Urea toxicity

Producer Question from 2010

Q:  How much urea can be consumed daily by range cattle on low quality pasture grass before urea toxicity becomes a concern? (January 21, 2010)

A:  Urea supplies part of the protein equivalent in many of the commercial supplements formulated for beef cattle today. Urea is a non-protein nitrogen (NPN) compound. The urea used in livestock feeds is a synthetic compound. Urea is also used in the fertilizer industry and when nitrogen fertilizer goes up, so does the price of urea.

Feeds are analyzed for nitrogen in the laboratory to determine their crude protein. Protein averages 16 percent nitrogen. Thus the percent nitrogen multiplied by 6-1/4 gives the percent crude protein analysis. Therefore, urea with 45 percent nitrogen has a protein equivalent of 281 percent (45 x 6-1/4) for ruminants. Urea with 42 percent nitrogen has a protein equivalent of 262 percent (42 x 6-1/4). This is the reason urea with 45 percent nitrogen or 42 percent nitrogen is often designated as "281" or "262" urea, respectively.

Urea is broken down in the rumen by enzymes and the NH group from urea is attached to the carbohydrate change to make bacterial protein. Microorganisms must have energy and carbohydrates to use urea to make protein. It is important to have ammonia released simultaneously with available energy and carbon skeletons for ammonia to be converted to microbial protein. Starch from grain is the best source of energy for urea utilization. Molasses is good, but its highly fermentable sugars are used up too quickly and may not get good utilization from the urea. Energy (carbohydrate chain needed by the rumen bugs to make bacterial protein) from roughages is made available slowly in the rumen and therefore the urea is only partially used in these diets. Thus cattle on high-grain rations can derive a larger percentage of their protein needs from urea and cattle on roughage diets cannot, although they can derive some.

My point with this discussion is that cows on forage diets that are protein deficient, only a small portion of that protein deficiency could be met using urea. So cows that are protein deficient when consuming forage diets, urea should only be a small part of the protein supplement and using this thought process, these cows should not experience urea toxicity.

The information that I found indicates that there are a lot of factors that need to be considered when determining urea toxicity, so its not easy to say at what amount of urea consumption it will be toxic. What cattle are eating, water consumption, if cattle are thin, how quickly the urea is being consumed, etc. Data suggest urea toxicity can occur when consumption is 15 grams per 100 lb of body weight and the consumption occurred in a 30 to 60 minute period.

Dr. Rick Rasby Dr. Rick Rasby, Professor of Animal Science
Animal Science, University of Nebraska - Lincoln, Lincoln, NE