Veterinary · Pre-clinical and foundation · also MSVE ANN-111
What feed is made of and how its value is measured: proximate analysis, energy partition, protein quality, minerals, vitamins, anti-nutritional factors and feed processing.
In the exams: ICAR AICE-JRF core group 05; ICAR AIEEA-PG 14 (Animal Nutrition); TNPSC Nutrition 20; RPSC (feeds of Rajasthan); EGEL Plus nutrition 13; ASRB NET (numerical problems, per coaching sources)
Animal nutrition is the study of what feed is made of, what happens to it inside the animal, and how much of each nutrient an animal needs to stay healthy, grow, reproduce, work or produce milk, meat, eggs and wool. Principles of Animal Nutrition and Feed Technology is the first half of that study: the chemistry of feeds, the methods used to measure their value, the partition of energy and protein, water, minerals and vitamins, the substances that interfere with nutrition, and the processing and formulation that turn raw materials into a ration.
Feed is usually the largest single cost of livestock production, commonly quoted as well over half of the cost of producing milk, meat or eggs, though the share varies by system and country. Many diseases a veterinarian sees are nutritional: milk fever, grass tetany, white muscle disease, swayback, urea poisoning and aflatoxicosis. A veterinarian who cannot read a feed analysis report, check a ration against a feeding standard or spot an anti-nutritional factor will miss causes that no drug can fix.
Indian postgraduate, PhD and state commission tests ask for Weende fractions, energy terms, protein quality indices, deficiency diseases, anti-nutritional factors and short calculations. Licensing examinations elsewhere test the same facts through cases: lambs with ataxia, piglets with anaemia. This manual covers every unit of MSVE course ANN-111, noting where practice or law differs between countries.
Every feed consists of water and dry matter (DM). Dry matter is divided into organic matter and inorganic matter (ash, the mineral residue left after burning). Organic matter holds the nitrogen-containing compounds (true protein and non-protein nitrogen such as amides, amino acids, nitrates and urea), lipids (fats, oils, waxes, phospholipids, fat-soluble vitamins), carbohydrates (sugars, starch, pectin, hemicellulose, cellulose) and the non-carbohydrate cell-wall polymer lignin. A nutrient is any food constituent that supports life; the six classes usually taught are water, carbohydrates, lipids, proteins, minerals and vitamins.
Because water content varies so much between feeds (green fodder may be 75-85% water, grains and oilseed cakes around 10%), all feeds are compared on a dry-matter basis. A value 'as fed' or 'fresh basis' includes the water; a value on 'DM basis' does not. The conversion is simple: nutrient on DM basis = nutrient as fed divided by the DM fraction. Feeding standards express requirements and feed composition on DM basis, so this conversion appears in nearly every ration problem.
The proximate analysis scheme was developed at the Weende Experiment Station, Germany, by Henneberg and Stohmann in the 1860s. It divides a feed into six fractions: moisture, crude protein, ether extract, crude fibre, total ash and nitrogen-free extract. It is 'proximate' because each fraction is a group of compounds, not a pure substance.
Moisture is found by drying a weighed sample in an oven, commonly at about 100-105 degrees C to constant weight. The loss in weight is moisture; volatile compounds such as the fatty acids in silage are lost too, so oven-drying underestimates silage DM, and toluene distillation or correction factors are used for such feeds.
Crude protein (CP) is not measured directly. Nitrogen is measured by the Kjeldahl method (digestion with concentrated sulphuric acid and a catalyst, distillation of ammonia into boric acid, titration) or by the Dumas combustion method, and multiplied by 6.25. The factor 6.25 assumes that protein contains 16% nitrogen (100/16 = 6.25). The assumption is imperfect; food analysts use 6.38 for milk. CP also counts non-protein nitrogen as if it were protein.
Ether extract (EE) is the material dissolved by petroleum ether or diethyl ether in a Soxhlet apparatus. It contains true fats but also waxes, pigments and fat-soluble vitamins, so in forages much of the EE has little energy value.
Crude fibre (CF) is the residue left after the fat-free sample is boiled successively in dilute acid and dilute alkali (1.25% sulphuric acid, then 1.25% sodium hydroxide, in the classical method), then dried, weighed and ashed; the loss on ashing is CF. It was meant to represent the indigestible cell-wall fraction.
Total ash is the residue after burning the sample in a muffle furnace, usually at about 550-600 degrees C. High ash in a forage often means soil contamination, measured as acid-insoluble ash (largely silica).
Nitrogen-free extract (NFE) is not analysed at all. It is calculated by difference: NFE = 100 - (moisture + CP + EE + CF + ash) on an as-fed basis, or 100 - (CP + EE + CF + ash) on a DM basis. It was intended to represent the soluble, readily digested carbohydrates, sugars and starch.
The system's main weakness lies here. During the acid and alkali boiling, a variable part of the lignin and much of the hemicellulose dissolve and are lost from CF, so they end up in NFE. Crude fibre therefore underestimates true fibre, and NFE, carrying all analytical errors and the dissolved lignin, is sometimes less digestible than CF in straws. Even so, Weende fractions remain the basis of TDN, feed labels and statutory feed standards.
Peter Van Soest, working with the United States Department of Agriculture in the 1960s, replaced crude fibre with a detergent scheme that divides plant material by nutritional availability. Boiling a sample in neutral detergent solution (sodium lauryl sulphate with EDTA and other reagents) dissolves the cell contents: sugars, starch, pectin, lipids, soluble protein and non-protein nitrogen, which are almost completely digestible. The insoluble residue is neutral detergent fibre (NDF), representing the plant cell wall: hemicellulose, cellulose and lignin (plus some cell-wall protein and ash).
Boiling in acid detergent solution (cetyl trimethylammonium bromide in about 1 N sulphuric acid) removes hemicellulose, leaving acid detergent fibre (ADF): cellulose, lignin and acid-insoluble ash. Treating ADF with 72% sulphuric acid dissolves cellulose and leaves acid detergent lignin (ADL) plus ash; ashing that residue gives lignin. By difference, hemicellulose = NDF - ADF and cellulose is approximately ADF - ADL.
NDF is related to bulk and the voluntary intake of forage: the higher the NDF, the lower the intake. ADF and lignin are related to digestibility. Nitrogen bound in the ADF residue (acid detergent insoluble nitrogen, ADIN) indicates heat-damaged protein unavailable to the animal.
