Both the rumen microbes and animals need protein for maintenance and growth. In reality, they both need amino acids, which are the building blocks of protein. Microbes can use amino acids from digestion of feed protein or can synthesize amino acids from non-protein nitrogen such as urea. The animal must use preformed amino acids, it cannot synthesize amino acids from non-protein nitrogen. Preformed amino acids can come from digestion of feed protein or microbial protein.
In ruminant animals feed protein is either degraded in the rumen by microbes, termed rumen degradable protein, or digested in the small intestine of the animal, termed rumen undegradable protein. All feeds have some rumen degradable protein and some rumen undegradable protein, but varying the proportion of each. For example, soybean meal protein is approximately 75% rumen degradable protein and 25% rumen undegradable protein, whereas, dried distillers grains protein is approximately 40% rumen degradable protein and 60% rumen undegradable protein. Feeds like urea that are termed non-protein nitrogen are 100% rumen degradable protein.
In animals, amino acids are not stored in the body like carbohydrates are stored as fat. If amino acids are not needed for protein synthesis, the amino acids are broken down in the liver and the nitrogen in the amino acids are excreted in the urine. However, ruminant animals have the ability to recycle this nitrogen back to the rumen where microbes can convert the nitrogen to amino acids again. In this way, extra amino acids can provide microbes with the nitrogen they need for maintenance and growth.
Research has demonstrated that feeding 2 times the requirement of rumen degradable protein every other day provides the same response in low-quality forage digestion and intake as feeding the daily requirement of rumen degradable protein every day. The same response is due to the ability of cattle to recycle the excess amino acids/nitrogen for the microbes to continue to growth on the non-feeding day. Additionally, rumen undegradable protein can also be recycled, and potentially more efficiently because of slower rates of digestion.
A recent study evaluated whether adding a source of rumen degradable protein in the form of urea to a high rumen undegradable protein feed improved performance of calves grazing low-quality (crude protein ~6%) corn residue. Adding urea to dried distillers grains provided no improvement in growth of calves (Figure 1; Trial 1). In a subsequent trial, feeding dried distillers grains resulted in the same average daily gain as feeding a combination of SoyPass and soybean meal (Figure 1; Trial 2). SoyPass is a modified soybean meal product to increase the rumen undegradable protein proportion to 75%. The SoyPass/soybean meal combination provided mathematically a balance of rumen degradable and undegradable protein without accounting for the nitrogen recycling.
Cattle have the ability to recycle nitrogen from both rumen degradable and rumen undegradable protein. Feeding adequate protein from dried distillers grains does not require additional rumen degradable protein.
