Modelling of nitrogen flow and excretion in dairy cows

<p>In the Netherlands, the dairy husbandry is held responsible for a major part of the N-pollution of the environment. The objective of this thesis was to develop a model that predicts N flow in the animal and N excretion in faeces, urine and milk in order to reduce unnecessary N losses. The model was based on the concepts of modern protein evaluation systems, using data from nylon bag incubations in the rumen and intestine. Protein evaluation systems from different countries were compared and validated using experimental data. Different methods to estimate feed protein degradation in the rumen and digestion in the small intestine were compared and data of individual feedstuffs obtained with the nylon bag methods were collected. Experiments were carried out to obtain additional information on the organic matter fermentation and digestion of protein in the small intestine of forages, and on the change in amino acid profile of feedstuffs during rumen fermentation and small intestinal digestion. The model described the N flow in four compartments: rumen, small intestine, large intestine and metabolism. Flows were validated with experimental data. Total duodenal flow could be accurately predicted, but to predict ileal N flow a negative digestibility coefficient for endogenous protein was necessary. The model slightly overestimated faecal and urinary N excretion; milk protein production could be accurately predicted. It was concluded that N losses in dairy cows can be reduced by balancing the ratio between energy and protein in the rumen and in the metabolism, and that more information on endogenous N excretion and digestion in the gastrointestinal tract is needed