How to harvest and process corn stover to maximize its quality as a fuel or industrial feedstock and minimize material losses are compelling issues in the industrial utilization of corn stover. The objectives of this investigation were to evaluate the variation in the chemical composition and energy content of aboveground components of the corn plant over time and to evaluate how composition changes after grain physiological maturity is reached and the plants are weathered while undergoing further field drying. Above ground biomass distribution and composition of two almost identical corn cultivars (Pioneer 32K61 and 32K64 Bt) were studied from an estimated 2 weeks before corn kernel physiological maturity until 4 weeks after the grain had already reached a moisture content suitable for combine harvesting. Compositional analysis of corn stover fractions gathered over the course of maturation, senescence, and weathering using NIR spectroscopy showed (1) a rapid drop in soluble glucan, (2) increase in lignin, and (3) increase in xylan. By day 151 after planting, about when grain from surrounding non-test plots was harvested at about 15.5% moisture, composition of the different fractions remained fairly constant. Since product yield in fermentation-based biomass conversion processes is proportional to the structural carbohydrate content of the feedstock, timing of stover collection and the proportion of anatomical fractions collected affect the quality of corn stover as fermentation feedstock. Since the energy content of corn stover anatomical fractions is shown to remain fairly constant over time and from one plant to another (16.7-20.9 kJ g(-1)), insofar as combustion processes are concerned, it apparently makes little difference which part of the plant is used, or at what time the material is harvested. (c) 2004 Elsevier Ltd. All rights reserved.