Estimating calcium and phosphorus requirements in broilers by inverting a mechanistic model of response to dietary calcium and phosphorus

In previous work, a comprehensive model describing broiler growth was expanded to include the dynamics of dietary calcium (Ca) and phosphorus (P), enabling the estimation of their metabolic fate. This approach accounts for the interaction between Ca and P and their effects on mineral retention and excretion. The model was ‘inverted’ in the sense that dietary Ca and P (originally the model inputs) were treated as unknown quantities to be determined through the inversion process, while the response traits served as the driving inputs. This allow developing a factorial approach to estimate Ca and P requirements. Maintenance requirements were endogenous losses, and growth requirements were based on mineral deposition in soft tissues and body ash. To obtain standardized total tract digestibility requirements, estimated (SID) faecal and urinary losses were accounted for. A digestibility coefficient of 50% was used to convert digestible Ca? to total Ca as no values are available for all feed ingredients. Model outputs were assessed via global sensitivity analysis and compared with Rostagno et al. (2024), Cobb-Vantress (2026), CVB (2018), Ross 308 guidelines (Aviagen, 2022). Sensitivity analysis identified the proportion of P in body protein and Ca in body ash as the most influential factors governing digestible P and total Ca requirements, respectively. Notably, Ca content in body ash explained 79% of the variation in total Ca requirement. Model-predicted digestible P requirements (% per kg of diet) aligned with published recommendations, estimating 0.49% for days 0 - 10, 0.41% for days 11 - 21 and 0.34% for days 22 - 35. Estimated total Ca requirements were 1.20%, 0.98% and 0.78% for the same periods. To reduce P excretion and enhance the sustainability of poultry production, understanding P metabolism and its interaction with Ca is essential. The proposed model offers a robust tool to estimate Ca and P requirements for optimal bone mineralization, supporting precision feeding strategies for more sustainable and efficient poultry production.