Veterinary · Pre-clinical and foundation · also MSVE ANN-211
Feeding pigs, poultry, rabbits, laboratory animals, fish and pets: amino-acid balancing, feed additives and their withdrawal, and mycotoxins in feed.
In the exams: ICAR AICE-JRF core group 05; ICAR AIEEA-PG 14 (Animal Nutrition)
This subject is about feeding animals that digest their food mainly with their own enzymes: the pig, the chicken and other poultry, the dog and cat, fish and the laboratory rodents, together with the hindgut fermenters, the rabbit and the horse, whose large caecum and colon change the picture. A non-ruminant cannot rely on rumen microbes to upgrade poor protein or to make B vitamins, so it must be given each indispensable amino acid in the right amount and proportion, and much of the subject is about doing exactly that at least cost.
Feed is the largest cost in pig, poultry and fish production, and the diet decides growth, feed conversion, nitrogen and phosphorus pollution and, increasingly, how much antimicrobial a farm uses. In companion animals the same science underlies obesity management and therapeutic diets.
Examinations treat this as a core animal nutrition subject. Indian postgraduate and PhD entrance tests (ICAR AIEEA-PG Animal Sciences and AICE-JRF/SRF core group 05) ask about feeding standards, ideal protein, energy systems, additives, mycotoxins and deficiency diseases, and like numerical problems such as the Pearson square. State veterinary officer examinations favour direct recall of deficiency diseases and feeding practice. Licensing examinations worldwide ask clinical cases: a pig with mulberry heart, a flock with curled-toe paralysis, a cat on a vegetarian diet, a horse that ate cattle feed. This manual notes where law or practice differs between countries.
A non-ruminant (monogastric) animal digests its food mainly with its own enzymes in the stomach and small intestine, before any large microbial population acts on it. The consequence for feeding is direct: the animal gets almost nothing from microbial protein synthesis or microbial B-vitamin synthesis in the upper gut, so the diet must supply every indispensable (essential) amino acid, the essential fatty acids and most vitamins in a form the animal's own enzymes can release. Fibre is fermented only in the large intestine, late in the tract, yielding far less energy than in a ruminant. A second group, the hindgut fermenters (the horse, the rabbit, the guinea pig), digests readily available nutrients in the small intestine like a pig but keeps a large fermentation chamber in the caecum and colon. Their hindgut microbial protein is largely lost in the faeces unless, as in the rabbit, the animal eats part of its faeces.
The pig has a simple stomach with a non-glandular pars oesophagea around the entrance of the oesophagus; this area has no protective mucus-secreting glands and is the site of gastric ulcers in growing pigs fed finely ground or pelleted diets. Starch is digested by salivary and pancreatic amylase and brush-border enzymes; protein by pepsin, trypsin, chymotrypsin and peptidases; fat by pancreatic lipase after emulsification by bile. The newborn piglet is adapted to milk: its lactase activity is high at birth, while pancreatic amylase, maltase, sucrase and protease activities are low and rise over the first weeks of life. This is why early-weaned piglets need highly digestible diets based on milk products, cooked cereals and high-quality protein, and why abrupt weaning onto a plant-based diet causes villous atrophy, poor intake and post-weaning diarrhoea. Protein fermented in the large intestine yields ammonia and amines, not usable amino acids.
The chicken has no teeth. Feed is stored and moistened in the crop, mixed with acid and pepsin in the glandular proventriculus, and ground in the muscular gizzard (ventriculus), which works best when the diet contains some coarse particles or whole grain. The small intestine is short and the transit time is a few hours, so digestion must be fast. Birds excrete nitrogen as uric acid, not urea, and urine and faeces are voided together through the cloaca as excreta. Urinary and faecal energy cannot be separated without surgery, so poultry feeds are rated on metabolisable, not digestible, energy. Birds have no functional urea cycle, so arginine is indispensable for them at all ages, and glycine (or serine) is needed in quantity by the young chick because it is consumed in uric acid synthesis.
Gross energy (GE) is the heat released when feed is completely burnt. Digestible energy (DE) is GE minus faecal energy. Metabolisable energy (ME) is DE minus energy in urine and combustible gases; in the pig, gas losses are small and ME is usually about 94 to 96 per cent of DE for typical diets, the ratio falling as dietary protein rises because surplus nitrogen is excreted as urea. Net energy (NE) is ME minus the heat increment of digestion and metabolism, and is the energy actually available for maintenance and production. Diets for pigs have traditionally been formulated on DE or ME, but NE systems, developed in France and the Netherlands and adopted in the NRC 2012 swine requirements, rank feeds more accurately: fat and starch have a high NE/ME efficiency, whereas protein and fermentable fibre have a lower one, so a high-protein or high-fibre diet is overvalued on a DE or ME basis. For poultry the working unit is apparent metabolisable energy corrected to zero nitrogen balance (AMEn). The correction removes the energy that retained nitrogen would have yielded had it been excreted as uric acid, conventionally 8.22 kcal (34.39 kJ) per gram of nitrogen retained. True metabolisable energy (TME) further corrects for endogenous excreta losses and is measured with a precision-feeding assay in adult cockerels. A bird eats 100 g of feed containing 4.0 kcal GE per gram, so energy intake is 400 kcal. Its excreta over the same period contain 100 kcal. Nitrogen intake is 3.5 g and excreta nitrogen 1.5 g, so 2.0 g of nitrogen is retained. AME is (400 minus 100) divided by 100 g, which is 3.00 kcal per gram. AMEn subtracts 8.22 kcal for each gram of nitrogen retained: (400 minus 100 minus 8.22 times 2.0) divided by 100, which is 2.84 kcal per gram, or about 2836 kcal per kg. AMEn is lower than AME in a growing bird because the bird is retaining nitrogen; in a bird losing nitrogen the correction would add energy back.
