Last week I received a call from a feed manufacturer client that was encountering some issues with a customer. The customer (a cow-calf operator) had purchased a feed supplement back in the fall and winter from the feed company and was now complaining that he had suffered some significant loss of performance because of the use of the product and the fact that the product included non-protein nitrogen in the form of urea. Upon reviewing the case and the letter sent to the producer by his veterinarian, it became obvious that even after all the years urea has been used as a source of nitrogen for the production of bacterial protein in the ruminant, many misunderstandings and misconceptions still exist about this commonly used feed ingredient. Additionally, since urea is, essentially, a by-product from the energy industry, its value and cost has been affected by recent developments in that industry. In response to both of these circumstances I felt it would be useful to revisit this topic.
Urea has been used for years in the feeding industry to provide an inexpensive source of nitrogen from which rumen bacteria can form protein. One thing that many cattlemen fail to understand is that much of the actual protein cattle digest and absorb comes from the bacteria which pass from the rumen and down into the lower digestive tract. Urea is an inorganic chemical that is used not only in livestock feeds but also as a source of nitrogen in some fertilizers. Like many materials found on the farm or ranch, especially those fed to cattle, urea consumption, especially excessive consumption, can create a host of problems, including toxic effects. Most producers, if they have not experienced it themselves, have heard the horror stories of someone who have had cattle over-consume a urea containing feed and had cattle become sick or even die. Unfortunately, what many producers fail to realize is that in most cases, it wasn't the presence of urea in the feed that created the problem, but was mis-handling or mismanaging the feeding program or the cattle themselves. Often it will be found that the bad reputation that urea has in certain areas is related to a poor experience by certain individual producers.
A statement needs to be made, and if this steps on any toes, I want to apologize in advance: in many situations, the feeding of urea or other generally considered safe ingredient is unjustifiably discouraged by the feed store, extension personnel, veterinarian or nutritionist simply because they have not done their homework or do not understand the ruminant digestive process, feed manufacturing technology or simple feeding management. As a producer, when you run across someone who makes a relatively strong statement (i.e., “feeding urea is bad for your cattle”) regarding a given feeding or management practice it is generally a good idea to check some other sources before implementing their advice. It has been said before that the feeding industry contains a tremendous amount of bad or inaccurate information. Virtually ANY feed component can cause problems if it is fed or managed incorrectly.
To truly understand urea feeding and urea toxicity, we need to discuss urea utilization a bit. Feed grade urea, at about 281 percent “crude protein”, is the most commonly used commercial source of non-protein nitrogen (NPN). The first question one typically asks is how can a product have 281 percent of anything. The 281 percent protein is more accurately termed “protein equivalent” and will vary depending on the source of the urea. In other words, one pound of urea can be used to make 2.81 lbs. of protein by rumen bacteria (one of the many beneficial functions rumen microbes perform). Enzymes from bacteria in the rumen, specifically urease, break down urea to carbon dioxide (CO2) and ammonia. Ruminal bacteria have the capacity to hydrolyze (break down) up to one gram of urea per liter (about a quart) per hour with diets high in urea. The ammonia which is liberated from urea is utilized in one of two ways. Part of the ammonia is ingested by the bacteria and is incorporated within the cell to form bacterial protein which is later digested by the animal as we discussed above. This bacterial protein accounts for a very significant portion of the protein actually absorbed and used by the animal to meet it's protein requirements. The remaining ammonia is absorbed directly into the rumen wall where it enters the blood. From there it is carried to the liver where it is transformed back to urea. This newly reformed urea is then recycled back to the rumen through saliva or is excreted in the urine. Excretion is the predominant means by which excess blood ammonia levels are handled and eliminated.
For both the breakdown of urea and its incorporation into microbial protein, a readily available supply of energy is required. This energy is derived from the breakdown of other feedstuffs within the rumen. When feeding urea it is desirable to utilize a readily fermentable source of energy such as a processed grain or molasses. Urea can only be utilized with the energy present from readily available feed sources in the rumen. Specifically, the ruminal bacteria must have a source of carbohydrate (starch) with which to combine the nitrogen obtained from the breakdown of the urea molecule in order to form bacterial protein. Urea metabolism cannot utilize animal stores of energy such as fat tissue from within the animal itself. In formulation of feeds and rations it is important to recognize the usefulness of urea or NPN as well as the limitations.
In a paper prepared by personnel at Iowa State University, a system was outlined describing the urea fermentation potential (UFP) of feeds. This system provides an index which can be used to calculate the amount of urea which can be transformed into microbial protein per unit of feed consumed. For example, it was reported that the UFP of corn is 4.72 gms of urea per pound of corn. Likewise the UFP of cane molasses is 6.85 gms of urea per pound of molasses (45 percent higher). By weight, molasses can provide better and more rapid utilization of urea than corn. From this we can see that molasses based liquid supplements provide an excellent carrier for urea when used properly.