Near-infrared reflectance spectroscopy (NIRS) now predicts composition rapidly once calibrated against wet chemistry; minerals are measured by atomic absorption or plasma spectroscopy.
Digestibility is the proportion of a feed, or of one of its nutrients, that is not excreted in the faeces and is therefore assumed to have been absorbed. The digestibility coefficient (DC) is calculated as: DC (%) = (nutrient intake - nutrient in faeces) / nutrient intake x 100.
This is apparent digestibility, because faeces contain not only undigested feed but also material of body origin: sloughed epithelial cells, digestive enzymes, mucus and microbial cells. This metabolic faecal fraction is most important for nitrogen (metabolic faecal nitrogen) and for fat. True digestibility subtracts the metabolic fraction from the faecal output: true DC = [intake - (faecal output - metabolic faecal output)] / intake x 100. True digestibility is always higher than apparent digestibility, and the gap is proportionally larger when the intake of the nutrient is low. Metabolic faecal nitrogen is estimated by feeding a nitrogen-free diet or by regression.
For pigs and poultry, amino acid digestibility is better measured at the end of the ileum (ileal digestibility), because the hind-gut microbes modify amino acids after absorption has finished, so faecal values mislead.
In the conventional or total-collection trial, animals of similar age and weight are kept in metabolism crates that allow separate collection of faeces and urine. The trial has a preliminary (adaptation) period, during which the animals adjust to the diet and residues of earlier feed are cleared from the gut, followed by a collection period. Ruminants need a longer preliminary period than pigs or poultry because the rumen holds feed for days; lengths vary between laboratories.
Feed offered and refused and all faeces are weighed and sampled daily (faecal aliquots are often acidified to prevent nitrogen loss). The DM, CP, EE, CF, NFE and energy of feed and faeces are analysed and digestibility coefficients calculated for each. If urine is also collected, nitrogen balance (intake minus faecal and urinary N) can be worked out, giving the nitrogen retained.
Total collection is laborious and impossible in grazing animals, so indicators (markers) are used. An ideal indicator is completely indigestible, not absorbed, harmless, evenly mixed with digesta and easily analysed. External indicators are added to the feed or dosed: chromic oxide (chromium sesquioxide) is the classical one; titanium dioxide and rare earths are also used, and alkanes for grazing. Internal indicators occur naturally in the feed: lignin, acid-insoluble ash and indigestible NDF.
Because the indicator is concentrated as the digestible part disappears: DM digestibility (%) = 100 - 100 x (indicator % in feed / indicator % in faeces). For a nutrient: DC = 100 - 100 x (indicator in feed / indicator in faeces) x (nutrient in faeces / nutrient in feed).
Some feeds, such as concentrates or protein supplements, cannot be fed alone to a ruminant. Their digestibility is found by difference: a basal roughage is tested alone, then basal plus test feed, assuming the basal diet's digestibility does not change. That assumption is often untrue because of associative effects, the interaction by which one feed changes the digestion of another (for example, starch depressing fibre digestion in the rumen).
Animal trials are slow and costly, so laboratory methods predict digestibility. The two-stage in vitro method of Tilley and Terry (1963) incubates a sample with rumen liquor under anaerobic conditions for 48 hours, then with acid pepsin for a further 48 hours; the loss of DM predicts in vivo digestibility closely. The in sacco or nylon bag technique, developed as a standard by Orskov and McDonald (1979), suspends feed in porous bags in the rumen of a fistulated animal and removes them at intervals to measure the rate and extent of degradation; it is the basis of the effective degradability of protein. The gas production technique of Menke measures fermentation from gas released.
Digestibility is affected by the chemical composition of the feed (lignin and silica reduce it; lignification rises as the plant matures), the level of feeding (digestibility falls as intake rises above maintenance because feed passes faster), processing (grinding increases rate of passage and can reduce fibre digestion in ruminants; heat increases starch availability), the animal species (ruminants digest fibre far better than pigs or poultry), the age and health of the animal, anti-nutritional factors, and the associative effects already described.
The international unit is the joule; 1 calorie = 4.184 joules. Feed energy is expressed in megajoules (MJ) in Europe, Australia and increasingly worldwide, and in megacalories (Mcal) or kilocalories in North America and India. 1 Mcal = 4.184 MJ.
Gross energy (GE) is the heat released when a feed is completely burnt in oxygen, measured in an adiabatic or isoperibol bomb calorimeter. Average values commonly given are about 4.15 kcal/g for carbohydrate, 5.65 kcal/g for protein and 9.4 kcal/g for fat. Fat therefore has about 2.25 times the energy of carbohydrate, the origin of the factor in the TDN equation. Most feeds have similar GE per kg of DM (roughly 18-19 MJ), so GE is nearly useless for ranking feeds: straw and maize grain have similar GE but very different value to the animal.
The energy of ingested feed is divided step by step. Digestible energy (DE) = GE - faecal energy. Faecal energy is the largest loss for most feeds and the most variable between feeds. Metabolisable energy (ME) = DE - urinary energy - energy of combustible gases, which is mainly methane from rumen fermentation. In ruminants, methane is a significant loss; values of several percent of GE intake are typical, varying with diet and intake level. In poultry, urine and faeces are voided together, so ME is measured directly and usually corrected to zero nitrogen balance (AMEn).
Net energy (NE) = ME - heat increment. Heat increment (HI) is the extra heat produced after a meal: heat of fermentation in the gut, energy spent on digestion, absorption, and the metabolic work of converting nutrients into body tissue, milk or urea. HI is high for fibrous diets in ruminants and for protein, which is why the heat increment of feeding matters in hot climates. NE is the energy actually used, either for maintenance (NEm: basal metabolism, voluntary activity, thermoregulation) or for production (NEp: growth, fattening, milk, eggs, wool, work, pregnancy).
In ruminant nutrition, ME is estimated from DE as approximately ME = 0.82 x DE, a relationship used by the NRC; it varies with diet. The ratio ME/GE is called metabolisability (q) and is used in the UK ME system. The efficiency with which ME is used differs by function: it is highest for maintenance, lower for lactation, and lowest for growth and fattening, and each falls as q falls.
