274 Determination of phosphorus requirement of lactating sows based on 24-hour urinary phosphorus excretion

The objective was to determine phosphorus (P) requirement of lactating sows using 24-hour urinary P excretion as the response criteria. The underlying assumption was that urinary P remains low and constant until the requirements are met then increases as P consumption increases. Thirty-six crossbred PIC Camborough sows (parity 3 to 7) were randomly assigned to 1 of 6 corn-soybean-meal diets with increasing dietary total P (tP) levels (0.40, 0.48, 0.56, 0.64, 0.72, and 0.80%) and a constant calcium (Ca) to tP ratio (1.25:1). Diets were fed from breeding until the end of lactation. Urine and blood samples were collected on days 4 and 18 of lactation and analyzed for P and Ca concentrations. Data were analyzed using MIXED and NLIN procedures of SAS. Phosphorus requirements were estimated using a broken-line regression model. Plasma Ca (ranging from 12.1 to 10.3 mg/dL) was not affected by dietary treatments, and was maintained within the normal physiological range on day 4 and 18 of lactation. Plasma P (ranging from 2.9 to 6.4 mg/dL) linearly increased (P < 0.05) with increasing dietary tP levels on day 4 and 18 of lactation. Only sows fed the 0.40% tP diet failed to maintain plasma P concentrations within the normal physiological range. Clinical signs of P and Ca deficiencies were not observed. Differences in sow and litter performance among treatments were not detected. A nonlinear response of urinary P excretion to dietary P intake was observed. Based on a broken-line linear model fit to 24-hour urinary P excretion, minimum tP requirements of sows at day 4 and 18 of lactation were 0.47 and 0.54%, respectively. In conclusion, 24-hour urine P excretion provided sensitive criteria for estimates of tP requirements in lactating sows. Efforts to adjust heteroscedasticity for animals fed dietary P above the estimated requirement need further evaluation.