Phosphorus does not naturally occur in nature because it is highly reactive and unstable. Instead, P is present in more stable forms such as phosphates and ortophosphates. Feed grade phosphates may be produced by the wet process method, which consists of the reaction of one of those stable forms of P, mainly phosphate rock obtained from either sedimentary or volcanic (igneous) mines, with sulfuric acid, to remove undesirable elements that can be harmful for the animal. The product of this reaction is phosphoric acid, which is used to produce feed phosphates after reaction with other minerals such as Ca, Mg, Na, or ammonium, which results in production of monocalcium phosphate, dicalcium phosphate, tricalcium phosphate, magnesium phosphate, mono-di sodium phosphate, or ammonium phosphate. However, dicalcium phosphate and monocalcium phosphate are the most used sources of feed phosphates in swine diets.
Phosphorus is needed in many essential physiological functions, including development and maintenance of bone tissue. Phosphorus deficient diets fed to pigs results in lower growth performance and reduced bone strength. Requirements for P are affected by body weight, the use of ractopamine and feed allowance. In the case of gestating sows, parity, litter size and day of gestation are the factors that may affect the requirement of P. Requirements for P may be expressed based on the standardized total tract digestibility (STTD) of P, which is believed to result in the most accurate provision of P in diets. The requirements for P for growing pigs from 5 to 135 kg, and for gestating and lactating sows and boars have been reported.
Several studies have been conducted to estimate requirements of P in growing pigs provides requirement estimates for different weight classes of pigs and the different stages of production. The requirement for P for growing and finishing pigs change with age and weight of the animal. As the weight of the pig increases, the requirement expressed as percentage of the diet decreases. But if the requirement is expressed as grams per day, it will increase as pig body weight increases. The estimated requirements of P used in formulation of diets results in maximum growth and efficiency of feed utilization, which is desired in pork production, but it may not always result in maximum bone mineralization because the requirement to maximize bone ash is greater than the requirement to maximize growth performance.
Gestating Sows
The requirement for P is not constant during pregnancy, especially for second parity or older sows. During the first 2 months of gestation, the requirement for P is primarily a result of the need to restore body reserves from the previous lactation. The mineralization of the fetus takes places mainly in the last month of pregnancy, which will increase the requirement for P during this time.
To determine the requirement for P for milk production in lactating sows, it is necessary to know both the quantity of milk produced and the P concentration in the milk. However, the milk yield of a sow is difficult to measure. Values reported in the literature are often results of equations to obtain an approximate value . An accurate estimation of the concentration of P in milk is also difficult to obtain. Jongbloed et al. reported values of 1.2 to 1.7 g of P per kg of milk, but Hughes and Hart reported values between 0.84 and 1.2 g/kg. Other studies have been conducted to estimate the nutritional value of sows milk, but with no values for P concentration. The current requirements by NRC are based on a growth model and takes into account the parity of the sow, the litter size, day of lactation, expected body weight after farrowing, and the expected mean daily weight gain of nursing pigs. These factors along with an estimate of the digestibility of the P in the milk can be used to estimate the requirement for P in 10 the sow to successfully support the growth of the litter with a minimum resorption of P from bones.
FUNCTIONS OF PHOSPHORUS
Bone Mineralization
Metabolic Functions
Deficiency of Phosphorus
PHOSPHORUS IN FEED INGREDIENTS
Plant Ingredients
Corn, one of the most used ingredients in diets for swine, has a concentration of P of approximately 0.23% with a STTD of P of 39% on average. The concentration of P in sorghum is approximately 0.29% and the STTD of P in sorghum is 39% . Wheat, rye, and barley have a greater concentration of P and a greater STTD of P than corn. In contrast, rice and rice co products have a greater concentration of P, but a lower STTD of P, except for broken rice, which has a lower concentration of P, but a greater STTD of P, than corn . However, in rice co-products, more than 65% of P is phytate-bound, which means that it will be unavailable to the animal if there is no exogenous phytase included in the diet.
Phytase. Phytase, also known as myo-inositol hexakisphosphate phosphohydrolase, is the enzyme that catalyses the release of phosphate from IP6. Although, this enzyme is present in some of the plant ingredients used in swine diets such as wheat, rye and, triticale, the amounts present are insufficient to hydrolyze all the IP6 in most diets. For other ingredients such as maize, legume seeds, and oilseed meals, the amount of phytase is low and hardly detectable. Nevertheless, intrinsic phytase can contribute to the breakdown of IP6 in the gastrointestinal tract of animals, assuming that the enzyme was not deactivated or destroyed in the process of manufacturing and processing the feed.
Exogenous microbial phytase has been used since 1971 to increase the availability of phytate-bound nutrients. This practice has been implemented in the industry by including approximately 500 units of phytase per kg, usually from Aspergillus niger or Escherichia coli, to hydrolyze approximately 50% of the phytate P in the diet. However, studies have been conducted where the inclusion of phytase is greater than 500 units per kg, resulting in a greater digestibility of P and in release of more than 60% of phytate bound P.
