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Small-scale Manufacture of Compound Animal Feed (NRI, 1988, 87 p.)[edit]

Chapter 4 - Outline of the feed manufacturing process[edit]


The process of manufacturing animal feed is a means whereby raw materials of widely ranging physical, chemical and nutritional composition can be converted into a homogenous mixture suitable for producing a desired nutritional response in the animal to which the mixture is fed. The process is basically a physical one and chemical changes are few. It should be remembered however that some raw materials will have undergone extensive processing prior to inclusion into a mixed feed, for example, extraction of oil from oilseeds by solvent or mechanical extraction, heat treatment of soya beans or other beans to denature anti-nutritive factors, or the production of fishmeal and meat meal. These processes will not be considered here and reference should be made to Appendix 6 for further information on these subjects.

The feed manufacturing process may be considered to be made up of several unit operations which, in almost all circumstances, include the following:

  • raw material, storage and selection
  • raw material weighing
  • raw material grinding
  • mixing of dry ingredients and addition of liquids
  • pelleting of mixed feed (optional)
  • blended feed bagging, storage and despatch.

Their sequence and the size and sophistication of equipment vary with the output of feed required as well as differences in manufacturer's design. For the purposes of illustration and for the development of cost models in Chapter 5, four levels of output will be considered as follows:

Production level

Typical design


500 kg per day

Shovel mixing


200 kg per hour

Cement mixer


1 tonne per hour

Farm-scale mill and mix plant


2.5 tonnes per hour

Small industrial-scale feed plant

Selection and layout of feed milling equipment[edit]

A number of manufacturers supply ranges of feed milling equipment and will advise on the selection of suitable models if provided with full information on the proposed operation. This must include the proposed capacity of the mill, the types of raw materials available, the types of livestock feed to be produced, and the characteristics of the power supply available. The chosen site for feed production should be readily accessible to transport, as near as possible to raw material sources and to the livestock owners, free from flooding, and with suitable power and water supplies available.

There are no set specifications for the layout of a feed milling operation, each being designed according to individual circumstances. The planning of larger mills requires the services of skilled engineers and draughtsmen, but small mills can usually be assembled from modules supplied by equipment manufacturers. Several manufacturers sell 'Mill + Mix' units which can be used for meal production, provided no difficult raw materials are to be used. In recent years there has been increasing interest in the concept of 'packaged' or 'containerized' feed mills where items of machinery are assembled within a space frame and wired up to a control panel at the factory. The unit is then shipped as a whole within a container. On arrival it is placed on a level (concrete) base, and the electricity supply connected to the control panel.

Raw material, storage and selection[edit]

In most circumstances the raw materials coming into a feed process area will have been requested by the nutritionist as being necessary to meet the nutrient requirements of the diet to be manufactured. In developing countries raw materials will normally be delivered or collected from a supplier in hessian, jute, cotton, paper, or possibly loosely woven polythene sacks. A standard size of sack may not be used for each consignment and care should be taken to check-weigh as many bags as possible since, for many small-scale operations, a weigh bridge for weighing a lorry before or after unloading may not be available. Bags are often man-handled, although the use of a small sack truck (see Figure 1) will considerably ease the burden of carrying heavy materials within a feed mill area. In some circumstances, and especially with larger feed mills, raw materials may be delivered in bulk, necessitating appropriate handling and storage facilities.

In order to ensure a continuous supply of raw materials at the mill, when some may only be seasonally available on the market, and to take advantage of price fluctuations, some form of storage will be necessary. The particular method chosen for raw material storage will depend on the local circumstances, but in areas where labour is cheap and plentiful and capital funds scarce, it is likely that storage in bags will be preferable. Raw materials should arrive in good condition and in sacks which have not been used for the storage of fertilizer, pesticides or chemicals. Contamination by string, large pieces of metal, wood or stones which could cause extensive damage to machinery can normally be removed on a coarse metal grid fitted over the sack tipping-in point of the feed mill, and permanent magnets will normally remove any tramp ferrous metal which may enter the system, particularly before entering the grinder, mixer or pelleter.

Storage areas must be waterproof and well-ventilated, and provide protection against infestation by insects and vermin which can quickly cause substantial losses in weight. If materials are to be stored in bags they should be kept in a building having a concrete floor. The roof and walls need only to be lightly constructed provided that they are pest and waterproof. The bags should be stacked a few inches above floor level, for example, on wooden pallets (see Figure 1), and away from walls. Raw materials may also be stored in bulk either in silos constructed from concrete or steel or in bins formed with partitions in conventional stores. Bulk storage normally entails a greater investment in capital equipment but lower operating costs. If raw materials are to be stored in this way it is essential that the bin manufacturers are informed of the raw materials to be handled, since some raw materials which have poor flow characteristics tend to form bridges of material in the bin base thus preventing their discharge. In general, raw materials of low bulk density have poor flow characteristics and those of high bulk density have good flow characteristics. Raw materials which have poor flow properties normally require large diameter augers for their transfer.

