Fundamentals of Animal Nutrition: Energy, Protein, Minerals, and Vitamins in Practice

Every formulation decision is, in essence, a decision about nutrients. Choosing one ingredient over another, defining the minimum level of a parameter, adjusting energy density of a growth phase, or reviewing mineral supplementation are choices that only make full sense when the formulator understands what that nutrient does inside the animal, how it interacts with other diet nutrients, and what the real consequences of errors in either direction are. This article covers the main nutrient groups with this applied-to-formulation perspective.

The objective is not to reproduce requirement tables that any reference system already provides. It is to explain the logic behind these numbers, the mechanisms that justify them, and the attention points that textbooks rarely present in operational language. For a broader view of the strategic and economic role of nutrition in competitive animal production, this article complements what we covered in nutrition for production animals: strategy, precision, and the real cost of formulating well.

Energy: the nutrient that organizes all the others

Energy is not a nutrient in the conventional chemical sense: it is not a molecule the animal absorbs and deposits directly. It is a property of organic nutrients, mainly carbohydrates, lipids, and proteins, released during metabolic processes and used to sustain vital functions, growth, and production. But despite being a property derived from other compounds, energy is the nutrient that most influences animal performance and, consequently, production cost.

The reason for this centrality is simple: in monogastrics, like poultry and swine, voluntary food intake is regulated primarily by dietary energy supply. The animal eats until satisfying its energy need and, when it does, stops eating, regardless of how much protein or mineral it still needs to consume. This means a diet with higher energy density will be consumed in smaller daily quantities, and all other nutrients must be present in that smaller amount of feed for total daily requirements to be met. An inaccuracy in diet energy value automatically compromises meeting all other nutrients.

Energy expression systems and their practical implications

Food energy can be expressed in different systems, each representing a different level of utilization by the animal. Understanding the differences between these systems is essential for correctly comparing ingredients and research results.

Gross energy (GE) represents the total energy released in complete combustion of food. It is the highest value and least useful for formulation, as it does not consider losses during digestion. Digestible energy (DE) discounts fecal losses, being the most used system for swine in many countries. Metabolizable energy (ME), which additionally discounts urinary and fermentation gas losses, is the standard system for poultry in Brazil, expressed in kcal/kg of feed. Net energy (NE), which also discounts heat produced during nutrient metabolism (heat increment), is the most precise system and is used for cattle and increasingly discussed for swine.

The practical point worth noting is that conversion between systems is neither linear nor the same for all ingredients. A starch-rich ingredient like corn has a relatively high ME/DE ratio because carbohydrates have low heat increment. A crude protein-rich ingredient like soybean meal has a lower ME/DE ratio because catabolism of excess amino acids generates more metabolic heat. This means comparing two ingredients by ME without considering their composition can lead to formulation errors, especially when ingredients have very different macronutrient profiles.

Energy density, temperature, and voluntary intake

The relationship between diet energy density and voluntary intake has a practical implication often underestimated: in hot environments, feed intake drops. This happens because food metabolism generates heat, and the animal under heat stress reduces intake to decrease internal heat production. For broilers raised in Brazil during summer, this consumption reduction can be significant and, if the diet is not reformulated with higher energy and protein density to compensate for lower intake volume, performance drops measurably.

Formulators working in hot climate regions or with buildings without temperature control need to incorporate ambient temperature as an active variable in defining diet density, especially in growth and finishing phases where protein requirement is high and intake volume plays a critical role in meeting requirements.

Protein and amino acids: from crude protein to ideal proteinProtein and amino acids: from the crude protein concept to ideal protein

For decades, crude protein was the central parameter of protein formulation, and it remains the most common in labels and analysis reports. But for those formulating high-performance monogastric feeds, working only with crude protein is like trying to hit a target looking only at the general direction, without aiming. The animal does not use crude protein: it uses amino acids, absorbed individually in the small intestine, each with its specific metabolic function and its own digestibility rate depending on source and processing.

The ideal protein concept and limiting amino acids

The ideal protein concept defines the diet in terms of the digestible amino acid profile needed to meet animal requirements with maximum efficiency, without excesses that would be catabolized and excreted as urinary nitrogen. The reference amino acid is digestible lysine, and all other amino acids are expressed as proportions of it, forming an ideal profile that varies according to species, production phase, and objective (muscle gain, egg production, reproduction).

