Prepared by Ashbell Gilad and Weinberg Zwi G.1


1. Introduction
2. The ensiling process
3. Silo types
4. Factors to be considered during ensiling, storage and unloading:
- harvesting & wilting
- transport
- filling the silo
- sealing
- unloading
5. Silage additives
6. Ensiling technology for the tropics
- tropical forages for ensiling
7. Silage for dairy cows
8. Use of other ensiling materials for cattle feeding
- orange peel
- poultry waste
9. Silage and health
10. General Information
- proceedings of silage conferences
- research centre publications
- journals
- websites
11. References
1. Introduction

In modern dairy husbandry forage crops are harvested at a stage when yields and nutritional value are maximal, and are then preserved in order to ensure continuous and consistent forage supply throughout the year. The major goal of preservation is to retain the original nutritional value of the feed at the highest value possible during storage. In the food industry for human consumption many preservation methods are available: cooling and freezing, preservation by heat (blanching, pasteurizing, commercial sterilization), drying, modified atmospheres, pickling and use of preservatives (Potter, 1978). For economical considerations and technical reasons many of these methods are not applicable for forage crop preservation. Forage crops are preserved either by hay making (field drying) or by ensiling. Hay is preserved because of its low water content which restricts detrimental microbial activity. Hay making is restricted to “thin” crops which can dry quickly and uniformly, and to areas without rainfall during the harvesting season.

Ensiling is a preservation method for large masses of moist crops. It is based on lactic acid fermentation under anaerobic conditions whereby lactic acid bacteria (LAB) convert water-soluble carbohydrates into organic acids, mainly to lactic acid. As a result the pH decreases and the moist crop is preserved. Air is detrimental to silage because it enables plant respiration and the activity of aerobic spoilage microorganisms such as yeasts and moulds (Woolford, 1990). Therefore, many practices applied during ensiling, storage and feeding are aimed at excluding air from silage. Other practices that enhance successful ensiling include the application of silage additives.

It is possible to ensile almost any plant material and by-products; the most important crops for ensiling worldwide are whole-crop corn, alfalfa and various grasses. Other crops include whole-crop wheat, sorghum and various legumes. Ensiling is a sophisticated and costly operation which is mainly used in developed countries. It is estimated that 200 million tons of dry matter are ensiled worldwide annually. The cost of production, which varies between 100 and 150 US dollars per ton of dry matter, comprises land and crop production (about 50%), harvesting and polyethylene (30%), silo (13%) and additives (7%) (Wilkins et al., 1999).

The ensiling process involves many steps which should be timed and controlled carefully, in order to ensure successful ensiling with minimal losses.

2. The ensiling process

The ensiling operation includes the following steps: harvesting the crops at the optimal stage of maturity, wilting (if needed and possible) to ensure adequate dry matter content for the solid- state lactic acid fermentation, chopping, loading into a silo, compaction and sealing to exclude air, storing and finally unloading for animal feeding. Additives can be applied at the chopping stage or when loading into the silo.

The biochemical and microbiological events during the ensiling fermentation can be divided into four distinct stages:

a. Aerobic stage - when air is still present between the plant particles and the pH is still high, 6.0-6.5. During this stage plant respiration continues, as well as proteolysis and the activity of aerobic microorganisms such as enterobacteria, fungi and yeasts.

b. Fermentation stage - which is carried out by a dynamic succession of LAB which change according to the conditions prevailing in silage, starting with enterococci and leuconostoc, followed by lactobacilli and pediococci (Woolford, 1984). Lactic acid and other organic acids accumulate and the pH decreases, below 5.0, depending on plant composition and its buffering capacity.

c. Storage stage - when the silage is sealed and no air penetrates few changes occur.

d. Unloading stage - for feeding when the silage is re-exposed to air which re-activates aerobic microorganisms, mainly yeasts and moulds, which might spoil the silage.

It is desirable to accelerate the fermentation step as far as possible in order to minimize the activity of detrimental microorganisms and stabilize the silage. It is also important to minimize the exposure of silage to air during storage and unloading.

Good quality silage has a pleasant odour, a typical colour and texture, and a high nutritional value. Silage should be prepared carefully in order to assure successful ensiling fermentation. Plant composition should be adequate for that purpose. This includes adequate dry matter and water-soluble carbohydrate content and low buffering capacity (resistance of the plants to pH decrease). High buffering capacity evolves from high soil, mineral and protein contents. If the crop is too moist or lacks enough fermentable sugars, or its buffering capacity is too high, secondary fermentation by clostridia will take place, resulting in increased pH (through the conversion of lactic into weaker butyric acid), spoilage and losses. Very moist crops (less than 30% DM) result in effluent which leads to loss of nutrients and to environmental pollution. Therefore, if possible, moist plants are wilted in the field to increase their DM content. If the crop is too dry fermentation will be too slow, consolidation might be incomplete and spoilage might occur by yeasts and molds.

Aerobic deterioration occurs when the silage is exposed to air which re-activates aerobic spoilage microorganisms such as yeasts and moulds. Some silages are more susceptible to aerobic spoilage than others. Aerobic deterioration is associated with loss of nutrients, decreased intake by animals, and sometimes, production of mycotoxins in the silage.

Table 1 summarizes the causes of undesirable processes and losses in silage (adapted from Zimmer, 1980).

Table 1. Energy losses in silage and causative factors
Process  Classification Approximate loss (%)  Causative factors
Residual respiration  Unavoidable 1-2 Plant enzymes
Fermentation Unavoidable  2-4  Microorganisms
Effluent or Mutually 5->7  Dry matter content is too low
Wilting losses unavoidable  2->5  Weather, technique, crop
Secondary fermentation Avoidable 0->5  Buffering capacity,Dry matter content is too low
Aerobic deterioration during storage Avoidable 0->10 Delayed filling and compaction, sealing, crop susceptibility
Aerobic deterioration after unloading  Avoidable 0->15  As above, DM content, season, unloading technique and rate
Total   7->40  

3. Silo Types

Silos for silage are the facilities in which the crops ferment and where they are stored until feeding. There are various types of silos which are selected according to operator preference and feeding circumstances. Silo capacity should be determined according to feeding needs (herd size), and its dimensions should be calculated according to feed-out rate, in order to minimize silage exposure to air. The most abundant types include stack (clamp) without retaining walls, tower silo, bunker silo, horizontal plastic sleeves and big bale (McDonald et al., 1991). (Figures 1-3).

