The high cost of feed grains and many high-protein grains may make the use of urea as a protein source very cost-effective in many cattle diets. Several issues must be considered, though, to make its use most effective.
Urea is a non-protein nitrogen compound. That is, the nitrogen portion of urea is used as the building block for the production of protein by rumen microbes. Most urea contains about 45% nitrogen, and protein contains 16% nitrogen. Therefore, when urea is converted to protein, the crude protein equivalent value of urea is about 281%. It must be recalled that urea contains no other useful feed components such as energy, minerals, or vitamins.
Cattle and other ruminants convert urea to protein through the production of ammonia and carbon dioxide. Sewell (1993) points out the ammonia that is released from urea can go two pathways in the animal. The first location is in the production of microbial protein, and the second is the liver where it is detoxified and excreted in the urine.
When too much ammonia escapes the rumen because the microbes are not able to utilize enough of it for protein, the capacity of the liver for excretion can be overwhelmed and a toxicity can occur. It is vitally important that the right level of urea is fed and that there will be sufficient bacterial action to produce protein. The level of bacterial action will be determined by the urea fermentation potential (UFP) of the total ration. When there is a higher level of fermentable energy available in the feed than what is required by the rumen to convert the ammonia in the feed to protein, the feed will have a positive UFP. If there is insufficient energy in the feed, or the feed already contains significant crude protein, the UFP will be negative. Thus, corn will have a positive UFP and fescue hay and soybean meal will both have a negative UFP. These results indicate feedlot rations high in energy will usually have the best potential for the use of urea as a protein source. However, combining several feeds that result in the dilution of urea as a protein source with other feeds can result in the formulation of a good protein supplement for many classes of cattle on high energy diets.
Precautions for Feeding Urea
As previously shown, urea can be harmful to cattle if the proper precautions are not taken to insure success. These precautions include:
- Formulate diets precisely with feed components with known energy and crude protein values from laboratory analysis.
- Use urea to provide no more than one-third of the total protein requirement in the ration.
- Feed the ration at least twice daily to prevent an ammonia overload.
- Weigh all feed components precisely and make sure the urea is mixed completely and uniformly in the ration.
- Do not start feeder cattle on diets with urea. Rather, use plant proteins in starting rations and then convert to urea-containing rations after 30 days of feeding. Cattle will often have a period of lower intake and lower weight gains during initial feeding of urea-containing rations.
- Urea contains only nitrogen for protein, so mineral, vitamin, and energy values should be adjusted with other diet components.
- Higher rumen-bypass proteins such as distillers grains (where the more of the protein is absorbed in the lower gut and outside the rumen) are complementary to urea in rations.