In Vitro Prediction of Standardized Total Tract Digestibility of Phosphorus Among Sources of Animal Protein Meals

In vitro digestibility of phosphorus (P) was determined in 13 sources of animal protein meals (2 blood meals, 1 chicken meal, 1 chicken byproduct meal, 2 feather meals, 2 meat meals, 3 meat and bone meals, 1 poultry by-product meal, and 1 poultry meal), and a mathematical model was developed to estimate the economic benefit of formulating diets with a more accurate P digestibility values of ingredients. In vitro enzymatic hydrolysis was conducted using a two-step assay in which pepsin and pancreatin were used to mimic gastric and small intestinal digestion. The amount of total P released by enzymatic digestion was analyzed by the ascorbic acid method. All assays were repeated 6 times. Data of in vitro P digestibility were analyzed using the GLM procedure in SAS. A simple linear regression analysis was conducted between in vitro P digestibility data and in vivo standardized total tract digestibility (STTD) of P, which was calculated based on apparent total tract digestibility in pigs from their respective sources from a previous experiment. Chicken meal had lower (P < 0.01) in vitro P digestibility (42.98%) compared with chicken by-product meal (62.24%), and there were no differences (P > 0.10) between poultry meal (33.52%) and poultry byproduct meal (32.21%). The average in vitro P digestibility of 3 sources of meat and bone meal was 33.21 ± 0.24%, and the average of 2 meat meal sources was 35.99 ± 1.56%. Animal protein meals with greater Ca:P tended to have a lower in vitro digestibility of P (in vitro P digestibility, % = -30.14 × Ca:P + 91.18; R² = 0.89). There was also a high correlation between in vitro P digestibility and in vivo STTD of P (STTD of P, % = 1.00 × in vitro P digestibility + 1.33; R² = 0.91). The mathematical model showed that when P digestibility is overestimated by 10% compared to its actual value in an ingredient, pigs fed P deficient diets may have a 30 g less average daily gain than pigs fed diets formulated with more accurate values from the in vitro assay. When P digestibility of ingredients is underestimated by 10% compared to its actual value, the cost of the feed may increase by $0.55/pig. These results suggest that the 2-step in vitro P digestibility assay can be used to reasonably predict in vivo P digestibility of animal protein by-products fed to swine.