Optimization of plastic covers for stack silos.
1988
Savoie P.
Recently introduced silage conservation methods rely on polyethylene film as a barrier against air infiltration. Round bale silage stored in 0.0100 cm thick individual bags requires about 5.5 kg of plastic per t of DM; stack silos covered with a 0.0150 cm thick film require 1.3 kg of plastic per t of DM. The quality of plastic used for stack silos is very important to limit respiration loss in the large mass being protected. A theoretical model was developed to predict the total cost of plastic and respiration losses due to air infiltration through the film. The optimum thickness (at min. cost) was derived as a function of storage period, silage density and DM content, film permeability and the relative value of plastic and silage. Simulation results indicated that monthly respiration losses vary between 2.4 and 0.16% for polyethylene thicknesses between 0.0025 and 0.0400 cm. Experimental data from small silage bags showed monthly loss levels of 0.2% under a perfect seal and 0.7% under an imperfect seal (1 hole of 1 cm diam.). The model predicted that commercially available thicknesses of 0.0100, 0.0150 and 0.0200 cm are optimum for storage periods of 3, 7 and 12 months, resp. The total cost curve is relatively flat near the optimum thickness; this suggests that it would be wise to choose the closest available size above rather than below the optimum to take advantage of the greater physical resistance to tear and puncture of thicker film.
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