The determination of acid-binding capacity of various limestones and subsequent impact on proximal gastrointestinal tract pH, calcium and phosphorous digestibility in broilers

Abstract

Limestone is a vital dietary ingredient in broiler diets and is the main source of Ca utilised in global feed formulations. However, it exhibits considerable variation in multiple physical and chemical characteristics. This study sought to quantify the variability of acid-binding capacity (ABC) in limestone, as well as the intrinsic limestone properties that may affect ABC. Furthermore, it aimed to investigate the relationship between limestone ABC and Ca and P digestibility. The trial utilised 40 different limestone samples that were measured for ABC at various timepoints, which were correlated with the various physical and chemical characteristics of the limestones. The in vivo trial used a 6 x 2 factorial design, consisting of 5 limestones of graded ABC values and diets with no added limestone. These diets were fed to broilers in the presence and absence of 1,000 FTU/kg phytase supplementation. A total of 648 broilers were allocated to one of 12 treatments, following a complete randomised block design. Birds were fed the treatment diets for 36 hours prior to being euthanised, and sampled for pH of the gizzard and proventriculus, as well as for ileal digesta. All data were captured and analysed using JMP 16.0 (JMP, 2022), using a full factorial analysis with block included as a random effect. Correlations were determined using Pearson correlation analysis, with significance set at P < 0.05. Correlation analysis reported the strongest correlations between GMD and ABC at 120 minutes (-0.43), and between 5-minute solubility and ABC at 120 minutes (0.48). Results from the in vivo study showed no effect of limestone ABC on the pH of the gizzard or proventriculus, but did report a main effect (P < 0.05) of phytase on gizzard pH. For digestibility, effects were reported for limestone ABC and phytase supplementation on the apparent ileal digestibility (AID) of Ca and P. Furthermore, an interaction was noted between limestone ABC and phytase supplementation on digestible P (dP). However, both Ca and P digestibility followed no clear response to limestone ABC, and one may hypothesise that the results may be more reflective of the limestone solubility profiles.