Nutritional Requirements of Pigs

Calcium and phosphorus are the most abundant elements in the body. However, 99% of Ca in the body, and 80% of P, is bound as hydroxyapatite crystals in mineralized tissues. Thus, as mineral crystals, Ca and P are not immediately exchangeable in physiological fluids, as discussed in the electrolyte section. Although Ca is the least abundant macromineral element when considered in a readily exchangeable ion, the Ca²⁺ ion is homeostatically regulated to maintain constant and distinct concentrations in fluid compartments. Extracellular Ca is 1,000 times more concentrated than intracellular Ca. Intracellular Ca concentration is 10^-7 M, yet intracellular Ca functions as a secondary messenger and serves critical functions in muscle contraction and neural signals. Mitochondria and microsomes contain 90% of intracellular Ca. Ca is lethal if intracellular concentration is increased to 10^-5 M; thus, Ca concentration is tightly regulated. 

In extracellular fluid, Ca is approximately 50% ionized, whereas 40% is bound to albumin and 10% is complexed with citrate and phosphate ions. The unique properties of Ca and the ability of the body to maintain vastly different concentrations among body compartments allow Ca to fill major roles in muscle contraction, neural and intercellular signaling, and blood coagulation. As a divalent cation, calcium strongly binds to numerous anions but must dissociate for absorption into the body. The relatively low (compared with Na and K) dissociation constant of Ca²⁺ ions contributes to the unique functional roles of Ca in biological systems.

Phosphorus has more biological functions than any other mineral in the body; however, less is known about P homeostasis than Ca homeostasis. Plasma P concentration is not as rigidly maintained as is that of Ca.

In mammalian cells, P is found complexed with oxygen as an orthophosphate. Pure phosphorus is too reactive to be found free in nature; metallic P ignites spontaneously and combusts when exposed to air. Herein, phosphorus and phosphate are used interchangeably and assumed to be phosphate. 

As with Ca, the majority (80%) of P in the body is bound in hydroxyapatite crystals that compose the ash content of bone. In soft tissue, P is an important buffer in the intracellular fluid compartments. Phosphate exists in a 4:1 ratio of dibasic and monobasic compounds at physiological pH. This ratio can be calculated using the Henderson-Hasselbalch equation. The ratio of dibasic and monobasic phosphate provides the basis of phosphate buffering at physiological pH and provides the chemical basis for the net elimination of excess H⁺ ions in urine measured as titratable acidity.

Although only a small percentage of total body P concentration, the phosphate ester bonds generated or cleaved during metabolism are the primary means of transferring chemical energy in compounds such as ATP. Covalent P bonds function as structural and key functional components of phospholipids, nucleic acids, and phosphorylated proteins. In these roles, P contributes to cell structure, function, and activation sites for enzymes.

The majority of Ca and P stores in the body are found in a complex molecule as hydroxyapatite-like crystals that comprise the mineralized matrix of bone. These crystals provide the structural integrity of bone to allow support for locomotion and protection of vital organs; however, the crystals also provide a vast storage reservoir of Ca and P as part of the homeostatic regulation of these ions.

Swine have well-developed physiological mechanisms to maintain constant Ca concentrations in soft tissue and blood. These immediate mechanisms involve the integration of parathyroid hormone (PTH), calcitonin, and vitamin D actions on 3 major organs: intestines, kidneys, and bones. Multiple systemic and local hormones and growth factors also modulate these responses. Dietary inputs and fecal and urine excreta are commonly assumed to balance increased demands for production beyond maintenance (eg, growth and milk production).

Minerals, especially Ca and P, are absorbed in various segments of the GI tract by active and passive transport mechanisms. If nutrients are limited, PTH, 1,25-(OH)₂D₃ (the active form of vitamin D), and FGF23 (fibroblast growth factor hormone produced in bone) induce transport proteins to enhance mineral uptake, downregulate renal excretion, and decrease bone mineralization. Likewise, if minerals are in excess, active transport proteins are downregulated, and pathways to eliminate excess minerals are upregulated to increase renal and intestinal tract excretion. Direct actions of these systemic hormones on bone tissues are not as clearly defined. Hormones initiate bone responses either to liberate Ca and P for soft tissue needs or to deposit Ca and P in the extracellular bone matrix.

