CHAPTER 4 - ENSILING CROP RESIDUES

Liu Jianxin
Zhejiang University
Guo Jun
China National Breeding Stock
Import and Export Corporation

Silage is the material produced by controlled fermentation of crop residues or forages with high moisture content. The purpose is to preserve forages by natural fermentation by achieving anaerobic conditions and discouraging clostridial growth. The ideal characteristics of material for silage preservation are: an adequate level of fermentable substrate (8-10 percent of DM) in the form of water soluble carbohydrate (WSC); a relatively low buffering capacity; and a DM content above 200 g/kg. The ensiling material should also ideally have, after harvesting and chopping, a physical form that allows easy compaction in the silo. Materials such as maize stover and grass can be ensiled successfully, while crop residues such as rice and wheat straw, with low WSC content, do not fulfil these requirements, and therefore pre-treatments, such as fine chopping or use of additives, or both, may be necessary.

There are plenty of materials suitable for ensilage in China. Whole maize and stover are the most common materials. Sweet potato vines are also usually ensiled after harvesting the tubers. In the provinces along the Yangtze River valley, large amounts of Chinese milk vetch are cultivated as green manure to improve soil fertility. Traditionally, farmers ensile surplus vetch for later use.

Types of silos

Tower silo

In China, tower silos are constructed from brick, and are several metres in diameter and 10-20 m in height. The advantages of this type of silo include: long life, small space required, low storage losses, and possibility for mechanization. Both the filling operation and daily extraction can be mechanized. However, tower silos are expensive, and therefore not widely used in China, with the exception of some state-owned farms.

Cellar silo

The cellar type is the most common silo on individual farms. Round or square concrete silos are usually built inside houses for protection from the weather. Advantages are lower cost and easy management. Size can be adjusted according to scale of production. Cellar silos are suitable for rural conditions in China. A disadvantage is high effluent loss, especially with clay walls.

Trench silo

This type is generally built underground or semi-underground, with two solid walls of 1.5-2 m in height. Advantages are similar to the cellar silo, but the trench silo is more suitable for mechanization. The tractor can be driven on top from one side to the other for compaction purposes. After compaction, it is covered with a plastic sheet pressed down with soil, sandbags or straw bales to maintain anaerobic conditions.

On many dairy farms, trench silos are built on the surface of ground. This type of trench silo resembles a bunker silo, but has vertical walls of 0.4-0.5 m in thickness and 3-4 m in height. This design makes mechanization more convenient, and may also prevent bottom leakage.

Stack silo

This type of silo implies a pile of material on the ground surface. On flat and dry ground, plastic sheet is placed underneath and the material is laid in a stack. The top is covered with plastic and sealed all round with soil. Sandbags or old tyres, or any other suitable objects, are placed on top to prevent the top cover from being blown away by the wind. The advantages of the stack silo are low cost and flexibility of placement.

Plate 4-1. Ensiling, building a trench on the ground

Plate 4-2. Ensiling, building a trench below ground surface

Plate 4-3. Silage bales wrapped with plastic

Plate 4-4. Tower silos, Shanxi, China

Plastic silo

Animal scientists from Neijiang, Sichuan Province, successfully ensiled sweet potato vines in plastic bags in 1978. The plastic silo is similar to the stack silo but it is covered with plastic sheets of polyvinyl chloride (PVC) or polyethylene. Alternately, the silo can be made in bags with sealed tops. The stack silo is also inexpensive and can be placed anywhere. However, labour requirements are high due to manual filling and handling. A specific machine has recently been developed by the Grassland Institute, Chinese Academy of Agricultural Science, to fill plastic bags with forage.

Silage making

Control of moisture content in raw materials

Ensilage can only be successful, with minimum DM and nutrient losses, when the moisture content of the raw material is kept to a suitable level. Although silage may be made within a large range of moisture contents, DM should be over 20 percent to assure silage quality.