Raw materials will vary from country to country and from region to region and will have widely ranging bulk densities (weight for a given volume). These differences in bulk density must be taken into account when determining the space required for the storage of raw materials and finished products. Appendix 4, Table XVIII lists typical bulk density values for common feed raw materials and indicates the areas required for their storage.

The proper storage of raw materials and of finished feeds is not only essential to prevent physical losses, but is also an important aspect of quality control which will be discussed in more detail later. Where the construction of stores is to be undertaken, it is recommended that advice be obtained either from relevant publications or from other appropriate sources such as the Storage Department of ODNRI.

Figure 1 - Sketches of feed milling equipment

Raw material weighing[edit]

The accurate weighing of raw materials according to the formulation for a given ration is perhaps the most important unit operation involved in feed manufacture, since no amount of mechanical processing can make up for any deficiencies in nutrients which have been omitted from the mixture. The point at which weighing occurs in the feed milling process will depend upon the design of the mill. Raw materials may be selected from store, weighed and then subjected to grinding and mixing, or materials may be pre-ground, then weighed and mixed. There are advantages and disadvantages in both approaches and their choice will depend upon the raw materials to be processed and the design considerations of machinery manufacturers. In small units, raw materials in sacks can be weighed individually on a platform scale with either a dial or lever-arm movement (see Figure 1), or if bags are known to be of accurate weight they can be counted and any excess needed for the formulation weighed on the scales. Lever-arm scales are cheaper to purchase than dial scales, tend to be more robust, but are less convenient in use. Where possible, it is advisable that all scales be fitted with an adjustable tare, so that operators do not need to make calculations when allowing for the weights of containers into which raw materials may be tipped for weighing.

Large bin-type weighers (see Figure 1) are often used for raw materials which have been pre-ground or are free flowing and discharge readily from storage bins or silos. Bin-type weighers may be mobile or stationary. Inline weighers which measure the quantity of material flowing over a small electronic sensor and volumetric dischargers are also available. Units which quantify raw material by volume tend to be more applicable to small feed units handling cereals of constant bulk density, and do not often find application in tropical countries where ingredients have diverse bulk-density characteristics. Designs of weighers are many and various but the above have been given to illustrate typical machines in use in feed mills.

The weighing of raw materials requires great care and inaccuracies must be kept to a minimum. It should be noted that errors in the weighing of small quantities of raw materials often have far greater influence on the growth performance of animals than errors in the weighing of large quantities of material, for example, the omission of say, 25 kg of bran from a mixture requiring 400 kg of bran is of much less significance nutritionally than the omission of 1.5 kg of vitamin pre-mix say from the same mixture requiring only 2.5 kg of pre-mix. It may therefore be necessary to purchase a scale to weigh small quantities, of up to 25 kg, with an accuracy of ±100g and a greater capacity scale, for example up to 500 kg with an accuracy of ± 2.0 kg. The use of accurate scales is of particular importance when handling expensive and/or potent raw materials such as vitamins and medicinal additives which are added at low inclusion rates.

Raw material grinding[edit]

In the sequence of unit operations involved in feed milling, raw material grinding may occur before or after weighing. It is a process with high power requirements which is often noisy and dusty. The design of machine most commonly found in the feed.) manufacturing industry is the hammer mill and the operation of such machines is illustrated in Figure 2. Inside the grinding chamber, hammers, which may be fixed rigidly to the central shaft, or more often swinging on steel pins, rotate at high speed. The impact of the raw material on the hammers and the continual high-velocity impact of particle on particle results in material breakdown until it is small enough in size to pass through a perforated screen. It is obvious that the smaller the screen size the more work will be required to reduce the particles to the desired size and the larger the grinder motor required. Raw materials also have different grinding properties somewhat related to their bulk density and flow characteristics. In general those of high bulk density grind more easily than fluffy, fibrous low-bulk density materials. Grinders are most efficient when they are running at maximum capacity for a given raw material and screen size.

Because of difficulties experienced in feeding certain raw materials (for example, brans, cottonseed cake) through a grinder, many feed manufacturers pre-blend ingredients before grinding in order that the more easily ground materials will act as carriers or flow aids to those offering resistance to grinding.

The grinding operation can generate considerable quantities of heat and dust and temperatures of raw materials may increase by at least 10-20°C. For these reasons the process may be a fire- or even an explosion risk particularly if the grinder is not protected against the entry of metal, stones, glass and other objects which can cause sparking. For safety reasons large grinders are often sited in separate brick-built stores on the outside walls of feed mills. If ground material is to be stored in bins or sacks before further processing it is essential that the heat generated during grinding be dissipated. Cooling normally occurs as air is drawn into the grinding chamber, and during the pneumatic conveying of ground material from the grinding screen to its point of discharge, which may be through a cyclone into a bin or mixer. Many small grinders have suction fans fitted to the grinder shaft which bring about cooling and conveying of ground material in one operation. Other grinders discharge directly into conveyors and the air drawn in during grinding is released through filter bags. Grinders may operate in a horizontal or vertical direction according to design.

