Renewable energy continues to be of interest in the USA due to concerns about the long-termed availability and the environmental impact of fossil fuels. Miscanthus � giganteus is a warm-season perennial bioenergy feedstock grass that has a long growing season, is high yielding, requires limited inputs, and will likely become an important renewable energy crop in the USA. This research studied the effects of N application rates and harvest timings on yields and biomass quality of a 4-year-old stand of M. � giganteus in Urbana, IL, USA. Plots received 0, 56, 112, 156, or 224 kg N ha<
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, and harvests were conducted at five dates between August and March in the 2009, 2010, and 2011 growing seasons. Miscanthus � giganteus production increased with N applications up to 112 kg ha<
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. Harvesting biomass before senescence, August through November, produced significantly more biomass compared to harvesting biomass after senescence. However, early harvest reduced biomass yields in the following years. Here, nitrogen fertilization compensated for the yield losses, but the yield gap between early and late harvest continually increased. Harvesting M. � giganteus after senescence optimized long-term productivity, reduced the need for N fertilization, and increased carbon and ash content in the harvested biomass, while harvesting before senescence not only removes N from the plant but also reduces remobilization of other nutrients and carbohydrates in autumn.