When we think about protein in a ration, we typically think of it as the most expensive nutrient from a feed cost standpoint, but it can also be expensive from a cost of gain perspective if we do not provide enough in a ration. Understanding how we define protein and its role in animal growth and production can help nutritionists and feedlot operators manage rations and feed inventory.
What is protein?
Historically, protein has been expressed in feedstuffs and total diet as crude protein (CP), which is calculated by multiplying nitrogen concentration by 6.25, assuming the average nitrogen concentration of a protein molecule is 16%. However, there is more to understand about CP when managing protein in rations. Protein can be categorized as non-protein nitrogen (NPN) and true protein. NPN, such as urea, is quickly degraded in the rumen and supplies nitrogen to rumen microbes to make microbial protein. True protein has an amino acid profile and can be broken down into two categories: rumen-degradable protein (RDP), which is broken down in the rumen, and rumen-undegradable protein (RUP), which is not broken down in the rumen but can be broken down and absorbed in the small intestine.
RDP provides peptides, free amino acids and ammonia that microorganisms use for growth and synthesis of microbial crude protein (MCP). MCP is the most important source of protein to a ruminant and can provide 50% to 100% of metabolizable protein to the animal. Metabolizable protein is the total amount of protein absorbed by the animal. In 1996, the National Research Council introduced protein requirements using the metabolizable protein basis; previously, it was expressed on the CP basis. This system allows us to account for both MCP and RUP more correctly, thus accounting for differences in extent of protein degradation among protein sources.
The extent of ruminal protein degradation is dependent on several factors, such as concentrate-to-forage ratio, dilution rate, passage rate, ruminal pH, feed processing, nutrient interactions and, as explained above, protein structure. High-RUP feeds consist of fish, feather and blood meal, and distillers grains (50% to 60% RUP). High-RDP feeds include alfalfa, small-grain forages, soybean meal and other oil seed meals, corn gluten feed and urea.
Why is it important?
Protein, or more specifically amino acids, are essential for maintenance, growth, gestation and lactation. Commonly known as having large roles in tissue growth and repair, protein also plays a big role in enzymatic activity, genetic storage, immune function, rumen function, molecule transport and cell differentiation. To summarize, protein is critical to support normal bodily function. Ruminants are also unique in that they can recycle nitrogen. Absorbed ammonia can be synthesized into urea nitrogen in the liver and either excreted or returned to the gut to supplement microbial CP synthesis. Our goals when managing rations is to supply minimum dietary CP but sufficient RDP to ensure proper rumen function and overall production of the animal on feed.
How requirements are determined
Age, sex, breed, body condition score, production level, health and overall nutritional management of the animal all play a role in determining an animal’s protein requirement. Although we typically balance on a percentage basis, animal protein requirements are determined based on intake. Therefore, when we balance a diet, it is extremely important to know and understand the average intake of the pen. To calculate an animal’s intake and intake as a percentage of bodyweight, use the following equations:
- Delivered as-fed pounds per pen divided by the number of head in the pen = as-fed delivered per head
- As-fed delivered per head multiplied by diet DM = DM delivered per head
- DM delivered per head divided by average bodyweight = DMI as percent bodyweight
General nutritional guidelines can be followed when cattle are consuming between 2% and 3% of their bodyweight as DMI. If an animal falls below 1%, often during the receiving phase, rations need to be formulated to compensate. Low-stress, healthy receiving calves will consume, on average, 1.6% of their bodyweight in DMI their first week. Higher-stress, “morbid” calves, on the other hand, are consuming less than 1%, and it can take them up to two weeks to reach the intake of what a healthy animal was consuming its first week. In addition to low intake, receiving animals incur stress and immune challenges that theoretically increase protein demand to mitigate negative effects. Additionally, receiving calves are still rapidly growing and thus require a higher amount of MCP. Therefore, a receiving and growing ration is typically balanced with a higher percentage of protein than finishing rations. Based on a recent survey of consulting nutritionists, on average, receiving diets are formulated with 14.5% protein and finishing at 13.4% CP of DM to account for intake and higher requirements of a receiving animal.
When managing a ration for proper protein consumption, it is advisable to pay attention to the RDP of the diet and how the percentage of RDP should change with the starch source. A deficiency in RDP hinders rumen function and can potentially decrease energetic efficiency of a diet. Ruminal starch digestion is the most accurate predictor of MCP and therefore is a guide for an RDP requirement. Typical Midwest finishing diets are either based on dry rolled corn, high-moisture corn or steam-flaked corn. We know as we add moisture and processing, rumen starch digestion increases. Therefore, RDP requirements should increase from dry rolled corn to steam-flaked corn to high-moisture corn. Much research has supported a 6.8% RDP diet when feeding a 90% concentrated dry rolled corn-based diet. High-moisture corn seems to have the highest requirement (around 9% to 10% RDP). Steam-flaked is intermediate, with industry on average formulating to 8.3% RDP.
Protein is an essential nutrient required for growth and production of cattle. A ruminant can synthesize high-quality MCP from NPN and RDP. Provided rations are formulated with adequate RDP in combination with RUP, ruminants can meet their metabolizable protein requirements for the given stage of production. Work with your nutritionist to ensure you are balancing for proper protein consumption based on your animals' stage of production and available feedstuffs.