If ground material is conveyed pneumatically, the air and material are separated in a cyclone (see Figure 2). This simple device, which is similar to an inverted cone, causes air to swirl around its walls depositing the ground material at the base of the cone while the air exits at the top of the cyclone through a filter. Cyclones are normally only 95% efficient at separating ground particles and air, and a cloth or other type of filter is necessary as a dust barrier.

It should also be noted that the desired fineness of grind will be influenced by the livestock to which the feed must be fed, or by other processes following grinding. Raw materials for poultry should be more finely ground than for cattle or pigs and raw materials to be pelleted are usually more finely ground than the equivalent feed as meal.

Figure 2 - Action of hammer grinder

Effect of moisture content of raw materials

The moisture content of raw materials to be ground in a hammer mill should not normally exceed 13-14%. High-moisture feeds are plastic or malleable in character with few planes of impact weakness and may clog a conventional hammer mill designed for handling dry ingredients. Hammer mills and other designs of grinders may be obtained for handling moist or wet commodities, but these would not normally be used in a conventional feed mill.

Use of pre-crusher

General purpose hammer mills for small-scale feed mills are not designed to crush large chunks of raw materials to fine particles in a single pass operation. Large, lumpy, hard materials such as dried cassava roots and expeller oil cakes should be pre-crushed in a cake breaker to a particle size suitable for the dimensions of the hammer mill intake throat. It is important therefore that when requesting information on grinding machinery from suppliers, full details of raw materials be provided. It is advisable to provide samples of the largest, hardest and most fibrous materials likely to be encountered.

Mixing of dry ingredients and addition of liquids[edit]

It is the job of the mixer to produce a homogenous blend of all the raw materials desired in a formulation, such that at each feeding period each animal receives a balanced mixture of nutrients. The smaller and younger the animals to be fed, the greater is the need for good mixing. Not only are their requirements more demanding, but the daily nutrient intakes of those eating small amounts of feed will be subject to much greater variation as a result of poor mixing. Mixing often improves feed palatability if one or more of the raw materials is unpalatable to livestock.


Limited quantities of animal feed can be very adequately mixed (assuming the raw materials have been ground appropriately) on a concrete pad with a shovel, in a manner similar to the dry mixing of cement and sand. Raw materials should be layered one above each other and then mixed and turned to form an adjacent heap. An efficient shovelling and mixing of the heap at least three times should produce an acceptable product with the even distribution of small quantities of vitamins and minerals. Such a mix should be similar to a mixture obtained from a vertical mixer described later. The evenness of colour of the mixture will often give a fair indication as to the homogeneity of the mixed feed.

Concrete mixers

Small concrete mixers with electric or petrol engine drives are mobile lowcost machines suitable for the manufacture of mixtures of dry ingredients or mixtures of wet feeds, for example for pigs. Pre-ground raw materials should be mixed for a minimum of five minutes to achieve a satisfactory blend. For larger-scale feed mixing however it is advisable and probably cheaper to use one of the conventional feed mixers described below.

Conventional feed mixers

Two designs of mixers are most commonly found in the feed industry: the vertical (or fountain) mixer and the horizontal (or U-trough) mixer. A third less common type is the conveyor mixer. Each type is described in more detail below.

Vertical mixers

The vertical mixer is a slow action, long-dwell time mixer which relies upon the continuous tumbling and intermingling of raw materials as they are discharged in a fountain-type action from a vertically running screw of approximately 8-10" diameter as illustrated in Figure 3. Raw materials may enter the mixer either at the top, from a cyclone or auger feed from the grinder, or at the base of the screw at a sack tipping point. After mixing for a pre-determined time, normally 10-15 minutes (although this time may be shorter in some mixes), the mixture is discharged into a bag or conveyed by auger or bucket elevator to a storage bin or pelleter.

Since many raw materials are dusty it is often desirable to include materials such as molasses, oils and fats in the formulations to reduce dustiness as well as to provide a source of nutrients. Vertical mixers, because of their slow-running action, are generally less effective in distributing liquids throughout the mixture, and liquids tend to form beadlets or balls coated with fine particle material, rather than produce a surface coating on the solid material. For coarse cattle rations where large quantities of feeds are consumed per animal the need for a completely homogenous distribution of liquid is less critical than for poultry feeds or feeds to be pelleted, where it is desirable that liquids be well mixed with minimal lumping.

Vertical mixers have a general tendency to encourage particle size segregation, especially if too long mixing times are used. They are tall units which may not readily fit into buildings with low roofs or ceilings. However, they can be easily loaded manually at floor level, and are relatively low capital-cost machines widely used in feed manufacture where liquid addition is not required, or for blending raw materials prior to grinding.