There are many disadvantages to ensiling crops with high moisture content. First, ensiling of wet materials results in the generation of a large volume of effluent, which not only poses disposal problems, but also carries off valuable, highly digestible nutrients in solution. The amount of effluent increases with silo height, due to pressure. Effluent is produced when moisture is above 75 percent. Secondly, the critical pH value for clostridial growth varies directly with the moisture content of the plant material, and unless soluble carbohydrate levels are exceptionally high, ensiling wet crops will encourage clostridial fermentation, resulting in high losses and reduced nutritive value. Thirdly, even if the water-soluble carbohydrate (WSC) levels are adequate to ensure lactic fermentation, very wet silages may still be nutritionally undesirable because voluntary DM intake of these is frequently low. Finally, drier plant materials are preferred because they are easier to handle and a higher quantity of DM can be carried per trailer load.

Moisture content of forage and grasses is above 80 percent when harvested at a suitable stage. Therefore the moisture content should be reduced by field wilting. It takes 4 to 6 hours to wilt in dry regions such as the north western provinces and Inner Mongolia, and 6 to 10 hours in north eastern and northern areas. Longer periods may be needed in southern provinces, depending on climate and weather, but it is not desirable to exceed 24 hours. When weather conditions are unfavourable, field wilting should be avoided to prevent nutrient loss due to rain leaching. In these cases, other methods should be considered.

In contrast, the moisture content of cereal straw is generally too low to allow tight packing, so cereal straw and stover should be finely chopped. Sometimes water should be added to bring moisture content to a suitable level.

Sweet potato vines are high in moisture content and wilting is necessary. Chopped vines are usually mixed with finely chopped straw or bran prior to ensiling to increase overall DM content.

The moisture content of plant materials may be measured with instruments, but it is usually estimated manually on farm. Samples of chopped and minced grass or leguminous forage are grasped tightly by hand for one minute or so to estimate moisture content. If juice can be extracted, moisture content is above 75 percent. If the material remains together but without juice, moisture content is between 70-75 percent. If the material has elasticity and spreads out slowly, moisture content is 55-65 percent. If the material spreads out quickly, moisture content is about 55 percent. If the material breaks, moisture content may be below 55 percent.

Chopping, compaction and sealing

Prior to ensiling, plant materials should be chopped. The fineness of chopping varies with moisture content and nature of the material. The following guidelines can be used, but, in principle, rough and hard materials should be finely chopped, while delicate and soft materials can be roughly chopped.

What are the advantages of chopping? Firstly, chopping facilitates compaction and thus reaching the anaerobic stage. When most oxygen is removed, clostridial growth is discouraged and lactic acid fermentation encouraged. Secondly, chopping releases plant juices, stimulating the growth of lactic acid bacteria. Thirdly, chopping may increase silage intake by improving quality of fermentation and by accelerating rate of passage of feed particles through the rumen. However, very finely chopped silage reduces the rumination and may decrease milk fat content. Thus, 10-15 percent of the silage material should be above 25 mm in length in order to maintain an effective fibre function.

Factors influencing silage quality

Quality of silage fermentation is influenced by several factors, including moisture, WSC content of raw materials, degree of compaction and effectiveness of final sealing. Stage of maturity influences a forage's nutritive value and thus quality of silage. Leguminous forages at budding stage have optimal energy, protein and carotene contents, but DM yield and nutritive value decrease with maturity (Table 4-1). Grass at heading stage is highest in caloric value and protein content. When leguminous forages reach late flowering stage or grasses reach seed stage, the energy value has decreased by 70-75 percent, digestible CP by 67-83 percent, and carotene content by 75-84 percent.

Table 4-1. Yield of dry matter and nutrients in legumes and grasses by growth stage

Therefore, it is important to harvest forage crops at their optimal stages. Legumes should be harvested at budding stage, grasses at heading stage, and whole maize plant at late milky to early dough seed stage.

Grass is highest in protein at pre-heading, and highest energetically at dough seed stage. Many grasses can not head again after the first cutting, and the re-growth may be collected 4 to 5 weeks after the first harvest. The appropriate stages at which plants may be harvested are indicated in Table 4-2.

