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Silage may contain several agents that are potentially hazardous to animal health, the safety of milk or other animal food products, or both. This paper reviews published literature about microbial hazards, plant toxins, and chemical hazards. Microbial hazards include Clostridium botulinum, Bacillus cereus, Listeria monocytogenes, Shiga toxin-producing Escherichia coli, Mycobacterium bovis, and various mold species. High concentrations of C. botulinum in silage have been associated with cattle botulism. A high initial concentration of C. botulinum spores in forage in combination with poor silage fermentation conditions can promote the growth of C. botulinum in silage. The elevated pH level that is generally associated with aerobic deterioration of silage is a major factor influencing concentrations of L. monocytogenes, Shiga toxin-producing E. coli, and molds in silage and may also encourage survival and growth of M. bovis, the bacterium that causes bovine tuberculosis. Soil is a major source of B. cereus spores in silage; growth of this bacterium in silage appears to be limited. Hazards from plant toxins include pyrrolizidine, tropane and tropolone alkaloids, phytoestrogens, prussic acid, and mimosine, compounds that exist naturally in certain plant species that may contaminate forages at harvesting. Another group of toxins belonging to this category are ergot alkaloids, which are produced by endophytic fungal species in forages such as tall fescue grass, sorghum, and ryegrass. Varying effects of ensiling on the degradation of these plant toxins have been reported. Chemical hazards include nitrate, nitrite, and toxic oxide gases of nitrogen produced from nitrate and high levels of butyric acid, biogenic amines, and ammonia. Chemical and microbiological hazards are associated with poorly fermented silages, which can be avoided by using proper silage-making practices and creating conditions that promote a rapid and sufficient reduction of the silage pH and prevent aerobic deterioration., (Copyright © 2018 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.)

The process of preserving crops by fermentation in silos is under the control of the farmer to a much lesser degree compared to the level of control by the manufacturer over the production of other fermented foods, such as cheese and yoghurt. Additives designed to direct the extent and pattern of the fermentation are relatively unpopular in most countries, and their use is not guaranteed to remove the risk of undesirable components in silage. Hazards to animal health associated with silage fall into three categories: (1) undesirable micro-organisms e.g. Listeria, enterobacteria, clostridia and moulds; (2) undesirable chemicals, e.g. mycotoxins, and (3) excess acidity and other metabolic disorders. In some regions of Europe, the production of silage is discouraged or prohibited because of the risk of undesirable microbes. The princIpal risk in these areas is that of the secondary fermentation of cheese made from milk contaminated by bacterial spores, rather than a direct hazard of contaminated silage to animal health. With the possible exception of high dry matter silage conserved in large bales, respiratory hazards to animals from moulds and their spores generally are less from silage than hay. Mycotoxins and phytoestrogens may survive the ensiling period and constitute risks to animal health. Relatively little is known about the epidemiology of diseases that may be linked to undesirable chemicals and excess acidity in silage. Therefore, research is needed to define epidemiologically and mechanistically the risks to animal health and to the human food chain from silages contaminated with pathogenic bacteria and mycotoxins, and to understand more completely the relationships between the physical and chemical compositions of silage and metabolic disorders in animals.

Abstract: Highlights of progress in the production of silage over the past 50 years include the introduction of improved hybrids of maize (Zea mays L.), the forage harvester, the big baler, polyethylene covering for horizontal silos, stretch‐wrap film for bales and novel additives designed to improve the fermentation and aerobic stability of silage. The key biochemical pathways in the silage fermentation have been described together with the effects of microbial and chemical additives on fermentation and aerobic stability during the feed‐out phase. The significance of oxygen and water in silage fermentation has been quantified and efficacy of covering silos has been established, with recent progress in the development of oxygen barrier film. Future perspectives include improving food safety and animal health by increasing the hygienic quality of silage, reducing the environmental impact of silage by decreasing loss of nitrogen to soil and atmosphere, reducing methanogenesis in the rumen and increasing methane yield from silage as biofuel, and the use of silages as feedstocks for multiple end uses in biorefineries. [ABSTRACT FROM AUTHOR]