Horizontal mixers

As the name suggests, horizontal mixers operate with a horizontally turning mixing shaft. The shaft may carry paddles or agitators of various designs which come in very close proximity to the wall of a U-shaped trough. Raw materials are lifted, folded and abraded against each other resulting in a relatively short mixing time, typically of the order of 3-6 minutes, though it may vary depending on the nature of the mix. The mixer is suitable for blending up to 8% liquids into a dry mix and therefore offers greater versatility if a wide range of rations are to be offered from one feed mill unit. It is preferable that fats and molasses be warmed before addition to the raw materials in the mixer and they should be added as the last ingredients. Because the horizontal mixer is a faster mixing machine than a vertical mixer, two or perhaps three mixes can be achieved in the same time as one mix in a vertical mixer. A half-tonne capacity horizontal mixer for example could possibly replace a 1-tonne vertical mixer since two halftonne mixes could be made in a horizontal machine including loading and unloading in the same time as one tonne in a vertical mixer. A horizontal mixer is more sophisticated in terms of its engineering construction and thus more expensive to purchase than a vertical mixer of equivalent capacity.

Conveyor mixers

Conveyor mixers are also available, particularly for farm use, and consist of a trapezoid metal box in which mixing is effected by slats extending almost the full width of the machine and which are carried on a pair of endless chains. Like the vertical mixer this machine is limited in its ability to blend liquids thoroughly into the mixture.

Figure 3 - Mixers

Pelleting of mixed feed[edit]

The use of pelleted feed is often popular with farmers because it is convenient to handle and reduces dustiness (for example, in cassava-based feeds), but pelleting can have other advantages. It prevents segregation of raw materials during handling and selection by animals, especially poultry, during feeding. This may be particularly useful where less palatable raw materials are included in the formulation. Pellets also reduce feed losses during feeding, and may help to maintain, or increase, feed intake under certain conditions. The heat generated during pelleting can inactivate some pathogenic bacteria which may be present in raw materials. Finally, in some circumstances, pelleting can assist in preventing adulteration of feed by unscrupulous traders. However, pelleting increases the cost of feeds because the capital cost of pelleters is relatively high compared to grinders or mixers, the energy requirement is high, and additional care and skill is necessary for their maintenance and operation. Therefore the decision on whether to pellet has to be made in the light of individual circumstances.

Pelleting involves the compression of a mixed feed through holes in a hardened steel ring or plate (a die) by means of hardened steel rollers. The die forms the feed into pencil-like extrusions which are cut by knives into pellets of desired length on leaving the die. The principle of operation of a ring die is given in Figure 4. In a ring die pelleter, the rollers or the die may be driven but in a plate die pelleter the rollers only are driven. The die and rollers of a ring die pelleter may operate in a horizontal or vertical plane according to machine design. Pelleters with horizontally running dies are most commonly found in farm-scale feed mills. The pelleting process is very energy intensive, demanding up to 50% of the total power required for feed manufacture. The diameter of feed pellets is governed by the diameter of the holes in the die ring but the smaller the die holes the greater effort is required to force meal into these holes, hence the greater the power demand, that is, the smaller the pellet, the greater the cost of manufacture.

Figure 4 - Operation of a pelleter die head

Pelleters may also be divided into two further groups according to the pre-treatment of mixed feed prior to compression or extrusion in the die head. Pelleters may be considered as cold pelleters or conditioner pelleters.

Cold pelleters

In cold pelleting, mixed feed is fed directly from a bin or auger into the die head at ambient (normal atmospheric) temperatures. Some water may be added, preferably in the mixer if the meal is too dry to bring it to approximately 15-16% moisture, but there is no heat treatment of the mixed meal before it enters the die. The frictional forces generated during pellet extrusion cause the temperature of the pelleted feed to increase from ambient to up to 60-70°C. Pellets must be cooled to ambient temperatures before storage by spreading thinly over a large area of floor, or preferably cooled in a bin fitted with a cooling fan. During cooling the moisture content is reduced to approximately 12% by evaporation in order to reduce the risk of sweating and mould growth.

Cold pelleters for farm-scale use have outputs of up to 750 kg per hour of poultry pellets, or 1 tonne of dairy pellets per hour, depending upon ration formulation, particle size and moisture content of the meal and pellet diameter.

Conditioner pelleters

The term 'cold pelleting' is something of a misnomer since a considerable amount of heat is generated during the pelleting operation, but it serves to distinguish the process from conditioner pelleting which is the usual process in industrial pelleters. During conditioner pelleting, the mixed meal is directly pre-heated with dry steam (i.e. steam which is in vapour form and does not contain suspended droplets of condensed steam) in a small high-speed mixer called a conditioner or in a slow turning mixer called a kettle or ripener.