Total digestible nutrients (TDN) is the oldest practical energy measure still in use, promoted by Henry and Morrison in the United States: TDN = DCP + DCF + DNFE + (DEE x 2.25), where D stands for digestible. The ether extract is multiplied by 2.25 because fat supplies about 2.25 times as much energy per gram as carbohydrate. TDN is expressed in kg or as a percentage of the feed. It is an approximate DE value; NRC convention gives 1 kg TDN = 4.4 Mcal DE. It overvalues roughages, because it ignores methane and the higher heat increment of fibrous feeds, and it relies on the flawed CF and NFE fractions.
Kellner's starch equivalent (SE), developed in Germany, measured the fat-storing ability of a feed in adult steers relative to pure starch; it was the basis of British feeding for decades.
The nutritive ratio (NR) expresses the balance between protein and energy: NR = (TDN - DCP) / DCP, written as 1:x. A narrow ratio (for example 1:4) means a protein-rich feed suited to young growing animals and high producers; a wide ratio (for example 1:10) means a feed rich in energy relative to protein, suited to maintenance or fattening.
Modern feeding systems use ME or NE because these reflect the losses that TDN ignores. The UK Agricultural Research Council (ARC) introduced an ME system for ruminants and later versions were produced by AFRC and in the Feed into Milk project. The United States NRC (now National Academies of Sciences, Engineering, and Medicine, NASEM) uses NE for maintenance and gain (NEm, NEg) for beef cattle and NE for lactation (NEl) for dairy cattle. France (INRA) expresses energy as feed units for lactation (UFL) and meat production (UFV). Australia (CSIRO) and the Netherlands have their own systems. For pigs, DE, ME and NE systems are all in use, NE increasingly in Europe; for poultry, AMEn is standard. India's ICAR feeding standards give requirements in terms of DM, TDN or ME, CP and minerals. Table values depend on body weight, production and edition, so none are given here.
Proteins are chains of amino acids linked by peptide bonds. About twenty amino acids occur in proteins. An essential (indispensable) amino acid is one the animal cannot synthesise at all, or not fast enough, and must obtain from the diet. The ten classically listed for the growing rat and pig are arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Birds also need glycine (or serine) for growth, and their arginine requirement is high because they lack a working urea cycle. Cats need dietary arginine (a single arginine-free meal can cause hyperammonaemia) and taurine (deficiency causes central retinal degeneration and dilated cardiomyopathy).
The limiting amino acid is the essential amino acid in shortest supply relative to requirement; it limits protein synthesis however much of the others is fed. Lysine is usually first limiting in cereal-based pig diets; methionine is usually first limiting in maize-soybean meal poultry diets. Synthetic lysine, methionine, threonine and tryptophan allow nutritionists to meet requirements with less total protein and less nitrogen excreted. The ideal protein concept expresses each essential amino acid as a ratio to lysine, which is set at 100, and is used in pig and poultry formulation.
Crude protein (N x 6.25) measures quantity, not quality. Digestible crude protein (DCP) is the part of the CP that is apparently digested: DCP = CP x digestibility coefficient of CP / 100. DCP was the protein unit of most older feeding standards and still appears in Indian feeding tables and examinations alongside TDN. Its weakness in ruminants is that it says nothing about what happens to the protein in the rumen: two feeds of the same DCP may supply very different amounts of amino acids to the small intestine. For monogastric animals, amino acid content and ileal digestibility have replaced DCP in practice.
Biological value (BV), defined by Thomas and later Mitchell, is the proportion of absorbed nitrogen retained in the body: BV = N retained / N absorbed x 100, where N absorbed = N intake - faecal N and N retained = N intake - faecal N - urinary N. True BV corrects faecal N for metabolic faecal N and urinary N for endogenous urinary N. Egg and milk proteins have high BV; cereal proteins are low because they are short of lysine.
Net protein utilisation (NPU) is the proportion of intake nitrogen retained: NPU = N retained / N intake x 100 = BV x digestibility / 100. It combines digestibility and BV in one figure.
Protein efficiency ratio (PER), from Osborne and Mendel, is the weight gain of a growing animal (usually the young rat) per gram of protein eaten. It ignores maintenance protein. Chemical score compares the content of the most limiting amino acid in a test protein with that in a reference protein such as whole egg. Complementary proteins, such as cereals with pulses or oilseed meals, correct each other's deficiencies, which is the basis of mixing concentrate ingredients.
For ruminants, BV of the dietary protein has limited meaning because rumen microbes rebuild most dietary nitrogen into microbial protein, which has a fairly constant and good amino acid profile.
In a ruminant, dietary protein is attacked first by rumen microbes. The part broken down in the rumen is rumen-degradable protein (RDP); microbes hydrolyse it to peptides, amino acids and finally ammonia. Rumen bacteria use ammonia, together with energy from fermented carbohydrate and carbon skeletons, to build microbial protein. The part that escapes degradation and passes to the abomasum and small intestine is rumen-undegradable protein (UDP), also called rumen-undegraded protein (RUP), escape protein or bypass protein.
Microbial protein synthesis depends mainly on the supply of fermentable energy: if RDP exceeds what the microbes can capture with the energy available, surplus ammonia is absorbed through the rumen wall, converted to urea in the liver, and partly lost in urine, costing energy and money. If RDP is short, microbial growth and fibre digestion fall. The synchrony of energy and nitrogen release is therefore a practical goal.
Urea is recycled to the rumen through saliva and across the rumen wall, which lets ruminants survive on low-protein forages better than monogastric animals can.
Degradability is measured by the nylon bag method. The disappearance of protein at intervals is fitted to an exponential model (Orskov and McDonald), giving a rapidly soluble fraction a, a slowly degradable fraction b, and its rate constant c. Effective degradability (ED) combines these with the rate of passage k from the rumen: ED = a + b c / (c + k). The faster the rate of passage (high intake), the lower the effective degradability.
Metabolisable protein (MP) is the protein truly digested and absorbed as amino acids in the small intestine: it comes from digestible microbial true protein plus digestible UDP (and a small endogenous contribution in some systems). The UK metabolisable protein system (AFRC 1992), the NRC/NASEM systems for cattle, the French PDI (protein digestible in the intestine) system and the Cornell Net Carbohydrate and Protein System all work on this principle, though their equations differ. They replace DCP for high-producing ruminants. High producers need UDP of good amino acid profile in addition to microbial protein.