Figure 1. Horizontal bunker silo
Figure 2. Tower silo
Figure 3. Bale silage
[Click on thumbnails to view full pictures]


Bunker silos are constructed of concrete floor and walls. The silage parts adjacent to the walls and the top parts of the silage are the most susceptible to air penetration and to spoilage (Ashbell and Kashanchi, 1987; Ashbell, and Weinberg, 1992). Tower silos are made of metal or concrete and can be top or bottom unloaded. The metal silos are less air permeable than concrete silos. Bale silage (0.5-1.0 t) is prepared from forage crops which are not chopped at harvest and are wrapped by plastic sheeting. Baling enables flexible use of the silage in grazing sites.

Recently Ashbell et al., 2001 showed that is possible to ensile forage in small plastic bags (10-20 kg) which can be used by small holder cattle owners in Africa. It was hypothesized that although such bags are not air tight, the volatile fatty acids (e.g., acetic acid) that result from the fermentation accumulate in the bags and so inhibit aerobic deterioration. Lane (2000) earlier reported on the use of small plastic bags containing 6 kg of silage and Pariyar (2005) more recently used plastic bags holding 6 and 12 kg of silage.

4. Factors to be considered during ensiling, storage and unloading:

- harvesting and wilting
In some cases it is recommended to wilt the crop in the field in order to obtain an adequate dry matter content (300-400 g kg-1) necessary for desirable fermentation and in order to avoid effluent. If the DM content is too low and the buffering capacity of the plants is high, secondary fermentation will occur by clostridia during which lactic acid is converted to weaker butyric acid, followed by pH increase and further spoilage. The wilting depends mainly on solar radiation and for rapid wilting it is recommended to spread the biomass in wide windrows in order to increase the surface area exposed to the sun, and to decrease the thickness of the plant layer to facilitate moisture removal. Sometimes plants are conditioned to enhance loss of moisture. That means that they go through a device that bruises the waxy cuticle of the plants. Losses during wilting arise from continuing plant respiration, the activity of aerobic microorganisms, and the extent of losses during wilting depends on the temperature during wilting and the initial DM content. Mechanical losses depend on plant properties, the number of turning treatments and DM content. Legumes are of special concern, because the thin leaves which dry out faster than the stems become brittle and fall off during handling. This results in high nutritional losses.

The recommended chopping length is 1-2 cm. A short chopping length is useful for effective consolidation of the biomass, but some animal nutritionists feel that if the particles are too short, the silage loses its roughage fibrous properties that are necessary for rumen function. Moreover, a very short chopping length results in excess of silage effluent through which losses of nutritive matter occur, bunker corrosion is enhanced and the effluent might pollute water sources. Effluent results if the DM content of the crop is below 30%; its amount depends also on degree of consolidation, chopping length and bruising of the plants and acidic additives. For example, maize ensiled at 20% DM produced 53 and 27 litres ton-1 of effluent at chop length of 6 and 32 mm, respectively (McDonald et al., 1991).

Different crops have different harvesting practices: for example, wheat is harvested at the milk-dough maturation stage at 300 g kg-1 dry matter (DM) and is subjected to a short wilting period before pick-up (Figure 4). This practice is used in order to assure the best nutritional quality and optimal ensiling properties. In addition, simultaneous harvesting and chopping slows down the process considerably, and would result in appreciable grain loss (Honig, 1980). Legumes are wilted more intensively to make haylage, in order to overcome problems associated with their high buffering capacity. Corn is not possible to wilt because of its thick stem. Its DM content is determined by the timing of last irrigation. Combines for corn silage are equipped with a crushing devise to damage the dry grains in order to increase their digestibility in the rumen. Figure 5 shows the effect of stage of maturity of corn (referred to as milk line position) on silage quality. Figure 6 describes the stage of maturity of corn at harvest for silage.

Figure 4. Pick-up of chopped whole crop wheat for silage
Figure 5. The effect of stage of maturity of corn (position of the milk line) on silage quality
Figure 6. Milk line of corn grains
[Click on thumbnails to view full pictures]

The chopped crop can be transported from the field to the silo by a variety of trucks of various sizes. The filling and unloading time depends on truck size, and the duration of the trip and on the distance. Although transportation is a simple operation, it adds much to the cost of ensiling because of the high moisture content and the large volume of the chopped crop.

- filling the silo
Filling should be as fast as possible in order to quickly exclude air from the silage and minimize losses that are a result of plant respiration and activity of aerobic microorganisms. In bunker silos the crop is generally ensiled in a series of wedges which are laid one upon the other along the silo. The silage is shaped with a slope in order for rain to drain off. Compaction is accomplished by tractors which drive slowly and continuously back and forth during filling (Figures 7, 8). The depth of rolling effect of the tractor is 20-40 cm from the surface (Honig, 1991; Muck and Holmes, 2000). It should be noticed that the bunker width should be at least twice of the width of the tractor, so that every strip along the silage would be compacted. It is estimated that the density of well compacted wheat silage at 350 g kg-1 DM is 230 kg DM m-3 (Israeli Extension Service). The DM density of barley and grass silages was 232 and 260 kg DM m-3, respectively (Darby and Jofriet, 1993). Densities of alfalfa and corn silages in a Wisconsin study varied between 106 to 434 kg DM-3 (Muck and Holmes, 2000). The density was correlated with the thickness of crop layer being consolidated, tractor weight, packing time and DM content (Muck and Holmes, 2000). The higher the DM content and the longer the chopping length, the more difficult is the compaction because the crop is more resilient. Wheat at higher DM content is more difficult to compact because of its hollow stem which is full of air. The lower the silage density in the silo, the higher the DM losses (Pitt, 1986).

