Switchgrass Harvest System Affects Nutrient Management.

Switchgrass is being valued as a feedstock for the production of energy, especially ethanol.  It is being promoted as a crop that can grow well on marginal lands with low nutrient inputs.  A study, established to evaluate the effects of nitrogen rate and harvest management system, was used to evaluate the nutrient removal in harvested switchgrass.  Switchgrass was established by seeding in May, 2007.  Nitrogen rates of 39, 78 and 157 kg ha^-1 were applied in the spring of 2008 and 2009.  Three harvest systems were used; 1) a two-cut system with the first cutting at early anthesis and the second cutting two weeks after the first killing frost, 2)  in the fall two weeks after the first killing frost, 3) over-winter cut in March. Biomass tissue samples were collected for each N rate and harvest time each year.  The tissue samples were analyzed for N, P, K Ca, Mg, S, Na, B, Cu, Mn, Zn, Fe and Al.  Results were similar for the two site years.  Content of the nutrients, other than N, were not affected by N rate.  Potassium concentration was most affected by harvest time.  Across all N treatments and the two site years the average K concentration was 1.41 % and 0.51 % for the summer and fall two cut system, 0.40 % for the single fall cut biomass and only 0.05 % for the over-wintered spring cut biomass.  Much of the K is remobilized into the root system and/or leached from the plant tissue when the switchgrass is not harvested until late fall or over-wintered.  For the two site years K removal averaged 125, 44 and less than 5 kg ha^-1 for the summer cut, fall cut and over-winter cut Switchgrass, respectively.   Similar numbers for N at the 78 kg N ha^-1 rate were 92, 61 and 41 kg N ha^-1.  Nitrogen removal increased with N rate.  For the summer harvest N removal averaged 52, 68, 92 and 109 kg N ha^-1 for the four N rates over the two site years.  For the fall harvest after the first killing frost N removal averaged 48, 54, 61 and 73 kg N ha^-1.  Concentrations of Ca, Mg, Mn and Zn were highest in the two-cut fall biomass compared to the other harvests.  Switchgrass harvested when actively growing results in much more K and N being removed from the soil-plant system than when harvest is delayed until after the first killing frost.