Environmental Engineering Reference
In-Depth Information
herbage protein concentration (Rehm et al. 1977; Perry and Baltensperger 1979; Rehm 1984; Vogel
et al. 2002a) and in vitro dry matter digestibility (IVDMD) of switchgrass (Perry and Baltsenberger
1979; George et al. 1990).
Fertilizer application rates for switchgrass should be based on the difference between the
requirements of the crop and available soil N. However, time of harvest will have a significant
impact on the nutrients removed in the harvested biomass. For example, harvesting a switchgrass
field at anthesis that produces 11 Mg/ha of DM with a N concentration 1.2% N will remove about
130 kg of N/ha, whereas material harvested after a killing frost may remove only half of that
amount of N/ha. Harvesting after a killing frost may reduce N application by 30-40%. Sampling
soils to determine available N for switchgrass production must be taken to a depth of 1.5-2 m
because of the soil mineralization potential of some soils, atmospheric N deposition, residual soil N
from previous crops that may be distributed deep in the soil profile, and the deep rooting capability
of switchgrass (Mitchell et al. 2008).
On a strongly acid (pH 4.3-4.9), low P soil, unfertilized switchgrass and big bluestem (
Andropogon
gerardii
Vitman) produced 50% as much forage as that receiving a low level of nutrients (Jung
et al. 1988). When P declined from 35 to 5 mg/kg, switchgrass yields declined 12% compared to
C
3
grasses which declined 35% (Panciera and Jung 1984). On acidic, low water-holding capacity
soils, first-cut switchgrass yields were two to three times greater, and four times greater than for
tall fescue on sites with N and without N, respectively. Nitrogen-use efficiency was greater for
switchgrass than for tall fescue (Staley et al. 1991). The timing of N application is critical in the
maintenance of switchgrass stands. If N is applied too early in the spring or in the previous autumn,
cool-season plants will utilize it because switchgrass is not active. The stimulated C
3
invaders will
increase rapidly and utilize the soil moisture. Later, during the period of switchgrass growth, soil
moisture will be depleted and the vigor of switchgrass plants will decline and stands will be invaded
by additional C
3
plants which can result in the conversion of a switchgrass pasture into a mixed
species cool-season pasture.
22.5.3 h
arvESt
t
iming
and
f
rEquEncy
Cellulosic biomass of herbaceous plants can be used as a feedstock for the production of liquid fuels
such as ethanol (Lynd et al. 1991) and switchgrass has been identified as a promising species for
development into a herbaceous biomass fuel crop in the United States (Vogel 1996; Sanderson et al.
2007). Switchgrass has an array of desirable energy, conservation, environmental, and economic
attributes for its use as a bioenergy crop (McLaughlin et al. 2002). These include broad adaptation,
high yields on marginal and erosive croplands, harvestability with conventional forage equipment,
a very positive energy balance, and relatively easy seed processing.
Several trials have been conducted in the United States and other countries to optimize harvest
timing and frequency. In general, a single harvest when switchgrass is fully headed gives the
highest yields (Madakadze et al. 1999a, b; Sanderson et al. 1999; Christian et al. 2002; Vogel et al.
2002a). Biomass yield continues to increase up to anthesis, after which biomass yield decreases
up to 10-20% before killing frost (Vogel et al. 2002a). There are circumstances, some cultivars at
some locations, in which two harvests provide higher biomass yields than one harvest, but the extra
fossil fuels required to conduct two harvests may not warrant a two-harvest management system.
Harvests after a killing frost usually result in decreased biomass yields but may require lower inputs
of N fertilizer, because the plant is able to utilize N mobilized into roots for storage during winter
and recovery the following spring. Depending on location and cultivar, biomass yields of the best-
adapted cultivars ranged from 10 to more than 20 Mg/ha.
Optimal harvest management for switchgrass use in combustion conversion systems may require
delaying harvest until spring when most of the minerals have leached from the plant. Biomass yield
reductions during the winter averaged 40% in Pennsylvania (Adler et al. 2006), but this management
system may be capable of utilizing some internal N cycling, helping to reduce the N fertilizer
