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Article Excerpt
BIOMASS is a renewable resource that can be used to generate electricity, heat, or liquid fuels such as ethanol. A successful perennial grass-based bioenergy system requires reliable establishment and persistence, knowledge of optimum cultural and production practices, high yielding cultivars, and appropriate conversion technology. The Conservation Reserve Program (CRP) is a land retirement program established by the Food Security Act of 1985. The main objectives of this program are to reduce soil erosion, reduce commodity surpluses, and to supplement farm income (Jewett et al., 1996). Native warm-season grasses such as switchgrass are permitted for use as permanent vegetation on CRP land. Rather than losing the environmental benefits and converting switchgrass CRP land to traditional crops when contracts expire, the herbaceous material could be used as a biomass feedstock.
Switchgrass can produce up to 14 Mg dry matter (DM) [ha.sup.-1] in parts of the USA, more than 60% of which is lignocellulose, which contains the desirable components for biomass energy (Vogel and Masters, 1998). At similar stages of maturity warm-season grasses such as switchgrass have higher lignocellulose concentrations than cool-season grasses and legumes (Reid et al., 1988) and, thus, are well suited as biomass feedstocks. In addition, lingo-cellulose concentrations increase as switchgrass matures primarily as a result of a decrease in the leaf-to-stem ratio (Griffin and Jung, 1983; Twidwell et al., 1988).
Switchgrass is sensitive to frequent defoliation because it elevates the apical meristems above ground during early vegetative development. New growth occurs from crown buds or aerial axillary meristems, and it has a high ratio of reproductive to vegetative tillers early in the growing season (Mitchell et al., 1998; Sanderson et al., 1999). Harvesting monoculture stands of switchgrass three times per year reduced yields by 51% as a result of reduced photosynthetic surface area on frequently defoliated plants (Cuomo et al., 1998). In the Midwest, Vogel et al. (2002) recommended harvesting switchgrass at the R3 to R5 stage of maturity (panicle fully emerged from boot to anthesis) to maximize biomass yields and allow sufficient regrowth to occur such that in years with adequate precipitation a second harvest (after a killing frost) may be obtained. Monoculture switchgrass cut once annually for 2 yr at anthesis, anthesis plus 3 wk, or anthesis plus 6 wk, produced average yields of 7.5, 7.3, and 8.6 Mg ha 1, respectively, while plots harvested two times per year (at anthesis and after a frost) produced lower yields (6.5 and 1.0 Mg [ha.sup.-1], respectively) (Balasko et al., 1984).
Switchgrass efficiently uses nitrogen and phosphorus (Vogel et al., 2002). Yearly nitrogen application up to 150 kg [ha.sup.-1] resulted in an average yield increase of 15 kg DM [kg.sup.-1] N applied (Berg, 1995). Others have also noted an increase in yield (Hall et al., 1982; George and Obermann, 1989; Madakadze et al., 1999) and TN (Jung et al., 1990; Brejda et al., 1995) in response to applied N to warm-season grasses.
Despite the potential for biomass production of switchgrass enrolled in CRP, data related to harvest and N management of this resource are lacking. Therefore, the objectives of this study were to: (i) determine the effects of harvest timing and N rate on yield and biomass characteristics of long-established switchgrass stands enrolled in or managed similarly to CRP and (ii) evaluate the impact of harvest management on species composition and persistence in these stands.
MATERIALS AND METHODS
This research was conducted at three locations in South Dakota. Location 1, in Moody County (96[degrees] 41' W; 44[degrees] 10' N), is a Wentworth-Egan soil (fine-silty, mixed, mesic Udic Haplustolls) with a 2 to 6% slope. Age of stand was 13 yr in 2000. Location 2, in Marshall County (97[degrees] 20' W; 45[degrees] 50' N), is situated on Buse (fine-loamy, mixed, frigid Typic Calciudolls) with a 6 to 9% slope. Age of stand was 12 yr in 2000. Location 3 is located in Gregory County (99[degrees] 45, W; 43[degrees] 43' N) on a Ree loam (fine-loamy, mixed, mesic Typic Argiustolls) with no slope. Age of stand was 9 yr in 2000. Locations 1 and 2 were enrolled in CRP while Location 3 has been managed for wildlife habitat (similar to CRP) since establishment. Detailed records for years in which these stands were harvested for hay (emergency CRP release on CRP land) or burned were unavailable; however, to our knowledge none of the herbage from any of the locations was routinely removed during the stand life. These sites were selected in 2000 and mowed at a 10- to 15-cm height in the autumn before imposing treatments in 2001. Selected soil chemical and physical properties in the top 15 cm for each location are shown in Table 1. Phosphorus (2.5 g [m.sup.-2]) was broadcast across the entire experimental area at each location during spring 2001 to bring soil fertility to recommended levels.
The experimental design was a factorial arrangement of two harvest dates and five N rates within a randomized complete block with four replicates. Plots 1.9 m wide and 6.1 m long were harvested once per year in 2001, 2002, and 2003 at anthesis or post-killing frost. Five levels of N fertilization consisted of a spring application of 0, 56, 112, and 224 kg [ha.sup.-1] and split application of 224 kg [ha.sup.-1] (one-half applied in spring and the other half applied after the anthesis or killing-frost harvests). The nitrogen source was ammonium nitrate, which was preweighed and hand-broadcast onto each plot on the dates shown in Table 2.
All locations were treated with herbicides as needed to help control broadleaf and grassy weeds. In 2003, glyphosate [N-(phosphonomethyl) glycine] was applied on 12 May at Location 1, 2,4-D amine [isooctyl (2-ethylhexyl) ester of 2,4-dichlorophenoxyactic acid] and atrazine (2-chloro-4-ethylamine-6-isopropylamino-s-triazine) were applied 12 May at Location 3, and clopyralid (3,6-dichloro-2-pyridinecarboxylic acid, monethanolamine salt) was applied 12 June at Location 2.
Yield was determined from a 1.1-m-wide by 4.8-m-long swath through the center of each plot with a sickle bar mower at a cutting height of 10 to 15 cm on the dates listed in Table 2. One 0.19-[m.sup.2] subsample was hand-clipped from each plot before mowing for the anthesis and killing-frost harvests in 2001 and anthesis harvest in 2002. Two 0.19-[m.sup.2] subsamples per plot were taken at...
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