Animal Health Alert #1/2002: Urea Poisoning of Range Cattle

From the Arizona Veterinary Diagnostic Laboratory

May 20, 2002

Arizona ranchers and large animal veterinarians are well aware of the difficult times facing range livestock this summer with the current drought. This situation has been made worse for several well-intentioned ranchers who have lost range cows after they attempted to supplement them using feed supplements that contained urea. Urea-containing supplements can be dangerous when formulated or used improperly and it is our intention to clarify this issue here.

Urea or ammonium salts are added to protein supplements as non-protein sources of nitrogen (NPN), which can be used, under the right circumstances, by ruminant livestock to provide up to 40 percent of their protein nitrogen requirements. NPN sources such as urea can be converted to protein and amino acids by rumen microorganisms in NPN-adapted ruminant livestock when energy intakes are adequate. They are commonly used because they are often less expensive than natural protein sources. When NPN supplements are improperly used, fatal ammonia intoxication can occur from too-rapid breakdown of urea (or ammonium salt)-containing products to ammonia from NPN sources by microorganisms in the paunch (rumen) of cattle or sheep. In appropriate amounts, ammonia can be utilized as a nitrogen source for protein and amino acid formation but is poisonous when formed in the rumen in excessive amounts.

Clinical signs of urea (ammonia) intoxication can occur in as little as 10 minutes to as much as 4 hours after consumption and include frothy salivation, grinding of the teeth, rapid breathing, bloat, muscle tremors, incoordination, bellowing, and seizure-like activity just prior to death. More often, clinical signs go unobserved in range cattle and they are simply found dead, sometimes in significant numbers.

Urea-containing or other NPN supplements should not be fed to range cattle when:

1. the main forage intake is dry, weathered range grass, which, during a drought or early summer is low in energy and protein and high in fiber. There must be an adequate intake of energy for ruminants to make efficient use of NPN sources of nitrogen such as urea for protein formation.
2. range cattle are on poor range or have been starved. These cattle are typically very hungry, are unaccustomed to NPN sources, and can easily become intoxicated if suddenly given access to NPN (urea) sources which can be rapidly consumed such as soft, easily chewed molasses-containing protein blocks. Range cattle are often supplemented only intermittently, which makes NPN ineffective as a nitrogen source for protein and amino-acid formation.
3. cattle are dehydrated. Cattle that have had an insufficient water supply are more susceptible to poisoning and should not be given NPN supplements.
4. cattle are not adapted to NPN sources. These animals are more susceptible to ammonia poisoning when suddenly given access to supplements containing urea or ammonium salts. Again, crumbly, easily consumed supplements that are suddenly given to unaccustomed, hungry cattle are often responsible for deaths because they are consumed too rapidly.

A high level of management is required for safe and effective use of urea-containing or other NPN supplements. A ruminant animal must be fed the NPN-containing supplement continuously at appropriate levels for several days before adaptation of ruminal microorganisms occurs and there is efficient utilization of the product as a protein supplement. If feeding of the supplement is stopped or for some reason animals go off feed, adaptation is readily lost.

Unquestionably range livestock need feed supplements at this time of the year and this is especially true given the present drought conditions on Arizona rangelands. For greater safety we suggest that ranchers consider using salt-limited feeding of all-natural protein supplements that do not contain urea or ammonium salts to avoid the risk of urea intoxication.

Check the label ingredient list of any products you are considering as supplements or ask for a formulation sheet from the vendor. Feed labels should state "for ruminants only" if the product contains an NPN source. Do not buy unlabeled products. All-natural protein salt-limited supplements are usually suitable for intermittent (i.e. every few days) supplementation in range cattle and are safer to use unless cattle are salt or water-deprived. Always follow label directions for any product used.

For more specific information on the use of livestock supplements, a qualified livestock nutritionist or large animal veterinarian should be consulted.

Diagnosis of NPN-related mortality in range cattle can be made by determining ammonia levels in rumen contents or blood (heparinized whole blood is preferable). Samples, however, must be from freshly dead animals and be iced down immediately after collection and during transport to the lab to prevent additional ammonia formation from decomposition. A full necropsy should be conducted to rule out other causes of death. Feed samples can be analyzed for urea if mixing errors are suspected.

Note: Horses, although less susceptible than cattle to urea intoxication, can be poisoned if they eat enough of a urea-containing supplement. Horses are roughly equivalent to cattle in susceptibility to poisoning by other types of NPN (e.g. ammonium salts).

By:

T.H. Noon, DVM, diagnostician, AZVDL
G.A. Bradley, DVM, diagnostician, AZVDL
H.F. Frederick Ph.D., livestock nutrition consultant
S. Peder Cuneo, Extension Veterinarian, University of Arizona

Reference: Clinical and Diagnostic Veterinary Toxicology, 3^rd Edition, by GD Osweiler, et.al. 1985, Kendall/Hunt Publishing Co.