In tower silos the consolidation is obtained by the forage weight itself. The density of the silage in tower silo depends on silage depth and moisture content (Harrison and Fransen, 1991).

- sealing
Sealing of bunker silos is usually obtained by using plastic sheeting, usually polyethylene of various thickness (0.1-0.3 mm). The plastic can be anchored to the silage with used tyres, paper pulp or other materials that keep it from flapping (Figure 9). The plastic protects the surface of the silage against air penetration, and the thicker it is then the less it is oxygen permeable. The film should be UV resistant in order to withstand prolonged exposure to sunlight. Dickerson et al. (1991) showed that in the top 250 mm, silage losses were 78 and 8% in uncovered and covered bunkers, respectively. Savoie (1988) showed that the longer the storage period of the silage, the thicker the plastic that should be used in order to balance losses vs. plastic costs. The thicker the plastic and denser the tyres are, the less physical tears by birds or rodents were caused, and smaller top losses were obtained (Ashbell and Weinberg, 1992).

- unloading
Losses during unloading depend on duration of silage exposure to air, ambient temperature and on the aerobic stability of the silage. Air ingress into the silage from the unloading face depends on silage density. Silage should maintain its consolidation during unloading and the technology used affects this factor to a great extent: unloading with a front-mounted bucket tractor tends to produce a rough silage face which is more porous and with larger surface area, and therefore it is susceptible to more air penetration and increased DM loss. Whereas a scraper type of unloader leaves a smooth face. Figure 10 shows a self-propelled silage scraper connected to a mixing wagon through a conveyer. Block cutters leave the unloaded silage compact and therefore such blocks can be stored longer. Air penetration during storage and unloading can be measured by inserting thin plastic pipes for gas sampling on the day of filling with outlets outside the silo. Gas composition can be determined by gas chromatography. Results of such experiments indicate that air ingress in normally compacted silage is 1-2 m from the face (Honig, 1991; Weinberg and Ashbell, 1994). In order to minimize the exposure of silage to air during feed-out it is recommended to renew the face often.

Figure 7. Filling horizontal bunker silo with whole crop wheat
Figure 8. Compaction of silage in horizontal silo
Figure 9. Horizontal silo sealed with polyethylene sheeting anchored by used tyres
Figure 10. Self propelled silage scrappers connected to mixing wagons
[Click on thumbnails to view full pictures]

5. Silage additives

Silage additives can be used in order to enhance silage fermentation and their nutritional quality. They are classified according to their function: fermentation stimulants, fermentation inhibitors, aerobic deterioration inhibitors, nutrients and absorbents (McDonald et al., 1991). Additives should be used according to needs and silage properties. It should be emphasized that additives can improve silage quality and minimize losses, but cannot compensate for poor silage making and management. There is a long list of available additives (e.g. see Bolsen and Heidker, 1985) which come in a variety of forms: liquid, powders or suspensions. Additives can be applied during the harvesting chopping operation, or during filling of the silo. For application of an additive suspension during chopping (bacterial inoculant for example), a special device should be mounted on the combine.

Bacterial inoculants are used in order to enhance the ensiling fermentation. They are safe, easy to use, non-corrosive to machinery and regarded as natural products. Most commercial inoculants for silage include homofermentative lactic acid bacteria (e.g. Lactobacillus plantarum, Enterococcus faecium and Pediococcus spp.). They are used because they are fast producers of lactic acid. However, in whole crop cereal silages such inoculants resulted in spoilage upon aerobic exposure because their fermentation did not produce enough volatile fatty acids such as acetic acid, that inhibit fungi. The new types of inoculants are tailored according to silage properties, and for whole crop cereal silage heterofermentative LAB is included, such as L. buchneri (Weinberg and Muck, 1996).

Chemical additives include various organic and mineral acids that lower the pH artificially and inhibit specific microbial populations. For very moist crops formaldehyde can be used at 2-4 g l-1. Other chemicals are available as well (e.g. sulphur based chemicals that inhibit fungi).

In some countries there are voluntary schemes by which new silage additives are tested by independent institutes. The approval categories of the additives are according to their declared functions (e.g., fermentation improvement, reducing losses, improving aerobic stability and enhancing animal intake, weight gain and milk production).

6. Ensiling technology for the tropics

 In many tropical countries, such as in Africa, most of the farmers are smallholder cattle owners who own two to five dairy cows. Usually the cows rely on natural pasture which is abundant during the rainy season. However, in the dry season that can last for 6 months, the cows survive only on remaining dry pasture and on body reserves. Since lactating cows need extra feed for milk production, the lack of forage during the dry season is an obstacle to the development of dairy husbandry in these areas. Under such conditions the weak cows are unable to survive, and those with better body conditions do not produce any milk (Dube, 1995; Smith, 1995). Therefore, preservation of forage crops would enable cows to be fed throughout the year, increase milk yields considerably and this might have a great economical and social impact in these areas.

