Urea and
Urea-Molasses Blocks (UMB)

Urea is a nitrogen-based waste product secreted by the kidneys. It is formed during the breakdown of protein in the body. Whilst urea is not a protein, it does contain nitrogen and can be used by the microbial population in the rumen to synthesize protein. Along with amonia, urea is therefore classified as non-protein nitrogen (NPN). Urea is toxic and dangerous to use. Legumes and tree forages also have the capacity to provide needed protein inputs into sheep and goat production systems, whilst at the same time benefiting the environment through nitrogen fixation and organic matter.

Urea Molasses-Urea Blocks (UMB) Treating straw to increase its nutritive value   See also: Crop and Industrial By-Products Crop and Industrial By-Products: Sugarcane Digestive Systems and Digestion of Feeds References and Further Reading

There is no store in the body for nitrogen, as there is for carbohydrates of fats. As a result any nitrogen that is ingested in excess of what is required by the organism has to be excreted. This excess nitrogen, derived from protein metabolism, is first converted to amonia and then dealt with in different ways by different groups of animals. Many aquatic animals, for example, simply excrete the amonia directly into the surrounding water. Birds (and reptiles) excrete the amonia in the form of uric acid. Mammals convert the amonia into urea before this is excreted.

In the bidy, urea is synthesized in the liver and is then secreted into the bloodstream. From there it is taken up by the kidneys and excreted in the urine. Although they are not proteins, both urea and amonia contain nitrogen.

When fed as a part of the diet to ruminants, urea is converted to ammonia by the microflora in the rumen. Microorganisms in the rumen use ammonia to make microbial proteins, which are then digested by the animal. Urea therefore has a value that is equivalent to protein for ruminants. (For more details on the process of digestion in ruminants see Digestive Systems and Digestion of Feeds).

Poor quality roughages, e.g. cereal straws, poor quality hay, or dry-season forage that contains a lot of stem but not much leaf, can be deficient in crude protein (N). Insufficient N in the diet means that rumen microorganisms cannot make enough microbial protein for the maintenance or growth of the animal.

Commercial Production of Urea

The commercial synthesis of urea is a by-product of the petro-chemical industry and involves the combination of ammonia and carbon dioxide at high pressure to form ammonium carbamate which is subsequently dehydrated by the application of heat to form urea and water. Urea is granulated to produce a white granular product for use in industry and agriculture. As a white crystalline solid containing 46% nitrogen, urea widely used in the agricultural industry both as an animal feed additive and as a fertilizer.

On its own, urea is bitter and unpalatable. Combining it with something like molasses or grain (or both) makes the urea more palatable. In addition, the molasses or grain will provide the necessary energy to assist the digestive process.

	Urea can therefore replace part of the protein, as long as it is avaiable and if its price is favourable compared with alternatives.
	Urea can be used in combination with other ingredients in a urea-molasses mineral block (UMB).
	Urea can also be added to low quality feeds such as straw in order to increase the nutritive value.
	Trials show you need to feed urea for more than a month to gain any economic advantage, because it may take that long for an animal's rumen to adjust to it.

Urea is Toxic Urea in high levels is toxic to livestock. Do not feed urea at levels greater than 1% of a total ration (for example presented as grain and hay), or no more than 15 gram of urea per 50 kg live weight in a single feeding. Do not handle urea without specialised equipment and facilities. Do not feed urea to non-ruminant livestock.

Molasses is a major by-product of the sugarcane industry. It is a source of energy and a widely available concentrated form of fermentable carbohydrate. For more information, see the page on Crop and Industrial By-Products: Sugarcane.

You will find these referred to either as Molasses-Urea blocks or as Urea-Molasses blocks.

Molasses, urea and other ingredients are used in the manufacture of molasses/urea feeds that are prepared as blocks, pastes or licks. The rate of feed intake is reduced when the molasses is prepared in this way (compared with a straight molasses lick for example, but intakes should still be controlled when the blocks are first introduced. These preparations are an excellent way of providing readily degradable protein and readily fermentable energy to ruminant animals, and they help increase the protein supply to the animal. However, goats are particularly sensitive to urea toxicity and so it is essential that the blocks are accurately made so that goats are not fed too much urea at one time.

The blocks can be made from a variety of components depending on their local availability, nutritive value, price, existing facilities for their use and their influence on the quality of blocks. They can also include specific components.

• Molasses provides fermentable substrate and various minerals and trace elements (but low amounts of phosphorous). Because of its pleasant taste and smell, it makes the block very attractive and palatable to animals. The degree Brix of the molasses should be as high as possible, and preferably higher than 85, to ensure solidification. (Degrees Brix is a hydrometer scale for sugar solutions graduated so that readings at a specified temperature represent percentages by weight of sugar in the solution, so 85° Brix is equivalent to 85% sugar).
  