The steam preheats or conditions the meal to the preferred temperature and moisture content for pelleting according to the formulation of the mixture, for example, 65°C and 15% moisture. During pelleting the temperature of the meal rises by approximately 10°C, hence the final temperature of pellets from a conditioner pelleter is similar to that of pellets from a cold pelleter. Coolers for these machines may be of vertical or horizontal design. Cold air is drawn through a moving mass of pellets either as they fall through the vertical machine, or as they pass along an open mesh belt through a horizontal cooler.

In terms of energy requirements for a given output, the energy required for manufacturing half a tonne per hour of pellets in a cold pelleter is approximately equivalent to the sum of energy required to manufacture the same quantity of pellets in a conditioner pelleter plus the energy required to produce the steam for the conditioner. Practical experience shows that for a given pelleter motor size, the output of the pellets will be approximately doubled if meal is pre-conditioned prior to pelleting, or, conversely a cold pelleter of say 25 horse-power will produce only half the output of a conditioner pelleter of 25 horse-power if the energy required to raise the steam is not taken into account.

Generally, the quality of pellets (that is, resistance to break-down after pelleting and during handling) of a given mixture from a conditioner pelleter is marginally better than that from a cold pelleter, but the conditioner pelleter requires a boiler and associated water treatment plant to treat the feed water for the boiler.

Pellet quality

Pellets should have a desired degree of hardness, and should also show high resistance to abrasion during handling and transport. Pellet quality depends largely on the amount and nature of starch and protein in the raw materials. Their binding effect is modified by a number of other factors including the moisture content, fibre content, oil content, and fineness of grinding of the raw materials. Various types of dies are available for dealing with different mixes. Instruments can be obtained for testing pellet hardness and resistance to abrasion.

Pellet binders

Some mixtures of raw materials do not bind well together when pelleted and require the addition of special binding agents. Molasses is often added at 2-5% to aid binding, but other binders include bentonite clays and lignosulphonates, and are added at the suppliers' recommended dosage levels, usually about 1-2%.

Augers, bucket elevators and conveyors[edit]

Augers and bucket elevators are used to move raw materials or meals from one feed mill operation to another. Augers are steel tubes containing a continuous screw which conveys meals along its length as it is driven by a motor. Various designs and diameters of augers are available, and care should be taken in their selection since augers designed for conveying materials of high bulk density may not readily convey low-bulk density materials. Augers may be used in a horizontal or inclined position, but are not suitable for the vertical movement of materials. This job is best undertaken by a bucket elevator.

The bucket elevator consists of a tall metal or wooden box in which runs an endless chain fitted with buckets. Buckets are filled at the base of the elevator and discharged at the top. Bucket elevators have a gentle lifting and tipping action and are therefore suitable for lifting pellets to a cooler, whereas an auger may well damage and break the pellets.

For the horizontal movement of large quantities of feed materials conveyors may be used. Again many designs are employed, but their action is similar to that of the bucket elevator, with the exception that the buckets are replaced by slats, chains or baffles to drag material from one process operation to another.


Compound feeds, whether in meal or pellet form, are usually distributed in sacks in developing countries, although for on-farm use or for distribution to a large livestock unit distribution could be in bins or trucks. Bags may be filled directly from mixers or from holding bins and may be weighed on a scale balance or through an automatic pre-set weigher and bagging unit set to weigh, for example, 25 kg of meal per bag. Bags may be of jute, cotton or paper and can be hand- or machine-stitched or tied with a string or metal tie. Stitching machines do not stand up to abuse and require a constant supply of appropriate needles and thread and are therefore more applicable to the larger feed mill models in this bulletin. Polythene bags are not normally recommended for storing animal feeds because of the risk of sweating and mould growth. If old bags are re-used, care should be taken that they have not been used previously for the storage of fertilizers, pesticides, or other chemicals.

Other requirements[edit]

For the successful manufacture of compound feeds several other requirements must be fulfilled: these are discussed below.


The buildings to house the manufacturing plant will depend to a large extent on the particular circumstances of the mill, but generally they must be capable of being kept clean, and provision should be made for keeping the dust level as low as possible since it can affect the operation of machinery. Excessive dust is also a fire and explosion hazard. In some environments, machinery can be housed in a light structure and where the climate is suitable it may even stand in the open. However, consideration may need to be given to local building regulations and to special precautions necessary for occasional adverse climatic conditions, for example, hurricanes. A concrete floor which can be swept is usual, but should be laid down to the manufacturer's plans as some pits and floor fixings may be required. Where flooding may occur, as during a monsoon period, the floor must be above the high level water mark. The machinery usually has its own supports which are supplied by the manufacturer or can be made locally to his specifications.


The power to drive feed milling equipment is generally obtained from electrically driven motors. Some small-scale processes can be undertaken by hand or by using direct driven machinery. Grinders, mixers and pelleters can be obtained which are driven by petrol or diesel engines directly, or from a tractor power take off (PTO). However, for most situations, electric motors provide the simplest and most convenient method of driving machinery. If grid ('mains electricity') is not available, a diesel-generating set can be used instead so that electricity is produced independently of the grid.