A decrease in plasma Ca concentration stimulates PTH release from the parathyroid gland and activation of 25-OH vitamin D₃ to 1,25-(OH)₂D₃. Concurrently, these hormones act to increase Ca absorption in the small intestine, decrease renal Ca excretion, and initially decrease Ca deposition in bone.

If serum Ca is high, PTH secretion decreases, and renal 1,25 hydroxylase activity is decreased. Calcitonin hormone is released from the thyroid gland, which suppresses bone resorption and enhances renal Ca excretion.

As a nutrient, vitamin D (vitamin D₂ and D₃) is metabolized in the body to active 1,25-(OH)₂D₃ that actually functions as a hormone. 1,25-(OH)₂D₃ is produced primarily in the kidneys and targets responses in other organs and tissues to stimulate gene transcription for synthesis of proteins involved in Ca and P homeostasis. This active vitamin D hormone enters the nucleus of a target cell and binds to a receptor complex (RXR, retinoid X receptor) that can then bind directly to DNA to increase or decrease gene transcription. Observations that retinoic acid must first bind to the RXR receptor before 1,25-(OH)₂D₃ can bind and excess vitamin A can inhibit vitamin D from binding are the presumed basis for interactions between vitamin A and vitamin D.

Vitamin D₃ can be produced from 7-dehydrocholesterol in skin by exposure to UV light, or vitamin D₃ can be supplied as a dietary supplement. Vitamin D₃ is a fat-soluble vitamin, so its derivatives are either stored in the liver and muscle tissues or converted in the liver to 25-OH vitamin D₃ by an enzyme, 25-hydroxylase. The 25-OH D₃ is transported via circulation to the kidney. In the kidney, another OH group is added by an enzyme, 1-alpha hydroxylase, to produce 1,25-(OH)₂ D₃, which acts on target tissues in the bones, kidneys, and intestines to regulate Ca and P homeostasis.

Elevated 1-alpha,25-(OH)₂D₃ targets the small intestine to stimulate active absorption of Ca and P via upregulation of proteins involved in Ca transport, including calmodulin (CaM), calbindin (CaBP), Ca transport protein 1 (TRPV6), and Ca ATPase (CaATPase).

The discovery of FGF23, a peptide hormone produced in osteocytes (bone cells embedded in a calcified matrix), contributed to the understanding of a major endocrine role of bone tissue in regulation of P and vitamin D homeostasis. FGF23 regulates renal P reabsorption, thus impacting the efficiency of P use. Diets oversupplemented with P, or even potentially superdoses of phytase, have potential, via FGF23 regulation, to increase renal P excretion for maintenance of plasma P within physiological ranges. Under the conditions of excess P supplements, the loss of P via renal pathways is not detected in assessments of dietary adequacy based on measures of standardized total tract digestible (STTD) P criteria. Likewise, the stimulation of FGF23 release downregulates the renal activation of vitamin D from 25-OH to 1,25-(OH)₂D₃. This downregulation of vitamin D activation may result in an inadequate vitamin D status even though serum 25-OH is within normal range.

Both the amounts and ratios of Ca and P are important in swine nutrition. An imbalanced Ca:P ratio affects the homeostatic processes involved and compromises efficient nutrient use. The required amounts and ratios have been established under the assumption that the animal has adequate vitamin D.

Dietary Ca:P ratio is influenced by the amounts stored in bone and soft tissue. In bone ash, Ca:P exists in a 2.1:1 ratio as hydroxyapatite-like crystalline structures. This ratio in bone ash is not altered by dietary inputs. Ratios of Ca:P expressed as a percentage of bone tissue, not bone ash, will reflect a wider range that varies based on the variable components of fat and protein matrix in the bone tissue. Soft tissue contains more P and limited amounts of Ca. A Ca:P ratio calculated for the whole animal is typically 1.7:1, depending on the animal's lean-to-fat composition.