A feeding standard is a published table of nutrient requirements or recommended dietary concentrations for an animal of stated class, weight and performance. The most cited for pigs is the US National Research Council's Nutrient Requirements of Swine (11th revised edition, 2012), which gives requirements as standardized ileal digestible (SID) amino acids and as DE, ME and NE, generated by a growth model. Europe uses NE-based national tables (French INRA, Dutch CVB), India uses ICAR standards, and breeding companies publish specifications for their genotypes. Requirements differ between these sources because they assume different genotypes, intakes, environments and response criteria (maximum gain, best feed conversion or least cost per kg gain). The headline pattern is consistent across all standards: the requirement per kg of feed for energy density stays fairly steady while the requirement for amino acids per kg of feed falls steeply with age, because intake rises and the proportion of gain that is lean tissue declines.
Piglets are born with low iron reserves, sow's milk is poor in iron, and pigs reared indoors have no soil to root in. Iron-deficiency anaemia, with pale skin, poor growth, laboured breathing (thumps) and sudden death in fast-growing piglets, follows within weeks unless iron is given. The usual prevention is an intramuscular injection of an iron dextran or similar iron complex in the first days of life; a commonly cited dose is 100 to 200 mg iron per piglet — confirm against the product label and your national formulary. Creep feed is a highly palatable, highly digestible diet offered to sucking piglets in the farrowing pen from about the second or third week. It accustoms the gut to solid food so the shock of weaning is smaller, though intake before weaning is small and variable.
Weaning age varies with law and system: commercial weaning commonly falls between three and five weeks, and in the European Union piglets may not normally be weaned before 28 days (21 days with specialised housing) under Council Directive 2008/120/EC. Pre-starter and starter diets for weaners are dense in energy and SID lysine and are built from ingredients the young gut digests: lactose or whey, heat-processed cereals, fishmeal, spray-dried plasma and processed soybean products with low antigen content. Crude protein is kept moderate and the amino-acid requirement is met with free amino acids, because undigested protein reaching the hindgut feeds proteolytic bacteria and favours post-weaning diarrhoea. Feeding pharmacological concentrations of zinc oxide (in the order of 2,500 to 3,000 mg zinc per kg feed) to control post-weaning diarrhoea was widespread, but it raises environmental zinc and co-selects for antimicrobial resistance; the European Union ended its use at medicinal levels in June 2022, and other countries regulate it differently.
From roughly 25 kg to slaughter, pigs are usually fed ad libitum in two to four diet phases, each with a lower SID lysine to energy ratio than the last. In NRC 2012 the SID lysine concentration for a pig of high lean growth potential falls from around 1.4 to 1.5 per cent of the diet in the youngest weaned pig to around 0.6 to 0.7 per cent near the end of finishing; the exact figure depends on genotype, sex, intake and energy density. Phase feeding, and its limit case blend feeding, in which two diets are mixed in changing proportions each week, reduces nitrogen and phosphorus excretion because the pig is never fed far above its requirement. Entire males deposit more lean and need more amino acids per unit energy than castrates, which eat more and fatten sooner; gilts lie between. Water must be available at all times; water deprivation followed by sudden access causes salt poisoning (sodium ion toxicosis) with nervous signs.
Gestating sows are fed a restricted allowance, adjusted to body condition, because ad libitum feeding in pregnancy makes sows fat, which lowers feed intake in the following lactation and increases farrowing problems. Restricted sows are hungry, so high-fibre diets are used to increase satiety; EU law requires that dry sows and gilts receive enough bulky or high-fibre food as well as high-energy food. In lactation the sow's requirement for energy and lysine rises steeply with litter size and milk yield, and she is fed to appetite, with intake built up over the first days after farrowing. Poor lactation intake causes heavy weight loss, a longer weaning-to-oestrus interval and smaller subsequent litters. Gilts are grown more slowly than finishers, with more calcium and phosphorus, for skeletal soundness at first mating.
Several pig diseases are diagnosed by thinking about the diet. Iron-deficiency anaemia of piglets has been described. Vitamin E and selenium deficiency causes mulberry heart disease (sudden death with haemorrhagic myocardium) and hepatosis dietetica (hepatic necrosis), mainly in rapidly growing pigs; diets high in polyunsaturated fat or made from grain grown on selenium-poor soils increase the risk. Parakeratosis, a crusting dermatosis, is caused by zinc deficiency and is made worse by high dietary calcium and phytate, which reduce zinc absorption. Rickets in young pigs and osteomalacia in older ones follow deficiency or imbalance of calcium, phosphorus or vitamin D; lameness and fractures in sows at the end of lactation may reflect depleted skeletal reserves. Gastric ulceration of the pars oesophagea is linked with fine grinding and pelleting as well as stress and irregular feeding. Salt poisoning follows water deprivation. Mycotoxins (Chapter 8) complete the differential list.
The classic reference is the US National Research Council's Nutrient Requirements of Poultry (9th revised edition, 1994). It is now dated, and most commercial diets follow the specifications of the primary breeding companies for their strains, adjusted for local ingredients and prices. National standards exist as well; in India, for example, the Bureau of Indian Standards publishes a specification for poultry feeds (IS 1374), and ICAR has published requirements. All of these sources express energy as ME (usually AMEn) and increasingly express amino acids on a digestible basis. Because birds eat to meet their energy need within limits, nutrient concentrations are usually set as ratios to energy: if dietary energy is raised, intake falls and every other nutrient must be raised in proportion, otherwise the bird is under-supplied. Optimum amino-acid levels also differ with sex, temperature and the response paid for (gain, feed conversion or breast meat yield).
Modern broilers reach slaughter weight in about five to seven weeks and are typically fed a starter, a grower and one or two finisher diets, often with a pre-starter crumb in the first days. The starter is the most concentrated in amino acids; breeding company specifications place digestible lysine for the starter in the region of 1.2 to 1.3 per cent of the diet, with the concentration falling in each subsequent phase while ME rises. Starter diets are crumbled and later diets pelleted, because pelleting increases intake and growth. In maize-soybean meal diets, methionine is the first limiting amino acid for broilers, lysine second and threonine third; synthetic DL-methionine or its hydroxy analogue, L-lysine and L-threonine are routinely added. A withdrawal diet without in-feed medicines with a withdrawal period is fed for the last days before slaughter so that residues fall below maximum residue limits.