Table 4-2. Appropriate harvest stages for grasses and forage crops

Because straw and stover are harvested after grain collection, their nutritional value is generally low. It has been recently shown that yield and grain quality do not change when harvest is shifted to an earlier stage (by 7-10 days). However, this shift may be beneficial in terms of improved nutritional value of straw and stover.

Silage additives

The purpose of using additives is to ensure silage quality by encouraging lactic acid fermentation, by inhibiting undesirable microbes or by improving its nutritional value. Common silage additives include bacterial cultures, acids, inhibitors of aerobic damage, and nutrients.

Bacterial cultures

A dominant lactic acid fermentation is the key to making good silage. Lactic acid bacteria are normally present on harvested crops together with clostridial bacteria in a ratio of about 10 to 1. Considerable nutrient loss usually occurs at initial stages of ensiling when oxygen is still present. Addition of lactic acid bacterial cultures during filling-up increases their population rapidly, encouraging lactic acid fermentation and pH reduction to a level that inhibits clostridial development. Different strains of lactic acid bacteria look similar under the microscope, but their biological activities are very different. Only those acid-tolerant strains that possess a homo-fermentative pathway, producing the maximum amount of lactic acid from hexose sugars readily available, and a growth temperature range extending to 50°C, should be used as a silage additive.

The environment under which lactic acid bacteria multiply favourably is also important. Lactic acid bacteria are anaerobic, and hence air should be removed and the silo should be kept airtight. These micro-organisms ferment soluble sugars to a mixture of acids, but predominantly lactic acid. Plant materials should contain at least 2 percent WSC, otherwise soluble sugars (e.g. molasses) should be added. Starch may also be added along with amylase in order to provide lactic acid bacteria with soluble sugars.

Cereal straws and stovers are high in lignocellulose. A mixture of inoculum or enzymes, or both, containing cellulase and xylanase is often used. Enzymes degrade lignocellulose, liberating soluble sugars for bacterial use. A number of commercial preparations are available from foreign companies, and some have been registered by government authorities and can be sold in China. During the Ninth Five-Year Plan, Chinese scientists successful produced a special additive for ensiling fresh cereal straws and stovers.

Mineral or organic acids

The original proposal to use acids as silage additives dates back to 1885. In the late 1920s, Virtanen from Finland, adopted this approach and recommended the rapid acidification of the crop with mineral acids (AIV process) to a pH of about 3.5, which was originally thought to inhibit microbial and plant enzyme activity. This AIV process was widely used in Scandinavia until quite recently. Due to the difficulties in handling corrosive acids, organic acids were later used as silage additives. When acids are added, plant materials sink quickly and are easy to consolidate. Acidity may arrest plant respiration and reduce heat production and nutrient loss. Rapid acidification may also inhibit clostridia. However, addition of acids increases effluent and can be potentially toxic to animals. Furthermore, acids are corrosive to people, animals and machinery. Reduction of moisture content may minimize effluent, and addition of calcium carbonate can be used to adjust silage acidity. Appropriate concentrations of different acids as silage additives are recommended as follows:

• sulphuric and hydrochloric acids: 50-80 litre of dilute acid (concentrated acid diluted with water at 1:5 v/v) per tonne

• formic acid: about 3 kg/ton

• acetic acid: 5-20 kg/ton

• propionic acid: 1 litre/m² of surface to prevent mould development.

Inhibitors of aerobic deterioration

The most common inhibitors of aerobic deterioration are sodium nitrate, sodium nitrite, sodium formate and formaldehyde. These chemicals do not contribute to the improvement of fermentation, but are effective in preventing silage deterioration. Some plant parts, such as larch leaves, contain natural bactericides that may safely function as antiseptics. Formaldehyde is a well-known sterilizing agent and is commercially available as formalin, which contains 40 percent of the gas in aqueous solution. Scientists are interested in the additives because of their bacteriostatic properties, and because of their known properties of protecting plant proteins from rumen microbial degradation. Adding formaldehyde at 3-15 kg/ton may result in a well-preserved silage.

