Molasses/urea blocks

The blocks can be made from a variety of components depending on their availability locally, 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.

• Urea, which provides fermentable nitrogen, is the most important component of the block. Urea may increase the intake of straw by cattle by about 40 percent and its digestibility by 8 units (or 20 percent). 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 and high rates of degradation of fibre. Blocks are an excellent way of controlling intake and allow continual access.

• Wheat or rice bran has a multiple purpose in the blocks. It provides some key nutrients including fat, protein and phosphorus, it acts as an absorbent for the moisture contained in molasses and gives structure to the block. I may be replaced by other fibrous materials such as dry and fine bagasse or groundnut hulls which are finely ground but some loss of nutritive value occurs.
• Minerals may be added where appropriate. Common salt is generally added because this is often deficient in the diet and it is inexpensive. Calcium is supplied by molasses and by the gelling agent, calcium oxide or cement. Although phosphorus is deficient, there is no evidence that its addition is beneficial where animals are at below maintenance 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 and in these cases phosphorus should be included in that supplement.
• A gelling agent or binder is necessary in order to solidify the blocks. Although the mechanism of gelling is unknown, various products have been tried successfully: magnesium oxide, bentonite, calcium oxide, calcium hydroxide and cement. The use of cement has raised some questions, from various nutritionists and extension workers, about possible negative effects on animals. In fact, research on the use of cement or its by-product, cement kiln dust, as a mineral supplement have not shown such adverse effects at levels of 1 to 3 percent of the total diet dry matter. (Nevertheless, 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 (e.g. anti-protozoal agents, ionophores) can be added to the molasses blocks which can be an excellent carrier for these products.
• Recent work has shown that the addition of small amount of rumen-insoluble calcium salts of long chain fatty acids could further increase the efficiency of the use of fibrous residues.

Finally, the formulae may vary according to the process adopted in manufacturing the block (Table 1).

Table 1. Examples of formulae according to manufacturing process

Different processes have been tried and can be grouped in three categories:

1. The "hot" process

This is the process which was first recommended in Australia. The molasses (60 percent) and urea (10 percent) were cooked with magnesium oxide (5 percent), calcium carbonate (4 percent) and bentonite (1 percent) at a temperature of 100-120 deg C for about 10 minutes. The content was brought to a temperature of about 70 deg C and mixture was left to cool slowly which enhanced solidification. It settled after some hours. The cooking was done in a double-jacketed rotating boiler with circulating water and steam.

2. The "warm" process

The molasses (55 percent) was heated to bring the temperature to about 40-50 deg C and the urea without water (7.5 percent) is dissolved in the molasses (Choo, 1985). The gelling agent was calcium oxide (10 percent). The rest was made up of common salt (5 percent) and bran (22.5 percent).

The inconvenience of these processes, particularly the "hot" one, is the necessity for providing energy for heating. However, if it is possible to use the hot molasses as it leaves the sugar factory or if an excess of steam is available, the cost of energy may be acceptable. The advantages are the reduction of time for setting and the finals product is not hygroscopic.

3 The "cold" process

It has been noted that, in tropical conditions, it was not necessary to heat the molasses in order to obtain a good block when 10 percent of calcium oxide was used as a gelling agent. This observation is of primary importance when blocks are manufactured in a unit separate from the sugar factory as was the case in Senegal.

The "cold" process involves a horizontal paddle mixer, with double axes, which is used to mix, in the following order of introduction, molasses (50 percent), urea (10 percent), salt (5 percent), calcium oxide (10 percent) and bran (25 percent). The mixture is then poured into moulds (plastic mason's pails or a frame made of four boards 2.5 m x 0.2 m). After about 15 hours, blocks may be removed from the mould and they may be transported by truck after 2 days.

Calcium oxide may be replaced by cement, but when cement is used it is important to mix it previously with about 40 percent of its weight in water, and common salt to be included in the block. This ensures its binding action, as the water in molasses does not seem to be available for the cement. The quality of the cement is of primary importance. Mixing the salt with cement accelerates hardening.

The disadvantage of the "cold" process is that it needs some time to set and the final product is somewhat hygroscopic. The advantages are the saving in energy, and the simplicity and ease of manufacture.

Independent of the process, the hardness of the block is affected by the nature and proportion of the various ingredients. High levels of molasses and urea tend to decrease solidification. The concentration of gelling agents and bran is highly important in the hardness of the final product. For example if the urea percentage is as high as 20 percent, molasses should be reduced to 40-45 percent and the gelling agent needs to be increased. Quick lime produces harder blocks than cement.