Swithgrass Plant Population Decline As Affected By Seeding and Nitrogen Rates.

Proper nitrogen fertilization is important for adequate stand survival and maximum biomass yield for potential use as a biofuel source. Switchgrass seeding rate and nitrogen rate experiments began in 2004 on four soil landscape positions. Seeding rates were 2.8 kg N/ha, 5.6 kg N/ha, 8.4 kg N/ha, 11.2 kg N/ha and 14.0 kg N/ha with each seeding rate split in the second year with nitrogen rates of 0, 67, 134, 201 kg N/ha. Initial switchgrass populations were adequate across all seeding rates. Since 2004 a slow decline in switchgrass population has occurred on the two highest nitrogen rates of 134 and 210 kg N/ha. This has resulted in increases in the amount of bare surface on these plots. Bare soil percentages have been rated as high as 70% with the lower rates having below 25%. This plant population loss has not affected yield with the remaining clumps of switchgrass at the higher rates being more robust and taller. The reason for stand loss at excessive nitrogen rates is not understood. Yields in 2014-2015 were higher than 0 kg N/ha at the 60, 120, 180 kg N/ha. However, even though stand loss was occurring at rates above 60, yields have been unaffected by the higher nitrogen rates. Changes in plant structure including much thicker stems and taller swords at the higher nitrogen rates are compensating for stand loss. Data on conversion efficiency relative to plant structural changes will be shown.