Preservation of forage crops in the tropics might be problematic: if the crops are harvested at the end of the rainy season, it is impossible to dry them to hay, because of rain interruption. If the harvest is postponed to the beginning of the dry season then the nutritional value of the crops decreases considerably, and they are much less digestible (Maclaurin and Wood, 1987). Ensiling is an alternative preservation method which can be practiced during the entire growing season and may yield higher quality and quantities of preserved forage. However, conventional ensiling technology requires large capital investments in silos and machinery, which the smallholder cattle owner could not afford. Therefore, special ensiling technology should be developed for such farms, to meet the needs and to be economically feasible. In any case the new technology should be adapted to the local farmers through extension programmes. In some areas farmers are reluctant to use ensiling and prefer to use low quality hay or even straw during the dry season. Ensiling experiments using small plastic bags have indicated that it is possible to ensile various crops (grass, wheat, maize) in such units (Lane, 2000; Shariffah et al., 2000; Ashbell et al., 2001) (Figure 11). There are many advantages for this technology: the bags are relatively inexpensive, the ensiling can be done manually by a few workers (Figure 12), and the bag units can be used individually according to feeding requirements. Experimental results indicate that the quality of the silage obtained within the bags is quite good. It is hypothesized that the bags should not be completely air impermeable and that the volatile fatty acids which are produced during the fermentation are retained within the bags and inhibit spoilage yeasts and moulds (Ashbell et al., 2001). Another possibility is to ensile in a pit dug in the soil which is lined with plastic sheeting and covered with plastic sheeting and soil (Figure 13 and Kebe, 2004). Figure 14 shows a clamp silage comprising chopped crop covered with dried sugarcane stalks.

Figure 11. Small bag silage
Figure 12. Manual chopping of crop for silage
Figure 13. Pit dug in the soil for ensiling
Figure 14. Clamp silage covered with dried sugarcane stalks
Figure 15. Maize for silage
Figure 16. Forage sorghum
[Click on thumbnails to view full pictures]

- tropical forages for ensiling

Maize (corn, Zea mays L.) (Figure 15) and sorghum (Sorghum bicolor L. Moench) (Figure 16) are summer forage crops that grow in tropical areas but they are also common in other climates (subtropical and temperate). Whole-plant maize is considered a perfect forage crop for ensiling due to its ensiling properties and high nutritional value for cattle. Sorghum has advantages because it is drought and saline resistant, and also needs less fertilization. Therefore, it is more suitable for semi-arid areas. Kipnis et al. (1994) showed that dry matter yields and digestibility increased in forage sorghum with decreasing water supply, whereas high-grain sorghum was not affected by irrigation. Many new sorghum cultivars have been developed with improved nutritional value which approaches that of maize (Dickerson,1986).

Other forage crops which are unique to tropical areas are available. They include mainly pasture grasses and legumes that can be harvested and ensiled during the rainy season when they are abundant. The following section summarizes various reports dealing with ensiling tropical grasses and legumes and mentions some of the problems:

Tropical grasses and legumes are not easy to ensile because of their low DM and WSC (water-soluble carbohydrates) content. For example Kikuyu grass contains only about 30 g kg-1 DM as compared with 350 for maize (Titterton and Bareeba, 2000). A possible solution is row intercropping of maize with legumes and grasses, and their co-ensiling (Titterton and Bareeba, 2000). In such systems special attention should be given to planting patterns and timing of sowing, so that one crop does not suppress the other. Co-ensiling maize with legumes at 50:50 by volume resulted in acceptable silage with pH values of 3.7-4.5 and low ammonia nitrogen ratio. Ashbell et al. (1999) also obtained improved silage when forage and grain sorghum were mixed at that ratio.

Silage additives to be used on smallholder cattle farms should be cost effective and easy to use. The role of silage additives in the tropics should be to increase DM and WSC in order to enhance the ensiling fermentation. Molasses are an inexpensive source of WSC often used in sugarcane producing areas. Using lactic acid bacterial inoculants for silage of wilted grasses or legumes is a possibility but they might be too expensive and require special application devices.

Aminah et al. (2000) studied the composition and ensiling characteristics of several tropical grasses and forage crops in Malaysia (Table 2). The setaria and Napier grass resulted in acceptable silage. In some samples the DM and WSC were too low for adequate ensiling and they could be improved by applying 4% molasses at ensiling.

Snijders and Woulters (2000) ensiled wilted (to 30% DM) Napier grass and Columbus grass in pits sealed with polyethylene sheeting, and obtained good quality silage with little mould.

Table 2. Composition of some tropical grasses and forage crops and their silage (% in DM) characteristics

Species DM (fresh)  WSC (fresh) Silage pH  Lactic acid
Setaria sphacelata 15.3  6.2  4.1  2.5
Brachiaria decumbens  20.4  8.6 5.1 1.1
Brachiaria humidicola 20.9 2.4 5.3  1.3
Digitaria setivalva 18.2 1.3  4.3 1.5
Pennisetum purpureum  15.8  9.9 4.0 2.5
Panicum maximum 19.4 3.0  4.7 1.9
Zea mays  21.2 23.0  3.7 2.7
Sorghum bicolor  21.4  11.7 3.7 3.8

Regan (2000) reports that in northern Australia bale silage was prepared from wilted pasture with Pangola grass (Digitaria eriantha subsp. eriantha) and two legumes, namely Cavalcade centurion (Centrosema pascuorum) and Wynn Cassia (Chamaecrista rotundifolia). Wynn Cassia is a plant which is difficult to make hay from because of its leafiness, and when dry the leaves shatter. Silage made from this plant is of good quality and readily eaten by cattle. It is also easy to make a mixed silage from Wynn Cassia and grass. The DM content of the silages made from the wilted plants ranged from 42-57%, the in vitro digestibility from 55-58% and the metabolizable energy from 8.5-9.0 MJ kg-1DM. In another experiment the Pangola grass and the Cavalcade were wilted more completely (up to 63.7% DM). The fermentation profile depended on wilting degree, and the haylage contained less lactic and volatile fermentation products, as expected. 

Kikuyu grass (Pennisetum clandestinum) is an important summer pasture in the tropical coastal areas of Australia. Kaiser et al. (2000) studied its composition in terms of ensiling: it is low in DM (mean 20%), relatively low in WSC (5% in DM), high in starch (4%) and has an intermediate buffering capacity. It was recommended that this grass is wilted to at least 30% DM for ensiling. At noon the WSC content was at a peak, due to photosynthesis, but afternoon cutting could have a negative effect on wilting speed. Therefore, it was concluded that delaying cutting to the afternoon is not beneficial.