Replacement pullets are grown on starter, grower and developer diets designed to reach a target body weight and uniformity at the onset of lay, not maximum growth; body weight at first egg affects egg size and persistence. A pre-lay diet with extra calcium is fed for a short period before the first eggs to build medullary bone, a labile calcium store inside the long bones that the hen uses for shell formation at night. Layer diets must supply a large amount of calcium: a hen in production needs roughly 4 g of calcium per day, so diets usually contain about 3.5 to 4.5 per cent calcium depending on intake, with part supplied as coarse particulate limestone or shell grit so that it stays in the gizzard and releases calcium during the night when the shell is being calcified. Too much calcium in a pullet's diet before lay damages the kidneys. Layers are often phase-fed, with protein and phosphorus reduced and calcium increased as the flock ages and eggs grow larger and shells thinner. Breeder flocks of broiler parent stock must be feed-restricted from early life, because their genetic appetite would otherwise make them obese and poorly fertile.
Many classic deficiency syndromes were described in the chick and are still examined. Vitamin E deficiency produces three syndromes: nutritional encephalomalacia (crazy chick disease, with ataxia and cerebellar haemorrhage and softening), exudative diathesis (green-blue subcutaneous oedema from increased capillary permeability, also prevented by selenium) and nutritional muscular dystrophy (white striations in breast muscle, prevented by vitamin E, selenium and adequate sulphur amino acids). Riboflavin deficiency causes curled-toe paralysis. Thiamine deficiency causes polyneuritis with head retraction (star-gazing). Manganese deficiency causes perosis (slipped tendon), and perosis can also follow deficiency of choline, biotin, niacin or folic acid. Vitamin A deficiency causes white pustules in the oesophageal mucosa and urate deposits in the kidneys, and lowers resistance to infection. Vitamin D, calcium or phosphorus deficiency causes rickets in growing birds and cage layer fatigue (osteoporosis and paralysis) in laying hens. Vitamin K deficiency causes haemorrhages. Biotin deficiency causes dermatitis of the feet and around the beak and eyes. Excess energy in caged layers is associated with fatty liver haemorrhagic syndrome. Visceral gout, with chalky urate on the serosae, follows kidney damage, which can result from excess dietary calcium in growing birds, vitamin A deficiency, mycotoxins or infectious agents, and water deprivation.
Of the twenty amino acids in body protein, ten are usually listed as indispensable for the growing pig and chick: lysine, methionine, threonine, tryptophan, valine, isoleucine, leucine, histidine, phenylalanine and arginine (arginine is partly synthesised by the young pig but not enough for maximal growth; it is strictly indispensable in birds). Cysteine can be made from methionine and tyrosine from phenylalanine, so requirements are often stated as methionine plus cysteine and phenylalanine plus tyrosine. Protein synthesis stops when any one amino acid runs out, so growth is set by the amino acid in shortest supply relative to need, the first limiting amino acid. Cereals are poor in lysine; maize is also poor in tryptophan; soybean meal is rich in lysine but relatively poor in methionine. In maize-soybean meal diets lysine is first limiting for pigs, followed by threonine, tryptophan and methionine in an order that shifts with ingredients; for broilers methionine is first, then lysine, then threonine. Feeding crude protein above need to cover the first limiting amino acid wastes the others, which are deaminated, raise nitrogen excretion and cost energy to excrete.
Ideal protein is a dietary protein whose indispensable amino acids are present in exactly the proportions the animal needs, with no excess or deficit, so that every amino acid becomes limiting at the same moment. Each amino acid is expressed as a ratio to lysine, set at 100. Lysine is the reference because it is used almost entirely for protein deposition, is easy to analyse, has the best-studied requirement and is cheap in feed-grade form. Once the lysine requirement of a given animal has been set, the other amino acids follow from the ratios; that is why so much effort goes into the lysine requirement. Typical ratios for the growing pig, on an SID basis, place methionine plus cysteine at roughly 55 to 60, threonine at roughly 60 to 67, tryptophan at roughly 17 to 20 and valine at roughly 65 to 70; for broilers methionine plus cysteine is higher, roughly 72 to 76, reflecting the need for feather protein, which is rich in cysteine. The published ratios differ by source, age and response criterion, and threonine rises relative to lysine as the pig grows because maintenance (gut mucin and endogenous secretions, which are rich in threonine) becomes a larger share of the requirement.
Total amino acids in a feed overstate what the animal receives, and by different amounts for different ingredients: heat-damaged meals, meat and bone meal, feather meal and some oilseed cakes are less digestible than soybean meal or synthetic amino acids. Digestibility is measured at the end of the ileum, not in faeces, because microbes in the large intestine both destroy and synthesise amino acids and the amino acids that disappear there are not absorbed as such. Apparent ileal digestibility (AID) is intake minus ileal outflow, divided by intake; it underestimates true digestibility, especially for low-protein feeds, because ileal digesta contain endogenous protein from enzymes, mucin and sloughed cells. Standardized ileal digestibility (SID) corrects only for basal endogenous losses, those that occur whatever the diet, so SID values of individual feeds are additive in a mixed diet; this is why SID is the basis of NRC 2012 and most modern pig formulation. True ileal digestibility (TID) corrects for total endogenous losses, including the diet-specific part caused by fibre and anti-nutritional factors, which are hard to measure routinely. Poultry formulation increasingly uses standardized ileal digestible values too.
Crystalline amino acids allow the nutritionist to lower crude protein while keeping the ideal pattern, which cuts feed cost when protein meals are dear, reduces nitrogen excretion and ammonia in the house, and improves gut health in weaners. Feed-grade L-lysine hydrochloride is about 78 to 79 per cent lysine (lysine is about 80 per cent of the molecular weight of the hydrochloride, and the product is about 98.5 per cent pure); DL-methionine is about 99 per cent methionine; L-threonine, L-tryptophan and L-valine are also sold. Worked example: a finisher diet supplies 0.85 per cent SID lysine and the target is 0.95 per cent. The gap is 0.10 per cent, which is 1.0 kg lysine per tonne. Because L-lysine HCl is about 78.8 per cent lysine and synthetic lysine is taken as fully digestible, the addition is 1.0 divided by 0.788, which is 1.27 kg per tonne. A common slip is to add 1.0 kg of the product, which falls about 21 per cent short. The limit to cutting protein is reached when the next amino acid in line (threonine, tryptophan, valine, isoleucine) becomes limiting; each extra synthetic amino acid opens another step down in crude protein.
