This review summarizes the evolution of methods to measure the availability of P to formulate swine and poultry diet more accurately. Moreover, the knowledge about P availability may help to optimize feed efficiency, economic results, and reduce phosphorus excretion by animals.
In vivo digestibility trials or bioavailability trials ?
P nutritional value in feed ingredients can be determined using in vivo digestibility trials or bioavailability trials. This latter was traditionally determined by measuring bone ash, bone P, bone strength or growth differences among animals fed graded levels of different test P sources. In general, P digestibility or bioavailability values are expressed as a Relative Biological Value (RBV), that means comparing values with a standard phosphate source highly available (purified grade) (Nelson et al., 1990; Cromwell et al., 1972). Indeed, the RBV of a standard P source is normally equal to 100%. The digestibility or bioavailability of test P is estimated as a relative % to the standard P source using the slope ratio method (Petersen et al., 2011).
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What is the difference between the ileal digestibility and the total GI tract digestibility ?
Because P is mostly absorbed in the small intestine, there is no significant differences in available P measured at the ileum and over the entire digestive tract (Ajakaiye et al., 2003; Bohlke et al., 2005; Zhang et al., 2016). However, it is much easier to measure total GI tract digestibility than ileal digestibility of nutrients in swine.
On the contrary, it is not easy to measure total tract digestibility of nutrients in poultry because feces and urine are excreted together. Therefore, in recent years ileal digestibility of P is the recommended method for estimating availability of P in feed ingredients fed to poultry, to exclude urinary excretion in the excreta (Rodehutscord et al., 2012; Shastak and Rodehutscord, 2013; WPSA, 2013).
The ileal or total GI tract digestibilities can be expressed in apparent or standardized digestibilities.
What is the difference between the apparent digestibility and the standardized digestibility ?
The total collection procedure and the indicator procedure are the most commonly used approaches to determine the apparent total tract digestibility (ATTD) of P or the apparent ileal digestibility (AID) of P, depending on the species concerned (swine or poultry). If the total collection procedure is used, then the ATTD or AID (%) of P in each diet can be calculated according to the following equation (Almeida et Stein, 2010):
- For pigs: ATTD = [(Pintake – Pfeces)/Pintake] x 100
- For poultry: AID = [(Pintake – Pileal digesta)/Pintake] x 100
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If the indicator procedure is used, then the ATTD or AID (%) of P in each diet can be calculated according to the following equation (Wu et al, 2008):
- For pigs: ATTD = 1 − [(Mdiet/Mfeces) × (Pfeces/Pdiet)],
- For poultry: AID = 1 − [(Mdiet/Mileal digesta) × (Pileal digesta/Pdiet)], where Mdiet, Mfeces and Mileal digesta are the concentrations of the marker used in the diet, feces and ileal digesta, respectively.
Some endogenous P is secreted in the GI tract, some of which may escape re-absorption, leading to underestimation of P digestibility if apparent digestibility assay is used to estimate P digestibility. Also, apparent digestibility values of P for feedstuffs are dependent on the level of P in the feedstuffs or diet (Johnston et al., 2013), and are not additive (She et al., 2017). Hence, it is necessary to correct for the endogenous loss of P (EPL) while estimating P digestibility values of feedstuffs for formulating feeds for non-ruminants. The endogenous P excretion can be divided into basal EPL and total EPL (Fan et al., 2001). Basal EPL represent the minimum losses of P from the animal’s body, whereas total EPL represent both basal EPL and EPL from the diet.
It is not easy to measure total EPL and the values vary among studies (NRC, 2012). However, it is easy to measure basal EPL (She et al., 2017). Basal EPL values do not significantly vary among studies (NRC, 2012). Thus, the standardized digestibility can be calculated from the apparent digestibility by taking basal EPL into consideration and is recommended for estimating P digestibility values of feedstuffs for non-ruminants (NRC, 2012).
The standardized digestibility of P values at ileal (SID for broiler) or total tract level (STTD for pigs) can be estimated by regression method or by direct method also called the “P-free diet method” (She et al., 2017).
What is the difference between the regression method and the direct method ?
The direct method
The direct method involves calculating standardized digestibility of P values by correcting apparent digestibility values for basal EPL values from animals fed P-free diet. In other words, this method only requires a P-free diet and a diet where a test P source is the sole source.
- Basal EPL (mg/kg of DMI) = [(Pfeces or ileal digesta/Feed intake) x 1000 x 1000]
- For pigs: STTD = [(Pintake-(Pfeces- Basal EPL)/Pintake)] x 100
- For poultry: SID = [(Pintake-(Pileal digesta – Basal EPL)/Pintake)] x 100
If the indicator procedure was used to calculate ATTD or AID, then, the SID or STTD can be calculated as follows:
- Basal EPL = Pileal digesta or feces ⨯ (Mdiet/M ileal digesta or feces)
- For pigs: STTD = ATTD + Basal EPL/Pdiet
- For poultry: SID = AID + Basal EPL/Pdiet
The regression method
The WPSA (2013) method involves a regression-based approach for determining ileal P. The regression method measures the linear relationship between fecal P excretion and the dietary intake of total P. Endogenous loss is determined from the intercept of the linear regression equation. The slope of the linear regression equation represents total P digestibility. Using the regression procedure, the following equations are used:
- For poultry: Pileal digesta = (SID × Pintake) – Basal EPL
- For pigs: Pfeces = (STTD × Pintake) – Basal EPL
What is the best method ?
Each method has limitations and disadvantages. In the direct method, it is necessary to formulate a P-free diet to determine EPL for the subsequent correction of the apparent value to the true value. In the regression method, EPL is determined from the intercept of the linear regression equation (Anwar et al., 2018), but this method is more costly and laborious, owing to the need to formulate and produce at least 3 diets to evaluate a single ingredient.
A few published studies have compared the application of the different methods to determine the digestibility of P. In swine, differences were found between the direct and the regression methods, which may be associated with the tested ingredient (Mutucumarana et Ravindran, 2016). In poultry, it appears that there were no significative differences between both methods. Finally, more research is needed to continue evaluate and compare the effectiveness of both methods in non-ruminants’ species.
Each method has limitations and disadvantages. In the direct method, it is necessary to formulate a P-free diet to determine EPL for the subsequent correction of the apparent value to the true value. In the regression method, EPL is determined from the intercept of the linear regression equation (Anwar et al., 2018), but this method is more costly and laborious, owing to the need to formulate and produce at least 3 diets to evaluate a single ingredient.
A few published studies have compared the application of the different methods to determine the digestibility of P. In swine, differences were found between the direct and the regression methods, which may be associated with the tested ingredient (Mutucumarana et Ravindran, 2016). In poultry, it appears that there were no significative differences between both methods. Finally, more research is needed to continue evaluate and compare the effectiveness of both methods in non-ruminants’ species.
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