Electricity supply

For small processes with a connected motor load of a few kilowatts (that is, the sum of the motor powers), operation from a single-phase electrical supply might be possible. However, it is normal for industrial/commercial premises and sometimes for agricultural premises to have a 3-phase supply. It is essential to determine the likely electrical load for the machinery and then to determine what type and quantity of electricity can be made available. If grid electricity is used, contact should be made with the local electricity supply authority. If a generating set is to be used, then it is the responsibility of the user to specify the requirements.

The characteristics of the supply which need to be known include:

  • the number of phases (1 or 3) and whether a neutral is available for the 3-phase supply,
  • the nominal voltage and frequency,
  • the variations in voltage and frequency,
  • the maximum demand in kVA,
  • the maximum starting load permissible,
  • the arrangements for earthing,
  • the arrangements for short-circuit protection.

Electrical equipment

Electrical equipment is designed to operate within prescribed limits of voltage and frequency, and under specific conditions. Any abnormal conditions such as high ambient temperatures, high humidities, high altitude, or dusty or wet environments, can affect the satisfactory operation of motors and equipment. These factors should be stated to suppliers of machinery as well as information on the electrical supply. If, for instance, equipment is to be used outside, it should be specified for tropical outdoor use.

All electrically operated process machinery should have a method of starting and stopping. This is usually achieved by operating pushbuttons on a starter. Some method of isolating each machine from the supply should also be incorporated to allow maintenance and cleaning to be undertaken safely. The starter and isolators (motor control gear) can be supplied with the machinery or obtained separately. Direct-on-line starting of small motors is normally used. However, with larger motors (typically 4 kW or 7.5 kW depending upon the local electricity supply undertaking) some method of reducing the current surge on starting is usually necessary such as star-delta starting. In fact many undertakings insist upon this so as to minimize voltage dips in the supply.

Depending upon the size of the installation, a main fuseboard and isolator (distribution gear) may be necessary as well as additional facilities such as lighting, socket outlets and ventilation. Sometimes it is necessary to provide power factor correction equipment.


Water is required for steam raising if the feed mill has a steam conditioner, or may be added to the mixer to raise the moisture content of the meal to a level suitable for pelleting. Water supplies should be of potable quality and uncontaminated with effluent or sediment.


Although feed mills are not factories for the production of human food, they should be kept as clean as practicable. Dusty conditions are unpleasant to work in and are ideal for the development of contaminating insects, micro-organisms and scavenging vermin which may introduce disease to animals and reduce animal productivity. Gross infestation by moth larvae in particular may well bring about blockage of augers, elevators or outlets to bins which are used only periodically due to excessive build-up of insect webbing.

Dusty conditions also demonstrate that quantities of expensive raw materials are being lost and wasted. Cleaning does not involve complicated procedures and can be fitted easily into the normal working schedule. Care should particularly be taken when cleaning process plant which has been used for the inclusion of veterinary compounds such as drugs since crosscontamination from one ration into another for a different species of animal may prove fatal.

Routine maintenance

All mechanical equipment is subject to wear and tear and regular maintenance should form part of the working schedule. Machinery manufacturers will give advice on maintenance programmes and a supply of spare parts should be kept in stock, a list of typical spare parts being given below.

Grinder screen and hammers Auger and elevator bearings Belts and bushes Spare motors Pelleter dies and rollers Dust filter socks Elbows and bends in ducting which may be prone to wear Miscellaneous nuts and bolts Electrical spares, etc.

It is important therefore to budget for spare parts when purchasing new equipment or when determining annual inputs for an established feed mill.

Importance of power factor[edit]

Most AC electrical machines draw from the supply apparent power in terms of kilovolt amperes (kVA) which is in excess of the useful power, measured in kilowatts (kW), required by the machine. The ratio of these quantities is known as the power factor of the load, and is dependent upon the type of machine in use. Assuming a constant supply voltage, this implies that more current is drawn from the electricity authority than is actually required.

Power factor = (true power) / (apparent power) = kW / kVA

A large proportion of the electrical machinery used in industry has an inherently low power factor, which means that the supply authorities have to generate much more current than is theoretically required. This excess current flows through generators, cables, and transformers in the same manner as the useful current. The motive power requirements are generally greater than the resistive loads such as lighting and heating. If steps are not taken to improve the power factor of the load, all the equipment from the power station to the factory sub-circuit wiring has to be larger than necessary. This results in increased capital expenditure and higher transmission and distribution losses throughout the whole supply network.

To overcome this problem, and at the same time to ensure that generators and cables are not overloaded with wattless current (as this excess current is termed), the supply authorities often offer reduced terms to consumers whose power factor is high, or impose penalties on those with low power factor. Most supply authorities insist that a power factor of at least 0.90 is achieved. Improving the power factor helps to reduce the overall consumption of electricity.


