Quantitative relationships between standardized total tract digestible phosphorus and calcium intake and its retention and excretion in growing pigs fed corn-soybean meal diets

Abstract Text:

An experiment was conducted to determine the quantitative relationships between standardized total tract digestible P (STTD P) and Ca intake with their retention and excretion by growing pigs fed corn-soybean meal diets. Forty-eight barrows (BW = 22.7 ± 0.2 kg) were allotted to 1 of 8 diets, housed individually in pens for 3 wk, then moved to metabolism crates and allowed 4 d adaptation and 5 d for collection of urine and fecal samples. Eight corn-soybean meal diets were formulated for similar NE, fat, and AA concentrations, but with increasing STTD P from 0.16% to 0.62% using monocalcium phosphate. The Ca:STTD P ratios were maintained constant among treatments. The STTD P intake increased (P<0.001) from 64% to 242% of the daily requirement (4.59 g/d of STTD P). Basal urinary P excretion of 0.03 g P/d was observed up to 4.96 g of STTD P intake/d, after which the urinary P excretion increased (P<0.001). Excretion of Ca in urine decreased (P<0.001) with intake, reaching a basal urinary excretion of 0.40 g/d at 17.97 g/d of Ca intake. The daily intake of STTD P and Ca moderately explained the variation in urinary excretion of P (R²=0.41) and Ca (R²=0.64). The absorption and retention of P increased linearly (P<0.001) with dietary P intake, whereas absorption and retention of Ca increased quadratically (P<0.001). Absorption and retention of P and Ca were highly predictable from the STTD P and Ca intake, with R² of 0.87 and 0.90, respectively. The femur mineral content (FMC, g) increased by 2.71 g of FMC per g/d of STTD P intake, but reached a plateau (29.54 g) at 8.84 g/d of STTD P intake. The FMC was highly predictable from the STTD P intake (R²=0.89). The FMC affected the urinary P excretion (P<0.01), but moderately explained (R²=0.19) the variation in urinary P. In conclusion, constant excretion of P in urine was observed, but increased linearly at STTD P intake levels above the requirement for maximum growth. The FMC increased with STTD P intake, but reached a plateau at a greater STTD P intake level than the requirement for maximum growth. Dietary STTD P was therefore absorbed and used for growth, but excess P was accumulated in bones, until a plateau was reached, and excreted in urine. The predictability of P and Ca excretion in urine from the dietary STTD P and Ca intake was moderate.

Keywords: phosphorus, calcium, urine