The amino acids most frequently limiting performance in practical poultry and swine diets are lysine, methionine, threonine, and tryptophan, in that general order, though this sequence can vary with the ingredients used. When one of these amino acids is below requirement in its digestible form, performance is limited by it regardless of how much the other amino acids are present. It is Liebig's law of the minimum applied to nutrition: the most deficient amino acid determines the performance ceiling.

In formulation practice, this means reducing diet crude protein level is possible, and often economically advantageous, as long as limiting amino acids are supplemented in synthetic form (DL-methionine, L-Lysine HCl, L-threonine, L-tryptophan) to ensure the diet's digestible amino acid profile is complete. This strategy of reducing crude protein with industrial amino acid supplementation has been widely adopted because it reduces the cost of expensive protein ingredients, decreases nitrogen excretion, and in hot climates reduces the diet's heat increment, which favors voluntary intake.

Amino acid digestibility: why processing matters

Amino acid digestibility is not a fixed property of the ingredient. It depends on the processing the ingredient underwent, storage conditions, presence of antinutritional factors, and in some cases interaction with other diet components. Understanding these factors is essential to avoid formulation errors that do not appear in conventional bromatological analysis.

The most classic example is soybean meal. Raw soybean contains trypsin inhibitors that severely reduce protein digestibility. The toasting process inactivates these inhibitors, making soybean meal one of the most digestible protein ingredients available for monogastrics. However, if the toasting process is excessive, Maillard reactions between amino acids and carbohydrates reduce lysine bioavailability, which preferentially reacts under these conditions. The result is a soybean meal with normal crude protein, low PDI (Protein Dispersibility Index), and reduced reactive lysine, directly affecting the performance of animals consuming it without this appearing in crude protein or even total amino acid analysis.

Animal-origin meals show even wider digestibility variability because the manufacturing process varies between plants and raw materials are inherently heterogeneous. A well-processed meat and bone meal can have protein digestibility above 80%, while a meal produced at excessive temperature or with very high bone proportion can have digestibility below 60%. Working with total amino acid values for these ingredients, without considering digestibility, is a frequent source of discrepancy between performance predicted in formulation and results obtained on the farm.

Protein in ruminants: the complexity of ruminal fermentation

For cattle, the digestible amino acid approach used in monogastrics does not directly apply because the rumen substantially modifies the protein entering the diet before it reaches the small intestine. What the formulator needs to control is not just the amount of protein entering, but the fraction that will be degraded in the rumen (RDP, rumen-degradable protein) and the fraction that will escape degradation and reach the intestine as rumen-undegradable protein (RUP, or bypass protein).

Microbial protein synthesis in the rumen is the most important source of metabolizable protein for cattle fed medium-production-level diets. This synthesis is limited primarily by the availability of fermentable energy in the rumen: without rapidly fermentable carbohydrates, microorganisms cannot grow and synthesize protein efficiently, even when RDP is at adequate levels. For high-production cows, where metabolizable protein requirements exceed what microbial synthesis can provide, it is necessary to supplement with sources of RUP with high intestinal digestibility, such as low-rumen-degradability soybean meal, fish meal, or protein sources treated with formaldehyde for ruminal protection.

Minerals: macro-elements, micro-elements, and the bioavailability variable

Minerals are inorganic nutrients participating in essential structural, metabolic, and regulatory functions in the animal organism. They divide into macro-elements, required in relatively larger quantities and expressed in grams per kilogram or as percentage of diet, such as calcium, phosphorus, sodium, chlorine, potassium, magnesium, and sulfur, and micro-elements (trace elements), required in much smaller quantities, expressed in milligrams or micrograms per kilogram, such as zinc, manganese, copper, iron, iodine, selenium, cobalt, and chromium.

The characteristic distinguishing mineral formulation from all other nutrients is the critical importance of source bioavailability. Two ingredients can contain the same amount of a mineral in chemical analysis and have completely different bioavailabilities, resulting in very different actual supply to the animal. Ignoring bioavailability and working with minerals only through gross analysis is one of the most frequent causes of subclinical deficiencies that never appear clearly diagnosed, but that reduce conversion efficiency, compromise immunity, and increase mortality diffusely.

Calcium and phosphorus: the most critical and most supplemented pair

Calcium and phosphorus are the minerals with the highest quantitative requirement and those most frequently requiring formulation attention, because the common ingredients in monogastric diets do not provide them in adequate proportions, requiring supplementation with inorganic sources such as calcium carbonate, dicalcium phosphate, and monocalcium phosphate.

