Basal endogenous phosphorus losses in pigs are affected by both body weight and feeding level

Two similar experiments were conducted to determine the separate effects of feeding level (FL) and BW on basal endogenous phosphorous losses (EPL) from the digestive tract and minimal urinary P content in growing-finishing (GF) pigs and sows. After an adaptation period, 16 GF pigs (initial BW 85 kg) and 16 gestating sows (initial BW 201 kg), both individually housed and divided in 2 groups of 8, received the same semipurified low-P diet at a feeding level (FL) of 2 or 3 kg/d during a 22-d period. Grab samples of feces and spot samples of urine were collected 2 times per day on d 6-8, 13-15, and 20-22 of feeding the low-P diet in GF pigs and on d 20-22 in sows. Phosphorus content was determined and excretion in the feces (in g/d, g/kg DMI, and g/(kg BW.d)) was calculated using titanium dioxide as indigestible marker. In GF pigs, the fecal P excretion (g/d) decreased over time from the first to the third collection period (P < 0.001). Fecal P excretion (g/d) increased with increasing FL in both GF pigs (P < 0.001) and sows (P = 0.003). In sows, the fecal P excretion was approximately 2 times higher compared to GF pigs in the third collection period, 499 versus 237 mg/kg DMI. The differences between sows and GF pigs were much smaller when P excretion was expressed per kilogram BW, 5.6 and 4.6 mg/(kg BW.d), respectively. The urinary P concentration was below the detection limit in nearly all samples of GF pigs and sows. The results of this study indicate that EPL in pigs increase with increasing FL, and BW and EPL in gestating sows exceed these in GF pigs. Inevitable urinary P losses seem extremely low, both in sows and GF pigs.