Protein can be protected from rumen degradation by heat treatment (roasting, extrusion, toasting), by treatment with formaldehyde or tannins that form reversible bonds with protein at rumen pH but release it at abomasal pH, by encapsulation, and by choosing naturally less degradable feeds. Excessive heating is harmful: the Maillard reaction between lysine and reducing sugars makes the protein indigestible as well as undegradable, detectable as raised ADIN.
Rumen-protected amino acids, especially methionine and lysine, are coated or chemically modified to pass the rumen and are used to balance MP in dairy diets.
Because rumen microbes build protein from ammonia, non-protein nitrogen (NPN) sources can replace part of the dietary protein in ruminants with a functional rumen. Urea is the commonest; biuret and ammonium salts are others. Feed-grade urea contains about 46% N, so its CP equivalent is about 46 x 6.25 = 287.5%. Urea has no energy value and no minerals, so it must be fed with readily fermentable carbohydrate (molasses, grain) and with sulphur, which microbes need to make methionine and cysteine; an N:S ratio of roughly 10-15:1 is commonly advised.
Rules of thumb for safe inclusion vary between textbooks and countries; commonly quoted limits are about 1% of total ration DM, about 3% of the concentrate mixture, or not more than about a third of dietary nitrogen. Animals must be adapted gradually. Urea must not be fed to monogastric animals, horses (limited use) or calves before the rumen functions.
Urea toxicity follows sudden intake of a large amount: rumen ammonia rises, rumen pH rises, ammonia crosses the rumen wall and overwhelms the liver. Signs develop within about half an hour to a few hours: restlessness, muscle tremor, frothy salivation, incoordination, bloat, tetany and death. Emergency treatment is oral dilute acetic acid (vinegar) with large volumes of cold water; rumenotomy can empty the rumen.
Water is the nutrient an animal can least do without: animals survive losing most of their body fat and about half their protein, but die after losing roughly a tenth to a fifth of body water. Water is the solvent in which nutrients are transported and reactions occur, it takes part in hydrolysis, it regulates body temperature through its high specific heat, high latent heat of evaporation and high conductivity, it lubricates joints and cushions the nervous system, and it is the major component of milk.
Body water is highest in the newborn (commonly around three-quarters or more of body weight) and falls as the animal ages and fattens, because adipose tissue holds little water while lean tissue holds a lot.
Water comes from three sources: drinking water, water in feed (a large source when green fodder or silage is fed), and metabolic water produced when nutrients are oxidised. Metabolic water yields per gram oxidised are about 1.07 g for fat, about 0.56 g for starch and about 0.40 g for protein. Metabolic water matters most in desert animals such as camels and some rodents, but even in them it cannot replace drinking water except in special cases.
Water is lost in urine, faeces, milk, eggs, sweat, and by evaporation from lungs and skin (insensible loss).
Water requirement is not a fixed figure. It rises with dry matter intake, with milk yield, with environmental temperature and humidity, with salt and protein intake, and falls when succulent feed is eaten. Feeding standards express it relative to DM intake or milk yield, and the multipliers vary by species, climate and source, so free access to clean water is the practical rule. Restricted water quickly depresses feed intake and milk yield.
Water quality is judged by total dissolved solids (salinity), nitrate, sulphate, fluoride, heavy metals, pH, hardness, algal blooms (some cyanobacteria are toxic) and microbial contamination. High sulphate in water is a recognised cause of polioencephalomalacia in cattle; high fluoride in groundwater causes fluorosis in parts of India, Africa and elsewhere; nitrate from fertilised land adds to feed nitrate. Water deprivation followed by free access in pigs and poultry fed salty diets causes salt poisoning (sodium ion toxicosis), with nervous signs in pigs.
About twenty mineral elements are known to be essential. Macrominerals are needed in amounts expressed as grams per day or percent of diet: calcium, phosphorus, magnesium, sodium, potassium, chlorine and sulphur. Microminerals or trace elements are needed in milligrams or micrograms and expressed as mg/kg (ppm): iron, copper, zinc, manganese, cobalt, iodine, selenium, molybdenum, and in some lists chromium and fluorine. Minerals build the skeleton and teeth, maintain acid-base and osmotic balance and nerve and muscle excitability, and act as parts of enzymes, hormones and vitamins (iron in haemoglobin, iodine in thyroxine, cobalt in vitamin B12, selenium in glutathione peroxidase, zinc in carbonic anhydrase and many other enzymes).
Mineral nutrition is full of interactions. Molybdenum and sulphur form thiomolybdates in the rumen that bind copper, so copper deficiency occurs on normal copper intakes when molybdenum is high; high iron and zinc also reduce copper absorption. Phytate binds phosphorus, zinc and other minerals. Deficiency is often regional because it follows soil and water, so area-specific mineral mixtures are recommended in India and elsewhere.
About 99% of body calcium and about 80% of body phosphorus are in bones and teeth. Vitamin D, parathyroid hormone and calcitonin regulate calcium. A dietary Ca:P ratio of about 1:1 to 2:1 is generally recommended for most species; laying hens need far more calcium for eggshell. Deficiency of calcium, phosphorus or vitamin D in young animals causes rickets (enlarged joints, beaded ribs, bowed legs); in adults it causes osteomalacia. Phosphorus deficiency is common in grazing cattle on phosphorus-poor soils, producing pica (eating bones, wood, soil), poor fertility and, in the old term, aphosphorosis; bone chewing can lead to botulism. Post-parturient haemoglobinuria of buffaloes and cattle is associated with hypophosphataemia. Horses fed high-phosphorus bran diets develop nutritional secondary hyperparathyroidism (big head).
Milk fever (parturient paresis) is acute hypocalcaemia around calving in high-yielding cows, when calcium drain into colostrum exceeds mobilisation; it is prevented by low-calcium or negative dietary cation-anion difference (DCAD) diets before calving. Grass tetany (hypomagnesaemic tetany) occurs in cattle and sheep grazing lush, potassium-rich spring pasture, because magnesium has little body reserve and potassium reduces its absorption; calves fed only milk develop a similar tetany.
