Date of switchgrass harvest affects biomass yield and quality, and both of these affect biofuel production and its associated economics. Balancing these components may vary depending on conversion systems. A single harvest after flowering is the preferred timing, but harvesting any time between early flowering and just after frost generally produces the highest switchgrass biomass yield.
If harvest of the standing crop is delayed until the next March, a decrease in biomass yield could reach 40 percent. The main reasons for this decrease are: 1) decrease in tiller weight, 2) increased biomass lost during the harvesting process, and 3) losses due to weathering over winter. Spring harvesting, however, provides habitat for wildlife during winter and may be preferred over burning to clean up the field.
Number of Harvests
No more than two harvests per production season should be taken or a reduction in biomass yield and stand density could occur. Switchgrass stubble height should be no less than 6 inches. The tall stubble helps maintain effective growing points and reduces tire punctures by allowing equipment tires to push over the stubble. This cutting height also helps trap winter precipitation in the form of snow and lessens the chance of winter kill. Switchgrass stand survival, vigor, and yield can be increased by harvesting only during the recommended harvest windows and at the appropriate stubble height.
Equipment and Storage
Switchgrass can be harvested with traditional hay swathers and balers — large round bales or large square bales (See Figures 6-1 and 6-2). The preferred package is the large square bales because they are easier to manage for transportation and long-term storage. Baled switchgrass may be stored unprotected outside in dry areas. In areas with high rainfall (> 30 inches), a significant amount of dry matter loss should be expected. Storage in barns reduces biomass losses but increases overall production costs.
Switchgrass quality is another important trait affecting efficient biofuel production. Lignocellulose yield is important for fermentation and gasifi cation processes and is favored by fall harvest due to the high biomass yields. For direct combustion as pellets, spring harvest is favored due to its lower mineral concentration because of leaching during winter. Moisture should be 15 percent or less to facilitate baling and transportation. This also ensures a higher quality feedstock. Switchgrass that is co-fi red in coal plants is burned at a moisture percentage of 12 to 13 percent. As with hay, the lower the moisture content, the higher the handling losses while swathing and baling.
Windrows made with a traditional hay swather.
Harvesting switchgrass in a windrow with a round baler.
Grazing and Hay Production
A possible option is to graze or hay switchgrass in the spring when switchgrass plants have substantial growth and the forage nutritive value is relatively high. After the spring harvest, subsequent regrowth could be harvested for biomass at the end or after the growing season. This dual purpose option of switchgrass may reduce biomass yield, but could increase farmers’ income, especially in the early stage of introducing switchgrass into the existing farming landscape.
Grazing and haying are traditional harvest methods for native perennial grasses and may remain important for switchgrass grown in monocultures. While the switchgrass biofuels industry is being developed, grazing may offer opportunities to some producers who do not want to devote land for two years without opportunities for revenue. Switchgrass forage has good quality for grazing before it begins heading. Once seed heads are formed, nutritive value for grazing or hay falls dramatically.
Spring-planted warm-season grasses such as switchgrass, are slow to establish and grazing is not usually an option during the seeding year. With good weed control during the seeding year, grazing may be an option the following year. If there was no effective weed control or weeds were just clipped the fi rst year, it may take two years to realize any appreciable grazing. This underscores the importance of beginning with a weed-free seedbed.
It may be possible to use switchgrass as a forage crop early in the season and use the regrowth for biofuel, but care must be exercised. Regrowth potential following grazing for tall, upright plants, such as switchgrass, is low. This is because the growing points are easily removed. Any new growth must come from the crown of the plant.
There are two critical
heights to remember about grazing switchgrass.
• The first — grazing should not begin until switchgrass reaches at least 18 inches tall.
• The second — it is important to leave a stubble height of no less than 6 inches when
grazing or haying.