Animal growth is affected by even marginal deficiencies of P, earlier than any detrimental effect of low Ca or vitamin D. Body weight gain is also more susceptible to deficiencies in P than deficiencies in Ca, even though deficiencies of both nutrients will alter skeletal growth.

Although used primarily in skeletal growth, Ca and P play important metabolic roles in the body and are essential for all stages of growth, gestation, and lactation. The NRC estimates requirements of 0.66% Ca and 0.56% total P for growing pigs of 25–50 kg body weight. Requirements are higher for younger pigs and lower for finishing pigs; however, the optimum dietary Ca:P ratios are assumed to be the same for all weight groups, especially for corn-soybean meal diets. These levels are adequate for maximal growth (rate and efficiency of gain), but they do not allow for maximal bone mineralization. Generally, maximal bone ash and strength can be achieved by including 0.1–0.15% additional dietary Ca and P.

For gestating and lactating sows, Ca and P requirements are influenced by stage of gestation (the first 90 days versus the final 25 days of gestation), parity, milk production, and other factors (see the table Reproductive Measures and Dietary Nutrient Requirements of Gestating and Lactating Sows). The higher requirements for Ca and P during late gestation are attributed to rapid development of fetuses. Swine producers may choose to feed slightly higher levels of Ca and P to sows to ensure adequacy of these minerals and to prevent posterior paralysis in heavy milking sows. The Ca and P requirements listed are based on daily feed intakes of 2.1–2.6 kg (4.7–5.7 pounds) during gestation and 5.9–6.6 kg (13.1–14.6 pounds) during lactation (these amounts include 5% wastage). If less feed is consumed per day, the percentages of Ca and P may need to be adjusted upward.

The ratio of total calcium:total phosphorus should be kept between 1.25:1 and 1:1 for maximal utilization of both minerals. A wide Ca:P ratio decreases P absorption, especially if the diet is marginal in P. The ratio is less critical if the diet contains excess P. When based on digestible phosphorus, the ideal ratio of calcium:digestible phosphorus is between 2:1 and 2.5:1.

Most P in cereal grains and oilseed meals is in the form of phytic acid (organically bound phosphorus) and is poorly available to pigs, whereas the P in protein sources of animal origin (eg, meat meal, meat and bone meal, and fish meal) is in an inorganic form and is highly available to pigs. Even in cereal grains, availability of P varies. For example, the P in corn is only 10–20% available, whereas the P in wheat is 50% available. Therefore, swine diets should be formulated on an “available phosphorus” basis to ensure that the P requirement is met. The NRC publication expresses the digestible phosphorus requirements as apparent total tract digestible (ATTD) and standardized total tract digestible (STTD) phosphorus. ATTD phosphorus represents the P digested, and STTD phosphorus is the digestible P corrected for endogenous P excretions. Neither of the digestible methods consider the variable excretion of P by the kidneys.

Phosphorus supplements such as monocalcium or dicalcium phosphate, defluorinated phosphate, and steamed bone meal are excellent sources of highly available P. These supplements are also good sources of Ca. Ground limestone is another excellent source of Ca.

Phosphorus is considered a potential environmental pollutant, so many swine producers feed diets with less excess P than in the past to decrease P excretion. Supplemental phytase, an enzyme that degrades some of the phytic acid in feedstuffs, is commonly added to diets to further decrease P excretion. The general recommendation is that dietary Ca and P can both be decreased by 0.05–0.1% when ≥ 500 units of phytase per kg of diet are included.

Major changes that have altered diet recommendations for Ca, P, and vitamin D involve inclusion of phytase and a shift to P and Ca digestibility as the basis for establishing feed ingredient inclusions. Along with these changes in approach to meet the nutrient needs of Ca, P, and vitamin D, novel insights into the physiological regulation of these nutrients have emerged. These insights include the role of skeletal tissue as an endocrine gland to regulate renal P homeostasis and vitamin D metabolism, and the identification of cellular signals that act as local paracrine signals to regulate endochondral ossification.

