New concepts on baled silages.

Throughout the world, forage crops are harvested as silage on intensive dairy farms to reduce feeding costs. Among the various silage conservation methods, wrapped bales are commonly used in Europe to preserve the quality of rotational and perennial grass and legume forages (Wilkinson and Toivonen, 2003; McEniry et al., 2007) and have been gaining popularity in the US over the last decade (Han et al., 2006; Arriola et al., 2015). Silage in wrapped bales offers advantages over hay production, such as a more flexible harvest date, less weather dependency and a greater flexibility in ration formulation (Savoie and Jofriet, 2003; Shinners et al., 2009a). Big bale silage is now a well-established conservation system for storing excellent quality forage, and can provide an opportunity to maintain the high feeding value of young herbage (Forristall and O’Kiely, 2005). The making of big bale silage involves several mechanical treatments, ranging from harvesting to storage, to achieve high quality forage in terms of nutritive value and hygienic characteristics. Baled silage is often made from herbage that is wilted more extensively and undergoes more limited fermentation than conventional bunker silage, in order to reduce the number of bales per hectare, the plastic consumption and the costs; moreover, it can be more convenient when fed to animals (Han et al., 2006; McEniry et al., 2007; Tabacco et al., 2013). Unfortunately, the increased dry matter (DM) content also tends to increase fungal growth in wrapped forages (O’Brien et al., 2008; Tabacco et al., 2013), thus increasing hygienic issues as well as the risk of mycotoxicosis (O’Brien et al., 2007; McElhinney et al., 2016) and Listeria contamination (Fenlon et al., 1989; Nucera et al., 2016). Even though the baled silage system is based on a well-established procedure, the fact that the incidence of mold spoilage can be relatively high in baled silage (O’Brien et al., 2008; Borreani and Tabacco, 2010) suggests that the current baled silage making practices can be considered only partially satisfactory (McEniry et al., 2011). In bale silages, more than 40% of the stored forage DM is within 120 mm of the film cover, and the reduced total thickness of the combined layers of stretch-film on the bale side, usually 70 μm (four layers) to 105 μm (six layers), could be expected to make individually wrapped bales more susceptible to oxygen ingress (Forristal and O’Kiely, 2005). Even small holes that can occur on farms, due to both mechanical and wildlife factors, can result in quantitative DM losses because of mold growth, especially in conserved forages with higher DM contents (McNamara et al., 2001; Müller et al., 2007). Air penetrating into the silage stimulates aerobic bacteria, yeasts and molds and causes aerobic deterioration (Borreani and Tabacco, 2008; O’Brien et al., 2007). Moreover, under farm conditions, the improper mixing of different parts of the baled silage in the feed-mixer could enhance the final fungal and mycotoxin feed contamination. Farm surveys conducted in Ireland to establish the incidence of fungal growth on baled grass silage have shown that up to 90% of the examined bales had visible fungal growth (O’Brien et al., 2008). Furthermore, Borreani and Tabacco (2010) observed, in a temperate environment, that the development of molds in the peripheral areas of a bale could involve more than 10% of the bale surface, when the conservation period was longer than five months. Therefore, to keep the molded surface as low as possible and to ensure good, stable bale silage conservation, air-tightness has to be maintained for extended conservation periods. A significant reduction in mold growth and an improvement in silage conservation quality are obtained when six or eight layers of film are applied, compared to four (Keller et al., 1998; Müller, 2005). This is more evident when high DM forages are ensiled in wrapped bales and conserved for periods of more than 8 months (especially for alfalfa, Medicago sativa L., Borreani and Tabacco, 2008). More layers of stretch film assure a better airtight cover, but involve prohibitive increases in costs, in plastic usage and in environmental concerns, due to necessity of disposing of the additional plastic (Lingvall, 1995). Furthermore, the cover can easily be damaged, especially for dry alfalfa, where stems can puncture the plastic film in the corners (shoulders) of the bale, and this leads to large DM and quality losses (Bisaglia et al., 2011). As with all forms of silage, moist bales must be sealed rapidly to deplete the remaining O2 and initiate fermentation. Sealing was originally performed manually by placing a large plastic bag over each bale (Savoie and Jofriet, 2003). Today, specialized machines (wrappers) provide a seal by applying a stretchable, thin, plastic film around the bale (Savoie and Jofriet, 2003). The wrapping method has not been changed since 1984, when the first bale wrapper model was introduced in Europe by the Norwegian company Kverneland-Underharg (Anonymous, 1995). Two main types of round bale wrapper are currently available: the rotating table and the rotating arm (Lingvall, 1995). Combined baler-wrapper units that can only wrap in the field have become common (Forristall and O’Kiely, 2005). However, the stretch polyethylene wrapping system has shown some limits, with regards to sealing efficiency (Jacobsson et al., 2002), concerning the high permeability to oxygen of stretch films (Borreani and Tabacco, 2008; 2010) and the non-uniform distribution of plastic films between the ends and the curved surface of the bale (Borreani et al., 2007). These problems have lead to undesirable air exchanges over the conservation period, and it has been suggested that an increasing number of plastic film layers is required. For these reasons, great efforts have been made to reduce the possibility of damage to the plastic and of air ingress through the plastic cover during the conservation period.