Sodium and chlorine are supplied as common salt, which should always be available; herbivores on plant diets are commonly short of sodium, and deficiency shows as a craving for salt, licking of soil and sweat, and reduced intake and production. Excess salt with restricted water causes salt poisoning. Potassium is usually abundant in forages, so deficiency is rare in herbivores, but excess potassium impairs magnesium absorption. Sulphur is needed by rumen microbes to make sulphur amino acids, especially when urea is fed; excess sulphur causes polioencephalomalacia and aggravates copper deficiency.
Iron: piglets are born with small iron reserves and sow's milk is poor in iron, so housed piglets develop iron-deficiency anaemia (pale, thumps, poor growth) unless given iron, usually an injection of iron dextran in the first days of life. Copper: deficiency causes enzootic ataxia (swayback) in lambs from ewes deficient during pregnancy, loss of hair and wool pigment (achromotrichia, 'spectacle eye' in cattle), steely wool, anaemia, diarrhoea and bone fragility; sheep are highly susceptible to chronic copper poisoning, with a haemolytic crisis, so mixtures for cattle or pigs must not be fed to sheep. Zinc: deficiency causes parakeratosis in pigs (worse with high calcium and phytate) and poor growth, skin and feather lesions. Manganese: deficiency causes perosis (slipped tendon) in chicks, which also follows choline or biotin deficiency, and poor fertility. Cobalt: ruminants need it for rumen microbes to synthesise vitamin B12; deficiency causes wasting (enzootic marasmus, pine) with anaemia. Iodine: deficiency causes goitre, stillbirth and hairless newborn piglets and lambs; goitrogens in brassicas and Leucaena make it worse. Selenium: deficiency, together with vitamin E deficiency, causes nutritional muscular dystrophy (white muscle disease) in calves and lambs, exudative diathesis in chicks, mulberry heart disease and hepatosis dietetica in pigs, and retained placenta; excess from seleniferous soils and accumulator plants causes chronic selenosis (alkali disease: hoof deformity and hair loss) and acute blind staggers. Fluorine is best known for its toxicity, fluorosis, with mottled teeth and bony exostoses in areas with fluoride-rich water or rock phosphate. Molybdenum excess (teart pastures in England) causes scouring by inducing copper deficiency.
Chelated or organic trace minerals (bound to amino acids or proteins) are sometimes more available than inorganic salts; their value relative to cost is debated and depends on species and diet.
Vitamins are organic compounds needed in small amounts that the animal cannot make in sufficient quantity. Fat-soluble vitamins (A, D, E, K) are absorbed with fat and stored in the body, so excesses can be toxic; water-soluble vitamins (B complex and C) are stored little, except B12.
Vitamin A (retinol) is found preformed only in animal products; plants supply carotenoids, chiefly beta-carotene, which most species convert to vitamin A in the intestinal wall. Cats cannot make this conversion and need preformed vitamin A. Cattle absorb much carotene unchanged, which gives their fat and milk a yellow colour; buffaloes, sheep and goats convert it efficiently and have white fat and milk. Green fodder is rich in carotene; hay loses it on storage and sun-drying, and straw has almost none. Deficiency causes night blindness (loss of rhodopsin), xerophthalmia and keratinisation of epithelia, reproductive failure, abortion and weak or blind calves, and in chicks nutritional roup.
Vitamin D is formed in skin exposed to ultraviolet light (D3, cholecalciferol) or supplied as irradiated plant sterols (D2, ergocalciferol) in sun-cured hay. It is converted in liver and kidney to 1,25-dihydroxycholecalciferol, which increases calcium and phosphorus absorption. Poultry use D2 poorly, so D3 is used for them. Deficiency causes rickets and osteomalacia; excess causes calcification of soft tissues, as also happens with calcinogenic plants such as Cestrum and Solanum species.
Vitamin E (tocopherols) is the main fat-soluble antioxidant and works with selenium. Deficiency causes white muscle disease, encephalomalacia (crazy chick disease) and exudative diathesis in chicks, and reduced immunity. Requirement rises with polyunsaturated fat in the diet. Vitamin K is needed for clotting factors; rumen and hindgut microbes make it, so deficiency is uncommon except in poultry with sulphonamide or coccidiostat use, and in dicoumarol poisoning from mouldy sweet clover, which acts as a vitamin K antagonist.
Thiamin (B1): deficiency causes polyneuritis in birds (star-gazing, head retraction) and, in ruminants, polioencephalomalacia (cerebrocortical necrosis), linked to thiaminase in bracken fern and in some rumen conditions, to high sulphur, and to the thiamin analogue amprolium. Horses eating bracken develop thiamin deficiency. Chastek paralysis occurs in foxes and mink fed raw fish containing thiaminase.
Riboflavin (B2): curled-toe paralysis in chicks. Niacin: black tongue in dogs and pellagra in humans; tryptophan can be converted to niacin in most species but poorly in cats. Pantothenic acid: goose-stepping gait in pigs, dermatitis in chicks. Pyridoxine (B6): convulsions and anaemia. Biotin: foot-pad dermatitis and perosis in poultry, fatty liver and kidney syndrome in broilers, and hoof lesions in pigs; raw egg white contains avidin, which binds biotin. Folic acid: anaemia. Choline: perosis and fatty liver in poultry; it is needed in grams rather than milligrams and is sometimes not counted as a true vitamin. Vitamin B12 (cobalamin) contains cobalt; ruminants depend on rumen synthesis, so cobalt deficiency produces B12 deficiency.
Vitamin C (ascorbic acid) is made from glucose by most animals, but guinea pigs, primates and some fish and birds lack the enzyme gulonolactone oxidase and develop scurvy without dietary vitamin C.
Rumen microbes synthesise the B vitamins and vitamin K, so a healthy adult ruminant generally needs only vitamins A, D and E from the diet, although supplementary niacin, biotin and thiamin are used in some high-producing or sick animals. Pigs, poultry, fish, dogs and cats need a full vitamin supply, provided as a vitamin premix.
Feeds are commonly divided into roughages and concentrates. Roughages are bulky feeds with more than about 18% crude fibre on a DM basis and low TDN: dry roughages (hay, straw, stover) and succulent roughages (green fodders, pasture, silage, root crops). Green fodders may be leguminous (lucerne, berseem, cowpea: high in protein and calcium) or non-leguminous (maize, sorghum, oats, Napier and other grasses). Concentrates contain less than about 18% crude fibre and high TDN: energy-rich concentrates (cereal grains, milling by-products, molasses, roots and tubers) with CP below 20%, and protein-rich concentrates with 20% or more CP, of plant origin (oilseed cakes and meals) or animal origin (fish meal, meat and bone meal, blood meal, milk products). Mineral and vitamin supplements and feed additives form the remaining groups.
