Effects of a slow-release urea source on absorption of ammonia and endogenous production of urea by cattle

Three experiments were conducted with Angus or Holstein steers to evaluate effects of dietary urea-calcium (a slow rumen-release urea source) on absorption of ammonia N from the gut and urea N production in the liver. Steers were fed a high-grain diet (Experiment 1) or an all-forage diet (Experiments 2 and 3). Urea or urea-calcium (0.25 g/kg body weight) was dosed into the esophagus (Experiments 1 and 2) or rumen (Experiment 3), and blood samples were serially collected for 180 min. Blood concentrations of ammonia N and urea N were measured in all experiments, and net flux of metabolites across splanchnic tissues was measured in Experiment 3. Compared to urea, urea-calcium reduced (P<0.05) plasma concentrations of ammonia N in steers fed all-forage diets, and tended (P<0.06) to reduce arterial glucose concentrations in Experiment 3. Plasma concentrations of urea N were not affected by treatment in any experiment. Treatment and time post-dosing interactions (P<0.05) in Experiment 3 were due to increased ruminal fluid concentrations of ammonia N, net release of ammonia N by portal-drained viscera and total splanchnic tissues with urea versus urea-calcium treatment shortly after dosing. Similar interactions (P<0.05) indicated that urea caused higher hepatic glucose release and increased l-lactate release by total splanchnic tissues after dosing than urea-calcium. Urea-calcium was effective in mitigating rapid ammonia release in the rumen and subsequent effects on glucose and lactate metabolism.