In the case of phosphorus, there is an additional complication for monogastrics: most phosphorus present in plant-origin ingredients is in phytate form, a compound where phosphorus is bound to phytic acid and is practically unavailable for poultry and swine, which do not produce phytase endogenously. This means total phosphorus of a plant ingredient is not a useful number for formulation: what matters is available phosphorus, corresponding to the non-phytate fraction plus any release by exogenous phytase added to the diet.

The addition of exogenous phytase to poultry and swine diets is now a standard industry practice, and its use has direct formulation implications: it releases previously unavailable phosphorus, reducing the need for inorganic phosphate supplementation, and also improves digestibility of other nutrients complexed with phytate, such as zinc, calcium, and amino acids. To correctly take advantage of this benefit in formulation, the nutritionist must work with a phytase nutritional matrix that reflects the actual expected release of each nutrient for that specific dose and diet, something that varies between commercial products and requires periodic updating as enzyme manufacturers revise their performance data.

Trace elements: inorganic versus organic forms

The discussion about organic versus inorganic trace element forms has gained significant space in formulation in recent decades, driven by evidence that minerals chelated or complexed with amino acids, hydrolyzed proteins, or yeast have higher bioavailability than conventional inorganic sources like sulfates and oxides. The practical difference is that, to deliver the same effective absorbed mineral intake, smaller doses of organic forms can be used, with less excretion and lower environmental impact.

For the formulator, the decision between organic and inorganic sources involves a cost-benefit analysis considering the price differential, the required inclusion level, and specific performance objectives. Organic zinc, for example, has been associated with improvements in hoof integrity in swine dams and carcass quality in broilers, while also allowing lower inclusions for the same biological effect, which reduces zinc excretion — a mineral that accumulates in soils when manure is used as fertilizer. Organic copper plays an important role in piglet intestinal health and antibiotic use reduction programs. Organic selenium, in selenomethionine form, has higher bioavailability and tissue retention than sodium selenite, with impact on selenium transfer to egg and milk.

Electrolyte balance and the role of sodium and potassium

The diet electrolyte balance, expressed by the cation-anion difference (CAD = Na+ + K+ - Cl-, in mEq/kg), is a parameter frequently neglected in poultry feed formulation but with real impact on the animal's acid-base balance and, consequently, on nutrient digestibility, amino acid utilization, and eggshell quality in layers. Diets with excessively low CAD induce mild metabolic acidosis that reduces protein deposition efficiency. Diets with very high CAD can impair fecal consistency and increase litter moisture, with health and welfare implications. The ideal value for most broiler production situations is between 240 and 260 mEq/kg, and small adjustments of sodium bicarbonate or ammonium chloride inclusion can be used to correct deviations.

Vitamins: stability, interactions, and the cost of excess

Vitamins are organic compounds required in very small quantities but with essential metabolic functions that cannot be replaced by other nutrients. They divide into fat-soluble vitamins A, D, E, and K, stored in body fat tissues, and water-soluble vitamins, the B-complex vitamins and vitamin C, which do not accumulate significantly and must be regularly supplied by diet.

From a formulation standpoint, vitamin challenges concentrate in three areas: stability during processing and storage, variation in biological response according to chemical form used, and defining supplementation levels that balance performance and cost.

Stability and processing losses

Vitamins are compounds sensitive to heat, light, oxidation, and moisture, to varying degrees depending on type. Vitamin C is the most labile of all, practically destroyed by the thermal processing of conventional pelleting, making the protected (stabilized) form necessary when ensuring it reaches the animal in useful quantities. Vitamin A is sensitive to oxidation and the presence of pro-oxidant minerals in the diet. B-group vitamins vary in stability, with thiamine (B1) and folic acid among the most sensitive and riboflavin among the most stable.

The pelleting process, especially with steam conditioning above 80°C, is the greatest point of vitamin loss in feed processing. The extent of this loss depends on temperature, exposure time, moisture, and the chemical form of vitamins used. Quality vitamin premixes use encapsulated or stabilized forms that reduce processing losses, but there is significant quality variation between suppliers. To ensure vitamin levels declared on the label are present in the final product consumed by the animal, it is necessary to know the loss correction factors for each vitamin and specific process, and apply appropriate inclusion overages.

