Long term differentiated phosphorus supply from below to above requirement affects nutrient balance and retention, body weight gain and bone growth in growing-finishing pigs

Dietary phosphorus (P) supply has attained increased interest in farm animal production during the last decades due to environmental concern and to limited global phosphate resources. The objective was to study the long-term effect of low (LP; 4.1 g/kg dry matter (DM)), medium (MP; 6.2 g/kg DM) or high dietary P (HP; 8.9 g/kg DM) on the absorption, retention and utilisation of P, calcium (Ca), and protein [N (nitrogen) x 6.25] during the growing-finishing phase (40-100 kg BW) in addition to daily weight gain, insulin-like growth factor 1 (IGF-1) concentration in serum, and bone length and weight at slaughter. All pigs were subjected to three balances and most traits (daily feed intake, P and Ca but not N balances) were significantly affected by the interaction between diet and balance (P < 0.001). Low dietary P caused reduced feed intake compared to the MP diet. The Ca, P and N absorption, excretion and retention were lower in pigs fed the LP diet compared to pigs fed the MP and HP diets. Oppositely, HP caused increased retention and excretion of P and Ca compared to the MP fed pigs. The absorptive capacity of P, Ca and N increased with age whereas the apparent total tract digestibility (ATTD) of P, Ca and N remained constant irrespective of P supply. The length and wet weight of humerus and femur were not affected by dietary P supply, though the defatted dry bone weight was significantly reduced in the LP fed pigs, but similar in the MP and HP fed pigs (P < 0.01). A dietary ratio of 1.4:1 may be tentatively suggested as a proper dietary Ca:P based on both digestible P and Ca and not total P and Ca. The ratio between P:N (retained) of 1:4 along with Ca:N (retained) of 1:3 may also be valuable indicators of sufficient daily P supply in pigs fed adequate protein and amino acids. In conclusion, a daily supply of about 4.6 and 6.7 g digestible P seems to be sufficient to fulfil the physiological need for P to sustain a growth rate of 800-1100 g/day in lean young and growing-finishing pigs, respectively. Deficient as well as excessive P supply resulted in low P utilisation and a relatively higher total P excretion compared to the sufficient dietary P supply. The relative distribution of P excreted through the urine or the faeces depended on the dietary P content.