Crude protein is simply the percentage of nitrogen multiplied by 6.25. For beef cows, this simple measurement is adequate for formulating nutrient requirements. However, for growing animals it is important to understand that not all proteins are alike or preform the same. Crude protein in a ruminant animal can be broken down in the rumen (digestible protein), passed through the rumen to the other stomachs or to the hindgut for digestion (bypass protein), or is passed through the feces (undigestible protein).
When feeding ruminants, you are actually feeding two animals. The first being the microorganisms in the rumen and then feeding the animal. The microbes in the rumen require the protein to work efficiently at breaking down and digesting feedstuffs. The protein sources that feed the animal are the bypass protein, the volatile fatty acids and ammonia from microbe digestion, and the dead microbes themselves can be used as a protein in the hindgut.
Each feed ingredient has a certain amount of bypass protein that it contributes to the animal. Ingredients such as soybean meal, alfalfa hay, sunflower meal, feather meal, canola meal, peanut meal, and corn gluten feed contain only ten to thirty percent bypass protein. These feeds are commonly used when degradable protein is needed or when cost effective. A perfect time when one of these supplements is needed is in a situation with really low-quality forage (less than 4-5% crude protein), such as mature and dormant forage. The extra degradable protein these supplements provide will meet the protein requirements of the microbes.
Cottonseed meal and linseed meal are two very common ingredients used in ration formulation, and they contain between thirty to sixty percent bypass protein. We commonly compare soybean meal with cottonseed meal because of their availability and high levels of crude protein, but the degradable fractions of that protein are drastically different.
The feed ingredients with high levels of bypass protein are blood meal, fish meal, brewers grains, and distillers grains. The amount of bypass protein available is these supplements is the reason that they have become popular with producers growing stocker or feeder calves. They supply adequate amounts of protein to the rumen but also supply ample amounts of bypass protein to the animal for growth.
Ingredients such as urea and biuret are almost 100% degradable protein and provide no bypass protein to the animal. These ingredients do not supply any protein in the form of amino acids or peptides but only supply nitrogen to the microbes. Typically, animals consuming grain, silage, alfalfa, or lush pasture do not need to be supplement with rumen degradable protein.
So, when you are comparing supplements for your animals just remember that not all protein is the same. Find the protein source that best fits your situation at the most economical cost. If you have any questions about feeds and feeding, please contact your county’s OSU Extension office. (Source: Earl Ward, OSU Extension Area Livestock Specialist)
Reference: New Protein Values for Ingredients Used in Growing Cattle Rations. Fact Sheet G84-694. https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1292&context=extensionhist
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