Predicting the amount of urea nitrogen recycled and used for anabolism in growing cattle

In ruminants, urea nitrogen (N) produced by the liver and recycled to the gastrointestinal tract (GIT) provides a source of N for microbial growth and also conserves N. In this respect, it buffers the dietary supply of N available for microbial growth and microbial protein supply. The equation for recycled N in the National Research Council's (NRC 1996, 2000) beef model is based on relationships between ruminal ammonia and plasma urea N concentrations. The objective of the current paper was to estimate recycled N available for anabolism (i.e., urea N used for anabolism, UUA) using available kinetic data. A meta-analysis was conducted using results reported in nine publications that measured urea N kinetics using the dual-labelled urea technique in growing cattle. Diets used in these experiments were predominantly forage-based. Urea production was linearly related to N intake (NI, g/day). Growing cattle converted 74.5% of the incremental NI to urea N. As NI increased, a smaller proportion of the urea produced was recycled to the GIT. On average, 54.4% of the urea N recycled to the GIT was used for anabolism; however, this percentage was not constant. As NI or dietary crude protein (CP) increased (g/kg dry matter, DM), proportionately less of the urea produced was used for anabolism. Nonlinear equations were developed to predict UUA based on NI or dietary CP in the current database and simulated at 5 or 10 kg of daily DM intake (DMI) over the same range of NI (g/day) and therefore, for diets differing in CP content (g/kg DM). The equation based on NI had a quadratic behaviour and the same estimated UUA for both levels of DMI. The equation based on CP showed a relatively small increase in UUA at low DMI and increased UUA at the higher DMI as NI increased. For both equations and both DMI, the pattern suggested a limit to use of recycled N for anabolism.