-inch stubble can facilitate snow accumulation and reduce winter injury. Furthermore, particularly with the coarser-stemmed lowland cultivars, shorter stubble can puncture tires of implements.

Summary

In this paper, we have primarily emphasized results from recent work. The growing literature on the history, biology, agronomy, and improvement of SG as an energy crop has been reviewed extensively in the last few years [e.g., (23,24,28,31,32,33,40)]. See also the review by Fike et al. (12), which puts SG and energy cropping in a broader context and identifies several likely difficulties to be encountered when trying to beat plowshares into oil wells.

A number of SG-for-biomass management guides have been developed by Extension personnel and by other non-commercial and commercial sources [e.g., (27,29,30,37,41)]. Such guides are region-specific – as they should be. One would always do well to seek locally developed recommendations on cultivar selection, planting dates, weed control, pests, and general SG management.

In sum, practitioners who have familiarity with managing SG as forage can adapt that knowledge to a new use for the species – bioenergy. However, our knowledge of the species is still quite incomplete; it has been looked at in monocultures for only three or four decades (28). Some of what we know about SG as forage does not translate readily to SG as energy crop; but, as we gain greater understanding of the species, it may well prove an ally in our search for renewable energy.

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