Nutrients

Nutrient additives are defined as substances which, when added to ensilage materials, contribute significantly to the nutritional value of the silage. Most of nutrient additives can also favour lactic acid fermentation. A number of materials are considered to be in this category.

Nitrogenous compounds

Certain crops, such as maize and most cereal straw, are nutritionally deficient in nitrogen, and when fed to ruminants as silage require supplementing with a protein supplement. An alternative approach is to improve the CP content of the silage by adding urea during ensiling. When applied to maize, urea produces silages with higher pH values and fermentation acid contents than in untreated silages. Urea addition has also a marked effect on nitrogenous components of the silages, resulting in higher CP, true protein, free amino acids and ammonia. Addition of urea to cereal straw and stover may also have an ammoniation effect, which is associated with higher CP and lower fibre content. However, attention should be paid to the rapid release of ammonia from urea in the rumen. High concentrations of ammonia in the rumen may cause ammonia poisoning. One solution is to add the urea or other nitrogenous compounds together with soluble sugar sources, such as molasses and starchy grains.

Urea may be added at 0.5 percent of fresh materials. When whole maize plant was added with urea at 0.5 percent, the CP of the resulting silage increased to 12.9 from 8.7 percent in the untreated silage. Urea may also be added prior to feeding the animals.

Carbohydrate-rich materials

Carbohydrate-rich materials are added to silage crops in order to increase the supply of available energy for the growth of lactic acid bacteria, and are of particular importance in crops such as legumes, which are deficient in soluble carbohydrate content. Materials that have been used for this purpose include molasses and cereals. Molasses is a by-product of the sugar industry, and has a DM content of 70-75 percent and a soluble carbohydrate content of about 65 percent of DM. In order to obtain maximum benefit, it should be used at 4 percent (w/w) for grass silage and 6 percent (w/w) for legume silage. Cereals have been used as additives in an attempt to improve both the fermentation quality and the nutritional value of silages. Cereals contain 50-55 percent of starch that can be hydrolysed to soluble sugars and then utilized by lactic acid bacteria. If amylase or amylase-rich materials, such as malt, are added with the cereal to ensiled crops, they will be more effective as fermentable carbohydrate sources.

Minerals

In addition to being nutritionally deficient in N, most straw and stover is a poor source of Ca and many micro-elements. Limestone is sometimes added to silages as Ca supplement and to alleviate silage acidity. Calcium carbonate may be added at 0.45-0.50 percent to obtain maximum benefit. Common salt has a high osmotic pressure to which clostridia are sensitive, but lactic acid bacteria are not. Addition of salt may increase lactate content, decrease acetate and butyrate, resulting in silage with good quality and palatability.

Other minerals may be used in ensiled materials. Examples of mineral sources include copper sulphate (2.5 ppm), manganese sulphate (5 ppm), zinc sulphate (2 ppm), cobalt chloride (1 ppm) and potassium iodide (0.1 ppm).

Finally, it has to be pointed out that recommendations on the use of silage additives must be based not only on the results of scientific research, but also on sound economic return.

Evaluation of silage quality

The nutritive value and the quality of silage should be accurately evaluated. Working on behalf of the Bureau of Animal Production and Health (BAPH), MOA, researchers at Zhejiang University drafted methods of evaluating nutritive value and quality of silage, which have been tested in China since 1996. This handbook includes subjective methods (on-farm) and chemical methods (for use in laboratory).

Subjective methods of evaluation

The pH value and certain simple subjective criteria such as colour, smell and texture are used to evaluate the quality of silage on-farm. These criteria are briefly reviewed below.

pH value

The pH is the simplest and quickest way of evaluating silage quality, and may be determined on-farm using wide-range pH test papers such as bromophenol blue (range 2.8-4.4), bromocresol green (range 4.2-5.6) and methyl red (range 5.4-7.0). The classification of silage based on pH value is:

Colour

Good silage usually preserves well the original colour of the standing plant. When green raw material produces silage with green or yellow colour, it can be considered of good quality. Temperature is one of the important factors affecting silage colour. The lower the temperature during ensilage, the less colour change. Above 30°C, grass silage becomes dark yellow. Above 45 to 60°C, the colour becomes closer to brown. Beyond 60°C, the colour darkens towards black due to caramelization of sugars in the forage.