Argel et al. (2000) ensiled the legume Cratylia argentea in Costa Rica in a stack type silo covered with plastic sheeting, with addition of molasses, or mixed with King grass and inoculated. The silage successfully replaced expensive concentrates in rations for lactating cows during the dry season.

In Zimbabwe (Titterton et al., 2000) silage was made in plastic bags from forage sorghum mixed with Napier grass or with local legume – dolichos bean (Lablab purpureus). The DM content of the silages was 27.5-32.8%. The fermentation quality of all silages tried was acceptable with pH values below 5.0 and the ammonia ratio was less than 10% of total nitrogen. DM losses in the bag silages were lower for the forage sorghum silages (<10%), and higher for the other crops or mixed silages (up to 20% for the Napier grass silages).

Maturation of Napier grass (30 vs. 80 days) decreased CP from 11.9 to 7.3 % DM; NDF and ADF increased from 64.2 and 37.7 to 70.2 and 43.9%, respecively. Digestibility of DM decreased from 70.5 to 66.8%.

Weinberg et al. (1995) studied the ensiling properties of direct-cut (20% DM), mildly wilted (27% DM) and heavily wilted (45% DM) pearl millet (Pennisetum americanum). The final pH values of the silages were 3.7, 3.9 and 4.0, and lactic acid 8.2, 5.5 and 2.6 % in DM, respectively for the three wilting stages. In the aerobic stability test the direct cut and the mildly wilted silages deteriorated whereas the more wilted (drier) remained relatively stable.

7. Silage for dairy cows

The husbandry of feeding dairy cows is mainly based on the quality standard of the feed. Of course such a standard is influenced by several factors, including: climate, body condition, days in lactation, milk production, pregnancy, etc. In general, the diet must be highly nutritious under all conditions. Usually the ration for dairy cows comprises a combination of concentrate feed and forage, and the ratio between these two sources depends on economic and physiological factors.

In most cases, the farmer produces and preserves the forage, and will prefer to use his/her own product rather than buying concentrates. Dairy cow intake of silage is usually less than that of fresh grass or hay. The characteristic silage components such as volatile fatty acids and amines have negative effects on the intake of the animal. The proportions of these components, especially butyric acid and ethanol, are higher in low-quality silage, and there is a linear correlation between silage quality and intake. When good forage has been properly harvested and stored, it can supply as much as 60% of the dry matter requirements in high-lactating dairy herds (McCullough, 1991). In modern farming, dairy cows are fed between 10 and 20 kg of wet silage per day; where silage is the only forage, cows will eat about 3 kg of silage per 50 kg of body weight. Increased silage intake can be achieved by feeding the silage twice a day and by adding some hay. Silage is better suited (in comparison to hay) for inclusion in total mixed rations (TMR) (Figure 17). It has been experimentally shown (Argel, 2000) that replacement of concentrate with Cratylia agentea silage for Jersey dairy cows is a viable option for small-scale dairy farmers; it was found to replace expensive concentrates economically, with no effect on milk yield, and to result in a milk yield with a higher percentage of fat.

Figure 17. Dry matter losses as affected by preservation technology
[Click to view full image]

It is common knowledge that three factors are the primary determinants of silage intake: fineness of chopping, dry matter content and quality of preservation. Fine chopping improves the fermentation process and aerobic stability, but it slows down production and sometimes has a negative effect on the rumination process. The influence of fineness of chopping on the intake of silage by sheep has been confirmed experimentally (Deswysen et al., 1978). The increased intake associated with fine chopping results partly from improvement in conservation quality, but also from the reduction in forage particle size, per se. The effect of fineness of chopping is less evident for cattle than for sheep as was shown in six comparisons between heifers and sheep (Duphy et al., 1984). Compared with the intake of the initial fresh forage, the intakes of long-chopped silage by these two types of animal were 87 and 53%, respectively, compared with 98 and 86%, respectively, for precision-chopped silages. It should be noted also that flail machines introduce soil into the silage, which results in a further drop in intake (Zimmer and Wilkinson, 1984).

In most feeding experiments intake of silage increased with increasing DM content. The exact causes of this effect of DM content on intake are difficult to determine, and they probably vary. When the DM content increases, the forage contains less water, but its grain content increases, the proportion of cell wall decreases, fermentation is less intense and the proteins undergo weaker breakdown (Sarwatt et al., 1989). Conservation quality improves as the ammonia and volatile fatty acid contents decrease (McLeod and Edwards, 1976). The effects of the three main factors – fineness of chopping, dry matter content and conservation quality – are given in Duphy, 1979. Other factors may also be involved in the determination of intake, since they affect the intake of fresh forage. Thus, the intake of maize silage may increase with increasing starch content – a factor that depends on stage of maturity, variety and plant population – or with diminishing lignin content. In contrast, as a result of protein hydrolysis during silage making, the intake of silages with low protein contents will be limited compared with the intake of corresponding fresh forages. Thus, a nitrogen supply may either be due to normal cellulolytic activity of rumen microorganisms, by improvement in the organic matter digestibility or as a result of increased total protein content contributed by the addition of additives or urea to maize silages (Andrieu and Demarquilly, 1974). Increased intake of certain poor-quality grass silages has been achieved by feeding casein or methionine into the abomasum (Barry et al., 1978) or by the addition of fishmeal to feeds (Gill et al., 1988). A review on digestibility and voluntary intake of conserved forages can be found in (Duphy and Dermarquilly, 1991).

