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Extension > Environment > Agroforestry > Native Perennial Grasses in CRP Lands - Biomass yield, energy values, and chemical composition

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Native Perennial Grasses in CRP Lands - Biomass yield, energy values, and chemical composition

Diomy Zamora and Gary Wyatt

Why Native Grasses for Energy?

Native perennial grasses are potential energy sources because they produce significant amount of biomass while providing a plethora of ecological services. Perennial native grasses reduce the frequency of, and potential soil and environmental degradation associated with annual tillage. They also capture solar radiation for a longer portion of the year compared to annual species, as well as help stabilize soils, thus reducing erosion, improving water quality and wildlife habitat, while sequestering carbon.

Project Objectives

Biomass production of native grasses is not being adopted due to lack of knowledge and markets. Past studies were more focused on individual forage instead of mixed native perennial plants as bioenergy feedstocks, and on the effect of agronomic practices on perennial grass productivity. This factsheet series # 1 aims to share our study results comparing the biomass and energy values between monoculture and mixture of grasses and forbs in Conservation Reserve Program (CRP) sites in Minnesota and examined how seasonal time of harvest affected biomass yield and chemical composition for energy production.

The Study

The study was conducted in four established (polyculture native/forbs perennial grasses) CRP sites located in the Minnesota River watershed encompassing Watonwan and Cottonwood counties in Southwestern, Minnesota. Each site, which varied in species composition, was designated as (i) monoculture of switchgrass, MS, (ii) mix of four grasses with little forbs, 4G + F, (iii) mix of 5 grasses and 13 forbs, 5G + 13F, and (iv) 5 grasses mix, 5G. Prior to converting into CRP using native mix grasses, each site was under row crop production systems. Soil characteristics of each site varied.

Six strips in each site were established with a total of 24 strips for the entire study. Each strip measuring 8 x 400 was mowed using disk mower and harvested grasses were then baled after harvesting. Prior to baling, four grass samples were systematically collected in the strip such that samples were collected from 50, 150, 250 and 350 away from the start of the strip. Biomass production and its associated energy values were assessed based on time of harvest. There were 3 harvests in each of the 4 fields. Strips were harvested in May 2009 (Spring Harvest), strips harvested in May 2009 were then allowed to regrow and harvested in the Fall of 2009 (Fall Regrowth), and the Fall only strip was harvested in the Fall of 2009 (Fall Harvest). Harvested biomass was determined for each time of harvest and compared with each other. Baled materials were weighed to determine biomass (ton ha-1). Mowing and harvesting equipment was standard field scale.

The native perennial grasses potential ethanol yield was determined from chemical composition (cellulose, hemicellulose, and lignin) and was calculated based on chemical composition analysis using the USDA National Research Energy Laboratory (NREL) online theoretical ethanol yield calculator.

Summary of Results and Implications

Biomass Production

Among the native grasses, the 5 grass mixture (5G) produced the highest amount of biomass (7.9 Mg ha-1), the largest theoretical ethanol yield (425 L Mg-1 of dry biomass), and the highest energy value of 16,731 kJ kg-1 of dry grass after 7 years of growth (Fig 1A, 1B, and 1C). 5G also contained mass fraction of 36% cellulose, 28% hemicellulose, 20% lignin, and had the lowest ash content at 6.04% (Fig 1D). The high biomass yield and energy content observed in 5G is also partly explained by the high site fertility in terms of NO3-1 availability that was highest in the 5 grass mixture. Reduced ash concentration, which was true for 5G, is related to increased biofuel quantity, as high ash content of a plant part makes it less desirable as fuel.

Fig 1 - Grass biomass production (A) of different Conservation Reserve Program sites in Southwestern, Minnesota, USA that were evaluated for the study and their corresponding theoretical ethanol yield (B), energy value/content (C), and ash content (D). Each site differed in species composition. Means with the same letter are not significantly different at α=0.05 level of significance.

Energy Content

Fig 4 - Chemical composition (cellulose, hemicellulose, and lignin) of grasses harvested for biomass from the Conservation Reserve Program (CRP) with varying species composition.

Seasonal timing of native grass harvests significantly affected ethanol yield and energy content. Strips of native grasses harvested in Spring 2009 that were allowed to re-grow and harvested in Fall 2009 showed greater potential for ethanol production than those harvested in Spring 2009 (Fig 2). In fact, both theoretical ethanol yield and energy content were similar between the Fall Regrowth and Fall harvests (Fig 4). However, combining biomass harvested in Spring and Regrowth (harvested in Fall) would have higher biomass yield than Fall harvest alone (Fig 2). Harvesting biomass in Fall produces more energy than in Spring because of higher quality feedstock as changes in mineral concentrations occur in plants after Fall, and most of these minerals are translocated to the belowground biomass. Hence, timing of biomass harvest for energy is important and biomass production in grassland areas could be improved if such area is harvested twice a year.

Plantings of grass mixtures generally produce an average of 1.7 times more biomass than monocultures (Cardinale et al., 2007). Sites with varied species composition produce more ethanol per ton of biomass primarily because of the higher photosynthetic pathway of the species capable of producing more biomass. Compared with hybrid poplars (see Factsheet Series # 2), grassland biomass has lower lignin content than hemicellulose (Fig 4). Grass with less lignin content has greater potential for energy production of biofuels since lignin significantly inhibits release of fermentable sugars in biomass.

Fig 2 - Grass biomass production based on time of harvest of the 5G CRP site used for this study. Means with the same letter are not significantly different statistically.

Fig 3 - Grass theoretical ethanol yield. Means with the same letter are not significantly different statistically.

Implications

These results have implications on developing alternative (low-input high diversity systems) uses of CRP lands. Plant diversity has long been suggested as a way to maximize sustainable biomass production of prairies for biofuel production. Mixed prairie systems also provide ecological services that monoculture crops do not. Plots with a higher percentage of grasses displayed higher biomass yields. The 2 harvests in one year combined for the highest biomass yield harvested in this study (spring and regrowth harvests). The practice of a spring and regrowth fall harvest may not be sustainable. This was a one year study of established CRP fields that were seeded and managed differently. Changes in the intended seeding mix changed over time due to many factors including broadleaf herbicide use to control thistles. More research is needed to determine the best management practices and perennial grass/forbs mixes that produce quality biomass energy and yields that can be sustained for a long period of time.

Contact Information

Diomy Zamora, zamo015@umn.edu, 612-626-9272
Gary Wyatt, wyatt@umn.edu, 507-389-6748

Note: Information presented in this factsheet series was taken from a peer reviewed article:
Zamora D, Wyatt G, Apostol K, Tschirner U. 2013. Biomass yield, energy values, and chemical composition of hybrid poplar in short rotation woody crops production and native perennial grasses in Minnesota, USA. Biomass and Bioenergy. Vol. 49. pp. 222-230.

Acknowledgement
Funding was provided by the Productive Writing Conservation Lands Program of the Tree Rivers RC&D

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