The charges for electricity are based on various tariffs which vary both in structure and cost from place to place. Various standing charges and a connection charge are also made. Typically, the electricity charged for will be based on:

(i) a standing charge based on the total kilowattage of the installed motors or on the kilowattage of the largest installed motor;

(ii) on the number of units consumed;

(iii) an extra charge for units when an agreed maximum level is exceeded - referred to as the maximum demand charge.

The standing charge (i) is applied irrespective of the amount of electricity consumed or of how often the equipment is used. The charge (ii) is an accumulative charge to take account of the quantity of electricity used in a particular period. Not all units are necessarily charged at the same rate. A meter is provided by the supply undertaking for this. The maximum demand charge (iii) is a penalty charge which is applied if the amount of electricity used in a specified period (usually 0.5 hours) exceeds a level which has been previously agreed between the supplier and user. It is intended to level out demand by discouraging users from consuming a large amount of electricity for just a short time. A separate meter is provided for this; it measures kVA rather than kW. Some authorities offer reduced tariffs depending upon how and when the electricity is used.


If grid electricity is not available or not suitable in some way, an alternative method of obtaining electricity is to use a generating set. Small sets of a few kVA capacity can be petrol driven, but normally they are diesel-engine driven. The size of the set required depends upon the output required and upon the starting characteristics of the various items of equipment. The supplier of the feed mill machinery can usually advise on the size most suitable for the particular installation. When the installation consists of a number of small motors, then a set slightly larger than the sum total of the motor kilowattages is usually adequate, but expressed in kVA based on a power factor normally of 0.8. If however just one of the motors is large in comparison with the total load, a larger generating set is necessary so as to prevent undue voltage dips occurring when that particular motor is started, as such dips will effect equipment already running. For satisfactory operation, the diesel engine will require regular maintenance.

Quality control[edit]

Quality control is essential at all stages in the production of compound feed if the maximum and most efficient returns are to be obtained by the feed compounder and livestock producer. In some countries the control of feed quality is regulated by government legislation, while in others there is no such provision. In either case, omission of any serious attempt at quality control is false economy in the longer term.

The achievement of good quality control is frequently difficult in developing countries. Locally available raw materials may be highly variable in composition, and for this reason routine analysis should be carried out on as many batches as possible. However, the equipment for setting up a basic quality control laboratory costs around £30,000 at 1986 prices and is therefore a relatively expensive operation, especially for small-scale feed milling operations, and suitably trained staff may not be available. In some cases it may be possible for a limited number of samples to be analysed by government laboratories or by independent chemical analysts. Not all larger laboratories will have facilities for some of the more specialized analyses for example, for amino acids, which may be required.

Fairly simple and inexpensive equipment is available for the rapid determination of moisture content and should be available in all feed manufacturing operations. If further facilities can be established, the next most basic analyses are crude protein and fibre. The microscopical examination of raw materials can provide a valuable check on their identity and the presence, or otherwise, of adulterants. The cost of the relevant equipment (microscope, etc.) is fairly modest, but some experience is necessary before individual materials can be identified with confidence. Training courses in the technique are available.

The quality of raw materials can be affected by growing, harvesting, and post-harvest handling and processing, but at the feed mill the quality control function usually begins with the receipt of raw materials. They should arrive in good condition in sacks, or other containers, which should not have been used for the storage of fertilizer, pesticides, or other chemicals. They should not be lumpy or mouldy or heavily infested with insects. The moisture content should not be excessive and should be closely monitored if the raw materials are stored. The control of moisture content is one of the most important aspects of quality control.

Moisture content

Moisture content of stored produce is closely related to ambient relative humidity. Oil-free materials such as grains have higher moisture contents than those containing oil, in equilibrium with the same ambient relative humidity. However, differences in moisture content/relative humidity relationships are small for oil-free feed materials, and it is possible to generalize for these to some extent with moisture contents which are critical for different types of biological activity. Moisture content in equilibrium with a given relative humidity varies with temperature, and for a 10°C rise decreases by 0.6-0.7% for the oil-free material.

The moisture content in equilibrium with a given relative humidity is also affected by the so-called 'hysteresis' effect. Due to this, feed materials absorbing water to achieve a given equilibrium relative humidity, have lower moisture contents than those drying out to the same equilibrium relative humidity. Biological activity both within the materials and of pests is greatly affected by moisture content. Insect pests will not develop on feedingstuffs at relative humidities outside the range 30-90%, while bacteria will only develop at relative humidities of over 90%. Fungi generally grow only at relative humidities of over 70%, while seed germination normally requires relative humidities of more than 95%. Expressing these in terms of approximate moisture contents of oil-free material stored at temperatures of 20-30°C, the following can be anticipated:

(i) up to 8% moisture (30% relative humidity): no significant biological activity;

(ii) 8-14% (30-70% relative humidity): insect infestation possible; mites can infest at relative humidities of over 60%;

(iii) 14-20% moisture (70-90% relative humidity): insect infestation and mould growth can occur;

(iv) 20-25% moisture (90-95% relative humidity): mould and bacterial growth possible;

(v) above 25% moisture (more than 95% relative humidity): bacterial growth and seed germination possible.