Palatability and intake of silage for dairy cows have been predicted by estimation of the following parameters: pH, lactic plus acetic acid, total free acids, acetic acid, ammonia plus amines, and proportion of ammonia plus amines in soluble protein (Miettinen et al., 1991). In the last few years sorghum has become an important summer forage crop, replacing maize silage because of its lower requirements for water and fertilizer. The quality of sorghum silage can be high and can provide good feed for high-lactating dairy cows (Ashbell et al., 2001). Silage dry matter intake (SDMI) is a function of the intrinsic intake potential of the ensiled herbage, its fermentation quality and supplementary feeding. Digestibility and the cell wall content in the forage mainly determine the intrinsic intake potential, and the extent to which the intake potential is achieved depends on the modifications to the carbohydrate and nitrogen fractions that occur during in-silo fermentation. The intake of badly fermented silage can be markedly lower than that of well-fermented material. The relative SDMI index can be estimated from silage digestibility and fermentation characteristics, and it can be used as a tool to improve silage quality in the farm and in ration formulation. Using silage dry matter content or the metabolic energy intake index can be used to estimate by how much the concentrate content of the feed should be increased, to maintain milk production when the silage intake potential is lower than expected, or, conversely, to estimate the benefits to be expected, in increased production or saving of concentrates, through the use of high-quality silages. Factors affecting silage intake, including digestibility, fermentation and protein content, are discussed by Huhtanen (2001). 

8. Use of other ensiling materials for cattle feeding

The terms “by-products” and “residues” may be misleading. Usually in the food industry and in agricultural production there is a main product, which is economically the most important, while the rest of the output is referred to as residue or by-product. Sometimes the residues have even higher nutritional value than the main product, and sometimes the designations “main product” or “residues” alternate according to the market. With regard to silage and this paper “by-products” are considered to be materials that remain from the food processing industry and are not used for human consumption and the term “residues” is used to refer mainly to animal excreta.

In many cases ruminants can utilize by-products, especially from the vegetable and fruit industry, but they are seasonal and might be produced in amounts greater than can be used immediately. Because most such by-products are high in moisture, nitrogen and carbohydrates, they spoil rapidly and cannot be stored in the raw form for long periods. If they are not used, the accumulation of such sensitive materials might create environmental problems (water pollution, unpleasant odours, and the attraction of undesirable rodents and insects). In material that goes mouldy, mycotoxins might be produced, posing serious health hazards for both humans and animals. Therefore, there is a double benefit in the use of by-products for ensiling: providing animal feed and reducing environmental pollution.

Preservation of wet material can be done by drying or by ensiling. Drying is an expensive procedure, involving technology whose cost depends on the price of energy. Preservation by ensiling is much cheaper, but requires material of appropriate composition. The advantage of dry material is that it can be transported easily and the unit cost of transportation of dry matter is relatively low. A basic condition for using a by-product is low cost, therefore, in most cases, preservation of by-products by ensiling is the only economical option.

From basic knowledge of silage making it is known that the composition of the raw material strongly affects the fermentation process, and the quality and nutritional value of the silage. With respect to suitability for ensiling, most of the wet by-products contain too much water and in some cases are low in fermentable carbohydrates. It is possible to mix several by-products in order to obtain an appropriate composition for a suitable fermentation, but the preparation of a correct mixture requires knowledge of the composition and quality of the by-products and their mixtures.

Among the many wet by-products that can be ensiled are: apple pomace, beet pulp, brewer’s mash, citrus pulp, grape pomace, molasses, whey, tomato pomace, corn cannery waste, pineapple greenchop, snap bean cannery waste, sugar cane tops, sugar cane bagasse, and cull fruit and vegetables (Figures 18, 19, 20). In some areas of the world residues and by-products of sugar cane form the most important source of cattle feed. These wet by-products can be mixed with dry by-products such as: bakery waste, grain screenings, rice, wheat and cotton residues, and animal residue (Huber, 1980). Utilization and preservation of by-products is especially important in developing countries, where most farmers are smallholder cattle owners and don’t have enough capital to invest in modern means of making silage (Machin, 2000; Chedly and Lee, 2000; Caluya, 2000). 

The preservation (ensiling) of orange peel, as an example of a wet by-product, and of poultry manure, as an example of an animal residue are described in detail below. The general considerations for these materials are applicable to many others with similar properties. The possibility of mixing them together and the resulting advantages lead to simplification of the preservation process and result in a superior product for feeding cattle.

Figure 18. Tomato by-product
Figure 19. Banana rejects
Figure 20. Fresh citrus peels
[Click on thumbnails to view full pictures]

- orange peel:

Orange peel is a by-product of the orange-juice industry; peel comprises more than 50% of the whole fruit, by weight. The peel is available only during the picking season, which lasts no more than a few months. During this period peel is produced at a rate greater than that required for use directly as fodder, therefore conservation is necessary to ensure full utilization of the material (Figure 21). Drying by heating is not economically feasible, mainly because of the price of fuel, although it is practiced in some countries (e.g. Brazil, USA) mainly because of the needs of transportation from the orange growing regions to cattle-raising areas. Preservation (ensiling) of the peels “as they are” entails high losses (over 50% of the dry matter), deterioration and creation of environmental pollution by the release of large volumes of effluents (Ashbell and Donahay, 1984, 1986) (Figure 22). Chemical analysis of the peel indicates low dry matter content (17-21%), high WSC content (21-35%) and over 90% digestibility (Ashbell and Lisker, 1987). The microbiological content of the peel is very high; it comprises mainly yeasts, probably because of the richness in WSCs, and is in the range of 103 to 105 colony forming units (CFUs) per gram of DM (Ashbell and Weinberg, 1988). To be able to take the correct steps to ensile any forage or by-product, it is important to be aware of its ensilability characteristics, which result from its chemical and microbiological composition. Most of this important knowledge was lacking with regard to orange peel, and an earlier study by Ashbell et al. (1987) aimed to provide the missing information. The results indicated high DM losses, copious release of seepage, that started from day 2 of ensiling, great similarity in the chemical and microbiological contents of the peel and of the effluent, much ethanol production (Figure 23), and the connection of most of the losses to CO2 production – a feature that was new to the authors. The production of ethanol and CO2 raised the possibility that such products probably resulted from yeast activity. If this hypothesis was correct, it would mean that reduction of the yeast activity would improve preservation; this was tested by technological and chemical treatments. The ensiled peels were blanched (Ashbell et al., 1988) with live steam for 2.5, 5.0 or 7.5 minutes, and it was found that the longer the blanching time, the greater the reduction in the yeast count and the better the preservation. Addition of sorbic acid in three concentrations (0.025, 0.05 and 0.1% w/w) yielded similar results: increasing the acid concentration decreased the yeast population and improved the preservation (Weinberg et al., 1989). These two technologies are still too expensive to be economically feasible, although orange peels are high in nutritional value; on a dry matter basis orange peel is comparable with concentrates such as sorghum, barley and corn (Table 3).

