5. SUGGESTED METHODS OF FEEDING UREA AND OTHER NPN COMPOUNDS

Urea in concentrate mixtures

In the United States most feed-grade urea is mixed into commercially distributed concentrate feeds for growing or lactating dairy cattle or for finishing beef cattle. Concentrate mixtures containing 12 to 20 percent crude protein, designed for direct feeding to dairy cattle, usually contain 1.0 to 2.0 percent urea to replace an equal amount of nitrogen from oilseed meals or protein-rich by-product feeds. These mixtures usually contain large amounts of cereal grains or by-products rich in starch. When as much as 3 percent urea has been incorporated into meal mixtures of cereal grains, palatability has sometimes been lower than desirable for high-producing cows, even when 7 percent cane molasses was included. When such mixtures were pelleted (cubed), dairy cattle accepted these feeds, with urea or other NPN compounds, as well as they consumed mixtures derived from vegetable protein sources only (Reaves, Bush and Stout, 1966). It is important to maintain a high digestible energy content in meal mixtures containing urea in order to ensure a performance similar to that from rations containing only vegetable proteins. Urea is most frequently prepared into a premix or high-protein supplement which may be hand-fed in restricted amounts, in addition to other concentrate feeds, or mixed with cereals. Table 4 lists the formulas of three mixtures containing urea and having 32, 40 and 50 percent crude protein.

Table 4. - Examples of protein supplements

Table 5 lists formulas of concentrate prepared by mixing the supplements shown in Table 4 with cereal grains and by-product feeds. These feeds all contain approximately 16 percent crude protein and they can be fed either as meals or pellets.

Table5. - Examples of 16 percent protein dairy feeds using the supplements from Table 4

¹ Maize grain and cob ground together.

Other examples of concentrate mixtures which have been used with growing and lactating dairy cattle are shown in Table 6.

Table 6. - Other examples of dairy feeds containing urea

Urea in high-protein supplements

It is often convenient and less expensive to use a higher protein feed because smaller quantities will then satisfy the requirements of cattle or sheep. Beeson (1965), Burroughs (1965) and Conrad and Hibbs (1966) have reported good results with a number of high-protein supplements containing urea. The ingredients used in these mixtures are listed in Tables 7 and 8.

Care must be exercised to limit the amounts of high-protein supplements containing urea which are fed to animals so that they do not cause toxicity. Mixtures containing as much as 80 percent or more crude protein but with a high urea content (Table 8) should be fed at a level not exceeding 0.3 to 0.5 kilogram daily to cattle weighing 350 kilograms or more and they should be fed along with other concentrates. Small animals should receive correspondingly less of these supplements. Animals that have not been fed previously, and especially underfed animals receiving only poor quality hay or pasturage, may be unusually sensitive to urea toxicity and they must gradually be adapted to urea feeding. Conrad and Hibbs (1966) fed the previously mentioned 100 percent protein mixture, pelleted, to milking dairy cows as a supplement to maize grain and maize silage. The supplement containing the dehydrated alfalfa meal gave better results than mixtures without alfalfa. They recommended feeding not more than 1.4 kilograms daily in 2 or 3 individual lots. Animals did not eat the pellets well unless they were mixed with other feeds. Other studies confirmed these favorable results.

Table 7. - Protein supplements for beef cattle

Note. May add 10 million IU of vitamin A and 2 million IU of vitamin D to mix A and twice these levels to mixtures B and C.

In studies with growing and finishing beef cattle the high-protein urea mixtures have given slightly slower rates of gain than equal nitrogen from soybean meal, but the feed cost was appreciably lower in the United States.