However, silage quality can be misjudged by on a colour basis. For example, silage from red clover or Chinese milk vetch is often dark brown instead of light brown. Despite its excellent quality, it may be considered a failure due to colour. A more useful indicator is colour of the silage juices. It can generally be said that the lighter the colour of the juice, the greater the success.

Smell

Good silage usually has a mild, slightly acidic and fruity smell, resembling that of cut bread and of tobacco (due to the lactic acid). A rancid and nauseous smell denotes the presence of butyric acid and signifies a failed silage. A musty smell is a sign of deficient compaction and presence of oxygen. A distinctive unpleasant smell, of sow's urine and faecal matter, is always indicative of marked protein degradation during ensilage.

Texture

Plant structures (stems and leaves) should be completely recognizable in the silage. A destroyed structure is a sign of severe putrefaction. A viscous, slimy appearance reveals the activity of pectolytic (sporulating) micro-organisms.

Taste

This test is more suitable for specialists. It is of little value to the farmer, whose basis is the palatability to farm animals.

Integrative evaluation

It is obvious that silage quality can not be satisfactorily evaluated on any one of the above subjective indicators. The following methods of integrated evaluation (BAPH, 1996) have been on trial. Tables 4-3, 4-4 and 4-5 present indicators of integrated silage evaluation from maize stover, Chinese milk vetch or alfalfa, and sweet potato vines, respectively.

From overall evaluation scores (based on the parameters given below in Tables 4-3 to 4-5), silage quality can be classified as follows:

Table 4-3. Integrated evaluation of silage from maize stover using scores

NOTES: (1) The figures in parentheses represent the scores.
(2) The pH value is determined using wide-range pH test paper.

Table 4-4. Integrated evaluation of silage from sweet potato vines using scores

NOTES: (1) The figures in parentheses represent the scores.
(2) The pH value is determined using wide-range pH test paper.

Table 4-5. Integrated evaluation of silage from Chinese milk vetch or alfalfa

NOTES: (1) The figures in parentheses represent the scores.
(2) The pH value is determined using wide-range pH test paper.

From the above review it can be seen that subjective criteria by themselves are insufficient to determine quality, and that an objective laboratory analysis is necessary.

Chemical methods of evaluation (laboratory methods)

Evaluation of silage in the laboratory is mainly based on chemical analysis. It includes pH determination and assay of organic acids (acetic, propionic, butyric and lactic), which are the main fermentation metabolites in silage. Fermentation characteristics may be estimated based on total acids or on proportions of individual acids. Free ammonia assay, and better still the ratio of ammonia nitrogen to total nitrogen, is the most valid criterion for protein degradation.

Before discussing the value of such analyses, it is necessary to emphasize the necessity for the sample to be truly representative. Samples must be taken with a probe from several places at different layers in the silos. The sampling points should be more than 30 cm from the edge of the silo to prevent misleading results. Samples should preferably be sent to the laboratory in sealed plastic bags or glass bottles to avoid aerobic deterioration (secondary fermentation). Ammonia nitrogen and pH should be determined as soon as possible.

Silage evaluation by pH

The pH of silage should be measured in the laboratory using a precise pH meter. For fresh grass silage (moisture above 75 percent) and maize silage at all DM contents, the pH is both the simplest and quickest method of evaluation. Research has shown that there is a very close relationship between pH value, fermentation quality and DM during ensiling. The criteria for evaluation of silage from pH values have been shown in the preceding section.

Silage evaluation by the ratio of ammonia-N to total N

The ratio of ammonia-N to total N (NH₃-N/TN) in silage provides information on the stage of protein degradation and it undeniably constitutes a test of the state of conservation of the ensiled proteins. The higher the ratio, the more protein has been degraded, and the worse the quality. In the system proposed, a maximum of 50 points is given for a ratio lower than 5 percent, and points are deducted for a ratio higher than 35 percent. Table 4-6 presents the point scale for the evaluation of silage from ammonia N.

