Discussion

The specific gravity of particles changed with their size. Intact grain was much denser than coarsely chopped stover components. However, when material was finely ground, the density of all components increased and no further difference in density was observed. The results suggest that stalk and leaf have more micropores than husk and cob while grain had the least micropores. In principle, fine chopping and processing would con­tribute to an increase in the density of all components and the proportion of stover that sinks with grain in a water separation process.

In experiment 2, most of the corn grain sunk rapidly. Between the first and the eighth water separation, the amount of sunk grain increased by only 0.4% for the Prairie-du-Sac silage, 0.9% for the Binversie silage, 1.5% for the Ziegler silage, and 5.9% for the Arlington silage. Therefore, only one or two water separations would be needed to separate most grain.

When fresh silage was separated in water, the highest grain concen­tration achieved in the sunk material was 75%; this was observed with processed and relatively dry corn silage (64% MC). Short chopped mate­rial (8 mm) actually had a higher grain concentration (68%) than long material (17 mm), whose grain concentration was only 41% largely because of a high initial MC (74%). The moisture content had a greater impact than the physical form in the range that was observed (8- to 17-mm MPL, processed or not processed).

The effect of MC was even more apparent in experiment 3 when fresh material was compared with material partially dried (10 or 20 percentage units of moisture removal) or completely dried prior to water separation. Greater than 99% grain concentration was observed with bone-dry mate­rial. As corn silage becomes drier, stover pieces are likely to become more buoyant because of their large area compared to grain. Oven-drying is therefore a good pretreatment followed by water separation to concen­trate grain from corn silage. Because of the large amount of water in silage and the high cost to dry this material, the procedure could be used for small samples in the laboratory but would not likely be feasible in an industrial setting.

Without any pretreatment, hydrodynamic separation could allow the production of a concentrate of about 75% grain and 25% stover. It is difficult to achieve a higher grain concentration without having to par­tially dry or sieve the silage. Sieving increased the grain concentration in the sunk material to 79%. From an industrial point of view, water separa­tion would require recuperation of considerable amounts of soluble and deposited fine particles (18% of original DM after one separation, and between 21 and 26% after eight separations). Hydrodynamic separation could provide a feedstock with a high concentration in grain (75-80%), but it could not provide a stover-free feedstock without drying. Com­pared to pneumatic separation and sieving alone, hydrodynamic separa­tion is likely to require less energy when drying is not used. If pure corn

components are needed, the traditional combine thresher is more suitable for providing pure grain than any poststorage separation method applied to silage. Various harvest devices can be designed to collect the stover either simultaneously with threshing or afterward with another pass machine.

Acknowledgments

This research was partially supported by the USDA-ARS, UW Gradu­ate School, John Deere Technical Center, and Wisconsin Corn Promotion Board. We also acknowledge support from the Natural Science and Engi­neering Research Council of Canada and Agriculture and Agri-Food Canada.

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