The international feed classification scheme of Harris and colleagues, adopted by the NRC, places each feed in one of eight classes: 1 dry forages and roughages; 2 pasture, range plants and forages fed green; 3 silages; 4 energy feeds; 5 protein supplements; 6 mineral supplements; 7 vitamin supplements; and 8 additives. Each feed has a six-digit International Feed Number, the first digit being its class. Tree leaves (top feeds) such as Prosopis cineraria (khejri), Ziziphus, Leucaena and Moringa are key dry-season feeds in arid regions, including Rajasthan and the Sahel.
A feed additive is a substance added in small amounts, not mainly to supply nutrients, but to improve the feed, the animal's performance or health, or the product. The main classes are: buffers and alkalisers (sodium bicarbonate, magnesium oxide) that stabilise rumen pH on high-concentrate diets; ionophores (such as monensin and lasalocid) that shift rumen fermentation toward propionate, reduce methane and control coccidia, but are highly toxic to horses; enzymes such as phytase (releasing phytate phosphorus in pigs and poultry and reducing phosphorus pollution) and xylanase and beta-glucanase (lowering the viscosity of wheat, rye and barley diets in poultry); probiotics or direct-fed microbials (Lactobacillus, Bacillus and Saccharomyces species) and prebiotics (oligosaccharides that favour beneficial bacteria); acidifiers (organic acids) in piglet diets; antioxidants that prevent rancidity of fats; mould inhibitors such as propionic acid; binders of mycotoxins (clays and yeast cell-wall products); pellet binders; flavours and sweeteners; pigmenting agents (xanthophylls) for egg yolk and broiler skin; and coccidiostats.
Low doses of antibiotics in feed improve growth and feed conversion, probably by suppressing subclinical infection and gut bacteria that compete for nutrients. Because this selects for antimicrobial resistance, the European Union banned antibiotic growth promoters from 1 January 2006, and many other countries have restricted them. In the United States, medically important antimicrobials in feed require veterinary oversight under the Veterinary Feed Directive and may no longer be used for growth promotion. India and other countries have restricted particular drugs (for example colistin as a growth promoter) and the WHO, WOAH and FAO promote a One Health approach to reduce use. Rules differ and change; check current national law. Feed additives are regulated in the EU under Regulation (EC) No 1831/2003, with authorisation, withdrawal periods and maximum levels.
Anti-nutritional factors (ANFs) are compounds in feeds that reduce nutrient use or intake, or harm health, without being classical poisons. Trypsin inhibitors in raw soybean and other legumes reduce protein digestion and cause pancreatic hypertrophy in monogastric animals; they are destroyed by proper heating, checked by the urease activity test. Lectins (haemagglutinins) in raw beans damage the gut wall; ricin in castor bean is a very potent lectin, so castor cake must be detoxified. Gossypol, a yellow polyphenol in cottonseed, is toxic to pigs, poultry and young calves (cardiac failure, poor growth) and causes olive-green discoloured yolks in eggs; mature ruminants tolerate more because rumen proteins bind free gossypol, and adding iron salts binds it in monogastric diets. Glucosinolates in rapeseed and mustard cake yield goitrogens; canola types are low in them. Tannins bind proteins and reduce palatability and digestibility; low levels can protect protein in the rumen and prevent bloat, high levels (sal seed meal, some tree leaves) depress performance; polyethylene glycol binds tannins.
Mimosine in Leucaena leucocephala (subabul) causes hair loss and goitre; rumen bacteria that degrade its goitrogenic metabolite DHP (Synergistes jonesii) protect adapted ruminants. Cyanogenic glycosides (dhurrin in young sorghum, linamarin in cassava) release hydrocyanic acid; young, drought-stressed or frost-damaged sorghum is dangerous, and cattle die rapidly with bright red blood. Nitrates accumulate in heavily fertilised or stressed forages and form methaemoglobin (chocolate-brown blood). Oxalates in some tropical grasses (Setaria, Napier, Pennisetum) bind calcium, causing hypocalcaemia and, in horses, osteodystrophy (big head). Saponins in lucerne contribute to frothy bloat. Phytic acid binds phosphorus and zinc. Lathyrus sativus (khesari) contains beta-ODAP (BOAA), which causes neurolathyrism. Oestrogenic isoflavones in subterranean clover cause clover disease (infertility) in ewes.
Mycotoxins are toxic fungal metabolites that contaminate feed in the field or in storage. Aflatoxins from Aspergillus flavus and A. parasiticus, favoured by warm humid storage of groundnut cake, maize and cottonseed, are hepatotoxic and carcinogenic; aflatoxin B1 is converted to aflatoxin M1 and excreted in milk, so dairy feeds have strict limits, set differently by each country. Ducklings and turkey poults are especially sensitive (turkey X disease in England in 1960 was traced to aflatoxin in groundnut meal). Ochratoxin damages kidneys; zearalenone is oestrogenic and causes vulvovaginitis and prolapse in pigs; deoxynivalenol (vomitoxin) causes feed refusal and vomiting in pigs; T-2 toxin causes oral lesions in poultry; ergot alkaloids cause gangrene and reduced milk; fumonisins cause equine leucoencephalomalacia and porcine pulmonary oedema. Control relies on good harvesting and drying, storage at low moisture, mould inhibitors, testing and binders.
Unconventional feeds are those not traditionally used for livestock, whose use grows when conventional feeds are scarce or costly. They include agro-industrial by-products such as brewers' and distillers' grains (distillers dried grains with solubles), sugarcane bagasse and tops, molasses, citrus and fruit pulps, tomato and apple pomace, rice and wheat straw, cottonseed hulls and cassava (tapioca) waste; forest by-products such as sal seed meal, mango seed kernel, mahua cake, rubber seed cake, neem and karanj cakes and Prosopis juliflora pods; poultry litter and feather meal; and insect meal (black soldier fly larvae), single-cell protein, algae and Azolla. Many need processing to remove an ANF (neem cake contains bitter limonoids such as azadirachtin; karanj cake contains karanjin; mango seed kernel and sal seed meal contain tannins; mahua cake contains saponins), and inclusion should start low and be tested on the target species. Guar meal, a by-product of guar gum production important in Rajasthan, is protein-rich but contains residual gum, trypsin inhibitor and saponins, so its level in poultry diets is limited.
