Urea is not protein, but for the ruminant (cattle, sheep, deer, goats) it does have the potential equivalent of 281% crude protein under certain circumstances. The items to consider bringing this about deal with the animal’s age, the quantity of fermentable carbohydrates provided in the animal’s diet and the other forms of protein in the diet of the animal. Careless use of urea as a feedstuff can bring on poor feed efficiency, metabolic troubles (especially seen at parturition in cows), central nervous system disorders and death.
Age is probably not a completely accurate classification as much as it is a function of the adaptability. The newborn calf for instance is not born with a functional rumen. This comes about over time with the introduction of solid feed along with exposure to the cow and environment which inoculate the rumen and stimulate rumen development and function. Generally it is a good rule of thumb to hold off using urea as a protein supplement for cattle until they are about 600 lbs. in order to ensure all systems are up and working. Likewise, an animal which has not had urea in it’s diet also requires a bit of adaptation time. Therefore, it is usually wise to hold back on urea use in receiving rations for purchased cattle or cattle just removed from pasture.
Sugar, starch, pectin, fiber and a number of derivatives of these items provide a source of fermentable carbohydrate that rumen micro flora can degrade and with the addition of a nitrogen source such as urea form a high quality protein that that animal can use. All fermentable carbohydrates are not created equal however. The fiber portion or neutral detergent fiber (NDF) fraction on a feed analysis ferments slower than the other carbohydrates listed, which would appear as the non fiber carbohydrate (NFC) on the feed test and limits the use of urea in the diet because urea is very readily available. Therefore a good quantity of NFC must be present to utilize the urea properly since the rumen micro flora can rapidly utilize this fraction and capture the nitrogen (ammonia) released from the urea. It is tempting to use urea to add crude protein to poor forage, but do so cautiously since without the readily available carbohydrate the urea may reduce energy availability of the forage to the animal since the excess urea (ammonia) must be excreted into manure in order to prevent toxicity and this requires energy. Urea could be used in this situation quite effectively though if a small quantity of grain or molasses would be added. A estimate of urea inclusion here would be about 1½ to 3% of the grain mix.
Other Protein in Diet
Urea is in a class of feed nutrients called nonprotein nitrogen (NPN). This classification of nutrients along with excess protein and highly rumen degradable proteins and their derivatives are converted to ammonia in the rumen and supply a source of nitrogen to rumen micro flora. Urea can be added to a point where this pool of nitrogen for microbial use is satisfied. Further additions lead to diet energy loss because of excretion or ammonia toxicity. With this in mind, note that nitrates, high quality cool season forage, raw soybeans and ammoniated feeds when present in a diet limit the amount of urea that can be used.
Generally rations containing high levels of grain or composed of warm season grasses like corn silage, corn stalks or sorghum would utilize urea quite well since this forage tends to have a higher NFC level and lower rumen degradable protein fraction. This does not hold up though if these forages or the animal water source contains high concentrations of nitrate. Proper urea use can be calculated mathematically and it is highly recommended that software like the ISU BRaNDS program be pressed into duty before making ration recommendations that have urea as an ingredient. With contemporary formulations the ISU Beef Center recommends balancing the protein requirement in terms of metabolizable protein. To satisfy this requirement urea can be used in the diet up to the point where the rumen degradable intake protein (DIP) meets 100% of the available carbohydrate. If the DIP level exceeds 100%, no urea or NPN source should be used. Below are a couple examples of what the above article tried to convey.
Rations for 3rd Trimester Stock Cows
Corn silage 60 lbs
Urea @ .22 lbs and ration is balanced
Corn stalks = 30 lbs
Urea allowed @ .1 lb (ration is not balanced)
If 5 lbs of dry corn is added >>> urea now allowed @ .2 lbs and the ration is now balanced
CRP Hay ration =35l bs
Urea should not be used since this hay has a higher DIP and low NFC ( ration is not balanced)
If 4 lbs of dry corn is added >>> urea is allowed @ .1 lb and ration is now balanced.
Source: Garland Dahlke, IBC assistant scientist