Residues of dead cover crops can suppress weeds by providing a mulch on the soil surface. The cover crop usually is killed with herbicides, but a mechanical method is desirable in systems intended to reduce chemical use. We designed and built an undercutter to kill cover crops by severing their roots while flattening the intact aboveground biomass on the surface of raised beds. We studied which cover crop species could be killed with the undercutter and compared the weed control potential of cover crop residues after flail mowing, sicklebar mowing, and undercutting.Whether a species was killed by the undercutter depended primarily on growth stage. Species that were in mid- to late bloom or beyond, including rye, hairy vetch, bigflower vetch, crimson clover, barley, and subterranean clover, were easily killed by undercutting. There were no differences in dry weights of broadleaf weeds between the undercut and simulated sicklebar mowed treatments, both of which had less weed biomass than the clean-tilled or flail-mowed plots.

The upward vertical flow of oil‐water mixtures has been investigated in a 37‐ft. length of cellulose acetate butyrate tubing of inside diameter 1.038‐in. Flow pattern, holdup and pressure drop data were obtained for water mixtures with 0.936, 20.1 and 150 centipoise oils at superficial water velocities ranging from 0.10 to 10.0 ft./sec. The oil‐water mixtures exhibited a behavior similar to that of air‐water mixtures studied previously. The flow patterns observed at constant superficial water velocity with increasing oil‐water ratio were: drops of oil in water, slugs of oil in water, froth, and drops of water in oil. Holdup of the phase forming the continuous medium was observed but to a much lesser extent than with the air‐water system. Curves of pressure drop versus oil‐water ratio exhibited a minimum, a maximum and a second minimum at low water velocities; a single minimum at intermediate water velocities; and a steady increase at superficial water velocities above about 5 ft./sec. A friction factor based upon the properties and the superficial velocity of the water is correlated with the superficial velocity of the oil and a Reynolds number based on the properties and superficial velocity of the water. This shows that the pressure drop due to friction and other irreversibilities is essentially independent of the viscosity of the oil except under conditions where the oil is the continuous phase.

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