Effects of Reduced Switchgrass Seeding Rates On Dry Matter Yield & Plant Population.

Switchgrass (Panicum virgatum L.) may become an important bioenergy crop; however widespread use may be limited by slow and erratic establishment.  Recommended seeding rates for establishing lowland switchgrass vary from 2.24 kg pure live seed (PLS) ha^-1 and 8.96 kg PLS ha^-1. Previous Noble Foundation experiments indicated that after successful establishment and with accepted cultural best management practices (BMPs), initial seeding rates above 2.24 kg PLS ha^-1 had no significant effects on crop cover and dry matter yields by the end of the second and third seasons after establishment. Minimal information is currently available regarding switchgrass performance and rate of compensation towards full stands over time when planted at seeding rates below 2.24 kg PLS ha^-1.  Producers could benefit from guidelines to assist in determining whether or not to re-seed switchgrass when less than favorable conditions during establishment result in partial stands.  A set of randomized complete block design field experiments were initiated in south central Oklahoma during 2010 and repeated in 2011 to evaluate the effects of reduced seeding rates on stand establishment and dry matter yields over time.  Objectives of these studies are to determine the minimum seedling density required for production and critical density below which re-planting is justified.  Three soils (sandy loam, loam, clay loam) were used for the experiments.  EG1101 lowland switchgrass was planted at three locations on 35.6 cm row spacing in early May of 2010 and 2011 at seeding rates of 0.11, 0.28, 0.56, 1.12 and 2.24 kg PLS ha^-1 respectively. Response variables evaluated included seedling counts m^-2 30 days after planting (DAP), end of season (EOS) plant population and dry matter (DM) yield, and EOS visual estimates of percent switchgrass cover. With no location x treatment interaction detected for 30 dap seedling counts, average seedling counts across all locations for both establishment years were 7, 14, 23, 42, and 65 m^-2 for the 0.11, 0.28, 0.56, 1.12 and 2.24 kg PLS ha^-1 seeding rates, respectively. Establishment year mean EOS plant population and DM yield for the highest seeding rate (2.24 kg PLS ha^-1) were both significantly higher than those means for all other seeding rates. DM yield mean separations for 2010 increased linear as seeding rate increased. Data collection and interpretation from both years is ongoing and these trials will be repeated at the same locations again during 2012 to gain additional data to assist producers in determining minimal seedling threshold for replanting purposes.