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Understanding your protein supplement

Nutrition with John McKinnon, beef cattle nutritionist

cows at a mineral feeding trough

In my last column, I wrote about what a crazy year it has been for growing and harvesting hay. The variability in this year’s hay crop will force many producers to purchase supplemental feed in order to meet the nutrient requirements of their cattle this winter. Last month we looked at alternate feed sources that can be used to supplement energy and protein. With this column I want to look specifically at commercial protein supplements and examine some of the issues you need to consider when purchasing this class of supplement.

Let’s start by examining the nutrient composition of the supplement. The feed tag will provide you with basic information on nutrient content, the form of specific nutrients, recommended intake level and specifics on medications such as ionophores. For example, most protein supplements are not only a source of protein but also of a variety of macro (i.e. calcium, phosphorus, magnesium) and micro or trace (i.e. copper, zinc, manganese, selenium) minerals and vitamins (i.e. A, D and E). The content of macro minerals such as calcium and phosphorus in the supplement are given as percentages while trace minerals are expressed as milligrams per kilogram. Vitamin levels are expressed as international units (IU) per kilogram of supplement.

In the case of a commercial protein supplement, the tag will indicate a specific minimum crude protein content. Commercial supplements can be formulated to a wide range of crude protein levels (i.e. eight to 32 per cent), using all natural protein sources (i.e. canola or soybean meal) or by incorporating a non-protein nitrogen source such as urea. Compounds that supply non-protein nitrogen are not protein sources per se; rather they provide rumen bacteria with a source of nitrogen for amino acid and protein synthesis. This bacterial protein ultimately benefits the animal as it leaves the rumen with undigested feed and is absorbed in the lower gut and used for maintenance and productive purposes (i.e. pregnancy, growth).

In order to understand the difference between natural and non-protein nitrogen sources of crude protein, it is necessary to understand the nature of the term crude protein. Crude protein is an all-encompassing term that reflects the amount of nitrogen in a feed. This nitrogen can originate from natural protein sources such as that found in forages, cereal grains or oilseed meals or from non-protein nitrogen sources as in the case of urea. With natural protein sources, the nitrogen originates primarily from amino acids. This amino acid nitrogen is used by rumen bacteria for protein synthesis as discussed above or if the protein bypasses the rumen, the amino acids can be used directly by the animal for productive purposes. On the other hand, nitrogen from urea is only used by rumen bacteria for protein synthesis and is of no direct value to the animal and in fact can be toxic if fed in excess.

How do you know the type of protein supplement you are purchasing? Again, the feed tag provides critical information that can guide you in making the appropriate decision. For example, a tag that gives the protein level as 32(0) per cent indicates a 32 per cent supplement where all the protein is derived from natural sources, while a 32(14) tag indicates a 32 per cent protein supplement where a maximum of 14 per cent or almost half of the protein is derived from non-protein nitrogen sources. How much urea does this supplement contain? Urea is 281 per cent crude protein (45 per cent nitrogen). A supplement where 14 per cent of the protein comes from urea will contain five per cent urea (i.e. 14% / 2.81 = 5%). All things being equal, the 32(14) supplement should be less expensive than the 32(0) supplement.

Does the form of protein matter? The answer to this question depends on the type of animal as well as the feeding situation. If cattle are going to efficiently use urea as a protein source, it is necessary that the diet provides adequate energy for rumen bacterial growth. This generally means a relatively high level of grain feeding. A good example would be the use of a urea-based supplement with a corn grain-based finishing diet. Such diets are typically low in crude protein and high in energy, a situation that optimizes the opportunity for rumen bacteria to capture nitrogen and utilize it to synthesize protein. In contrast, urea-based supplements are not recommended for starting calves on feed due to issues with palatability and low feed intake. As well, newly weaned calves are not typically fed high-energy diets, making for inefficient capture of urea nitrogen by the bacteria. For the same reason, urea-based supplements are not recommended for grazing cattle. However, in some pasture situations where the protein content of the grass is limiting, a protein supplement that is a combination of natural and urea-based sources can increase forage intake and utilization.

Commercial protein supplements can be purchased in a variety of forms including mash, pellets, blocks and tubs. As indicated above, in addition to protein most of these supplements supply other critical nutrients and, in some cases, medications such as ionophores. The decision to purchase a given supplement should be based on an evaluation of its contribution to meeting all essential nutrients, not just protein.

About the author


John McKinnon

John McKinnon is a Professor Emeritus at the University of Saskatchewan and a consulting nutritionist who can be reached at [email protected].



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