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

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.