Farm-scale bunker silos were filled with whole-crop maize and their top surfaces covered with either two layers of standard low-density polyethylene film of 150μm thickness, following normal practice, or a single layer of low-density High oxygen barrier (HOB) film of 45μm thickness. Total weight of film used per silo was 241.5 kg for standard film and 43.4kg for HOB film. Primary energy used per silo for the manufacture of the films was 18.9GJ for standard film and 3.39GJ for HOB film. Estimated global warming potential of film used per silo was 514.4kg CO2e for standard film and 92.3kg CO2e for HOB film. Mean composition of samples of silage taken from the top 30cm of each silo was similar between the two types of film. The use of HOB film reduced primary energy use and greenhouse gas emissions associated with film by 82% without affecting silage composition adversely.
A bunker silo of 40m length, 12m width and 2.5m height contains almost 20% of the original ensiled crop in the top 0.5m. Studies with 127 farm-scale silos in the USA over a four-year period revealed that loss of organic matter (OM) during the storage period was 470g kg-1 in the top 0.5m of uncovered silos, compared to113g kg-1 for the same silage 0.5m to 1m from the top surface (Bolsen, 1997). These losses illustrate the importance of maintaining an effective barrier to both water and oxygen throughout the storage period. Covering silos with polyethylene film reduces losses by protecting the crop from the effects of wind and rain and also by reducing, but not preventing, oxygen permeation into the silo. Normal practice in northern Europe is to line the side walls with a single sheet of film that overlaps the periphery of the top surface and to use two layers of film to cover the top surface itself.
Global consumption of low-density polyethylene film for silage was 582.5kt in 2012. We estimate that 368kt of polyethylene film are used annually worldwide to cover walled bunker and unwalled clamp silos and 156kt of stretch-film to wrap baled silage. The production of low-density polyethylene is associated with the consumption of 78.1 MJ primary energy kg-1 and with a global warming potential (GWP) of 2.13 kg CO2e kg-1 (Plastics Europe, 2008). Additional energy is used in the recovery and recycling of film and initiatives have been launched in some member states of the European Union to encourage the recycling of agricultural plastics. For example, in France farmers are charged €65 t-1 of film to support a recycling programme (Comité Français des Plastiques en Agriculture, 2012).
Losses of nutrients to the atmosphere through the aerobic deterioration of silage increase greenhouse gas (GHG) emissions per unit of animal product output and have a large negative impact on net return to labour and management (Van Schooten and Phillipsen, 2012). The use of high oxygen barrier (HOB) film to cover silos and bales is associated with reduced losses of organic matter during the storage period and increased aerobic stability of silage in the peripheral areas of silos, compared to standard polyethylene film (Wilkinson and Fenlon, 2013). In this paper the hypothesis was tested on farm-scale silos that the use of OB film reduced GHG emissions in silage production compared to standard polyethylene film.
Materials and methods
Two adjacent walled bunker silos of 40m length and 12m width at the Waiboerhoeve Research Farm of Wageningen UR Livestock Research, Lelystad, The Netherlands, were filled with chopped whole-crop forage maize (Zea mays L.) between 15 and 20 October 2012. Both silos were filled to an average height of 2.2m. Harvesting and ensiling equipment were identical for both silos and about 700t of fresh crop were ensiled in each silo. The top surface of one silo was covered immediately with two sheets of standard low-density polyethylene film (RKW ProAgri®, Michelstadt, Germany), following normal practice on the farm. Each sheet was of 50m length, 14m width, 150μm thickness and 0.92g cm-3 density. A third sheet of the same standard polyethylene film (50m length x 7m width x 150μm thickness, 0.92g cm-3 density) was used to line the side walls. The top surface of the other silo was covered immediately with a single sheet of OB film comprising low density polyethylene co-extruded with ethylene vinyl alcohol of 50m length, 14m width, 45μm thickness and 0.93g cm-3 density. A second sheet of the same hOB film (50m length and 5m width, 60μm thickness and 0.93g cm-3 density) was used to line the side walls. Woven polypropylene netting (Genap BV, ’s-Heerenberg, The Netherlands), weighed down by gravel bags, was placed above the top sheets of both silos. Six samples of silage of 1kg fresh weight were taken for compositional analysis (BLGG AgroXpertus, Wageningen, The Netherlands) to 30cm depth from the top surface and 2m from the outer walls from each silo during the feed-out periods; on 26 June 2013 (251 days post-ensiling) for the silo covered with HOB film and on 11 September 2013 (328 days post-ensiling) for the silo covered with standard film.
Primary energy use and GHG emissions, estimated as GWP, associated with the two films are shown in Table 1. The use of HOB film reduced total weight of film, primary energy use and GWP from film by 82% compared with standard film. Additional benefits of lower mass of film to be recycled, with further reductions in GHG, would also accrue.
Table 1. Weight of film used per silo, associated primary energy use and global warming potential (GWP): Standard film compared to HOB film.
The mean composition of the silages in the top 30cm stored under standard polyethylene or HOB film is shown in Table 2. Differences in mean composition between silages stored under standard and HOB film were relatively small. With the exception of water soluble carbohydrates, lactic acid and acetic acid, coefficients of variation for compositional parameters were less than 10% for silages stored under both types of film.
Table 2. Composition of silages (mean ± SD of six samples) in the top 30 cm stored under either standard polyethylene or high oxygen barrier film.
Covering ensiled forage maize with a single layer of HOB film gave large reductions in primary energy and GHG associated with polyethylene film, compared with normal practice of covering with two layers of standard film. The composition of silage in the top 30 cm was similar between the two types of film.