The course of phosphorus excretion in growing pigs fed continuously increasing phosphorus concentrations after a phosphorus depletion

A balance study was performed in order to quantify the effect of continuous ly increased phosphorus (P) intake on faecal and urinary P excretion. The a im was to quantify the level of intake where regulatory P excretion becomes relevant for comparative digestibility measurements on P, and when the pig adapts its urinary P excretion to increased P intake. Phosphorus intake of growing pips was continuously increased on a daily basis starting at a mar ginal level and P excretion via faeces and urine was continuously followed for 92 days. Two semi-synthetic diets were prepared with different proporti ons of Na2HPO4 resulting in 2.4 (diet 1) and 6.3 (diet 2) g P/kg DM. Concen tration of Ca was adapted to achieve a Ca supply of similar to 3.1 fold the digestible P supply. Six castrated male crossbred pigs (31 kg BW) were kep t individually in metabolism crates after they had undergone a 14 d P deple tion period during which they were fed diet 1 solely. Pigs received 1.04 kg of diet 1 per day throughout the experiment, and each day the amount of fe ed and P supplied to pips from diet 2 was increased by 12 g and 69 mg, resp ectively. ME supply was similar to 2.4 fold maintenance and average daily B W gain of pigs during the entire experiment was 690 +/- 30 g. While intake increased linearly, faecal excretion of P and Ca increased non -linearly and could be best described by third order polynomial functions. The proportion of ingested P not excreted via faeces followed a quadratic t ype of curve with a maximum of 81% at 25 days on experiment and P intake of 4.0 g/d. Thereafter, the proportion decreased continuously. The digestibil ity of P from diet 2, determined by the slope ratio technique, was constant and not affected by P intake up to a P intake of 5 g/d. Renal P excretion did not exceed inevitable losses until day 60 and increased exponentially t hereafter when body P reserves were restored. It is concluded, that an adap tation to surplus P supply occurred earlier on the intestinal than on the r enal level. While faecal P excretion appeared regulated depending on the ac tual requirement for P retention, the regulation via urine depended on the P status of the pig. Once the renal P excretion of growing pigs exceeds a l evel of 25 mg/d, intake of digestible P cannot be regarded sufficiently low to measure P digestibility as a capacity of the feedstuff.