When to harvest switchgrass

Switchgrass should be harvested once in the fall, after killing frost, for biofuel production.

Baling frost-killed switchgrass in the fall at the Great Lakes Bioenergy Research Center scale up field near Kellogg Biological Station.

Baling frost-killed switchgrass in the fall at the Great Lakes Bioenergy Research Center scale up field near Kellogg Biological Station.

Switchgrass (Panicum virgatum) is one of the most popular biomass feedstock candidates for future cellulosic ethanol production. Switchgrass is native, fast growing, has a long growing season, and will produce quality biomass for over 10 years. It grows well on marginal lands and nutrient-depleted crop fields, is water efficient, drought resistant, and helps boost soil organic matter. Harvest typically involves traditional forage harvesting equipment that results in large round or large rectangular bales. Timing of harvest can impact yield, moisture, stand longevity and fertility. This article examines current research to identify the optimal timing of switchgrass harvest for biofuel production.

A great deal of research has been done to examine whether switchgrass should be cut once or multiple times per year. Research from University of Florida’s Chae-in Na in 2013 found that multiple harvests for some crops reduced long term (starting in year three) biomass yield due to stand loss. In an article published by Wullschleger et al., a database of switchgrass biomass productivity studies was compiled and analyzed with 39 field sites in 17 states. The analysis showed that single harvest systems were the most practical and economically feasible for bioenergy systems. This is the generally accepted practice by production agronomists involved in bioenergy production. So the question gets down to when switchgrass should be harvested.

The study “Switchgrass Biomass Production in the Midwest USA: Harvest and Nitrogen Management” reported that switchgrass yields increase during plant growth up to anthesis (flowering), then decreases by 10-20 percent until killing frost. Thus, maximizing biomass yield would mean harvesting at anthesis. Upland ecotypes like Cave-in-Rock, planted in northern climates like Michigan, will senesce rapidly and go completely dormant within seven days after a killing frost, according to Rob Mitchell and Marty Schmer in “Switchgrass Harvest and Storage.” As plants senesce, they translocate nutrients from the leaves and stems to the roots for over winter storage. Switchgrass biomass yield and moisture tend to decrease over the winter. Danielle Wilson of Iowa State University found a 44 percent reduction in yield over the winter (4.46 tons per acre in October to 2.50 tons per acre in April). Lodging caused by heavy snow and wind is the primary cause for the overwinter yield loss. Cutting at least 4 inches high will help reduce injury to plant crowns which directly affects tillering in the spring.

Best management practices should include methods to maximize biomass yields, reduce risks to stand persistence and allow for harvest and storage systems that are adaptable and conducive to being able to supply biomass throughout the year to a biorefinery. While harvesting at anthesis may maximize yield, this practice will reduce translocation of nutrients to the roots and risk stand persistence and quality long term. Getting biomass dry enough after cutting to bale and store properly can be a challenge and is completely weather-dependent. Plants naturally senesce and dry down after killing frost. In order to make a single harvest effective, harvest should occur seven days after the first killing frost. As previously reported, there will be some yield loss, but overall this is Michigan State University Extension’s recommended practice for long-term biomass supply to a biorefinery.

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