A formulated diet must meet the animal's nutrient specification, stay within limits on ingredients that are unpalatable, poorly digested, high in anti-nutritional factors or unsafe above a certain inclusion, be mixable and pelletable, and cost as little as possible. Every calculation must use one basis (as fed or dry matter, total or digestible, one energy system); mixing bases is the commonest error. Ingredient composition should come from analysis of the actual lots where possible. The Pearson square mixes two ingredients to reach one target concentration of one nutrient. The target must lie between the two ingredient values. Draw a square, write the target in the centre and the two ingredients at the left corners; subtract diagonally, ignoring signs; the numbers obtained at the right corners are the parts of the ingredient on the same horizontal line. Worked example: maize contains 9 per cent crude protein and soybean meal 44 per cent, and a mixture with 18 per cent crude protein is needed. Maize parts are 44 minus 18, which is 26; soybean meal parts are 18 minus 9, which is 9. Total parts are 35, so the mixture is 26 divided by 35, 74.3 per cent maize, and 9 divided by 35, 25.7 per cent soybean meal. Check: 0.743 times 9 plus 0.257 times 44 equals 18.0. The method works equally for energy or any single nutrient, and two ingredients can each be a pre-mixed group, for example a cereal mix and a protein mix.
Real diets contain fixed items, such as a 5 per cent mineral-vitamin premix contributing no protein. The fixed part is set aside and the target is adjusted so that the remaining part carries all the protein. Worked example: a 20 per cent crude protein diet is wanted, containing 5 per cent premix with no protein, using maize at 8.5 per cent and soybean meal at 46 per cent. The 95 kg of maize and soybean meal must supply 20 kg of protein, so their own target is 20 divided by 0.95, which is 21.05 per cent. Maize parts are 46 minus 21.05, which is 24.95; soybean meal parts are 21.05 minus 8.5, which is 12.55; total 37.50. Scaled to 95 kg, maize is 63.2 kg and soybean meal 31.8 kg. The trap is to solve the square for 20 per cent and then add the premix, which dilutes the diet to 19 per cent.
When both energy and protein must be met exactly, two ingredients are not enough in general; three are needed, and the problem is solved with three simultaneous equations: the weights add up to the batch, the energy adds up to the target and the protein adds up to the target. Worked example per 100 kg: maize (ME 3,350 kcal per kg, 8.5 per cent CP), soybean meal (ME 2,230 kcal per kg, 46 per cent CP) and vegetable oil (ME 8,800 kcal per kg, no CP), with a target of 3,000 kcal ME per kg and 21 per cent CP. Solving gives about 66.1 kg maize, 33.4 kg soybean meal and 0.45 kg oil. If a solution comes out negative, the target cannot be reached with those ingredients and one must be exchanged. Hand methods stop here; with more nutrients and more ingredients, linear programming is needed.
Least-cost formulation states the problem as a linear programme: minimise the sum of each ingredient's price times its inclusion, subject to constraints that each nutrient is at least (and sometimes at most) a stated level, each ingredient lies within allowed bounds, and the inclusions add up to one. The solver finds the cheapest corner of the feasible region. Worked example, solved by enumerating the corners in a script: ingredients maize (3,350 kcal ME, 8.5 per cent CP, price 22 per kg in arbitrary currency units), soybean meal (2,230 kcal, 46 per cent, 45), de-oiled rice bran (1,900 kcal, 15 per cent, 12, maximum 15 per cent of the diet) and vegetable oil (8,800 kcal, 0 per cent, 110); constraints at least 2,950 kcal ME per kg and at least 20 per cent CP. The cheapest feasible diet is about 65.6 per cent maize, 29.9 per cent soybean meal and 4.5 per cent rice bran, with no oil, costing about 28.42 per kg, with both energy and protein exactly at their minimum. Maize and soybean meal alone, mixed by Pearson square to 20 per cent CP (69.3 per cent maize), give 3007 kcal per kg, comfortably above the energy floor, but cost 29.05 per kg; the programme spends that surplus energy on cheap rice bran. The output also reports shadow prices: the cost of tightening a constraint by one unit, and the price at which an unused ingredient would enter the formula. A least-cost formula is only as good as its nutrient matrix and its inclusion limits.
About two thirds of the phosphorus in cereals and oilseed meals is held as phytate (myo-inositol hexaphosphate), which pigs and poultry digest poorly because they have little intrinsic phytase. Unabsorbed phytate phosphorus is excreted and pollutes water; phytate also binds calcium, zinc, other minerals and some protein, reducing their availability. Microbial phytase added to the feed releases phosphate from phytate in the stomach and crop, so less inorganic phosphate (such as dicalcium phosphate) is needed and phosphorus excretion falls. Phytase activity is expressed in phytase units (FTU): one FTU is the amount of enzyme that releases one micromole of inorganic phosphate per minute from sodium phytate under standard conditions of pH 5.5 and 37 °C. Supplier matrix values credit each dose with available phosphorus, calcium and sometimes energy and amino acids; these credits are product-specific and should be used with caution. High doses (superdosing) aim to destroy phytate as an anti-nutrient rather than only to release phosphorus. Phytase activity is reduced by high pelleting temperatures unless the enzyme is thermostable or applied after pelleting.
Wheat, rye and triticale contain soluble arabinoxylans, and barley and oats contain soluble mixed-linkage beta-glucans. These non-starch polysaccharides (NSP) dissolve into viscous gels in the gut of young birds, slow the mixing of enzymes and digesta, trap nutrients, encourage bacterial overgrowth in the small intestine and produce wet, sticky droppings and dirty eggs. Xylanase is added to wheat- and rye-based diets and beta-glucanase to barley- and oat-based diets; both reduce viscosity and also break down cell walls that encapsulate starch and protein (the cage effect). Their benefit is greatest in young birds and in viscous-cereal diets and smaller in maize-soybean diets. Amylase, protease and alpha-galactosidase (for the oligosaccharides of legumes) are also used, alone or in combinations.
