Animal agricultural production systems are a major source of nitrogen (N) which may contribute to potential environmental pollution and one way to reduce losses of N to the environment is through feeding protein closer to requirements without overfeeding. This experiment was conducted to evaluate the effect of two rumen degraded protein (RDP) sources (non-protein N in the form of urea and amino acid-N in the form of casein) on microbial N (MN) flow, digestibility and production in lactating dairy cows. Eight ruminally and duodenally cannulated Holstein cows were fed one of four dietary treatments in a repeated 4x4 Latin square. The first diet (BASE) served as the negative control and contained 12.2% crude protein (CP). The remaining diets contained either urea (UREA), casein (CAS), or a combination of both (U+C) on an equal N basis and contained 15.0% CP. Cows were infused with Co-EDTA, Cr-mordanted NDF and 15N which were used as markers for liquid, solid and bacteria flow, respectively. Intake, duodenal MN flow, milk production, and digestibility were lower when cows were fed the BASE diet and there were differences in MN flow between the UREA, CAS or U+C diets. Ruminal starch digestibility was highest when cows were fed the U+C diet and NDF digestibility was higher when cows were fed the CAS and U+C diets. Therefore a source of RDP with amino acids was required to maximize both fiber and starch digestibility.

In the same study flow rates of various particle sizes from reticulum and duodenal samples were compared as well as the bacterial attachment to these particles as they flow through the digestive tract. Digesta collected from both the reticulum and the duodenum were poured over a set of sieves to allow for particle size separation. Flow rates of DM, NDF and N differed depending on particle size and the composition of the various sieve fractions differed but was still similar between reticulum and duodenal samples. Bacterial attachment differed depending on particle size and location in the digestive tract. These results indicate the importance of particle size passage from the rumen and the usefulness of flow markers to adjust for unrepresentative sampling from both the rumen and the duodenum.

A better understanding of the responses of MN flow due to RDP source can lead to improved diet formulation models which can be used to balance dairy cattle rations for optimum production yet minimize losses of N from the cow and therefore to the environment.