A simple indicator for ruling out inadequate phosphorus digestibility in growing pigs

Abstract

Species differences in calcium and phosphorus metabolism can be expressed via dietary and faecal calcium/phosphorus ratios. The aim of the present meta-analysis was to use faecal Ca/P ratios as an indicator of phosphorus digestibility in growing pigs as a simple diagnostic tool. This tool can be useful in cases of suspected phosphorus deficiency but adequate dietary calcium and phosphorus contents. Studies (n = 34) with phytase-supplemented (PHYT; n = 110) and non-supplemented control (CON; n = 106) diets were analysed for dietary intake, faecal excretion and apparent digestibility of calcium and phosphorus. A modified Lucas-test was used for both minerals plotting intake against faecal excretion (mg/kg body weight0.75). For calcium, there was no significant difference between PHYT and CON (p = 0.29) while in accordance with literature, the relative faecal phosphorus excretion was significantly lower in PHYT than CON (p < 0.01). Faecal calcium/phosphorus ratios were calculated and grouped according to the animals' body weight and apparent phosphorus digestibility (body weight ≤ 30 kg: apparent phosphorus digestibility ≤ 60 % and > 60 %; body weight > 30 kg: apparent phosphorus digestibility ≤ 40 % and > 40 %). Data distribution as displayed in a box plot shows that - given a dietary Ca/P ratio of > 1.2 - faecal Ca/P ratios of > 1.5 in pigs with a body weight of ≤ 30 kg and of > 1.2 in pigs with a body weight of > 30 kg indicate a high apparent digestibility of phosphorus of > 60 % and > 40 %, respectively. When faecal samples reveal a Ca/P ratio above the indicated thresholds, a low phosphorus digestibility is unlikely. No conclusion regarding the apparent phosphorus digestibility can be drawn from faecal Ca/P ratios below this threshold.