Losses of organic matter in the outer layers of bunker silos covered with conventional polyethylene ( PE) plastic can be substantial due to oxygen transmission through the plastic top-covering film during the post-ensiling storage period. The effect of two silo covering materials, oxygen barrier ( OB) film (45 μm thickness) and clear PE film (50 μm thickness), as underlays to a white-on-black PE plastic top cover (120 μm thickness), was assessed in the outer layer of whole-plant maize silage stored in three large bunker silos in the People's Republic of China. Samples of the crop at harvest and of silage from the upper 45 cm layer at 5 months post-ensiling, prior to removal of silage for feed-out, were analysed for DM, fermentation profile and chemical composition. Loss of OM was estimated from concentrations of ash in the crop at harvest and in the silage. Differences between underlay films in silage fermentation profile were small. Silage protected with OB underlay film had higher mean concentration of starch ( p < .008) and higher mean NDF digestibility ( p < .003) than silage under PE underlay film. Concentrations of ash were lower ( p < .001) for silage covered with OB film than for PE film in all three trials. Mean estimated losses of OM were 170 g/kg for OB underlay film and 232 g/kg for PE underlay film ( p < .001), and whole-silo estimated net economic benefits to OB underlay film ranged from 0.17 to 0.74 US $ per tonne fresh crop ensiled. [ABSTRACT FROM AUTHOR]

Implications of silage hygienic quality for animal production were investigated on forty-five dairy farms in South West England. Samples of grass and maize silages and of total mixed rations ( TMR) were obtained together with information on silage technology, herd size and animal production. Samples were analysed for mycotoxins, bacteria, yeasts, moulds and chemical composition. Thirteen mycotoxins were assayed, but none were detected in the samples of grass silage. However, mycotoxins were found in 0·9 of all maize and other silage samples, with deoxynivalenol and zearalenone predominating. There was no relationship between total mycotoxin concentration and mean lactation milk yield per cow. Enterobacteria counts tended to be higher in maize silage than in grass silage and higher still in TMR - a cause for concern. There were no relationships between mould counts and mycotoxin concentrations in silages, implying that mycotoxins may have been produced in the field pre-ensiling. [ABSTRACT FROM AUTHOR]

The concentration of crude protein in grazed New Zealand pasture is typically in excess of 200 g/kg DM, which exceeds the requirement of the dairy cow and is reflected in elevated levels of daily urinary nitrogen (N) excretion, estimated to average 262 g N/head or 751 kg N/ha. This has adverse environmental consequences, including leaching of nitrate from soil into water courses and atmospheric emissions of nitrous oxide. Problems associated with pasture as the sole feed for dairy cows include weight loss in early lactation, poor fertility and reduced lactation length. Low-protein supplements can reduce N excretion rates and increase N use efficiency. A simple feeding strategy is proposed in which pasture is supplemented with maize or whole-crop wheat silage at 5 kg DM/cow per day from the start of the pre-calving dry period on winter run-off pasture to 100 days post-calving, and from 250 days post-calving to the end of lactation. The expected response, at an assumed substitution rate of 0.7 kg decrease in pasture intake per kg silage DM, is an increase in metabolisable energy of 10 to 15 MJ/cow/day, equivalent to 33 to 48 g milk solids (MS)/kg DM of supplement. This strategy is expected to result in significantly lower urinary N excretion by the cow. Actual responses in daily milk output, from published experiments where grazing stocking rates were increased to take account of reduced herbage intake, ranged from 50 to 100 g MS/kg DM of supplement. Other benefits include early lactation bodyweight maintenance, higher percentage of cows calved which are pregnant at 150 days in milk and increased lactation period. Constraints for farmers to implement such changes include cost of silage, value of milk sold, failure to integrate forage maize or wheat in rotational cropping with pasture, and the relatively poor aerobic stability of maize and whole-crop wheat silages. [ABSTRACT FROM AUTHOR]

Silage making involves preserving forage crops by fermentation of water-soluble carbohydrates (sugars) to organic acids. The fermentation is relatively uncontrolled and inadequate acidification can allow the growth of undesirable microorganisms such as enterobacteria and clostridia. Ingress of oxygen to the outer layer of the silo or bale during the storage period encourages growth of yeasts and moulds. Mycobacterium bovis may survive in silages with pH above 5.0. Outgrowth of clostridia in aerobically deteriorating silage can lead to high counts of butyric acid spores in silage that can contaminate milk and cheese. Milk may be contaminated with Penicillium toxins and be rejected for containing antibiotics. Specific disease risks to livestock from poorly preserved silage include bacterial endotoxicosis, listeriosis, botulism, mycotoxicosis and nitric acid poisoning in addition to reduced feed value. Rapid acidification is essential once the crop is ensiled to restrict the growth of enterobacteria, listeria and clostridia. Oxygen barrier film, in addition to reducing loss of dry matter during the storage period, prevents clostridial outgrowth in the outer layer of the silo and is also likely to reduce the risk of listeriosis. [ABSTRACT FROM AUTHOR]

