Phosphorus digestibility of corn and soybean meal (SBM) in broilers : in search of a consistent and reliable model to quantify individual ingredient contribution
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Two experiments were conducted to determine true digestibility of Phosphorus (P) in feed ingredients commonly in commercial broiler diets. A second objective was to determine if the true digestible P values of corn and soybean meal (SBM) would be additive when the two ingredients are fed in a typical commercial diet and finally how phytase supplementation affects true digestible P values. In Experiment 1, P digestibility of individual ingredients with increasing doses of phytase supplementation in broilers was used to estimate true P digestibility (TPD) of corn, SBM and a formulated diet without or with phytase using the regression method approach in broilers, and verify the additivity of TPD of corn and SBM. Three diets with different inclusion levels each of corn, SBM, and a combination of corn- soybean meal (C-SBM) were prepared with and without phytase supplementation. Broiler chicks were raised on commercial starter diets in accordance to the World Poultry Science Association (WPSA) recommendations and with specific management and routine of the University of Missouri ACUC protocol. Then they were fed the experimental diets for a minimum of 7 d before content of the posterior half of the ileum was collected. Birds were randomly assigned to one of twenty treatments with eight replicate pens per treatment. All diets and digesta samples were analyzed in the same laboratory. Endogenous loss though not significant yielded high negative values for corn diets with and without phytase supplementation (being numerically less) for true ileal P digestibility from -281 to -196 mg/kg DMI and excreta P from -212 to -129 mg/kg DMIrespectively. True digestible P content vs P intake of corn, SBM and C-SBM diets supplemented with phytase were determined to be 30.4, 59.9, and 19.5% respectively. In Experiment 2, Phosphorus ileal digestibility of corn, SBM and dicalcium phosphate is affected by dietary calcium level in broilers whereas endogenous phosphorus losses are not. The main objective of this study was to determine if digestibility values of ingredients would be additive when using the direct method approach. Also, to study the effect of calcium (Ca) (using two fixed Ca levels 0.35 and 0.85%) on apparent ileal P digestibility (AIPD) values of corn and dicalcium phosphate, on AIPD and TIPD values of SBM and on the estimations of endogenous P losses. The results in this study showed that the TIPD of SBM with 0.85% Ca was estimated to be 59.3%, that is 23 points lower than the TIPD estimation of SBM with 0.35% Ca (82%). The TIPD estimate values are greater than the AIPD values obtained from the SBM diets supplemented at different inclusion level. Increasing SBM level from low to medium improved AIPD in both Ca concentrations however, increasing SBM from 40 to 60% didn't further increase AIPD. Overall apparent P digestibility in diets with 0.35% Ca was higher than a similar diet with 0.85% Ca. These studies indicate that estimates for true P digestibility should be higher than apparent P digestibility data. The negative Endogenous P Loss (EPL) values in Experiment 1 are a possible explanation for the lack of significant differences between AIPD and TIPD data for diets formulated with the fixed Ca:P as negative EPL. A possible consequence of regression equations that predict negative EPL also is the underestimation of true P utilization (TPU). Based on these findings, it is suggested that the Ca concentration in broiler diets should be maintained low as realistically possible to maximize the utilization of other nutrients. Given that the addition of microbial phytase is currently routine in poultry diets and since the effect of phytase is diet-dependent, measurement of true P digestibility with a background of microbial phytase is of practical relevance. Using the regression approach, dietary phytase supplementation improved the true P digestibility in corn and SBM and a dietary P increases digestibility tend to decrease.
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