Feed-grade urea contains 42 to 45 percent nitrogen (262 to 281 percent crude protein equivalent). Fertilizer-grade urea usually contains 46 percent nitrogen (290 percent crude protein) because smaller amounts of conditioners have been added to prevent lumping. One kilogram of urea plus 6 kilograms of maize or other grain furnishes the same amount of nitrogen as 7 kilograms of soybean meal or an equivalent high-protein feed, but it-may be lower in energy content since urea adds no useful energy.
The amount of urea included in concentrate mixtures for cattle or sheep should not exceed 3 percent and usually the addition of 1 to 1.5 percent will prove adequate. In the total ration, the amount of urea should not exceed 1 percent. At these levels of intake, urea has proved an effective replacement for vegetable proteins in rations for growing and fattening beef cattle and for dairy cattle in the United States, when liberal amounts of maize were fed. As a supplement to low-protein roughages, mixtures of urea and molasses have usually given results equal to or slightly inferior to vegetable proteins. Urea mixtures have been lower in cost and for this reason they have been widely used in rations for fattening cattle and milking dairy cows in the United States.
In Canada feed manufacturers are not permitted to use urea in greater amounts than will supply one third of the total nitrogen of the grain mixture for ruminant animals. This, in effect, limits the use of urea to supplying less than one sixth of the nitrogen in the total ration. It is generally considered that this maximum amount can be effectively used for beef cattle. Few, if any, feed manufacturers or feeders, however, use the maximum permissible amounts of urea for the feedlot finishing of beef cattle or for the feeding of high-producing dairy cows. Some dairymen feel that little urea should be used in the feeding of high-producing dairy cows, especially if the concentrate portion of the ration is being fed in a milking parlor where it must be consumed in a relatively short period of time.
Success from the use of urea has not been as great in Canada as in the United States. This may be due to the use of barley, oats, and wheat as the main high-energy feeds, whereas maize is the main grain in the United States.
Nearly all the literature in Australia and South Africa has been concerned with the use of urea to improve the nutritive value of low quality roughages with the general objective of converting a submaintenance to a maintenance ration.
In spite of the great number of experimental observations recorded, this review is forced to the conclusion that no evidence is yet available to support the contention that urea can be employed profitably with low-quality roughages in genuine pastoral conditions. This is regrettable because large quantities of urea preparations are at present being used. It would be wrong to assume from the magnitude of this commercial enterprise that the results are favorable in practice. Of course, the point must be made that lack of evidence cannot be taken to disprove the proposition that urea feeding is beneficial.
The harvesting, storage and feeding of low quality pasture hays or silage are dubious economic propositions because the costs involved will usually be greater (per unit of feed) than for a good hay crop in areas where the latter is possible. The expenditure on these operations has to be weighed against alternatives such as grain storage, forage crops, or silage. Also, as mentioned earlier, in ecosystems giving a higher yield per hectare of low-protein feed, it can reasonably be expected that agronomic improvements will, in due course, make the growing of superior plants possible. Even in semiarid pastoral areas there are possibilities for improving the quality of forage grown.
Besides being less palatable than the more commonly used protein supplements, urea may often be broken down to ammonia in the rumen at a rate faster than the rumen microflora can utilize, for the formation of proteins, amino acids, or other nitrogenous compounds. The result is that the excess of ammonia is absorbed through the rumen wall directly into the bloodstream and much of it may then be excreted in the urine if the intake is higher than the amount required. Methods are needed to reduce the rate of urea breakdown in the rumen. Coating the urea particles with waxes and similar substances has been tried but has been found ineffective. However, it should be possible to incorporate urea into some other material that breaks down slowly to release the urea at a sufficiently slow rate, in the same way that cobalt is slowly released from “cobalt bullets.” Research on slow-release pellets or on new compounds that would allow urea to be released in about five hours would seem warranted.
