Graduate Student Literature Review: Concepts and challenges of amino acid supply and nitrogen metabolism in dairy cattle.

Abstract

Nitrogen utilization in dairy cows is crucial for maximizing production efficiency and minimizing environmental impacts. Ruminants possess a unique digestive system filled with microorganisms that work synergically to degrade feed particles and provide nutrients for both the microorganism and host metabolism. Dietary N, supplied as true protein and NPN, undergoes different degradation processes: a portion is degraded in the rumen, and another portion escapes ruminal degradation (i.e., RUP). Rumen-degraded protein is essential for microbial growth, contributing to the formation of microbial CP (MCP). Amino acids present in MCP and RUP that are digested and absorbed into the bloodstream and become available for animal metabolism (e.g., maintenance, growth, and production) constitute MP. Accurately assessing protein degradability and AA availability remains a challenge due to the limitations of current evaluation methods. In situ techniques are the most commonly used technique for this purpose, but they are not meant to predict MCP, RUP, and consequently the metabolizable AA supply of feed ingredients, which compromises diet formulation. Microbial CP growth is enhanced when both fermentable energy and N is available in the rumen. However, the extent of N recycling in sustaining microbial growth under low-protein diets is not adequately captured by MCP prediction models. Due to these limitations, several biases are added to protein models, resulting in protein overfeeding in an attempt to supply MP requirements and increasing both costs and N overload to the environment. Therefore, this review aims to synthesize fundamental information on the complex interplay of factors influencing ruminal protein degradation, MCP synthesis, AA supply, N metabolism, and N efficiency, and highlight the knowledge gaps that must be addressed to improve the accuracy of prediction models of AA supply in dairy cattle.