Continuous improvements in the stability and actions of phytase have hampered definitive recommendations for Ca and P supplements in diets. The inclusion of exogenous microbial phytase as a feed additive in swine diets increases the bioavailability of phytate P and decreases the need for supplemental inorganic P. The digestibility of P from phytate-rich plant feed ingredients ranges from 10% to 30% due to the lack of a mammalian enzyme to cleave the p-ester bond in inositol phytate found in plants. Effective, feed-grade sources of phytase improve the digestibility of P to levels that may approach 50%.

The adoption of phytase has complicated approaches to formulating swine diets compared to the pre-phytase era when only inorganic sources supplemented mostly plant-based feed ingredients. These complications generated a plethora of research efforts to refine estimates of P availability with estimates of P based on digestibility of P in the plant bases and inorganic feed ingredients. Unfortunately, the estimates of P availability and digestibility values for feed ingredients are dependent on the specific source and amount of phytase added to diets. This variability also complicates efforts based on factorial modeling approaches to establish P requirements because these models are built on assumptions of body composition and accretion of various tissues (bone, muscle visceral, skin, etc) and also on assumptions of the P digestibility in feed ingredients.

The fat-soluble vitamin K is necessary to maintain normal blood clotting. The requirement for vitamin K is low: 0.5 mg/kg of diet. Bacterial synthesis of the vitamin and subsequent absorption, directly or by coprophagy, generally will meet the requirement for pigs. Although rare, hemorrhages have been reported in newborn as well as growing pigs, so supplemental vitamin K is recommended at 2 mg/kg of diet as a preventive measure. Generally, hemorrhaging can be traced back to feeding moldy grain or other ingredients that contain molds.

The active form of vitamin K is menadione. Precursors of menadione can be supplied by plant, microbial, or synthetic sources. Deficiencies of vitamin K cause functional failures of 6 of the proteins involved in the intricate steps of blood coagulation and result in bleeding.

Blood coagulation involves binding of Ca by serum proteins. Vitamin K also has a role in synthesis of a protein (osteocalcin) produced in bone tissues. The functional roles of vitamin K in both coagulation proteins and osteocalcin involves gamma carboxylation reactions to produce the active forms of these proteins. The reduced form of vitamin K provides the energy via oxidation-reduction reactions to add a carboxylate residue to the gamma carbon of glutamate residues in these proteins. The two carboxylate residues bind Ca ions to regulate the roles of these proteins. The understanding of the role of vitamin K in the gamma carboxylation reaction was initially discovered after the isolation of a compound, dicoumarol, from moldy sweet clover hay that was responsible for the inhibition of blood coagulation. Both dicoumarol and a chemical derivative, warfarin, act by inhibiting the reduction of vitamin K. The reduced form of vitamin K is essential for the gamma carboxylation reaction. Dicoumarol and warfarin compounds were initially used as rodenticides. An accidental overdose of these inhibitors can be counteracted by supplemental dosages of vitamin K.

Osteocalcin, a gamma carboxylated protein, is synthesized in the bone matrix. Elevated serum osteocalcin has been associated with increased bone formation. However, the functional role of osteocalcin in bone mineralization has not been established. Osteocalcin has been implicated in feedback pathways within the pancreas to regulate insulin release and thus glucose metabolism.

Iron and copper are involved in many enzyme systems. Both are necessary for the formation of Hgb and, therefore, for prevention of nutritional anemia. Because the amount of iron in milk is very low, suckling pigs should receive supplemental iron, preferably by IM injection of 100–200 mg in the form of iron dextran or gleptoferron during the first 3 days of life (see iron toxicosis in newborn pigs). Giving oral or injectable iron and copper to sows will not increase piglet stores at birth nor will it increase the iron in colostrum and milk sufficiently to prevent anemia in neonatal pigs.

