Review: Urea Feeding to Dairy Cattle: A Historical Perspective and Review

Abstract

Urea has been fed in ruminant rations for more than 100 yr. Its use in dairy cattle rations has fluctuated with protein and urea prices, with various values used in different formulation systems, and with mixed to negative experiences in experiments and field situations. In many studies, rations were not isocaloric when urea was added, and intake reduction occurred because of high dietary levels of urea. Some studies concluded that cows disliked the flavor or odor of urea, or that there was some metabolic component. A series of studies revealed that cows did not dislike the flavor or odor of urea per se, that they could identify different levels of urea in rations, that they developed a conditioned negative aversion to urea when urea was fed at higher levels and for several exposures, and that 135 g/cow daily did not reduce DMI. In older studies, and in even more recent ones, this level of urea intake has been exceeded by 50 and up to 300% with a corresponding DMI decrease, even when fed in TMR. Urea use has also been limited because of in vitro studies showing no benefit to adding urea if ammonia levels are at 5 mg/mL or dietary CP is at 13%. However, several in vivo and in situ studies have shown the optimal rumen ammonia level to be between 17 and 25 mg/mL when DM disappearance and nonammonia-N flow are the determining measurements. Several studies have shown that oils, especially more unsaturated oils, defaunate or reduce protozoa, which can increase microbial protein synthesis efficiency but reduce DMI. In one study, the authors speculated that addition of urea could be beneficial to counteract reduced rumen ammonia and pH. Although there is some belief that addition of urea to higher nitrate-containing diets exacerbates the situation, studies do not support this contention. A large-scale field study and a long-term feeding study did not show any meaningful negative effects on reproduction when feeding urea. Synchronizing rumen N available with carbohydrate fermentation has a theoretical benefit, but a recent review found this did not occur, most likely because of N recycling and because of the adaptability of rumen microorganisms to asynchronous N and energy supply. Various commercially processed urea products have been developed, but few studies have been published showing that the processing and feeding objectives were achieved. Adding urea sources to ensiled forages has increased final N content and reduced protein degradation of the silage. When urea was also added in the concentrate, no negative effects were seen if total supplemental NPN was less than 20% of total dietary N. Classic ammonia toxicity from too much dietary urea being provided in a short period is most closely related to rumen pH because urea hydrolysis elevates rumen pH, which then allows more rapid absorption of the now greater levels of rumen ammonia into the blood. Traditional recommendations for feeding urea to dairy cows have been excessive. More reasonable recommendations would be for not more than 1% in the concentrate, approximately 135 g/cow daily, and not more than 20% of total dietary CP coming from added urea-NPN sources.