Probiotics (called direct-fed microbials in North America) are live microorganisms which, when given in adequate amounts, confer a benefit on the host. Strains used in feed come mainly from Lactobacillus, Bifidobacterium, Enterococcus and Pediococcus, spore-forming Bacillus species, which survive pelleting well, and yeasts such as Saccharomyces cerevisiae. Proposed mechanisms include competitive exclusion, production of acids and bacteriocins, and immune modulation. Competitive exclusion products made from the caecal flora of healthy adult birds were developed to reduce Salmonella colonisation of newly hatched chicks. Prebiotics are non-digestible ingredients selectively fermented by beneficial microbes; fructo-oligosaccharides, galacto-oligosaccharides and mannan-oligosaccharides from yeast cell walls are examples, the last also binding bacteria with type-1 fimbriae. Synbiotics combine the two. Organic acids (formic, propionic, lactic, fumaric, citric and their salts) lower the pH of feed and of the stomach in young pigs, act against Gram-negative bacteria and preserve feed. Plant-derived additives (phytogenics) give less consistent responses. Responses to all of these are larger in young, stressed or challenged animals than on clean, high-performing farms.
Coccidiosis is controlled in broilers mainly by in-feed anticoccidials, either ionophores (polyether antibiotics such as monensin, salinomycin, narasin, lasalocid and maduramicin) or synthetic chemicals, often rotated or used in shuttle programmes, or by vaccination. Ionophores are classed as feed additives in the European Union and as drugs elsewhere, and their status relative to antibiotic-free claims differs between markets. Two toxicity facts are examined everywhere. Horses are extremely sensitive to ionophores: feed meant for poultry or cattle, or a mixing error at the mill, can kill horses through cardiac and skeletal muscle damage. And some ionophores interact with tiamulin, a pleuromutilin antibiotic used in pigs and poultry, which inhibits their metabolism and produces ionophore toxicity at normal doses. Feed mills therefore need clean-out and sequencing procedures between medicated and non-medicated batches.
Antibiotic growth promoters (AGPs) are antimicrobials fed at low, sub-therapeutic concentrations for long periods to improve growth rate and feed conversion, not to treat or prevent a diagnosed disease. Discovered in the late 1940s, they were used widely in pigs, poultry and feedlot cattle for half a century. The responses were largest in young animals and on farms with poor hygiene. The accepted mechanisms all act through the gut microbiota: suppression of subclinical infection, reduced competition by microbes for nutrients, reduced production of growth-depressing microbial metabolites such as ammonia and amines, less microbial breakdown of bile salts (improving fat digestion), and a thinner intestinal wall with lower maintenance cost. Germ-free animals do not respond to AGPs.
Low-dose, long-term exposure of large animal populations selects for resistant bacteria and resistance genes, which can pass to humans through food, the environment and direct contact, and some AGPs belong to classes used in human medicine. The example that drove policy was avoparcin, a glycopeptide used as a growth promoter in Europe, whose use was linked with vancomycin-resistant enterococci in animals and people; the European Union banned avoparcin in 1997. Sweden had banned all AGPs in 1986 and Denmark ended them in the late 1990s. The European Union then banned all remaining antibiotics as growth promoters from 1 January 2006 under Regulation (EC) No 1831/2003. In the United States, the Food and Drug Administration's Guidance for Industry 209 and 213 ended the use of medically important antimicrobials for growth promotion from January 2017 and placed their in-feed use under the Veterinary Feed Directive, requiring veterinary authorisation. China banned growth-promoting antibiotics in feed from 2020. Following the discovery of the plasmid-borne colistin resistance gene mcr-1 in China in 2015, colistin use in animals has been restricted in many countries, and India prohibited colistin in food-producing animals, poultry, aquaculture and animal feed supplements in 2019. Policies continue to change and differ between countries, so a practitioner must check the current national rules.
The Scandinavian experience is the best documented. In finishing pigs and broilers, productivity was largely maintained once management adapted, with small losses in feed conversion in some periods. In weaned pigs, post-weaning diarrhoea and therapeutic antimicrobial use increased for several years before management and nutrition caught up. Growth promoters had been compensating for weaknesses in hygiene, weaning management and diet. The replacement is a package, not one product: better weaning management, lower crude protein with synthetic amino acids, digestible ingredients, acids, enzymes, probiotics, vaccination, all-in all-out flow and hygiene.
The word withdrawal is also used in a second sense that examiners mix up deliberately. A withdrawal period is the minimum time between the last administration of a veterinary medicine, including one given in feed or water, and slaughter or the use of eggs or milk for food, so that residues fall below the maximum residue limit (MRL). It is set on the product authorisation and differs by drug, species, product and country, so it must be read from the label or the prescription. On poultry farms the final days of feeding a broiler flock use a withdrawal feed free of such medicines. Withdrawing growth promoters as policy and observing a drug withdrawal period are separate ideas.
Mycotoxins are toxic secondary metabolites of fungi that grow on crops in the field or in storage. Field fungi, chiefly Fusarium species, infect grain before harvest under cool, wet conditions at flowering; storage fungi, chiefly Aspergillus and Penicillium, grow when grain is stored too moist or too warm, or is damaged by insects. Aspergillus flavus also infects maize and groundnuts in the field under heat and drought stress. Mycotoxins are chemically stable and survive most processing, including pelleting, so visual inspection of grain is unreliable: mouldy grain may be toxin-free and clean-looking grain may be contaminated. Contamination is patchy, so sampling error is usually larger than analytical error, and many samples carry more than one toxin. Concentrations are reported in micrograms per kilogram (µg/kg, equal to parts per billion, ppb) or milligrams per kilogram (mg/kg, equal to parts per million, ppm); 1 mg/kg equals 1,000 µg/kg.
