Adequacy of equations used to calculate urinary nitrogen contribution to total nitrogen excreted by ruminants

Nitrogen (N) concentration in the diet is the best sole predictor of the contribution of urinary N to total N in excreta. A large database was compiled containing feed and excreta variables from dairy cattle, beef cattle, sheep and deer, to evaluate the predictive ability of five linear models and four non-linear models to calculate the contribution of urinary N to total N in excreta. Species-specific linear models for beef cattle and sheep did not result in better predictions than those obtained from a generic linear model developed for all ruminant species. The predictive ability of a non-linear dairy cattle model was noticeably better than that of a linear model. The poor performance of a linear model along with a smaller number of observations from deer studies points to the merit of the construction of a new deer-specific model. This study provides support for the robustness of N partitioning towards urinary N in the animal sub-model of OverseerFM™ for a comprehensive range of diets with varying N concentration for beef cattle and sheep. In the future, the use of more sophisticated modelling approaches that involve partitioning of data for model development and evaluation may be required.