RESUMEN
In a series of experiments we investigated effects of several factors on intake of urea by lactating dairy cows. Cows given an unfamiliar ingredient or altered moisture in the ration reduced their intake, and this effect was attributed to a newness factor. Addition of urea to a ration may have a similar effect. An experimental design providing both no-choice and two-choice data was devised to minimize the effect of newness. When urea was isolated physically by pellets coated with ground corn, cows selected against urea-containing pellets on a two-choice basis and reduced intake on a no-choice basis. When the choice was between two urea-containing rations, cows preferred the pellets in which urea flavor and odor should have been most evident. Urea odor in the feedbox or urea in the drinking water did not reduce feed intake. Eating rate of a 2.5% urea-containing ration over two daily 30-min feeding periods was about one-half that of a non-urea ration. This effect was evident during the first 5-min eating interval. Administration of urea solution into the rumen prior to feeding a nonurea ration markedly reduced subsequent intake. Reticulum pH and ammonia indicated possible sublethal ammonia toxicity. Intake and eating rate were depressed and reticulum pH was elevated when cows were fed a ration with 2.5% urea versus 1% urea or nonurea rations. Elevated reticulum ammonia occurred on both 1 and 2.5% urea-containing rations. Cows not previously exposed to urea-containing rations reduced intake and eating rate when fed rations with 2.0 and 2.5% urea. Intake reduction was not comparable to that by cows preconditioned to urea rations. Upon third exposure to 2.5% urea in the ration, cows reduced and ceased intake but readily consumed a nonurea ration. Cows require preconditioning to develop a negative aversion to rations containing high urea, perhaps through a mechanism of sublethal ammonia toxicity.