Aflatoxins (B1, B2, G1, G2), from Aspergillus flavus and A. parasiticus, are hepatotoxic and carcinogenic; aflatoxin B1 is the most potent and is metabolised in the liver to a reactive epoxide. Ducklings and turkey poults are among the most sensitive species; Turkey X disease, which killed many thousands of turkeys in England in 1960, was traced to aflatoxin in imported groundnut meal and led to the discovery of the aflatoxins. Rainbow trout are very sensitive and develop liver tumours. Aflatoxin B1 eaten by dairy animals appears in milk as aflatoxin M1. Ochratoxin A, from Aspergillus ochraceus and Penicillium verrucosum, is nephrotoxic and causes mycotoxic nephropathy in pigs and poultry. Fumonisins, from Fusarium verticillioides and related species in maize, disrupt sphingolipid metabolism and cause equine leukoencephalomalacia, a fatal liquefaction of the cerebral white matter in horses, and porcine pulmonary oedema. Zearalenone, from Fusarium graminearum and others, is oestrogenic: in pigs it causes swelling and reddening of the vulva, vaginal and rectal prolapse in gilts, and reproductive failure. Deoxynivalenol (vomitoxin), a trichothecene from Fusarium graminearum and F. culmorum, causes feed refusal, reduced intake and vomiting, pigs being the most sensitive species. T-2 toxin, another trichothecene, causes oral and skin necrosis and haemorrhage, and lesions in the beak commissures and mouth of poultry. Ergot alkaloids, from the sclerotia of Claviceps purpurea on rye and other grasses and cereals, cause gangrene of extremities and, in sows, reduced prolactin and agalactia.
Most countries set legal maximum or guidance levels for aflatoxins in feed, and many set guidance values for deoxynivalenol, zearalenone, fumonisins and ochratoxin A; the values differ by country, toxin, species, age and whether the feed is complete or a single ingredient, so the current national limits must be checked. As one example, the United States Food and Drug Administration action level for aflatoxin is 20 µg/kg in feed for dairy animals and immature animals, with higher levels permitted for some finishing animals. Control runs from the field (resistant varieties, timely harvest, insect control) through drying to safe moisture and clean storage, to sampling, testing (immunoassay strips, ELISA, chromatography) and segregation of contaminated lots at the mill. Mycotoxin-binding agents are added to feed: hydrated sodium calcium aluminosilicates and other clays bind aflatoxin well but bind deoxynivalenol, zearalenone and fumonisins poorly, so other approaches (yeast cell wall products, and enzymes or microorganisms that transform specific toxins into less toxic metabolites) are used for those. Clinically, mycotoxicosis should be suspected when signs are non-contagious, linked to a batch of feed, not responsive to antimicrobials, and improve when the feed is changed; diagnosis rests on analysis of a representative sample of the feed actually eaten.
The dog is a carnivore by taxonomy but metabolically an omnivore: it digests cooked starch well, can make taurine from sulphur amino acids, converts beta-carotene to vitamin A and makes niacin from tryptophan. The cat is an obligate (strict) carnivore with several fixed adaptations to a prey diet. It has a high protein requirement because its liver enzymes for amino-acid catabolism stay highly active even on a low-protein diet. It requires dietary taurine, because its synthesis is limited and taurine is lost in bile acid conjugation; deficiency causes dilated cardiomyopathy, central retinal degeneration and reproductive failure. It requires arginine: a single arginine-free meal can cause hyperammonaemia within hours, because arginine is needed for the urea cycle. It needs preformed vitamin A (it cannot cleave beta-carotene), dietary niacin (it diverts tryptophan metabolism away from niacin) and dietary arachidonic acid (its delta-6 desaturase activity is low).
Pet food in the United States is regulated by states using the nutrient profiles and model regulations of the Association of American Feed Control Officials (AAFCO), and in Europe the industry federation FEDIAF publishes nutritional guidelines; the National Research Council's Nutrient Requirements of Dogs and Cats (2006) underpins both. Labels state whether a food is complete (it can be fed as the sole diet) or complementary, and for which life stage: growth and reproduction, adult maintenance, or all life stages. Wet and dry foods cannot be compared on the as-fed guaranteed analysis because their moisture differs; compare on a dry-matter basis or per 1,000 kcal. Worked example: a canned food with 78 per cent moisture and 10 per cent protein as fed has 22 per cent dry matter, so its protein on a dry-matter basis is 10 divided by 22, which is 45.5 per cent. WSAVA guidelines ask for a nutritional assessment at every consultation: body weight, body condition score (9-point scale, 4 to 5 ideal) and muscle condition score.
Resting energy requirement (RER) in kcal per day is estimated as 70 times body weight in kg raised to the power 0.75. A linear approximation, 30 times body weight plus 70, is acceptable only between about 2 and 45 kg. Maintenance energy requirement (MER) is RER times a factor; commonly quoted factors are about 1.6 for a neutered adult dog and 1.8 for an intact adult dog, about 1.2 for a neutered adult cat and 1.4 for an intact one, about 1.0 times RER at the target weight for weight loss, and higher multiples for growth, gestation, lactation and work. These factors are starting points with wide individual variation, and the amount fed is then adjusted by body weight and condition. Worked example: a 20 kg neutered adult dog has an RER of 70 times 20 to the 0.75, which is 662 kcal per day, and an MER of about 1.6 times that, 1059 kcal per day. A 4 kg neutered cat has an RER of 198 kcal per day and an MER of about 238 kcal per day. Obesity is the commonest nutritional disorder of dogs and cats in many countries.