• Urea, which provides fermentable nitrogen, is the most important component of the block. Urea may increase the intake of straw and other low quality forages as well as their digestibility. The intake of urea must be limited to avoid toxicity problems but sufficient to maintain ammonia levels in the rumen consistently above 200 mg N/l for growth of microorganisms in the rumen and high rates of degradation of fibre.
  
• Wheat or rice bran has a multiple purpose in the blocks. It provides some key nutrients including fat, protein and phosphorus (see Dietary Requirements). It also acts as an absorbent for the moisture contained in molasses and gives structure to the block. It may be replaced by other fibrous materials such as dry and fine bagasse (the residue from sugar cane processing) or groundnut hulls which are finely ground but some loss of nutritive value occurs.
  
• Other crop residues can also be included in Molasses-Urea blocks. Clearly this depends on availability, and some crop residues and by-products will provide more nutrients than others.
  
• Minerals may be added where appropriate. Common salt is generally added because this is often deficient in the diet and it is cheap. Calcium is supplied by molasses and by the gelling agent, calcium oxide or cement (see below). Although phosphorus is deficient, there is no evidence that its addition is beneficial where animals are at below maintenance requiremetns when grazing on dry mature pastures or fed low-quality forage. Mineral requirements are reduced at maintenance or survival levels. Deficiencies will generally become a problem only when production is increased, particularly when a bypass protein supplement is given (proteins that are not degraded by rumen microorganisms and are digested in the intestines, see Digestive Systems and Digestion of Feeds). In these cases phosphorus should be included in that supplement.
  
• A gelling agent or binder is necessary in order to solidify the blocks. Various products have been tried successfully: magnesium oxide, bentonite, calcium oxide, calcium hydroxide and cement. The use of cement has raised questions about possible negative effects on animals. Research on the use of cement or its by-product, cement kiln dust, as a mineral supplement have not shown adverse effects at levels of 1 to 3 per cent of the total diet dry matter. However, the USDA has restricted the use of cement kiln dust since it could cause a deposit of heavy metals in animal tissue.
  
• Various chemicals or drugs for the control of parasites or for manipulation of rumen fermentation can be added to the molasses blocks which can be an excellent carrier for these products.

When straws form a large component of the diet, the rumen micro-organisms grow very slowly because of the low availability of either energy or protein. This in turn lowers the digestibility of straw still further, as it is only in the rumen that the straw will be digested. Straw digestibility is increased a little (although the amount of feeding value obtained from straw is still extremely low) if the rumen micro-organisms are provided with some readily available energy and protein, so that they are able to grow. The increased microbial population is then able to digest the straw to a slightly greater extent. This is the rationale behind either treating straw with urea (to provide Nitrogen for the rumen microbes) or supplementing the diet with protein, urea or (preferably) both urea and molasses.

There has been a considerable amount of work done on the urea treatment of straw. The recommended treatment rate is 40 g urea/kg straw with the urea usually being added as a solution in water (40 g urea/l water) which is then sprinkled on the straw. The straw may then either be fed straight away, or ensiled to enable the urea to degrade the fibre to some extent. If the urea treated straw is fed straight away, then straw digestibility is increased by about 5 units, whereas if it is ensiled for ten days, the increase in digestibility is twice this. However, although there is considerable evidence of the beneficial effects of treating straw with urea, the uptake of this technology in all areas has been extremely low. A number of reasons have been given for this, which include:

1. The cost and availability of the urea
2. Lack of knowledge of the technology
3. Lack of benefit observed in farm situation
4. Difficulty of practising technology.

It should also be noted, when using this technology to improve the feeding value of straw for goats, that goats are extremely susceptible to urea toxicity and will die from urea toxicity at much lower dose rates than is the case with large ruminants such as cattle and buffalo. It is therefore important that if straw is treated with urea, the urea solution is dispersed throughout the straw, and no ‘hot spots’ of high urea concentration are produced.

A more applicable technology, perhaps, for smallstock keepers is to use straws and stovers as only a small part of the diet for sheep and goats (since they will need other supplementation anyway to meet their maintenance requirements). Supplementing the diet with green forage or legumes that will provide some protein will stimulate microbial growth in the rumen, and increase the digestibility of the straw to some extent. Cakes and meals from the processing of oilseeds (either industrially or on the farm) can also be used to help balance a straw-based diet. However, again, care should be taken when feeding these residues as they often contain a number of anti-nutritive factors.

Khan, M. A. S. and Chowdhury, M. A. R. (2002). Urea Molasses Blocks to improve milk production and reproductive performance of cross-bred dairy cattle under smallholder farm condition in Bangladesh. In: Responding to the increasing global demand for animal products. Proceedings of an International Conference organised by the British Society of Animal Science, the American Society of Animal Science and the Mexican Society of Animal Science. Merida, Mexico, 12-15 November 2002.