Development of a dynamic model of calcium and phosphorus flows in layers.

Abstract

Phosphorus (P) is involved in most metabolic activities of the body as well as in bone formation. P and calcium (Ca) are closely related so that a deficiency in one can interfere with proper utilization of the other. In layers, requirement for dietary P is mainly due to the need to store Ca in bones prior to eggshell formation. Ca requirement for eggshell formation is high. If at any time during the day Ca requirement exceeds the amount of Ca absorbed from the gut, layers mobilize Ca, and consequently P, from medullary bones. P is then excreted in urine, potentially causing environmental pollution. A model of Ca and P dynamics in the layer was developed to describe and evaluate flows of Ca and P during the day. The model comprises eight state variables representing Ca and P in the crop, stomachs (proventriculus and gizzard), plasma and bone. P is defined as P absorbable at the terminal ileum. Zero pools are assigned to Ca and P in the duodenum. Outflow of Ca and P from crop and stomachs is assumed to obey mass-action kinetics. A higher fractional Ca absorption rate from the duodenum is assumed during eggshell formation than at times when there is no such formation. Eggshell formation commences 20 h before oviposition and follows a sigmoidal pattern. Ca and P in plasma can be used for egg synthesis, accretion in bone and excretion in urine. Michaelis-Menten forms represent utilization and production of Ca and P for bone accretion and resorption. Rate of utilization of plasma Ca and P for accretion depends on the more limiting of the two minerals. Rate of bone resorption is inhibited by plasma Ca or P level when Ca or P requirement exceeds supply from the gut, and consequently is related to the lowest level of the two minerals. Ca and P excreted in urine is the sum of basal maintenance requirement for Ca and P and amount of Ca or P in plasma that cannot be utilized for bone accretion because the other mineral is lacking. In the simulations, a light period of 16 h/day is assumed, and feed intake occurs continuously and only during the light period. Time of laying varies between 1 and 7 h after light is switched on. Simulated Ca and P absorption from the gut rises to a plateau until light is switched off, upon which absorption declines rapidly. However, depending on time of oviposition, simulated Ca requirement for eggshell formation is highest between 14 and 21 h after light is switched on. Because of the simulated imbalance between Ca absorption from the gut and Ca requirement for eggshell synthesis, Ca and consequently P are mobilized from bone. The model may be used to evaluate feeding strategies aimed at reducing P excretion to the environment in poultry manure.