Experiment 1: Specific Gravity of Corn Components

Several stalks of whole-plant corn were cut with a scythe at 10 cm from the ground at full maturity (early December 2002) near Madison, WI. Plants were separated manually into five components: grain, stalk, leaf, husk, and cob. The components were oven-dried at 103°C for 24 h (8) to estimate moisture content and the respective proportions of dry matter (DM). Specific gravity was estimated with a PMI Automated Gas Pyc­nometer (Porous Materials, Ithaca, NY), which measured the pressure change in helium gas as it surrounded the crop component in an enclosed volume (9). Three iterations were done for each sample in the pycnom­eter, and three replications were done for each component in two states: intact grain or coarsely chopped stover, and ground components. Chop­ping was done with a laboratory chopper set at 13-mm theoretical length of cut. Grinding was carried out with a Model 4 Thomas Wiley Mill using a 1-mm screen (Thomas Scientific). The specific gravity was corrected to a dry basis by mass balance:

pwm ph2o (l00- MC) 100 p h2o- p wm mc

in which pDM is the corrected specific gravity on a dry matter basis, pWM is the wet matter specific gravity as measured experimentally, pH2O is the specific gravity of water (1000 kg/m3), and MC is the moisture content on a wet basis (%).