In practical terms, this means that moisture content should be kept as low as possible, but should not be allowed to exceed that which would be in equilibrium with relative humidities of 70% or more. Allowing a safety margin to take into account fluctuations in equilibrium equivalents, a maximum moisture content of around 1 3% for oil-free material would seem to be appropriate. Lastly it should be mentioned that moisture content can influence the degree to which certain chemical changes, which are not biologically induced, may occur. However, its greatest effect is on the biological changes already mentioned.


Almost all vegetable compound feed materials of tropical origin are liable to contamination by the aflatoxins, a group of highly toxic mould metabolites, produced by certain strains of the moulds Aspergillus flavus and Aspergillus parasiticus. The aflatoxins can be formed during the pre-and post-harvest stages of raw material production provided that a suitable environment for mould growth exists. The conditions required for mould growth are usually satisfied in tropical countries. Different commodities vary in their ability to support fungal colonization due to differences in the chemical composition of each commodity. Samples of oilseed cakes from groundnut, cottonseed, palm kernel and copra, together with cereals like maize have been found to contain high levels of aflatoxin, whereas the majority of samples of soya and fish meal which have been analysed for aflatoxin have been found to be free of the toxin.

The acute toxicity of the aflatoxins and their ability to induce liver cancer in animals varies according to the sex and age of the animal and a number of other factors. Young animals are more susceptible to aflatoxin intoxication than older animals, and males usually require a smaller dose of the toxin than females to produce a similar effect.

The aflatoxins can affect the cellular-immune system of animals and so decrease their ability to resist viral and bacterial infections. In addition, the aflatoxins have been reported to reduce the absorption of a number of essential feed constituents and drugs from the gut and this can affect animal health and productivity. Consequently, the amount of aflatoxin in the diet of animals should be restricted.

Many countries have introduced legislation to limit the amount of aflatoxin in animal feeds and some are restricting the levels of aflatoxin in compound feed ingredients imported from other countries. In the European Community (EC) the maximum level of aflatoxin permitted in a complete feed is 50 mg/kg and this is reduced to 10 mg/kg when the feed is to be given to dairy cattle because of the risk of aflatoxin derivatives reaching milk for human consumption. A variety of analytical and big-assay methods have been developed for determining the levels of aflatoxin in animal feeds. However, the efficiency of these methods is frequently compromised by the collection of an inadequate sample, or by the unsatisfactory preparation of the sample prior to analysis. Details of the methodology suitable for determining the levels of aflatoxin in animal feeds can be found in a manual prepared by ODNRI and used by trainees attending the aflatoxin training course held annually at ODNRI.

Other tests[edit]

In addition to the above factors, there are a number of other considerations to be borne in mind with specific types of materials. It is important to ensure that processed materials, particularly those of animal origin such as fish, meat and bone meal, do not contain any pathogenic bacteria which could cause diseases in animals to which they are fed. The most common pathogenic organism encountered is salmonella, and it is important that consignments, particularly from new suppliers of processed materials, be tested for this organism.

Protein concentrates which have undergone processing, for example, oilseed cake and meal and animal by-product meals which are to be included in feeds for monogastric animals, should be tested to ensure that the quality of the protein has not been reduced during processing. The most important form of damage recognized is the rendering of the amino acid Iysine unavailable for nutritional processes by excessive heating during processing. It is therefore important to test materials of this type in common usage for available Iysine content from time to time and to check any new materials which are offered.

Materials such as cottonseed cake which are prepared from seeds known to contain toxic substances (gossypol in the case of cottonseed), should be tested to ensure that they are of acceptably low toxicity for inclusion in feeds for the class of animals for which they are intended. For example, cottonseed cake should not be included in feeds for pigs or poultry unless the gossypol content is very low, whereas gossypol tolerance of mature ruminants is very much greater. Some toxicity problems may be overcome with chemical treatment, and ferrous sulphate has often been recommended for cottonseed. An indication of the various types of toxic factors which can be encountered are given in Appendix 3, Table XV.

Finished feeds[edit]

If the raw materials and processing conditions are of the correct standard, then the product should also be of the correct standard. However, variations and errors can arise in the weighing or accidental omission of an individual raw material. The omission of a small quantity of vitamin supplement may have a marked adverse effect on the health and growth rate of animals receiving the feed. For this reason, considerable care must be exercised in ensuring that the specified amounts of all raw materials are weighed out for each batch, and an appropriate system for checking this should be devised. It is important that representative samples of batches be taken for check analyses to monitor the composition of the finished feeds. If results show deviations from the required composition, the reasons for this must be sought and rectified. In some countries there may be statutory requirements for the composition of feed offered for sale.