Assigning phosphorus values
Phosphorus is an inorganic element important for development and maintenance of the skeletal system of swine. Preciseness on assigning the phosphorus value of a feed ingredient is important because supplemental sources of phosphorus are expensive and excess phosphorus increases its excretion in swine waste which imposes a negative effect on the environment.
Phosphorus can be expressed as total, digestible, or available. Available phosphorus represents the amount of phosphorus that is digested, absorbed, and available for utilization according to the slope-ratio method. Total phosphorus represents all phosphorus contained in the ingredient, including non-available phosphorus that is mostly bound to phytate. Digestible phosphorus represents the amount of phosphorus that is digested and absorbed, which is expressed as apparent (ATTD) or standardized (STTD) total tract digestible phosphorus. The basal endogenous losses of phosphorus are accounted for on STTD basis, but not on ATTD basis. The most commonly used method to formulate diets and estimate phosphorus digestibility is STTD.
Similar to the approach used for amino acids, there are two steps to practically assign STTD phosphorus values to feed ingredients (Gonçalves et al., 2017):
- Phosphorus analysis: The first step is to submit the feed ingredient to analysis of total phosphorus.
- Digestibility value: The second step is to assign STTD value to the phosphorus level in the feed ingredient. The most common approach is to use digestibility values from nutritional tables (NRC, INRA, or the Brazilian Tables), scientific literature, or university databases. Ideally, the digestibility of the same ingredient is used, but ingredients with similar characteristics can also serve as a reference for digestibility. In this case, it is important to pay attention to processing method, amount of phytate, and level of naturally occurring phytase, as these factors influence phosphorus digestibility in feed ingredients. EvaPig® accounts for processing methods and naturally occurring phytase on phosphorus digestibility of feed ingredients, while the NRC does not.