Large- and giant-breed puppies fed excess calcium or excess energy are at greater risk of developmental orthopaedic disease, because puppies cannot fully regulate calcium absorption; large-breed growth diets limit calcium and energy density, and calcium supplements should not be added to a complete diet. An all-meat diet is low in calcium relative to phosphorus and causes nutritional secondary hyperparathyroidism with bone demineralisation and pathological fractures. Cats fed mainly liver develop hypervitaminosis A with new bone formation on the cervical vertebrae; cats fed raw fish containing thiaminase develop thiamine deficiency with ventroflexion of the neck; cats fed diets rich in oily fish without enough vitamin E develop steatitis (yellow fat disease). An overweight cat that stops eating can develop hepatic lipidosis within days, so assisted feeding is started early. Toxic foods include chocolate (theobromine), grapes and raisins (kidney injury in dogs), onions and garlic (Heinz body haemolytic anaemia, cats especially sensitive) and xylitol (hypoglycaemia and liver failure in dogs). Therapeutic diets restrict phosphorus in chronic kidney disease, dissolve struvite uroliths, and use hydrolysed or novel proteins for elimination trials. A possible link between some grain-free, legume-rich dog diets and dilated cardiomyopathy has been investigated since 2018; the cause is not established and remains contested.
The horse is a hindgut fermenter with a small stomach, no gall bladder and a large caecum and colon in which microbes ferment fibre to volatile fatty acids. It evolved to graze for most of the day, so the diet should be based on forage. Total dry-matter intake is commonly about 2 to 2.5 per cent of body weight, and forage should provide at least about 1.5 per cent of body weight as dry matter (1 per cent is often quoted as the absolute minimum). For a 500 kg horse, 2 per cent is 10 kg dry matter a day and 1.5 per cent is 7.5 kg dry matter of forage, which is about 8.5 kg of hay at 88 per cent dry matter. Concentrates are added only when forage cannot meet energy needs, as in hard work, late pregnancy or lactation, and are split into small meals, because a large starch meal escapes small-intestinal digestion, ferments rapidly in the hindgut, lowers caecal pH and can cause colic and laminitis. Long periods without forage and high-starch feeding are associated with gastric ulcers. The NRC Nutrient Requirements of Horses (6th revised edition, 2007) is the main standard. Specific hazards include ionophores in feed meant for other species, fumonisins (leukoencephalomalacia), botulism from poor haylage, blister beetles (cantharidin) in lucerne hay, and obesity with insulin dysregulation (equine metabolic syndrome), a major risk factor for laminitis.
The rabbit is a hindgut fermenter with a very large caecum, and it solves the problem of microbial protein lost in the faeces by caecotrophy. The proximal colon separates digesta: large, poorly digestible fibre particles move quickly through and are excreted as hard, dry faecal pellets, while fine particles and fluid are moved back into the caecum for fermentation. Periodically, usually in the early morning or during rest, the caecum empties to produce caecotrophs (soft faeces), small mucus-coated clusters rich in microbial protein, B vitamins and vitamin K, which the rabbit eats directly from the anus. The behaviour is normal; uneaten caecotrophs on the floor signal a problem such as obesity, dental disease, pain or a low-fibre diet.
Rabbits need a high level of indigestible fibre for gut motility and caecal health, and a high level of long fibre or abrasive forage for dental wear. Their incisors and cheek teeth grow continuously, so a diet of soft pellets or muesli-type mixes without enough hay leads to overgrown cheek teeth, spurs that cut the tongue and cheeks, abscesses and anorexia. Low-fibre, high-starch diets, especially in young rabbits around weaning, allow starch to reach the caecum, disturb the microbial population and predispose to enteritis and enterotoxaemia associated with Clostridium spiroforme, and to the mucoid enteropathy complex. Gut stasis follows pain, stress, anorexia or low fibre and is an emergency, because anorexia leads to hepatic lipidosis. Meat rabbits get pelleted complete feeds whose fibre specification varies by country; a pet rabbit is best fed unlimited good grass hay with fresh greens and a limited quantity of pellets.
Rabbits absorb dietary calcium largely in proportion to intake rather than according to need and excrete the excess in urine, which is normally cloudy with calcium carbonate. On high-calcium diets, such as large amounts of lucerne, this predisposes to sludgy urine and urolithiasis, though calcium deficiency is also harmful, so calcium should be adequate without excess. Pregnancy toxaemia occurs in obese does around kindling and when feed intake falls. Rabbits are sensitive to some antimicrobials given orally (for example, oral lincosamides, penicillins and some cephalosporins), which can disrupt the caecal flora and cause fatal enterotoxaemia; this is a pharmacology point but it is examined with nutrition because it rests on the same caecal ecology.
In a laboratory, the diet is not only a welfare matter but an experimental variable: differences in its nutrients, contaminants and natural plant compounds can change the outcome of a study. The main reference for requirements is the National Research Council's Nutrient Requirements of Laboratory Animals (4th revised edition, 1995), and the Guide for the Care and Use of Laboratory Animals (8th edition, 2011) sets out good practice for feeding, storage and records. Diets fall into three classes. Natural-ingredient (cereal-based) diets are made from grains, oilseed meals, fishmeal, minerals and vitamins; they are cheap and well accepted, but their composition varies batch to batch with the ingredients. They may be open-formula, with the full formula published, or closed-formula, where only the guaranteed analysis is disclosed. Purified diets are made from refined ingredients such as casein, starch, sucrose, cellulose, a vegetable oil and defined mineral and vitamin mixes; the AIN-93G (growth, gestation and lactation) and AIN-93M (adult maintenance) diets of the American Institute of Nutrition are the reference formulas for rats and mice.
Guinea pigs and non-human primates require dietary vitamin C, because, like humans, they lack L-gulonolactone oxidase, the last enzyme of ascorbate synthesis; rats and mice make their own. Vitamin C deficiency in guinea pigs causes scurvy with joint swelling, haemorrhage, reluctance to move and poor teeth and gums, so guinea pig diets are fortified and used within their shelf life, or vitamin C is supplied separately. New World primates require vitamin D3 rather than D2. Rats, mice and other rodents practise coprophagy, which supplies some B vitamins and vitamin K and must be considered when studying these nutrients; special cages are used to prevent it. Rodent incisors grow continuously, and pellets must be hard enough to wear them. Phytoestrogens from soybean meal and alfalfa in natural-ingredient diets can affect reproduction, cancer and endocrine studies, so low-phytoestrogen or purified diets are chosen for such work.