The copper requirement for growing pigs is low (3–6 ppm) but higher for sows. The estimated copper requirement of 5 ppm for sows in the previous NRC publication was increased to 10 ppm for gestation and 20 ppm for lactation in the current edition.

Although manganese is essential for normal reproduction and growth, the quantitative dietary requirement is not well defined. Manganese in the diet is adequate for growth at 2–4 ppm, which is less than concentrations supplied by feed ingredients commonly fed. However, sows need a higher level, 25 ppm, during gestation and lactation, during which supplemental sources of Mn must be added to diets for sows.

For sows, the estimated zinc requirement was increased from 50 ppm in the previous NRC publication to 100 ppm in the current edition.

Zinc, copper, and manganese share common functions as cofactors for superoxide dismutase, which act as scavengers of free radicals. Zinc and copper superoxide dismutase is located within the cytosol, while manganese superoxide dismutase is located within the mitochondria. Additional functional roles of these minerals involve roles as cofactors for enzymes involved in the synthesis of compounds of the extracellular matrix.

The selenium content of soils and, ultimately uptake by crops is quite variable. In the US, areas west of the Mississippi River generally contain higher amounts of selenium, whereas areas east of the river tend to yield crops deficient in selenium. Under most practical conditions, 0.2–0.3 ppm of added selenium should meet the requirements. This trace mineral is regulated by the FDA, and the maximal amount of selenium that can be added to swine diets is 0.3 ppm. The biological role of selenium involves replacement of sulfur in cysteine to form selenocysteine, which is required for synthesis of selenoenzymes such as glutathione peroxidase, thioredoxin reductase, and other enzymes that contain selenium. These enzymes act to reduce free radicals and act as catalysts for oxidation-reduction reactions.

Chromium, a trace mineral and cofactor with insulin, is required by pigs, but the quantitative requirement has not been established. Chromium at a supplemental level of 200 mcg/kg (ppb) may improve carcass leanness in finishing pigs and may improve reproductive performance in gestating sows.

Cobalt is present in the vitamin B₁₂ molecule and has no benefit when added to swine diets in the elemental form.

Vitamin A is a fat-soluble vitamin essential for vision, reproduction, growth and maintenance of epithelial tissue, and mucous secretions. Vitamin A is found as carotenoid precursors in green plant material and yellow corn. Beta carotene is the most active form of the various carotenes. Unfortunately, only approximately one-fourth of the total carotene in yellow corn is in the form of beta carotene. The NRC suggests that for pigs, 1 mg of chemically determined carotene in corn or a corn-soybean mixture is equal to 267 IU of vitamin A.

Stabilized vitamin A is commonly used in manufactured feeds and in vitamin supplements or premixes. Concentrates containing natural vitamin A (fish oils, most often) may be used to fortify diets. Green forage, dehydrated alfalfa meal, and high-quality legume hays are also good sources of beta carotene. Both natural vitamin A and beta carotene are easily destroyed by air, light, high temperatures, rancid fats, organic acids, and certain mineral elements. For these reasons, natural feedstuffs probably should not be entirely relied on as sources of vitamin A, especially because synthetic vitamin A is very inexpensive. An international unit of vitamin A is equivalent to 0.30 mcg of retinol or 0.344 mcg of retinyl acetate.

Vitamin D, a fat-soluble vitamin, is necessary for proper bone growth and ossification and is required to prevent the development of rickets in growing pigs. Vitamin D occurs as the precursor sterols ergocalciferol (vitamin D₂) and cholecalciferol (vitamin D₃), which are converted to active vitamin D by UV radiation. Although pigs can use vitamin D₂ (irradiated plant sterol) or vitamin D₃ (irradiated animal sterol), they seem to preferentially use D₃. Some of the vitamin D requirement can be met by exposing pigs to direct sunlight for a short period each day. Sources of vitamin D include irradiated yeast, sun-cured hays, activated plant or animal sterols, fish oils, and vitamin premixes. For this vitamin, 1 IU is equivalent to 0.025 mg of cholecalciferol. The estimated vitamin D requirement of 200 IU/kg for gestating and lactating sows was increased to 800 IU/kg in the 2012 NRC publication.