Losses of dry matter and metabolisable energy (ME) occur in silage making due to delayed harvest and also between field and feed trough due to fermentation and aerobic spoilage. Farmers should aim to exert greater control of silage making operations to improve timeliness of harvest and to minimise losses of nutrients. Concentrate input per unit of milk production is often higher than optimal which is more often a reflection of low silage quality than of wasteful overfeeding. A week's delayed harvest of grass is estimated to incur a net cost of 13 pence per cow per day of winter feeding. Mowing a dry crop and spreading it out to maximise rate of water loss during the wilting period can reduce losses in the field. Consolidation of crop in the silo to achieve a minimum fresh weight density of 700 kg/m3 and dry matter (DM) density of 210 kg/m3 will help to minimise losses during the storage period, paying particular attention to the outermost layer which should be covered with an oxygen barrier film. A feed-out progression rate of at least 1 metre per week in winter and 1.5 to 2 metres per week in summer, coupled with harvesting a clean crop with low yeast and mould counts will help to reduce losses during feed-out. Many bunker silos are too old, too small, have no safety rail and are over-filled. Safety issues should be considered at all stages of silage making. Appropriate steps should be taken wherever possible to minimise risk of injury. [ABSTRACT FROM AUTHOR]

A meta-analysis was undertaken of 51 comparisons of standard polyethylene film with oxygen barrier ( OB) film in covering systems for bunker silos, unwalled clamp silos and bales. Mean losses of DM or OM during storage from the top 10 to 60 cm of bunker and clamp silos were 195 g kg−1 for standard film and 114 g kg−1 for OB film systems (41 sets of data, P < 0·001), while mean total losses of DM from baled silage were 76·8 g kg−1 for standard film and 45·6 g kg−1 for OB film systems (10 sets of data, P < 0·001). Top surface silage judged subjectively to be inedible was 107 and 29·6 g kg−1 for standard film and OB film systems respectively (5 sets of data, P = 0·02). Aerobic stability was 75 h for silage stored under standard film system and 135 h for silage stored under OB film system (11 sets of data, P = 0·001). It is concluded that the OB film system reduces losses from the outer layers of silos and from bales and increases the aerobic stability of silage in the outer layers of silos. [ABSTRACT FROM AUTHOR]

When silage is exposed to air on opening the silo, or after its removal from the silo, fermentation acids and other substrates are oxidized by aerobic bacteria, yeasts and moulds. The aerobic stability of silage is a key factor in ensuring that silage provides well-preserved nutrients to the animal with minimal amounts of mould spores and toxins. In this paper, key findings and recent developments are reviewed, and findings of recent research are integrated in terms of four themes: (i) the most significant biochemical and microbiological factors, (ii) physical and management factors, (iii) type of additive and (iv) silo sealing. The development of yeasts and moulds during plant growth, and during field wilting or storage, and the concentration of undissociated acetic acid in silage are important microbiological and biochemical factors affecting aerobic stability. Silage density and porosity are key physical factors that affect the rate of ingress of oxygen into the silage mass during the feed-out period. A target for potential silage aerobic stability is 7 d including time in the feed trough. To achieve this target, speed of harvest should be coordinated with packing tractor weight to achieve a minimum silage density by the time of feed-out of 210 kg DM m−3, maximum proportional porosity of 0·4 and a rate of silage removal, which matches or exceeds the depth of air penetration into the silo. The use of additives to increase aerobic stability is advisable when there is the risk of these objectives not being met. Novel microbial approaches to solving the problem of silage aerobic deterioration are needed. [ABSTRACT FROM AUTHOR]

Abstract The recycling of sewage sludge or biosolids to grassland is strategically important in the European Union (EU) and its use is tightly regulated to control the risk of pathogen transfer to animals and the food chain. Sewage sludges not only contain valuable concentrations of beneficial nutrients, but also elevated concentrations of potentially toxic metals (PTM) compared with average background concentrations in the soil. The EC and UK regulations refer to six PTM, Cd, Pb, Cu, Zn, Ni and Hg, with provisional regulations for Cr, that have to be controlled to prevent detrimental effects on soil and animal health. Despite these regulations, there is still a danger that grazing animals may ingest elevated concentrations of PTM. Biosolids may adhere to herbage after the surface application of sewage sludge to grassland. The repeated surface application of sewage sludge to grassland can lead to elevated concentrations of PTM at the soil surface that may be ingested, together with soil and herbage, by grazing ruminants. This may lead to accumulation of Cd or Pb in liver or kidney. The risk to the human food chain is considered to be low, but the impact on the environment is still unknown. There is little information, for example, on the amount of soil and PTMs that may become incorporated into conserved grass. At present EU and UK legislation and voluntary codes of practice have been developed to protect animal from pathogens in sewage sludge and to minimize any potential risks from accumulation of PTM. The background and implementation of the legislation are examined in this review, and the source and mechanisms of accumulation of PTM by the grazing animal are evaluated. [ABSTRACT FROM AUTHOR]