PSXI-18 - Prediction of fecal and urinary nitrogen excretion in growing pigs based on the ingredient composition

Abstract: The objective was to develop prediction models for estimating fecal and urinary nitrogen (N) excretion from growing pigs based on ingredient composition. Ninety-three observations were collected from 23 studies that determined fecal N excretion (ranging from 3.6 to 11.6 g/d) and urinary N excretion (2.5 to 54.7 g/d) from pigs with a mean body weight (BW) ranging from 20 to 105 kg. Average daily feed intake (ADFI; 0.7 to 3.2 kg/d) and ingredient composition data were also collected. Fecal N excretion can be estimated based on the apparent total tract digestibility (ATTD) of N in the ingredients used for a diet. As the NRC (2012) provides only apparent ileal digestibility (AID) of N, the ratio of ATTD of N to AID of N (N_ATTD:N_AID) is essential to accurately estimate fecal N excretion unless ATTD values of N are available. Based on the N_ATTD values for 93 diets obtained from the 23 studies in the literature, and the N_AID values for the same diets calculated using the N_AID for each ingredient in the NRC, N_ATTD:N_AID linearly increased with increasing ADFI: N_ATTD:N_AID = 0.088 × ADFI + 0.917 with r² = 0.42 and P < 0.001. To predict urinary N excretion, it is necessary to determine the limiting standardized ileal digestible (SID) indispensable amino acid (IDAA) order and convert the dietary concentrations of the first-limiting SID IDAA expressed as a percentage of the pig's requirement (IDAA^1st, %). The SID IDAA requirements in the 23 studies were calculated using the NRC models. The NRC feed ingredient tables were used to calculate total N, SID IDAA, and SID dispensable AA (DAA) in the diet for these studies. Utilized all SID AA can be estimated using the SID AA intake and urinary N excretion. Assuming that all SID IDAA are utilized up to 100% of their requirements when IDAA^1st exceeds 100% of the requirement and that all SID IDAA are utilized up to IDAA^1st of their requirements when IDAA^1st is lower than 100%, the urinary N excretion can be predicted by estimating the ratio of utilized SID IDAA to utilized all SID AA (SID IDAA:SID all AA, %). Based on the urinary N excretion, IDAA^1st (12.5 to 170%) and SID IDAA:SID all AA (31.6 to 84.6%) in the 93 diets, a prediction equation for SID IDAA:SID all AA was developed: SID IDAA:SID all AA = 52.0 + 0.510 × (54.7 – IDAA^1st) where IDAA^1st is less than 54.7%, with R² = 0.85. Overall, N_ATTD:N_AID increases linearly with an increase in ADFI. When the IDAA^1st exceeds 54.7%, the SID IDAA:SID all AA reaches a plateau of 52.0%, corresponding to a SID IDAA:SID DAA of 52:48.