Optimization of plastic covers for stack silos

Abstract

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 tonne of dry matter; stack silos covered with a 0·0150 cm thick film require 1·3 kg of plastic per tonne of dry matter. The quality of plastic used for stack silos is very important to limit respiration loss in the large mass being protected. A theoretical model is developed to predict the total cost of plastic and respiration losses due to air infiltration the film. The optimal thickness (at minimum cost) is derived as a function of storage period, silage density and dry matter content, film permeability, and the relative value of plastic and silage. Simulation results indicate 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 (one hole of 1 cm diameter). The model predicted that commercially available thicknesses of 0·0100, 0·0150 and 0·0200 cm are optimal for storage periods of 3, 7 and 12 months respectively. The total cost curve is relatively flat near the optimal 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.