Use of animal by-products is regulated: after bovine spongiform encephalopathy, the feeding of mammalian meat and bone meal to ruminants was banned in the EU, UK, USA and many other countries. Poultry litter can carry pathogens, drug residues and copper and must be heat-treated or ensiled where permitted.
Cereal straws are the main roughage for ruminants in much of Asia and Africa, but they are low in nitrogen, highly lignified and poorly digested. Physical treatments (chopping, grinding, soaking), chemical treatments and biological treatments can improve them. The alkali treatment of Beckmann used sodium hydroxide to break lignin-hemicellulose bonds; it works but the chemical is hazardous and adds sodium. Ammoniation with anhydrous ammonia or, more practically, with urea that is hydrolysed to ammonia by the straw's own urease, both improves digestibility and adds nitrogen. In the common farm method, urea dissolved in water is sprinkled on straw (a few percent of straw weight, often about 4%), the straw is packed airtight and left for a period that depends on temperature, commonly about three weeks, and aired before feeding. Supplementing straw with a urea-molasses mineral block or a small amount of protein supplement is often more practical than treating it.
Processing aims to increase digestibility or palatability, reduce wastage and selection, make feeds easier to handle, store and mix, destroy anti-nutritional factors and pathogens, and allow compound feeds to be made at scale. The gain must outweigh the cost, and the right process depends on the species.
Chaffing (chopping) of green and dry fodder with a chaff cutter reduces wastage and selective feeding, especially with thick-stemmed stovers such as maize and sorghum, and makes mixing with concentrates possible. Grinding in a hammer mill or roller mill breaks the seed coat of grain so enzymes and microbes can reach the starch. Pigs benefit from fine grinding, but very fine particles increase the risk of gastric (oesophagogastric) ulcers; poultry prefer a coarser crumb or pellet, and whole wheat is fed to broilers in some systems to stimulate the gizzard. Excess grinding of roughage for ruminants reduces particle size so much that rumination and saliva buffering fall and passage rate rises, reducing fibre digestion and milk fat. Soaking softens hard grains and cakes; crushing and rolling (dry-rolling) cracks grain for cattle.
Pelleting forces conditioned meal through a die. Conditioning with steam raises temperature and moisture, partly gelatinises starch and helps binding. Pellets increase bulk density, reduce dust and wastage, prevent birds and pigs from selecting ingredients, and usually improve feed intake and feed conversion in poultry and pigs; the heat also reduces Salmonella. Pellet quality (durability) depends on formulation, particle size, conditioning and die dimensions. Heat can damage vitamins and enzymes, which may be sprayed on after pelleting.
Extrusion uses higher heat, pressure and shear to cook starch fully; it is used for pet foods, fish feeds (floating or sinking) and full-fat soybean, where it also destroys trypsin inhibitors. Steam-flaking of maize or sorghum for feedlot cattle increases starch digestibility in the rumen. Roasting and micronising (infrared heating) are other heat treatments. Overheating causes Maillard damage to lysine.
A complete feed mixes roughage and concentrate into one feed, as mash, pellets or blocks, so each mouthful is balanced. A total mixed ration (TMR) is the same idea on the dairy farm: forages, concentrates, minerals and additives are weighed and mixed in a mixer wagon and fed together, which stabilises rumen fermentation and lets the cow eat a balanced diet at every meal.
Complete feed blocks compress chopped roughage with concentrate, molasses and minerals under a hydraulic press. They reduce transport and storage costs and are useful for drought and disaster relief. Urea-molasses mineral blocks (UMMB) are licks made of molasses, urea, minerals, a cereal bran and a binder (such as cement or calcium oxide, depending on formula) that let ruminants on straw take small amounts of NPN, energy and minerals throughout the day, which is safer than feeding urea loose.
A feeding standard is a table or model of the nutrient requirements of an animal of a given species, weight and production level, usually separated into maintenance and production. Early standards include Grouven's (1859), Wolff's German standard (1864), which used digestible nutrients, Kellner's starch equivalent standard, Armsby's net energy work, and Morrison's standard in the United States, based on TDN and DCP. Current standards include the NRC and NASEM series for dairy cattle, beef cattle, swine, poultry, horses, small ruminants, dogs and cats, and fish; the UK ARC and AFRC publications; the French INRA feeding system; the Australian CSIRO standards; and India's ICAR 'Nutrient Requirements of Animals' series, which covers cattle and buffalo, sheep, goat and rabbit, pig, poultry and other species under Indian conditions. ICAR standards express requirements for DM, CP or DCP, TDN or ME, and minerals.
Different standards give different numbers for similar animals because they use different energy and protein units, different experimental data and different safety margins. Standards are guides; the ration must be checked against the animal's response.
A ration is the feed allowed to an animal in 24 hours. A balanced ration supplies all nutrients in the right amounts and proportions for 24 hours. It should also be palatable, of adequate bulk, economical and free of toxins.
Formulation proceeds in steps. Define the animal (species, weight, production, stage). Look up requirements for DM, energy, protein, minerals and vitamins from a feeding standard. Estimate DM intake, commonly expressed as a percentage of body weight and varying with species, production and forage quality. List the available feeds with their composition and price. Decide the roughage share and the green:dry roughage mix. Fill the remaining requirement with a concentrate mixture. Check minerals, add a mineral mixture and salt. Finally convert the DM quantities to as-fed quantities by dividing by the DM fraction of each feed.
Maintenance requirements scale not with body weight but approximately with metabolic body weight, W to the power 0.75, following the work of Kleiber and Brody. A 400 kg cow has a metabolic weight of about 89 kg; a 100 kg animal has about 32 kg. Dry matter intake depends on the animal (size, production, physiological state, rumen capacity), the feed (NDF, digestibility, palatability, moisture of fermented feeds), and the environment (heat reduces intake). In ruminants on forage, physical fill limits intake; on concentrate diets and in monogastric animals, energy demand and chemostatic signals dominate.
