Effects of Calcium Feeding Strategy, Dietary Adaptation Period Length, and Diet Composition on Phosphorus Availability for the Determination of True Phosphorus Utilization in Growing Broilers

Abstract

Standard methods have been proposed for the determination of P availability using the true P utilization (TPU) protocol. However, reported TPU data have been variable between common feedstuffs. Therefore, a series of 4 experiments were conducted using broilers from 19 to 26 d of age to elucidate the effects different methodological factors have on measures of P availability for the determination of corn TPU. The first and second experiments assessed the effects of differences in diet type related to the inclusion of purified ingredients on measures of P availability and gastrointestinal tract (GIT) histology after short (< 5 d) dietary adaption period lengths (DAPL). Intestinal histology was not influenced by diet type, but DAPL affected the P availability of the different diet types, including a diet formulated as a control for DAPL. This indicates that DAPL effects may be associated with more than just differences in diet type. The third experiment evaluated the effects of different Ca feeding strategies (low, high, or fixed Ca:total P [tP] ratio) on diet apparent P digestibility and the resulting estimates of TPU. Different Ca feeding strategies affected estimates of TPU with low Ca diets having higher estimates of TPU. The high Ca diets had the lowest TPU. Negative endogenous P losses (EPL) were predicted when titration diets were formulated with a fixed Ca:tP ratio The fourth experiment was conducted to assess the effects of Ca feeding strategy and DAPL on the incidence of predicted negative EPL. Digesta samples from gizzard, jejunum, and ileum were collected to determine how phytate and an inert marker (TiO2) flowed through the GIT. Negative EPL were predicted for diets with a fixed Ca:tP ratio, but not predicted when diet Ca concentrations were fixed at 0.35%. Data also indicated that the flow of phytate and TiO2 varied in the anterior GIT resulting in inaccurate measurements of apparent phytate hydrolysis. These results indicated that DAPL, Ca feeding strategy, and the use of TiO2 may contribute to the variability of TPU estimates. Future research should continue to address these factors to refine the methods used for generating TPU of common feedstuffs.