The energy and nutrients (amino acids, minerals and vitamins) present in feedstuffs are not entirely available for use by pigs. Generally only a portion can be used for maintenance, growth, or reproduction. This is because feedstuffs are not completely digested and nutrients occur in forms that pigs are not able to use. The portion that is absorbed in a form suitable for use is termed bioavailable. High nutrient bioavailability results in more predictable animal performance and a smaller proportion of the diet lost in the manure.

A simple way to illustrate the concept of bioavailability is to examine the fate of energy in corn consumed by pigs.

Fig. 13 Calculation of apparent energy digestibility.

Energy estimates are obtained by measuring the amount of heat released from burning a known quantity of an ingredient or feces in a bomb calorimeter.

Fig. 15 Digestibility coefficients for selected amino acids in corn, sorghum, and skim milk.

Gross energy is the total amount of energy present in an ingredient, in this case corn. The proportion of the gross energy that is digestible by an animal is calculated from the equation in Figure 13. According to Figure 14, pigs digest about 83% of the gross energy in corn and sorghum. The same procedure is used to calculate amino acid and mineral digestibility in corn and other feedstuffs.

Fig. 14 Apparent digestible energy as a percent of gross energy in corn and sorghum for pigs.

The digestibility coefficients for four key amino acids in corn and sorghum is compared to that of skim milk (a very high quality protein source) in Figure 15. The digestibility of lysine, tryptophan and threonine in corn and sorghum is 15 to 25 percentage units lower than that in skim milk. Methionine digestibility in corn and sorghum is more similar to that in skim milk.

Fig. 16 The phytate molecule after it has bound calcium, zinc, iron, and magnesium.

The poor bioavailability of phosphorus in corn (14%) is also a concern to pork producers. Approximately 66 % of the phosphorus in corn is contained within the molecule phytate (Figure 16). The only way the phosphorus in phytate can be liberated and utilized by pigs is for the enzyme, phytase, to cleave the phosphorus groups from the molecule. Pigs lack sufficient qualities of phytase; therefore, most of the phosphorus in corn is not available to pigs. The digestibility of calcium, zinc, iron, and magnesium is thought to be reduced by phytate also.