Development of an indigestible dietary protein index to investigate the effects of dietary protein content in post-weaned pigs

Proteolytic fermentation induces negative effects on gut health and function, which may affect pig performance. The objective was to conduct a meta-analysis to develop an index of dietary indigestible dietary protein (IDP) to investigate growth performance outcomes of mixed-sex weanling pigs (average body weight of 7.59 kg). Eighty-nine articles reporting growth performance variables [average daily gain (ADG), average daily feed intake (ADFI), gain:feed ratio (GF), initial (IBW), and final body weight] in pigs fed different dietary protein (DP) content (from 12% to 33.6%) and protein sources (plant and animal) were included. Dietary protein and IDP index were calculated in all experiments using a common database, with the IDP index defined as the difference between total DP and standardized ileal digestible DP. A DP- and an IDP-based model were developed to predict the ADG, GF, and ADFI (by their relationship) of weaning pigs using a multivariable linear mixed model regression approach with estimates of variable effects obtained using the residual maximum likelihood method. Based on a stepwise manual forward selection, significant predictor variables with improvement of at least 2 points in the Bayesian information criterion were included in the final regression model. Statistical significance was set at P ≤ 0.05 and a trend at P < 0.10. Initial exploratory analysis of the database showed a quadratic increase (P < 0.01) in the IDP index with increasing inclusion of plant protein sources in diet formulation and a linear decrease (P < 0.01) in the IDP index with increasing synthetic amino acid inclusion. Regarding the models, the DP-based model could not account for the inclusion of protein sources compared to the IDP-based model. There was a tendency for DP to positively affect (P < 0.10) ADG and GF. Increasing the IDP index tended to negatively impact (P < 0.10) ADG while reducing (P < 0.05) ADFI. Using a practical and hypothetical feed formulation simulation, the final regression models predicted the expected negative impact of a high IDP index on newly weaned pig performance when compared to a low IDP diet. The IDP-based model predicted a stronger negative effect of high IDP when compared to the DP-based model. Results indicate that IDP may be an improved and more reliable index to investigate the impact of DP on pig performance in the post-weaning phase.