Diets for animals of defined microbiological status must be sterilised or pasteurised. Autoclaving with steam destroys heat-labile vitamins, especially thiamine and vitamin A, and can harden pellets or make them less palatable, so autoclavable diets are formulated with extra vitamins. Gamma irradiation causes fewer vitamin losses and is widely used for barrier facilities. Storage should be cool, dry and vermin-proof, and diets should be used within the manufacturer's recommended period; the Guide states that most natural-ingredient dry diets should be used within about six months of milling, and vitamin C-containing diets for guinea pigs and primates within about three months unless stabilised. Certified diets are analysed for contaminants such as heavy metals, pesticides and mycotoxins. Ad libitum-fed rodents become obese and develop more spontaneous disease than moderately restricted ones, which matters in long-term studies.
Fish are poikilotherms: their body temperature follows the water, so they spend no energy on keeping warm, and their maintenance energy needs are low compared with homeotherms. They excrete most of their nitrogen as ammonia across the gills, which is energetically cheap, and many species, especially carnivorous ones, use protein as a major energy source. As a result dietary protein requirements are high, roughly 30 to 55 per cent of the diet depending on species, size and protein quality, with carnivorous species such as salmon, trout and many marine fish at the upper end and omnivorous or herbivorous species such as carp, tilapia and many catfish lower. Carnivorous fish use dietary carbohydrate poorly. The same ten indispensable amino acids as for other animals apply, and lysine and methionine are often limiting in plant-based fish feeds. Feed intake, digestion and growth all depend on water temperature, and feeding rate, expressed as a percentage of body weight per day, falls as fish grow.
Fish need dietary essential fatty acids. Freshwater species such as carp and tilapia can convert the 18-carbon alpha-linolenic and linoleic acids into the long-chain highly unsaturated fatty acids, while most marine fish have limited ability to elongate and desaturate them and therefore require eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) directly, traditionally from fish oil. Most fish, like guinea pigs, cannot synthesise vitamin C, and deficiency causes spinal deformity (scoliosis and lordosis, broken-back syndrome), haemorrhages and poor wound healing; stable phosphorylated forms of vitamin C are used because ordinary ascorbic acid is destroyed in processing and storage. Fish absorb some calcium from water, but phosphorus must come from the diet.
Fishmeal and fish oil were the foundation of aquafeeds because of their protein quality, palatability and long-chain omega-3 content, but supply is finite and prices are high, so they are increasingly replaced by soybean and other oilseed meals, cereal and pulse protein concentrates, rendered animal proteins where permitted, insect meals, single-cell proteins and algal oils. Plant ingredients bring anti-nutritional factors and lower fillet omega-3 content. Feeds are pelleted or extruded; extrusion produces floating or slow-sinking pellets with good water stability, so that feeding can be watched and waste reduced. Feed conversion ratio in intensive salmon and trout farming is often about 1 to 1.5; protein efficiency ratio (weight gain divided by protein eaten) is used to compare diets in trials. Aflatoxin in feed is a classic cause of liver tumours in rainbow trout. The National Research Council's Nutrient Requirements of Fish and Shrimp (2011) is the main reference standard; requirements are species-specific and many cultured species remain poorly characterised.
Learn the gut first: why a pig and a hen need digestible amino acids, why birds are rated on AMEn, why a rabbit eats its caecotrophs. Then memorise the short lists that examiners love: the limiting amino acids in maize-soy diets for pigs and broilers, the chick deficiency syndromes and their nutrients, each mycotoxin with its fungus and target species, the obligate-carnivore needs of the cat. Practise the Pearson square until you can do it with a fixed premix, a synthetic lysine addition and a dry-matter conversion without hesitation, because these are free marks in entrance and civil-service papers and the same arithmetic recurs in clinics.
In pig and poultry practice you will be the first person asked why growth has dropped, why litters are pale, or why a flock has nervous chicks. Check the feed: delivery records, mixing, which diet each age group is actually receiving, water supply, and a properly taken feed sample for nutrient and mycotoxin analysis. In small-animal practice, record body condition and muscle score at every visit, calculate RER and MER for weight-loss plans, read labels on a dry-matter basis, and warn owners about home-made, raw and vegetarian diets for cats. In equine work, ask what feed was delivered whenever several horses fall ill at once, and think of ionophores and fumonisins.
At this stage the work is formulation and policy. You set nutrient specifications with the feed mill, decide when an NE or SID system is worth the change, review least-cost outputs and shadow prices before ingredient purchases, approve additive programmes and judge suppliers' matrix values sceptically. You lead the farm's response to antimicrobial reduction: weaning age, dietary protein, zinc and acid strategies, biosecurity and benchmarking of use. You write the mycotoxin sampling and rejection policy, train staff on withdrawal periods and medicated-feed cleanout, and advise companion-animal clinics on therapeutic diet selection and obesity programmes.
Research questions here are about precision and evidence. How should amino-acid requirements be modelled for individual animals fed by precision feeders? How large and reproducible are responses to probiotics, phytogenics and enzymes, and which mechanisms survive rigorous trials? How do NE and SID values transfer to new ingredients such as insect meals, single-cell proteins and fermented by-products? What are the interactions among co-occurring mycotoxins at low concentrations? A senior scientist designs dose-response and meta-analytic studies, insists on correct statistical units, publishes null results, and translates them into standards and regulations that differ by country.
This manual gives requirement values only as ranges or orders of magnitude, because commercial specifications change with genotype and are published by breeding companies; readers must use the current standard for their strain and country. It does not tabulate full requirement tables, ingredient composition matrices or mycotoxin legal limits, which differ by country and change. Ducks, turkeys, quail and other minor poultry species, ostriches, zoo animals, reptiles and exotic pets are not covered. Dosages of feed medicines are deliberately omitted except one common iron-injection range, which must be confirmed against the product label and national formulary. Shrimp and individual fish species' requirements are summarised only in general terms. The evidence on many gut-health additives is mixed and is described here in general terms rather than product by product.
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. Requirement values, legal limits and regulatory dates vary between countries and change; check current national standards.
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