Partitioning and Excretion of Dietary Phosphorus by Broilers

Understanding the dynamics of calcium (Ca) and phosphorus (P) metabolism is an important consideration when evaluating the requirements for both nutrients. Much more is known about the control of Ca than P metabolism (Li et al., 2017). Calcium elimination from the body is primarily through faeces, both unabsorbed dietary Ca and endogenous Ca. Kidney Ca elimination is controlled by endocrine factors and to a lesser extent by dietary Ca. In practical terms, dietary Ca may be available but not absorbed because of the animal’s Ca status, whereas a large portion of dietary P that is available will be absorbed and eliminated through the urine. Therefore, body content of P appears to be primarily regulated by urinary excretion, as there is limited control of the gut absorption of P compared to Ca. The partitioning and excretion of P was evaluated using ileal P and excreta P as surrogates for P absorption and P excretion, respectively.

Day-old, male Ross broilers were fed a starter (day 1-14) and grower (day 15-21) diets based on sorghum and soybean. The experimental diets contained the same level of Ca (10 and 9.0 g/kg diet for starter and grower, respectively) and graded levels of available P from 2.5 to 5.5 g/kg diet for starter; from 2.0 to 5.0 g/kg diet for grower in increment of 1.0 g/kg which covered a range to allow a “deficiency status” at the lowest level right through to the value recommended by NRC (1994) and current values used by industry. The study was a factorial design with or without phytase inclusion. Each treatment had 5 replicates and each replicate had 10 birds.

Phosphorus contents in both ileal digesta and excreta increased as dietary non-phytate phosphorus (NPP) increased. This resulted in an extra 19.1 and 46.9% of P being excreted from birds fed diets containing NPP of 4.5/4.0 (g/kg, starter/grower) and 5.5/5.0 (g/kg, starter/grower), respectively, compared to birds fed diets containing NPP of 3.5/3.0 (g/kg, starter/grower) without phytase supplementation. Phytase significantly reduced ileal P content (10.6 vs 12.7, P < 0.001), but showed no effect on excreta P content (14.2 vs 14.6, P > 0.05). Broilers fed on diets with lower NPP levels excreted much less P in excreta. There were more consistent and stronger relationships between dietary NPP and excreta P concentrations (R² = 0.9099) than dietary NPP and ileal P concentrations (R² = 0.4912), indicating high absorption of available P but rapid urinary excretion of absorbed P to maintain P homeostasis. A similar interpretation can be made from other published data (Leske and Coon, 2002), implying that measurement of P excretion may become the important strategy in formulating diets to meet exact P requirement and assess enzyme effectiveness in poultry.