Figure 21. A "lagoon" of citrus peels
Figure 22. Spoiling citrus peels
Figure 23. Fermentation products of citrus peels
[Click on thumbnails to view full pictures]

Table 3. Chemical compositions and metabolizable energy (ME) of citrus peel and common forage grains (on a DM percentage basis)
  Citrus peel1  Sorghum Barley Corn
Dry matter %  14-25  87  87  86 
Crude protein 3  10.5  12.1  9.9
Crude fibre3   12.5  3.4   6.3   2.9
Fat3  3.5  3.2 2.1  4.7
Ash 3 5.5  2.0  2.9 1.4
T.D.N. %   87 80  83  87
ME2  3.42 3.11  3.24 3.46 

1 WSC = 18-34%, and pectin 9-18%. 2 Mcal/kg. 3 % in DM

Thus it is evident that:

1. Orange (citrus) peel is a highly nutritive material.

2. Orange peel, like other wet by-products, deteriorates quickly.

3. The best way to preserve such material is by drying it; however, the technology is very costly and, in most cases, not economical.

4. It is possible to preserve wet by-products by ensiling, but it necessitates pretreatment to ensure good ensilability conditions.

One of the practical options to increase the DM content (of the wet by-product) is to mix it with a dry product (hulls, straw, poultry manure and others) in such proportions as to get the DM content required for ensiling. 

- poultry waste

Animal excreta may contain several nutrients and can be a source of feed (Ashbell, 1972), especially for sheep (Rasool et al., 2000) and cattle, but needs to be handled correctly. Such material can be especially valuable in developing countries, where raw material is expensive. Poultry waste from broilers (Figure 24) only comprises bedding material, excreta waste feed and feathers. In contrast the waste from caged birds (layers) contains less energy, has a high ash content, and is not recommended for feeding. The nutritional value of poultry waste is high, but it varies according to several factors, especially the quality of the bedding. The moisture content of the fresh manure, which is taken from the chicken coop, can vary between 25 and 35%. Such material will not be stable and will deteriorate mainly through the action of mould (Gilboa, 1991). Theoretically there are three options for the preservation of such material:

1. By drying to raise the dry matter content above 85%. Drying can be done by the sun or in an oven.

2. By adding water (or whey) to form a mixture with 50-60% DM, compacting it and treating it like silage for fermentation, with air penetration prevented.

3. By mixing it with a wet by-product, in this case orange peel, to obtain a mixture containing 50-60% DM, compacting it, and treating it like silage.

Mixing orange peel and poultry manure and ensiling the mixture for preservation is a very good solution for both materials (Figure 25). In terms of nutritional ingredients, the mixed composition is rich in energy, contributed by the peel, and crude protein and minerals, contributed by the poultry manure. The high WSC and the moisture from the peel ensure good fermentation. After a few weeks of fermentation the resulting product has a pleasant odour and good texture. It is almost impossible to distinguish between the two components, and the intake is very good. Caswell et al. (1978) and Klinger and Tagari (1976), who also showed the effect of decontamination during ensiling, have studied the technology of ensiling poultry manure with water. Ashbell et al. (1995) and Harari (1979) ensiled with a range of water contents and orange peel. 

Figure 24. A mixture of broiler litter and citrus peels before fermentation
Figure 25. "Silage" comprises a mixture of broiler litter and orange peels
[Click on thumbnails to view full pictures]

9. Silage and health

In the food industry – including dairy products, winemaking, brewing and pickling –fermentation processes are controlled and the quality of the final product is usually predictable. All stages are under control and defective products are rare. The situation is different in the silage industry, where silage quality depends on environmental conditions and is affected by epiphytic microorganisms. It is not rare to obtain silages of differing qualities, even thought they have been prepared in the same way. Not only may the quality vary, but, more importantly, the variations may also be associated with undesirable components. Hazards to health – of both animals and humans – associated with silage fall into three categories:

1. Undesirable microorganisms e.g. Listeria, enterobacteria, clostridia and moulds.

2. Undesirable chemicals, mainly toxins.

3. Excess acidity and other metabolic disorders.

Links between the food given to animals, their health, and the real potential hazard to human health are not well known, but awareness of this possibility increased in importance recently, particularly in Europe, following the epidemic of bovine spongiform encephalopathy (BSE).

Undesirable microorganisms that can be introduced into the silo with the forage may come from the soil and, especially, livestock waste, which contains large numbers of potential pathogens (Mawdsley et al., 1995). In addition to undesirable bacteria and fungi that can appear in the silage, there is evidence that protozoal pathogens such as Cryptosporidium parvum, which is sometimes detected in livestock wastes, can survive fairly adverse conditions in the silo (Merry et al., 1997). This finding has implications for both human and animal health, as the organism forms a very resistant oocyst, which is likely to survive in forage for considerable periods of time. The most important organism in the enterobacteria group, from the viewpoint of health hazards is Escherichia coli, which can cause acute diarrhea and death (Cooke, 1993). Clostridium species in silage are responsible for the secondary fermentation of glucose and lactic acid to butyric acid by saccharolytic and proteolytic bacteria (Weissbach, 1996) (Figure 26). Clostridium botulinum is an organism of great concern with regard to health, since several distinct types of botulism toxin (Types B, C and D) are pathogenic to animals. Relatively few incidents of botulism have been related directly to silage, even when it has been poorly preserved (Roberts, 1988), but the risk of botulism is increased considerably if the silage is contaminated with mammalian or avian remains. Botulism has been reported in cattle that received silage made from pasture to which poultry manure had been applied. It is, however, possible to vaccinate against butulinum (Smart et al., 1987).