Vitamin E, which is fat-soluble, serves as a natural antioxidant in feedstuffs. There are 8 naturally occurring forms of vitamin E, but d-alpha-tocopherol has the greatest biological activity. Vitamin E is required by pigs of all ages and is closely interrelated with selenium. The vitamin E requirement is 11–16 IU/kg of diet for growing pigs and 44 IU/kg for sows. Some nutritionists recommend higher dietary levels for sows in the eastern corn belt of the US, where selenium levels in feeds are likely to be low. Vitamin E supplementation can only partially address a selenium deficiency.

Green forage, legume hays and meals, cereal grains, and especially the germ of cereal grains contain appreciable amounts of vitamin E. Vitamin E activity is decreased in feedstuffs when exposed to heat, high-moisture conditions, rancid fat, organic acids, and high levels of certain trace elements. One IU of vitamin E activity is equivalent to 0.67 mg of d-alpha-tocopherol or 1 mg of dl-alpha-tocopherol acetate.

Riboflavin is a water-soluble constituent of two important enzyme systems involved with carbohydrate, protein, and fat metabolism. Swine diets are typically deficient in riboflavin, and the crystalline form is included in premixes. Natural sources include green forage, milk by-products, brewer’s yeast, legume meals, and some fermentation and distillery by-products.

Niacin is a component of coenzymes involved with metabolism of carbohydrates, fats, and protein. Pigs can convert excess tryptophan to niacin; however, conversion is inefficient. The niacin in most cereal grains is completely unavailable to pigs. Swine diets are typically deficient in niacin, and the crystalline form is included in premixes. Natural sources of niacin include fish and animal by-products, brewer’s yeast, and distillers solubles. The NRC niacin requirement is 30 ppm during all phases of growth.

Pantothenic acid is a component of coenzyme A, an important enzyme in energy metabolism. Swine diets are deficient in pantothenic acid, and the crystalline salt d-calcium pantothenate is included in vitamin premixes. Natural sources of pantothenic acid include green forage, legume meals, milk products, brewer’s yeast, fish solubles, and certain other by-products.

Grains and other feed ingredients supply ample amounts of thiamine to meet the requirement in pigs; no further supplementation is required.

A group of compounds called pyridoxines have vitamin B₆ activity and are important in amino acid metabolism. They are present in plentiful quantities in the natural feed ingredients usually fed to pigs. The requirement for vitamin B₆ in young pigs (5–25 kg) was increased by 3- to 4-fold in the current NRC publication compared with the previous edition.

Pigs can synthesize some choline from dietary methionine. Sufficient choline is found in natural dietary ingredients to meet the requirements of growing pigs. Natural sources of choline include fish solubles, fish meal, soybean meal, liver meal, brewer’s yeast, and meat meal. Choline chloride, which is 75% choline, is the common form of supplemental choline used in feeds. If choline is added as a supplement to sow diets, it should not be combined with other vitamins in a premix, especially if trace minerals are present, because choline chloride is hygroscopic and destroys some of the activity of vitamin A and other less stable vitamins.

Biotin is present in a highly available form in corn and soybean meal; however, the biotin in grain sorghum, oats, barley, and wheat is less available to pigs. When cereal grains are fed to swine, especially breeding animals, biotin may be marginal or deficient. Improvements in reproductive performance in sows has been observed with biotin additions. In some instances, biotin supplementation decreased footpad lesions in adult pigs. For insurance, biotin supplementation is recommended, especially for sow diets.

Folacin refers to a group of compounds with folic acid activity. Sufficient folacin is present in natural feedstuffs to meet the requirement for growth; however, supplemental sources of folic acid are required to meet requirements of sows because of the critical roles of folate in embryonic development.

Pigs are thought to synthesize vitamin C at a rapid enough rate to meet their needs under normal conditions. Dietary supplements of vitamin C are not required.