Research Note: Additivity of P digestibility values previously determined using 4 dietary Ca levels for plant-based...

Additivity of previously determined standardized ileal digestibility (SID) of P in individual feedstuffs at different dietary Ca levels was evaluated using nutrient-adequate broiler grower diets. The SID of P in corn, soybean meal, canola meal, corn fermented protein (CFP), and wheat middlings were previously determined in diets that contained 0.21 % P and 0.30, 0.45, 0.60, and 0.75 % Ca, and SID of P in monocalcium phosphate was determined in a diet that contained 0.65 % Ca. The same batch of these feedstuffs that were evaluated previously were utilized herein. Diet 1 was a corn-soybean meal-based diet that contained no inorganic P (0.84 % Ca, 0.12 % non-phytate P). Diet 2 was similar, except that monocalcium phosphate was added. A step-wise addition of 7 % canola meal, followed by 7 % CFP and 7 % wheat middlings, was performed for diets 3 through 5. Diets 2 through 5 contained 0.84 % Ca and 0.42 % non-phytate P. Diets were provided ad libitum to broiler chickens from 18 to 21 d-of-age. There were 7 pens of 5 chickens per treatment. In diet 1, SID of P was 33 %, which was lower (P < 0.05) than the values determined for all other treatments, which ranged from 57.1 to 57.4 %. Calculating SID of P in the test diets using previously determined SID of P values for individual plant-based ingredients at 0.75 % dietary Ca yielded values within 1 to 3 percentage units of the measured digestibility in the broiler grower diets which were close to or within the 95 % confidence interval for the measured SID of P. In contrast, individual ingredient values determined at 0.30 % dietary Ca overestimated SID of P in the complete broiler grower diets by 13 to 25 percentage units. These results demonstrate the importance of assessing Ca utilization of individual feedstuffs using test diets with Ca levels that are close to those that will be used in complete practical diets.