Calcium and phosphorus are essential in skeletal structure development and maintenance, lean tissue deposition, muscle contraction, and many other physiological functions. Calcium and phosphorus are the most abundant minerals in the pig and about 99% of the calcium and 80% of the phosphorus in the body are found in the skeleton. Consequently, deficiency of calcium and phosphorus results in impaired bone mineralization, reduced bone strength, and decreased growth. Clinical signs of deficiency include rickets in growing pigs and osteoporosis in sows, which are manifested as lameness and fractures. The deficiency is exacerbated during lactation as sows mobilize calcium from bone reserves to meet the demand for milk production, which can result in ‘downer sows’ in late lactation and post-weaning.

To prevent deficiency, swine diets must supply the individual requirements of calcium and phosphorus but also to provide an adequate ratio of one mineral to the other. The calcium:phosphorus ratio greatly influences the absorption and retention of both minerals. In general, wide calcium:phosphorus ratios or excessive calcium and deficient phosphorus concentrations interfere with phosphorus absorption (Reinhardt and Mahan, 1986).

Grains and oilseeds used in swine diets are typically low in calcium and have most of the phosphorus unavailable to the pig. Phytate is the storage form of phosphorus in feedstuffs of plant origin and the enzyme phytase is required to release phosphorus from phytate for absorption (Cowieson et al., 2016). As endogenous phytase activity is negligible in swine, exogenous microbial phytase is commonly used in swine diets to enhance phosphorus release from phytate. Phytase also releases calcium that can be bound to phytate. Furthermore, both calcium and phosphorus are supplemented in the diet by inorganic sources. Importantly, many inorganic sources supply both calcium and phosphorus, which require simultaneous adjustments of the amount of each source in the diet.

Calcium and phosphorus digestibility

The availability of calcium and phosphorus refers to the amount of minerals that are digested, absorbed, and available for use in a suitable form. Availability of calcium and phosphorus is traditionally estimated by the slope-ratio method, in which a standard source of calcium or phosphorus is given a value of 100% and the availability of calcium or phosphorus in an ingredient is estimated as a relative percentage to the standard source. Although the slope-ratio method seems to be more accurate to estimate availability of calcium and phosphorus, there is a concern about assuming 100% availability in the standard source. Alternatively, the availability of calcium and phosphorus is estimated by digestibility as it provides a fair estimate of the amount of minerals that are absorbed and available to the pig.

The digestibility of calcium and phosphorus is expressed as total tract digestibility or ileal digestibility:

• Total tract digestibility: measured by the difference between the amount of calcium or phosphorus ingested and the amount recovered from the feces.
• Ileal digestibility: measured by the difference between the amount of calcium or phosphorus ingested and the amount recovered from the digesta in the ileum.

The total tract digestibility is the most often used method because there is no net absorption or secretion of calcium or phosphorus in the large intestine. The total tract digestibility is expressed as apparent (ATTD), standardized (STTD), or true (TTTD) total tract digestibility, depending on how endogenous losses are considered in the measure of digestibility:

Endogenous losses: represent calcium and phosphorus synthesized for metabolic functions by the pig, such as digestive secretions and sloughed cells, that have not been absorbed in the digestive tract and are lost. Endogenous losses are divided into basal or specific losses. Basal losses represent the amount of minerals that are inevitably lost, whereas specific losses represent the losses of minerals abovethe basal losses that are influenced by specific diet composition. Endogenous losses underestimate the digestibility of calcium and phosphorus because the endogenous minerals in the feces are being accounted for as non-digested minerals from the diet.

• Apparent total tract digestibility (ATTD): measured by the difference between the amount of calcium or phosphorus ingested and the amount recovered from the feces without considering the endogenous amino acid losses.
• Standardized total tract digestibility (STTD): measured by the difference between the amount of calcium or phosphorus ingested, the amount recovered from the feces, and the basal endogenous losses.
• True total tract digestibility (TTTD): measured by the difference between the amount of calcium or phosphorus ingested, the amount recovered from the feces, and both basal and specific endogenous losses.

The values for STTD of ingredients are more likely to be additive in diets and some of the disadvantages and limitations of ATTD and TTTD are overcome: whereas the ATTD does not consider any of the endogenous losses, there is insufficient information available about TTTD and the ingredient-specific effects on endogenous losses.

The most commonly used method to formulate diets and estimate phosphorus digestibility is STTD. Calcium is most commonly expressed as total calcium, but efforts have been applied to determine the values of STTD calcium of ingredients and basal endogenous calcium losses.

Sodium and chloride are involved in nutrient absorption, electrolyte balance, and regulation of pH. Salt or sodium chloride is the most common source of sodium and chloride, composed of approximately 40% sodium and 60% chloride. The supplementation of diets with salt is essential because sodium and chloride are low in grains and oilseeds used in swine diets. However, dietary supplementation of salt is usually reduced in diets with spray-dried blood products, dried whey, or co-products from the food or pet food industry due to the high concentration of sodium and chloride in these ingredients.

The requirements for sodium and chloride are greater for nursery pigs and abruptly decrease for grow-finish pigs and sows (NRC, 2012; Shawk et al., 2018a,b). Diets deficient in salt result in decreased growth performance because of reduced feed intake and poor feed efficiency. Diets with high levels of salt are generally well tolerated if drinking water is available. However, toxicity quickly develops if drinking water is not available.

Grains and oilseeds used in swine diets typically provide magnesium and potassium in sufficient quantities to meet the requirements. Therefore, no other sources of magnesium and potassium are commonly used in swine diets.