Evaluation of Factors Responsible for Reduced Voluntary Intake of Urea Diets for Ruminants

Two experiments were conducted to evaluate factors responsible for depressed voluntary feed intake of cattle fed diets containing urea. In Experiment I four fistulated cows were used with four treatments in a latin square design with 14-day periods. Urea levels in complete blended diets were 1.0, 1.65, 2.30 and 3.0%. During each period urea was either fed in the diet (orally) or poured into the rumen as a solution (via rumen cannula) three times daily. Experiment II was similar in design with treatments as follows: (1) Control – 1.0% urea in diet; (2) 3.0% urea in diet; (3) 1.0% urea in diet with urea equal to 2.0% of diet placed in rumen as solution twice daily; (4) 1.0% urea in diet with urea equal to 2.0% of diet infused into rumen throughout each entire day. Urea appeared to depress total ration consumption when included in the diet at levels above 2% of the diet regardless of method of supplying urea to the animal. Either pouring or infusing urea into the rumen significantly depressed feed intake below that found when urea was consumed orally. Increasing levels of urea, regardless of method of administration, produced elevated concentrations of ruminai ammonia, venous and arterial urea and salivary urea. Blood ammonia was unaffected and no signs of ammonia toxicity were observed. The data suggest that feed intake depression with diets containing more than 1% urea is due to some physiological parameter other than undesirable taste.