Only animals with a functioning rumen can utilize urea; therefore it should not be given to young calves and monogastric animals. Unlike protein, urea does not contain energy, phosphorus or sulphur; hence a feed mixture containing urea should be supplemented to make up for these deficiencies. Poor results are usually experienced with urea when fats provide a substantial portion of the energy in the diet.
As a specific maximum amount of bacteria can be produced each day by the rumen, it is not possible to provide high-producing animals with the total requirement of protein in the form of nonprotein nitrogen.
An experiment with fattening bulls in Cuba (Preston, 1971) showed that an animal gaining 1 kg per day can be given a maximum of 60% of the required nitrogen in the form of urea. In this case the protein nitrogen was provided by insoluble Peruvian fish meal. The true protein required must be given in the form of insoluble or protected protein so that it will pass the rumen without being hydrolysed and reach the stomach unchanged. Generally, proteins that have been heated during processing (oil extraction or drying) are less soluble. For instance, soybean oil meal protein which has been heated to 80 C for ten minutes is only 67% as digestible as soybean oil meal protein processed at ambient temperatures; if heated to 120 C for fifteen minutes, the solubility decreases to about 20% of that of soybean oil meal processed at ambient temperature. At the optimum temperature the rumen digestibility is reduced while the digestibility in the true stomach remains high. Too high a temperature diminishes the biological value of the proteins by destroying essential amino acids. Proteins can also be protected from attack in the rumen by treatment with formaldehyde.
The energy source used with nonprotein nitrogen should easily ferment in the rumen. The carbohydrates in molasses are completely fermented in the rumen, whereas up to 30% of the starch in grain may pass undegraded to the stomach and thus be unavailable for synthesis.
There are two main ways of using nonprotein nitrogen:
1. Nonprotein nitrogen can be used alone or with a small amount of energy. Cattle consuming only poor-quality roughage usually have too low an intake of protein and energy. If extra nitrogen is provided, the intake of dry matter will usually increase and the nutritional status improve. Biuret can be provided alone, whereas urea should be mixed with a source of energy to avoid toxicity. The addition of starch alone or any other easily fermentable carbohydrate to a protein-deficient high-roughage diet decreases the digestibility of the roughage because of the competition for nitrogen between the fast-growing starch-consuming bacteria and the slow-growing cellulolytic bacteria. Additional nitrogen in the form of nonprotein nitrogen will thus increase the digestibility of roughage in such cases.
2. Nonprotein nitrogen (usually urea) can be used as a cheap substitute for true protein in balanced feeds.
Nonprotein nitrogen can be fed in the following ways:
(a) By spraying pastures with a mixture of molasses and urea. This method is now seldom used because of the high cost of labour and the high degree of waste.
(b) In supplement the crude protein in certain silages and hays. The silages supplemented with urea (mixed in during the silage-making process) are usually maize and sorghum silage and less frequently grass silage. Usually 0.5% urea, but occasionally close to 1% is added; adding urea to hay is seldom economical. Where possible, biuret should be used for safety.
(c) In dry feeds as a partial replacement for more expensive true protein. If mixed into dry feeds, free-flowing feed-grade urea should be used. It is possible, however, to use the cheaper fertilizer-grade urea if it is added in a suspension or a mixture with molasses. The feed should be thoroughly mixed so that no lumps of urea can be eaten by cattle. As the risk of toxicity from urea is greatest in dry feeds, usually no more than 3% urea is added to a concentrate and no more than 1 % to complete rations. Much higher concentrations have been used for adapted cows.
(d) As a block lick. These are often used under range conditions. A home-made block that has proved useful was developed by extension officers of the Queensland (Australia) Department of Primary industries (Alexander, 1971) using the following formula:
High palatability: Crushed grain 40%, Coarse salt 20%, Molasses 20%, Urea 10%, Bone meal 7%, Meat meal 5%
Low palatability: Crushed grain 32%, Coarse salt 32%, Molasses 15%, Urea 10%, Bone meal 7%, Meat meal 5%
The block contains about 34% crude protein. All block licks containing urea should be cast in a tin or hard box to prevent the animals from biting chunks off the block. They should also be protected from rain so that the animals do not drink a solution of urea. Cattle with no previous access to salt must be given salt for at least ten days before introducing a lick containing urea, as animals with a craving for salt may inadvertently take an overdose of urea.
(e) In liquid supplements and feeds (ref. 390). Liquid supplements contain a high percentage of urea - usually about 10%. As liquid feeds are intended for maximum utilization of molasses, they have a lower percentage of urea.
The proprietary mixed liquid supplement consists of a liquid vehicle, most often molasses of some type, fermentation liquors or propylene glycol, to which nonprotein nitrogen, minerals and vitamins have usually been added. Liquid supplements can be fed like dry-concentrate supplements at a rate of 0.5-2 kg together with roughage. They can also be given as a supplement to range cattle. The intake, generally 0.5-1.5 kg per day, is controlled by regulators in the feed (ethyl acetate, phosphoric acid or calcium chloride) or by mechanical devices. These devices are usually feeders with a wheel that turns as the animals lick the mixture from it, thereby exposing a fresh layer of liquid. The reason for the popularity of liquid supplements is their low cost, as they are made of inexpensive ingredients, and there are also savings in handling costs and waste. Liquid feeds have solved most of the problems associated with the use of urea. Because they contain molasses and are gradually consumed over a long period, the problems of palatability and toxicity are overcome and utilization is improved. The use of urea with molasses for maximum utilization of molasses is described in the Molasses datasheet.
(f) In ammoniated feeds. The ammoniation is usually performed on an industrial scale as efficient binding of ammonia to a feed requires high temperature and pressure. The most common ammoniated feeds are rice hulls, maize stovers, sugar-beet pulp and maize ears.