Table 4-6. Scale for silage evaluation from the ratio of ammonia-N to total N

Silage evaluation using various organic acids

The presence of lactic acid and various volatile fatty acids, especially acetic, propionic and butyric acids, is a reflection of the fermentation that has occurred. It is appropriate therefore to take into account, when judging silage success, both the type of acids and the amount present. The higher the proportion of lactic acid, the better the quality. The evaluation system is basically that of Flieg from 1938 with a maximum of 100 points given. Full points are given for lactic acid (25) above 68 percent, for acetic acid (25) below 20 percent and for butyric acid (50) below 0.1 percent. Individual acids are scored independently, and the total is the sum of the three acids. Table 4-7 presents the key for silage evaluation according to Flieg (0 - 20 = failure; 21 - 40 = poor; 41 - 60 = satisfactory; 61 - 80 = good; 81 - 100 = very good.

Table 4-7. Key for the evaluation of silage according to organic acids

SOURCE: Flieg, 1938.

Integrative silage evaluation from ammonia-N and organic acids

In order to integrate the information from both protein and carbohydrate in the silage, an evaluation system based on the points from ammonia-N and organic acids is proposed. There is a maximum of 100 points in the scale, 50 points for proteins and 50 points for carbohydrates. Points for carbohydrates are obtained by dividing the data in Table 4-8 by 2.0. The following table is used for evaluation of silage quality from protein and carbohydrate:

Feeding silage

Silage is suitable for feeding some 6-7 weeks after ensiling. The basic principle for silage use is to minimize the area exposed to air, with as little churning as possible. Silage should be taken from one side of the trench, and the exposed surface should be immediately covered to minimize re-entry of air and avoid secondary fermentations. Material taken out from the silo should be fed as soon as possible, since it rapidly deteriorates upon contact with air. Silage left in the trough must be cleaned away in time to prevent putrefaction.

Some animals may not like silage when offered for the first time. In these cases, some adaptive measures should be taken. For example, silage can be placed in the bottom of the trough, and covered with concentrates. When animals are adapted to silage, the offer can be increased.

Silage can be fed to all kinds of animals: replacement cattle, fattening cattle, dairy cows, sheep, goats and even pigs. The amount of silage offered depends on the animal and its age, as well as on the type and quality of silage. Taking cattle as an example, the recommended amount to be fed daily is:

Therefore a cow with liveweight of 500 kg may be daily fed 20 kg maize stover silage.

Sometimes the milk from cows fed on silage has a taint. Because this smell is only transmitted through the air, the following points should be observed to prevent it:

1. Handling Silage should never remain in the cowshed, and it should be offered in amounts exactly as required.

2. Feeding method Silage should not be given before milking. The effect on milk taste is more marked when the silage is fed 2 hours before milking, and least when given 6 hours after.

3. Hygiene and cleanliness Both floor and operators should be clean and the shed well ventilated.

4. Milk and milking Equipment should be kept clean, and milk cooled as soon as possible.

It is sometimes believed that ensiled forage has an adverse effect on the general health of the animal, especially on the skeleton of young animals. This criticism has no scientific evidence. It is certain that musty silage adversely affects animals, but such material should never be fed in the first place. It must be pointed out that silage is a good feed, but it is certainly not a "complete feed." For example, maize silage contains insufficient Ca and P, and should be supplemented. P content in ensiled alfalfa is sufficient for heifer growth and Ca more than enough. Animals fed on silage should be properly supplemented with the nutrients that are insufficient for the animals' requirements.

It must also be emphasized that the nutritional value of silage depends on the quality of the original forage, harvest time, conservation method, etc. Nutritional values of different silages therefore vary substantially. When it is impossible to analyse silage at the laboratory, one should refer to feed manuals and roughly assess the nutritional value of the silage to be fed, in order to formulate a ration that matches the animal's requirement.