Spoiled silage in the diet of livestock poses a health hazard and reduces animal performance.
Professor Mike Wilkinson of Nottingham University reviews research findings.
Introduction
The successful preservation of silage relies on the establishment and maintenance of anaerobic conditions during both the storage and feed-out periods. The common factor contributing to loss of nutrients is oxygen permeation into the outer surfaces of the silo or bale, which must be minimised by effective covering and rapid feed-out. Unfortunately, many bunkers are too wide to achieve a sufficiently rapid feed-out progression rate and the silage heats up behind the exposed feed-out face. Warm silage has probably been exposed to air for more than 48 hours and has lost about 5% of the original dry matter. The aerobic deterioration of silage has been reviewed in detail in Grass and Forage Science (see Wilkinson and Davies, 2012). In this article the emphasis is on surface spoilage during storage and on the adverse effects on the animal of silage aerobic deterioration.
Surface spoilage
A 1000t bunker silo of 30m length x 14m width x 3m height contains about 15% of the original ensiled crop in the top 0.5 metre. A round bale of 1.2m length x 1.2m diameter contains about 30% of the ensiled crop in the outermost 0.10m. Material in the peripheral areas of silos and bales, of lower density than the core, is prone to deterioration due to permeation of oxygen through standard polyethylene film covering and also due to damage to the film from wind, birds and animals. Consequently, a layer of grey or black spoiled silage is often seen at the periphery of the silo or bale, with visible mould growth. The material may also show signs of overheating with dark brown colouration and a smell of tobacco.
Covering silos with standard 500-gauge polyethylene film (125μm thickness) 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. Loss of organic matter (OM) is highest in the uppermost 0.5m. Studies with farm-scale silos in the USA over a four-year period revealed that loss of OM during the storage period can be very high in the top 0.5m of uncovered silos, compared to the same silage beneath the top 0.5m (Table1). The top half-metre of spoiled silage remaining after the storage period would originally have been a much thicker layer. Shrinkage due to the decomposition of OM during the storage period produces a black, foul-smelling, slimy layer with a mud-like texture in the outermost layer. The decrease in height due to top shrinkage depends on the effectiveness of the covering system and can be up to 0.9m in uncovered silos.
Table 1. Effect of covering ensiled forage maize and forage sorghum with standard polyethylene film and car tyres on loss of organic matter (OM) in the upper layers of 127 commercial farm silos in Kansas, USA (Bolsen 1997).
Top surface spoilage in bunker silos is most extensive nearest the walls where silage density is lowest. Spoiled silage judged to be unfit for use as animal feed (i.e. inedible) is normally discarded as waste material after the top surface is exposed prior to feed-out. However, spoiled silage can occasionally be given to livestock and the chances of including it in the diet are greatest when silos and bales are unloaded in the dark. The accidental inclusion of spoiled silage in the ration poses risks both to animal health and productivity.
Effects of spoiled silage on the animal
There has been very little research into the effects of exposure of silage to oxygen on its nutritional value to the animal. In an experiment with beef cattle fitted with ruminal cannulae, silage intake and digestibility were assessed when spoiled maize silage from the top 1m of an uncovered silo was mixed, at 25% of total silage dry matter (DM), with unspoiled silage from the same original crop but stored in an “AgBag” silo. Inclusion of spoiled silage in the ration appeared to destroy the integrity of the forage “mat” in the rumen with significant detrimental effects on intake and digestibility (Table 2).
Table 2. Effect of including spoiled silage in the diet of beef cattle (Whitlock et al., 2000).
Research in Germany showed an average 57% reduction in DM intake of maize silages differing in DM, chop length and density and exposed to air for 8 days prior to being offered to goats in a preference trial (Gerlach et al., 2013). In this trial the temperature of the silages was stable for the first 48 hours exposure to air. The mean composition and intakes of the silages exposed to air for 0, 4 and 8 days are shown in Table 3. Dry matter concentration, pH, and counts of yeasts, moulds and aerobic mesophilic bacteria increased during exposure to air whilst concentrations of fermentation products decreased, with the largest changes occurring between 4 and 8 days exposure. Accumulated increase in silage temperature above ambient during exposure to air was the best predictor of intake, intake being greatest in silages with the lowest accumulated temperature.
Table 3. Mean composition and intake of maize silage after 0, 4 or 8 days exposure to air (Gerlach et al., 2013).
The most common animal diseases associated with spoiled silage are mycotoxicosis and listeriosis. Mycotoxicosis is difficult to diagnose and there is a dearth of information on its prevalence. Signs of mycotoxicosis include decreased feed intake and fertility, reduced milk yield or growth rate and suppressed immune status with resulting increased incidence and severity of infectious diseases such as mastitis and diarrhoea. Listeria monocytogenes can develop in large numbers in wet mouldy silage, especially when oxygen ingress is significant due to physical damage to the film or stretch-wrap. Outbreaks of listeriosis are often sporadic, especially in flocks of sheep given contaminated silage.
Oxygen barrier film
Oxygen-barrier (OB) film (“Silostop”, B. Rimini Ltd.) reduces surface spoilage in the silage by restricting oxygen permeation and reducing the development of moulds and undesirable bacteria, including butyric acid bacterial spores, in the peripheral areas of the silo or bale during the storage period. The results of a meta-analysis of 51 comparisons (41 with bunker and clamp silos, 10 with baled silage) between standard polyethylene film and OB film are presented in Table 4. The OB film significantly reduced losses from the outer layers of the silo during the storage period and increased the aerobic stability of maize silage. This means that less labour is needed to discard inedible material and the risk of accidentally including spoiled silage in the animals’ diet is reduced. With bales, fewer layers of wrapping and less weight of film may be needed with OB stretch-wrap than with standard wrap and the process of wrapping bales may be speeded up. There is a lack of data on response to OB film with grass silages of different qualities, however the greatest responses are likely to be seen in drier crops that are more difficult to consolidate that wetter crops.
Table 4. Average losses, inedible silage and aerobic stability of silage in the top surface layer stored under standard or OB films (Wilkinson and Fenlon, 2013).
1. Number of comparisons
2. Includes drive-over piles and laboratory silos
3. All comparisons with maize silage
Conclusions
Inadequate covering of silage increases losses during the storage period. Spoiled silage reduces feed intake and digestibility and increases the risks of mycotoxicosis and listeriosis. Slow progression rates during the feed-out period can result in substantial losses of silage nutritional value. Oxygen barrier film reduces losses and increases aerobic stability in the peripheral areas of the silo.