Vitamin D: the form makes a difference

Vitamin D deserves special attention because the chemical form used has concrete impact on supplementation efficiency. Vitamin D3 (cholecalciferol) is the active form for mammals and birds, and is significantly more bioavailable than vitamin D2 (ergocalciferol) in these species. 25-hydroxycholecalciferol (25-OH-D3), a metabolite closer to the active form than conventional vitamin D3, has even higher bioavailability and can be used at lower doses for the same biological effect, with implications both economic and for situations of hepatic compromise where D3-to-25-OH-D3 conversion is limited.

Vitamin D plays a central role not only in bone mineralization (its classical function), but also in immune response modulation, hoof integrity in swine and heavy poultry, and reproductive performance. Marginal vitamin D deficiencies, which do not cause clinical rickets but are sufficient to compromise immunity and musculoskeletal system integrity, are more common than imagined in intensive systems with little sun exposure.

Vitamin E and selenium: the antioxidant duo

Vitamin E (alpha-tocopherol) and selenium act synergistically as an antioxidant system, protecting cell membranes and tissues against free radical damage generated by aerobic metabolism. Deficiencies in either compromise this protection and predispose animals to conditions such as white muscle disease in cattle and sheep, nutritional muscular dystrophy in broilers, sudden death in young swine, and immune function compromise in all species.

From a formulation standpoint, it is important to understand that vitamin E present in natural diet ingredients, especially vegetable oils, is subject to oxidation during storage. Feeds stored for long periods or based on ingredients that underwent lipid oxidation may have effective vitamin E content significantly below what was calculated in formulation, especially if there is no adequate antioxidant in the premix. The presence of oxidized fat in the diet additionally increases vitamin E demand, because free radicals generated by rancid fat consume the vitamin E available in tissues. This is a relevant attention point for plants working with lower-quality fats or feeds stored for longer periods.

Fiber: much more than the indigestible fraction

Dietary fiber was long treated in monogastric formulation as a problem to be minimized, a component that occupies feed space without providing available energy. This simplified view no longer withstands the accumulation of evidence about fiber's role in intestinal health, microbiota modulation, and welfare problem prevention.

In poultry, the inclusion of insoluble fiber, especially wheat straw, sunflower hulls, or cellulose, stimulates gizzard development, increases transit time, and improves digestibility of other nutrients by promoting better homogenization of digestive content. In heavy breeders with obesity tendency, low-energy-value insoluble fibers allow increasing feed volume without raising caloric density, reducing ingredient selection and pecking behavior in birds. In nursery piglets, fermentation of prebiotic soluble fibers in the large intestine produces short-chain fatty acids that feed the intestinal epithelium and positively modulate the microbiota, having a protective effect against enteric pathogens at a time when immunity is still developing.

For ruminants, physically effective fiber, expressed as peNDF, is a structural diet parameter. It is what sustains the ruminal mat, promotes rumination, stimulates buffering saliva production, and keeps rumen pH in the appropriate range for normal fermentation. The balance between peNDF and concentrate in feedlot cattle or high-production cow diets is one of the most delicate balancing acts in ruminant formulation, because excess concentrate with effective fiber deficiency is the main cause of subclinical ruminal acidosis, one of the conditions with the greatest hidden economic impact in intensive systems.

From nutrient understanding to formulation with real data

Understanding what each nutrient does and how it should be expressed in formulation is a necessary but not sufficient condition for formulating well. The link that closes the cycle is the quality of information about ingredients: knowing that digestible lysine is the reference amino acid for ideal protein is useless if the digestible lysine value used for soybean meal in the formula is a table value that does not reflect the actual digestibility of the ingredient being purchased.

For this reason, technical depth in nutrient knowledge needs to go hand in hand with the analytical quality of ingredient data. The two reinforce each other: a formulator who understands the mechanisms of lysine degradation during thermal processing will value a reactive lysine analysis by NIRS much more than a formulator who only knows that "lysine is an essential amino acid." And they will use that data much more precisely in formulation, because they know exactly what it represents and the consequences of ignoring it.

Formulation software like Formulamix was built with this premise: not just calculating the least-cost formula, but integrating laboratory analytical data into nutritional matrices in a structured way, so that the balancing equations operate with values that reflect the reality of available ingredients. When nutritionists with deep technical mastery work with tools that have the same data depth, the result is formulation that not only meets requirements on paper, but delivers predicted animal performance.