Cattle are adaptable to the feeding of urea. This simply means that given adequate time to adjust to urea feeding levels, the ruminal bacteria can be adapted to higher rates. This needs to be done by gradually increasing the amount in the feed, allowing the bacteria which process the urea to adjust to the increased concentration.
Urea toxicity has been documented many times and is characterized by an over-consumption of urea containing feeds or feeding of urea without a suitable fermentable carbohydrate source. Primary causes include:
1) Poor mixing of feed
2) Errors in ration formulation
3) Inadequate period of adaptation
4) Low intake of water
5) Feeding of urea in conjunction with poor-quality roughages
6) Low feed intake prior to exposure to feed containing urea
7) Rations that promote a high pH in ruminal fluid
In either situation, the breakdown of the urea and subsequent release of ammonia into the rumen exceeds the rumen microbe's ability to complex it into bacterial protein and thus the ammonia concentration in the rumen increases. This causes an increase in the rumen pH. This increase in the alkalinity of the rumen contents then facilitates the passage of the ammonia through the rumen wall. When this happens, excessive amounts of ammonia are absorbed into the blood stream. The increased alkalinity appears to be the most important factor contributing to high blood ammonia concentrations and related toxicity in animals consuming urea. This increase in blood ammonia concentration can exceed the liver's capability to process the ammonia back into urea for recycling in the salivary gland or the body's ability to excrete it through the urine. Concentrations of ammonia nitrogen approximating two to four milligrams (mg) per deciliter (dl) of blood are generally associated with urea-induced deaths. However, the chance of toxicity has been found to be high when blood ammonia nitrogen exceeds .8 mg/dl in 60 minutes following urea consumption. As in the rumen, when blood ammonia levels rise so does blood pH. This leads to interference in a number of normal physiological processes including normal cellular energy metabolism, increased uptake of ammonia by the brain and an upset of the central nervous system. Although not well defined, the precise cause of death in ammonia toxicity appears to be respiratory arrest. You will note that it is ammonia toxicity that creates the actual problem, not the urea itself.
Urea Toxicity Symptoms
Clinical signs are usually apparent within approximately 20 to 30 minutes following consumption of a toxic amount of urea. Untreated, death generally occurs in less than four hours. As you can see, time is of the essence in these situations. Symptoms include rapid breathing, tremors, and slight incoordination followed by severe incoordination, excessive salivation and labored breathing. Eventually, animals lose the ability to stand and tetany (muscle spasms) become increasingly apparent. Bloating tends to occur as rumen motility decreases.
Fortunately, when caught in the stages early after ingestion, urea toxicity can be treated. Treatment focuses on reducing ruminal pH levels, overall concentrations of ruminal ammonia as well as urea breakdown. A five percent solution of acetic acid (common vinegar) can be orally drenched to aid in reducing ruminal pH. A dosage of about one gallon for a 1,000 lb. cow will normally suffice if administered before tetany becomes severe. Also, additional quantities of cold water may be given orally to slow down urea hydrolysis (breakdown) and reduce the concentration already available for absorption. Once again, this is primarily effective only in the earlier stages.
As with the other compounds we discussed, the best treatment is good management and prevention. By managing cattle and our feeding program carefully, feeding of urea inclusive products is a cost effective practice. Some of the management considerations to keep in mind include:
1) Never provide urea inclusive feeds to excessively hungry cattle where over-consumption could take place.
2) Adapt cattle to feeds containing urea slowly, over a period of one to two weeks if at all possible.
3) Do not “slug feed” feeds containing urea. In situations where cattle are only provided supplement once or twice per week (a practice I strongly discourage for a number of reasons), this feed should not contain urea.
4) Provide access to plenty of fresh water and good quality roughage.
5) Urea containing feeds should not contain urea levels which exceed 1/3 of the total protein content of the product. In other words, if a feed contains 20 percent protein, no more than 6.6 percent should come from non-protein nitrogen.
6) Make sure that feeds containing urea are well mixed and evenly distributed in the bunk.
Urea is a valuable tool in the typical cattle feeding program and can be effectively used to help provide necessary nutrients as well as cut costs. If common sense is used and good management practices are implemented, the potential risks involved in use of this ingredient become minimal. Even “safe” ingredients can create problems if managed incorrectly. Taking the time to learn appropriate management steps and feeding practices will insure the safety of your program as well as increase your overall profitability.
Dr. Steve Blezinger is a management and nutritional consultant with an office in Sulphur Springs, TX. He can be reached at 667 CR 4711 Sulphur Springs, TX 75482, by phone at (903) 352-3475 or by e-mail at firstname.lastname@example.org. For more information please visit www.blnconsult.com.