Table 8. - High-protein mixtures

Nonprotein nitrogen additions to silage and hay

Favorable results have been recorded with dairy and beef cattle when urea has been added at the time of ensiling to green maize at the rate of 0.5 percent of the weight of the fresh forage. Dairy cows consumed less silage when more than 0.5 percent urea was added. With a high moisture silage less urea should be used than when a lower moisture silage is being made. The dry matter of the crop being ensiled should be 25 to 32 percent to prevent any seepage of moisture because such drainage greatly increases the loss of urea from the silage. For dairy cattle, the use of urea-treated maize silage and 1 percent urea in the concentrate gave results equal to vegetable protein mixtures. Urea should not be added to legume silages or mixed forages which contain 10 percent crude protein on an air dry basis.

Years ago, Virtanen introduced the use of ammonium sulfate (AIV salt) as a silage additive in Scandinavian countries. This chemical gave a less palatable silage than when molasses was used.

The Queensland Department of Agriculture and Stock (Anonymous, 1961) reported that the spraying of a low-protein bush hay containing 4.6 percent crude protein with a molasses-urea mixture sufficient to supply 40 grams of urea per head per day increased the hay consumption by almost 40 percent and the supplemented cattle gained whereas the control lost weight. One part of urea (dissolved in water) mixed with four parts of molasses was recommended for spraying on- to stored hay.

In contrast, steers fed 1.4 kilograms of lucerne hay per day twice each week as a supplement to the same pasture gained 55 kilograms more than the nonsupplemented steers. The superior performance of cattle fed supplements of vegetable proteins in comparison with urea supplements has been reported in many instances, but in some studies urea has given approximately equal performance, as pointed out previously.

As reviewed earlier in this report, extensive use has been made of the addition of urea and ammonium salts to low-protein forages, preserved as silage, in the U.S.S.R. and eastern European countries.

Salt licks containing urea or biuret

Homemade urea licks suggested by the Queensland Department of Agriculture and Stock, Brisbane, Australia may contain the following ingredients (Table 9):

Table 9. - Ingredients for two homemade urea lick mixtures

To prepare these mixtures, the urea is completely dissolved in 5 liters of hot water. The salt is then dissolved and the solution mixed with the molasses before the dry ingredients are added. A cement mixer has proved satisfactory for mixing the ingredients. The moist mixture should be packed into troughs or other containers and allowed to harden for several hours before cattle or sheep are permitted access to it. Urea lick A is more palatable than mixture B. Cattle should not eat more than 0.5 kilogram of either mixture per day, and to avoid danger of toxicity the compositions should be adjusted to prevent larger intakes being consumed. The addition of 2 or 3 kilograms of ground straw, rice hulls or maize cobs to the mixtures will decrease the amounts consumed. The licks should not be allowed to become wetted by rain or to become so soft that cattle may eat too much.

When sheep and cattle fed veld hay (4.1 percent protein) were allowed to consume ad libitum a lick containing 3 parts crude biuret, 2 parts bone meal and 1 part salt free choice (Mackenzie and Altona, 1964), loss of weight was prevented. Cattle consumed daily 33 to 95 grams per head of the lick and sheep averaged 54 grams. The crude biuret had the following analytical composition: 64 percent biuret, 5 percent urea, 3 percent cyanuric acid, 3 percent triuret, 25 percent water, and 30 percent nitrogen.

A number of urea salt licks are now produced and sold commercially. If they are used as prescribed by the manufacturers, they can be expected to give favorable responses with ruminants. Whether or not it is economically advisable to use them instead of another source of nitrogen must be determined by local conditions.

Precautions when using urea

There is no question but that urea and certain other nonprotein nitrogen substances can be fed safely to ruminants to replace part of the dietary vegetable protein. Favorable results can be expected when cereal grains are also included in the ration, but performance may be less satisfactory on forage alone.

Urea may cause toxicity and even death in ruminants if it is fed inadequately mixed with other feeds or in too large a dose. The toxic signs can easily be recognized.

Research by Kondos and McClymont (1965) suggests that high urea supplements should be withdrawn at least one half day before and after the administration of carbon tetrachloride, if the latter is being given as treatment against liver flukes and Haemonchus contortus infestations, because a concomitant absorption of ammonia increases the risks of toxic effects resulting from the drug.

Animals should never be permitted access to urea not mixed with other feeds.