Since the development of the corn-soybean meal diet, the use of animal origin ingredients in swine diets has been reduced and mainly concentrated in diets for weaning pigs. Plant based diets can deliver the required quantities of nutrients for growing-finishing pigs with the adequate supplementation of vitamins, minerals, and crystalline amino acids.
Feed Phosphates
The most widely used feed phosphates in swine diets are MCP and DCP, but, monosodium phosphate (MSP) and magnesium phosphate (MgP) can be used as well.
Dicalcium Phosphate and Monocalcium Phosphate
Although the process of producing feed grade MCP and DCP is designed to eliminate impurities that may be harmful to animals, such as Al, As, Cd, and Pb other minerals are usually present in feed phosphates. These minerals include ferrous phosphate (FePO4 + 2H2O), magnesium phosphate [Mg(H2PO4)2 + 4H2O], aluminum phosphate (AlPO4), and others.
Monosodium phosphate is produced from the reaction of phosphoric acid and sodium hydroxide or carbonate, and it is the feed phosphate produced in the largest quantity. The concentration of P in MSP is above 25% and the digestibility of P in MSP is greater than the digestibility in other feed phosphates , and MSP is, therefore often considered the ideal standard used in experiments aimed to determining the relative bioavailability of P in feed ingredients.
Magnesium phosphate is also used in animal nutrition, especially in ruminants, because deficiency of Mg is more common in diets for ruminants. Magnesium phosphate is highly soluble and has high digestibility, and the concentration of P is around 19%.
Phosphorus is absorbed in the inorganic form (phosphate) mainly in the duodenum and jejunum. Although results of some studies indicate absorption of P may also take place in the rumen of ruminants and in the cecum of swine. Phosphorus from animal ingredient sources tends to be absorbed in the upper section of the small intestine; whereas, phosphorus from plant ingredient sources is absorbed in the lower section . Absorption of P occurs by active transport that can be saturated and depends on Na, or by facilitated diffusion, which is the quantitatively most important route of absorption.
Relative availability of nutrients is a measure that estimates the absorbed proportion of that nutrient that can be used by the animal. To estimate relative bioavailability a standard substance is used, and a test source of the same nutrient is used to conduct bioavailability assays such as slope ratio, standard curve, mean ratio comparison, or the three-point comparison. To estimate the relative bioavailability of P, MSP is most often used as the standard because of the high digestibility of P in MSP, and ash content in bone, bone strength, or P in serum are evaluated to determine the value of relative bioavailability. However, a bioavailability assay implies the use of several treatments; therefore, more diets have to be formulated and more pigs have to be used. Moreover, to see results in the variable responses such as bone strength, the assays have to be longer than digestibility assays.
Digestibility is defined as the degree of disappearance of a nutrient after going through the gastrointestinal tract. It can be expressed as duodenal, ileal, or total tract digestibility, and may be calculated by subtracting the amount of nutrient excreted by the animal from the amount of nutrient ingested, and then divided by the amount of nutrient ingested. The values obtained from this calculation have to be considered apparent digestibility. Calculation of apparent digestibility assumes that the output of the nutrient comes entirely from the undigested portion of the food that was ingested by the animal. However, there is endogenous losses of most nutrients that originate from the gastrointestinal tract as a result of the natural process of digestion, and these losses may be basal endogenous losses or diet specific endogenous losses. Inclusion of the endogenous losses in the calculations of digestibility is accomplished by subtracting endogenous losses from the intake of the nutrient. This allows for calculation of the standardized or true digestibility of the nutrient.
ATTD of P (%) = [(P ingested-P in feces) / P ingested] × 100
Endogenous P loss (mg/kg dry of matter intake ) = [(P in feces/dry matter intake) × 1,000 × 1,000]
STTD of P (%) = {[P ingested - (P in feces - endogenous P loss)] / P ingested} × 100
CONCLUSIONS
Phosphorus is the most expensive macro mineral in swine nutrition and is needed for bone mineralization and in several metabolic pathways. Feed phosphates are included in most diets because P is deficient in most plant ingredients used to formulate diets for pigs. Requirements for P by pigs in different stages of production have been determined and the digestibility of P in most ingredients used in swine diets has been reported. The use of phytase increases the digestibility of P in plant ingredients, but even with phytase in the diet supplementation with P from feed phosphates is needed in most diets. The main feed phosphates used in swine diets, are MCP, DCP, and MSP. The raw material used to produce feed phosphates may originate from different rock deposits, which may affect the concentration of P and other minerals in feed phosphates. It is also possible that the origin of the rock phosphate influences the digestibility of P, but data to demonstrate this have not been reported. Therefore, more research is needed to determinate if there are significant differences among feed grade phosphates produced from different sources.