The Pearson square mixes two feeds to reach one target nutrient concentration. Write the target in the centre, the two feed values at the left corners; subtract diagonally, ignoring signs; the differences on the right are the parts of each feed. The target must lie between the two feed values. For example, to make 18% CP from maize (9%) and soybean meal (44%): 44 - 18 = 26 parts maize and 18 - 9 = 9 parts soybean meal, 35 parts in all, so soybean meal is 9/35 = 25.7% of the mixture. It balances only one nutrient at a time, though a fixed portion of other ingredients can be set aside first.
Simultaneous equations solve two or more nutrients at once. Trial and error, starting from a typical mix and adjusting, is common on farms. Least-cost formulation uses linear programming, which finds the cheapest combination of ingredients that meets all nutrient constraints (minimums and maximums) and ingredient limits; it is standard in the compound feed industry and needs accurate ingredient composition, because a low-cost solution built on wrong data is simply a cheap wrong feed. India's National Dairy Development Board runs a ration-balancing programme for smallholders on the same principles.
The compound feed industry manufactures complete feeds, concentrates and supplements from many ingredients. Global production is well over a billion tonnes a year according to industry surveys, dominated by poultry feed, followed by pig and ruminant feeds, and by aquafeed, the fastest-growing segment in many regions. In India, the organised compound feed sector serves mainly the poultry industry; much of the cattle feed is made by dairy cooperatives and private plants, while a large share of dairy animals still receives home-mixed concentrates, oilseed cakes and crop residues, so the scope for compound cattle feed is large.
A feed mill receives and tests ingredients, stores them in bins or silos, grinds, weighs (batching), mixes (horizontal ribbon or paddle mixers, with a tested mixing time), conditions and pellets, cools, crumbles or screens, and bags or bulk-loads the feed. Mixer uniformity is tested by the coefficient of variation of a marker.
Most countries regulate feeds. In India, the Bureau of Indian Standards publishes specifications for compound cattle feed (IS 2052) and poultry feed (IS 1374), among others; certain feeds have been brought under compulsory certification, and the Food Safety and Standards Authority and state authorities regulate residues in food. The European Union regulates feed hygiene, additives, undesirable substances (such as aflatoxin, dioxins and heavy metals) and labelling through a series of regulations. In the United States, the Food and Drug Administration regulates feed under the Food Safety Modernization Act, and the Association of American Feed Control Officials (AAFCO) publishes model ingredient definitions and pet food nutrient profiles that states adopt. The Codex Alimentarius Code of Practice on Good Animal Feeding sets international principles, and schemes based on HACCP (hazard analysis and critical control points) are used across the industry.
A veterinarian's role spans formulating or checking rations, investigating feed-related disease outbreaks, sampling feed properly (representative sampling, chain of custody), interpreting analysis reports, advising on withdrawal periods for medicated feed and on residue risks, and working with regulators on mycotoxin, dioxin or drug contamination incidents.
Quality control starts with raw materials: physical inspection (colour, smell, mould, insects, foreign matter), moisture, proximate analysis, specific tests such as urease activity for soybean meal, and aflatoxin testing, especially of groundnut cake and maize. Adulteration is a real problem: common examples are sand and ash in cakes and brans, urea added to raise apparent crude protein, and cheaper materials mixed into fish meal or oilseed meals. Because the Kjeldahl test counts urea nitrogen as crude protein, CP alone cannot detect urea adulteration; a urease test or direct urea determination is needed. Storage at low moisture, pest control and first-in first-out stock rotation prevent mould and rancidity.
Learn the Weende and Van Soest fractions until you can write the procedure and faults of each from memory, and practise the calculations until they are automatic: DM conversion, digestibility, DCP, TDN, ME from DE, BV, NPU and the Pearson square. Make a single sheet pairing each mineral and vitamin with its deficiency diseases and species, and another pairing each anti-nutritional factor with its feed and effect. These two sheets answer a large share of entrance and state commission questions. In practical classes, run one full proximate analysis yourself so that the numbers on a report mean something.
In practice this subject appears as herd problems. A dairy farmer with poor conception rates, retained placentas and milk fever needs a check of the dry-cow ration, mineral mixture and DCAD, not only treatment. A goat keeper feeding young sorghum, a pig unit with anaemic piglets, a sheep flock given cattle mineral mix, a feedlot using urea without adaptation: each is a nutrition diagnosis. Learn to take a feed history, collect representative feed and water samples, read a laboratory report on a DM basis, check a ration against the ICAR, NRC or local standard, and advise on safe use of urea, ionophores and medicated feeds with their withdrawal periods.
With experience the work moves to systems: designing area-specific mineral mixtures from soil, fodder and blood data; setting ingredient specifications and rejection limits for aflatoxin and adulteration at a feed plant or cooperative; running ration-balancing programmes for thousands of smallholders; deciding when a least-cost formula is too cheap to be safe; and leading investigations of feed contamination incidents with regulators. Senior nutritionists judge which feeding standard fits local breeds and climate, adjust for heat stress, and weigh the economics of bypass protein, protected fat or enzymes against the likely response, not the sales claim.
At research level the open questions include better prediction of intake and digestibility from composition, NIRS calibration for local feeds, protein and amino acid systems for buffaloes and indigenous cattle whose requirements differ from the temperate breeds behind NRC and ARC data, methane mitigation through diet and additives, nitrogen and phosphorus excretion, safe use of unconventional feeds and insect protein, mycotoxin risk under a changing climate, and alternatives to antibiotic growth promoters. A senior researcher designs digestibility and metabolism trials with proper preliminary periods, replication and statistics, publishes composition data openly, and contributes to revisions of national feeding standards.
This manual gives no feeding-standard table values, no nutrient requirement figures by body weight, and no mineral or vitamin requirement or toxicity thresholds, because these differ by standard, edition, species and production level; use the current ICAR, NRC/NASEM, INRA or national tables. Feed composition values for individual feeds are not tabulated. Rumen microbiology and fermentation, feeding of specific species, silage and hay making, pet food formulation and clinical nutrition are covered in the applied nutrition subjects. Regulatory details (mycotoxin limits, additive approvals, antibiotic rules) change often and are described only in outline; check the current national regulation. Detailed laboratory protocols (reagent concentrations, times) should be taken from AOAC or the laboratory's standard method.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions. Every numerical answer was computed by script; rules of thumb (such as urea limits) vary between textbooks and countries.
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