Mycotoxins are the products of fungal metabolism, and can be found in silage and any animal feed that has deteriorated during storage; temperatures above freezing point, relative humidity above 20%, and oxygen are critical environment requirement for the development of those fungi. Mycotoxins pose serious hazards to both animals and humans, therefore, setting safe or tolerable levels in silage is difficult. With regard to silage, the key factor is oxygen, which, if present, can promote mycotoxin production during storage (Gotlieb, 1997; Oldenburg, 1991). There is no evidence to indicate whether or not the fungus or its toxin can survive the ensiling process, but the possibility of survival cannot be ruled out, especially in material with a high dry matter content ensiled with little consolidation. 

The health risk of nitrate arises from the production of toxic oxides of nitrogen in the early phase of ensiling. Nitrate is reduced to nitric oxide (NO), a colourless gas that, on exposure to air is oxidized to nitrogen dioxide (NO2), a yellow-reddish-brown gas with an irritating odour. Nitric oxide and nitrogen dioxide react with water to form nitrous acid (HNO2) and nitric acid (HNO3), respectively. These gases and acids cause respiratory irritation by destroying the membranes of the respiratory tract. The role of nitrate in silage was reviewed by Hill (1999) and the role of silage in health by Wilkinson (1999) and Lindgren (1999).

Figure 26. Fermentation capacities of clostridia group
[Click to view full image]


10. General Information

Much information on silage, ensiling, silage research and related topics can be found in texts, proceedings of conferences, special publications of research institutes and research articles. The following is a partial list.

- Texts:

Bolsen, K.K. and Heidker, J.I. 1985. Silage Additives USA. Chalcombe Publications, UK.

Huber, J.T. (ed.) 1980. Upgrading Residues and By-Products for Animals. CRC Press, Inc., Boca Raton, FL, USA.

McDonald, P., Henderson, A.R. and Heron, S.J.E. (eds.) 1991. The Biochemistry of Silage, 2nd ed. Chalcombe Publications, UK. pp 9-19, 167-183.

Stark, B.A. and Wilkinson, J.M. (eds.) 1988. Silage and Health. Chalcombe Publications, UK.

Stark, B.A. and Wilkinson, J.M. (eds.) 1992. Whole Crop Cereals. Chalcombe Publications, Aberystwyth, UK.

Wilkins, R.J. 2001. Legume Silages for Animal Production. Increasing profits with forage legumes. (for details of booklet in English, Finnish and Swedish click here.)

Wilkinson, M. 1992. Silage UK, sixth ed. Chalcombe Publications, UK.

Wilkinson, J. M., Newman G. and D.M. Allen. 1998. Maize. Producing and Feeding Maize Silage. Chalcombe Publications, UK.

Woolford, M.K. (ed.) 1984. The Microbiology of Silage, The Silage Fermentation. Marcel Dekker Inc., New York, NY, USA.

- Proceedings of silage conferences:

Honig H. and Pahlow, G. (eds.) Forage 1991. Proc. of a conference on Conservation towards 2000. Braunschweig, Germany, January 23-25.

O’Kiely, P., O’Connell, M. and Murphy, J. (eds.) 1993. Proc. of the 10th International Conference on Silage Research, Silage Research 1993. Dublin City University, Dublin, Ireland. September 6-8.

Jones, D.I.H., Jones, R., Dewhurst, R., Merry, R. and Haigh, P.M. (eds.) 1996. Proc. of the 11th International Silage Conference. Aberystwyth, UK. September 8-11. 

Pauly, T. (ed.) 1999. Silage production in relation to animal performance, animal health, meat and milk quality. Proc. of the 12th International Conference on Silage, Swedish University of Agricultural Sciences, Uppsala, Sweden. July 5-7.

‘t Mannetje, L.(ed.) 2000. Silage making in the tropics with particular emphasis on smallholders. Proc. of the FAO Electronic Conference on Tropical Silage. 1 September – 15 December, 1999. 180pp.

Lyons, T.P. and Jacques, K.A. 2001. Science and technology in the feed industry. Proc. of Alltech’s 17th annual symposium. Nottingham University Press, Nottingham, England. (also previous proceedings).

Weinberg, Z.G. (ed.) 2001. Proc. of the Turkey-Israeli workshop on silage and by-products for high lactating cows. The Volcani Center, Bet Dagan, Israel. July 24-26.

- Publications of Research Centres:

Cattlemen’s Day. Agricultural Experimental Station, Kansas State University, Manhatten, KS, USA.

USDA-ARS, US Dairy Forage Research Center Research Summaries, Madison, WI, USA.

- Journals

J. of Appl. Bacteriology (Microbiology)

J. of Industrial Microbiology and Biotechnology (Microbiology)

Grass and Forage Science (general)

Animal Feed Science and Technology (general)

J. of Dairy Science (nutrition)

J. of Animal Science (nutrition).

Canadian Agricultural Engineering (Engineering aspects).

- Some Websites:

Canada – silage

Harvesting hay and silage

Silage production and storage

Kansas State University Silage website

DPI, Queensland

DPI Victoria silage

Silage fermentation and preservation

Grass silage

Corn silage management

Tips for top silage making

Silage management and production

TopFodder silage

Australian silage information network

Australian Fodder Industry Association Inc.

Silage/forage information system

11. References

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Ashbell, G. (1972). Examination of the nutritive value of poultry manure for poultry. M.Sc. Thesis, Hebrew University of Jerusalem (in Hebrew).

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1Ashbell Gilad and Weinberg Zwi G.
Forage Preservation and By-Products Research Unit
Agricultural Research Organization, The Volcani Center
Bet Dagan 50250, ISRAEL
Phone: 972-3-9683558; Fax: 972-3-9604428