Topsoil Thickness and Harvest Management Influence Switchgrass Production and Profitability

Abstract

Core Ideas Topsoil thickness, or depth to claypan, mainly affected the biomass harvest of the integrated system. The integrated system had greater weed cover, less yield, more N, P, and K removal, and less profit than the biomass only system. The only advantage of the integrated system was enhanced yield resiliency in an extreme drought year. Switchgrass (Panicum virgatum L.) is an attractive dual use forage and/or biomass crop option for eroded soils where corn (Zea mays L.) grain production is often not profitable. Topsoil thickness, especially on soils with a claypan, relates directly to crop productivity and nutrient removal, but knowledge is lacking on how it impacts switchgrass grown for summer forage and advanced biofuels. Therefore, a study was conducted near Columbia, MO, to determine how topsoil thickness, or depth to claypan (DTC) affects the production and profitability of integrated (forage and biomass) and biomass only switchgrass. Switchgrass was planted in 2009 on a range of DTC classified as exposed (<8 cm), shallow (8–15 cm), moderate (16–30 cm), or deep (>30 cm), and was annually harvested twice (pre‐anthesis and postdormancy) in the integrated system or once (postdormancy) in the biomass only system during 2011 to 2015. Results indicated that DTC mainly affected the second harvest of the integrated system; biomass yield increased 74% and nutrient removal by 50 to 102% from exposed to deep soil across years. Across DTC and years, the integrated system had more severe weed cover (46 vs. 6%), less yield (9.8 vs. 13.5 Mg ha−1), more N, P, and K removal (79 to 206%), and less profit (US$–155 vs. $137 ha−1 or $80 vs. $220 ha−1 without nutrient replacement costs) than the biomass only system. Therefore, improvements are needed before integrated systems are profitable, and biomass only systems may offer greater longevity, productivity, and profitability.