Not all Protein Sources are the Same

By Jerad Jaborek

Protein is one of the main macronutrients needed by cattle to survive and grow. As you may know, cattle are ruminants, and therefore have a four-compartment stomach that consists of the rumen, reticulum, omasum, and abomasum. The largest stomach compartment in ruminants is the rumen, which contains a vast diversity of microbes. In the rumen, ingested feedstuffs undergo microbial fermentation and breakdown allowing for nutrient absorption. The rumen microbes also use the dietary carbohydrates and protein consumed by cattle to maintain, grow, and reproduce themselves. As a result, the passage of microbes from the rumen to the lower gastrointestinal tract can provide cattle with two-thirds to three quarters of their protein requirements. We can think about the metabolizable protein requirements of cattle as the sum of rumen degradable protein (RDP), microbial protein (MCP), rumen undegradable protein (RUP), and small contributions from endogenous protein. Rumen degradable protein consists of dietary protein and amino acids, and non-protein nitrogen (NPN), such as urea, that are used by the rumen microbes to reproduce or replicate.  The microbes themselves provide the small intestine with MCP, in addition to dietary RUP that is not degraded by the rumen microbes, endogenous protein from sloughed cells within the digestive tract, and digestive enzymes reaching the small intestine.

Not all protein sources are the same because they are comprised of different concentrations of amino acids. Amino acids are the building blocks of protein, and the animal requires a certain concentration of each amino acid to meet its growth requirement. Therefore, if one amino acid is deficient or limiting, it can limit growth performance to a level that is less than when all the amino acids are supplied at their optimal concentration. Protein sources vary in digestibility and composition, but factors such diet forage to concentrate ratio, rumen pH, and passage rate of digesta out of the rumen can influence the site of protein digestion, thus making it very complicated to predict RDP, MCP, and RUP requirements and supply.

For growing and finishing cattle, the metabolizable protein requirements are quite different and are largely influenced by dry matter feed intake and muscle (i.e., protein) gain. Therefore, smaller, and typically younger cattle (less than 660 lb) are depositing more protein for lean muscle tissue than fat compared with larger, older, finishing cattle (greater than 660 lb). Additionally, smaller calves are capable of consuming 3.0 to 3.5% of their body weight in dry matter feed daily compared with about 2.0% for a mature finished steer or heifer. A greater feed and energy intake will result in a larger population of rumen microbes that can contribute to MCP supply in the small intestine. Considering these facts, the ratio of RDP to RUP needed in the diet of cattle increases as the cattle achieve a greater weight, where a growing steer calf (<660 lb) may require 65 to 85% RUP for a 2.2 to 3.3 lb/day gain, while a finishing steer (>660 lb) may only require 35 to 50% RUP for the same rate of gain.

In the Midwest, corn silage is a common feedstuff in growing cattle diets because of the greater energy content compared with other forage sources. Corn silage is about 8% crude protein, with most of the crude protein being RDP. Therefore, corn silage-based diets require supplemental protein to meet the protein needs of the growing calf. Research conducted by Oney and others from the University of Nebraska investigated supplying dietary protein at different RUP concentrations to determine the effects on growing crossbred steer performance. As the percentage of RUP increased in the corn silage-based growing diet from 0.4% to 5.5% (9 to 41% of the crude protein) the average daily gain (ADG) and feed efficiency of steers improved, especially during the first five weeks of the feeding period.

In another study investigating the protein requirements of growing steers, Zinn and Shen from the University of California, Davis supplied protein via urea, fishmeal, or soybean meal to growing crossbred Brahman-influenced steer calves weighing 500 lb. As the concentration of crude protein and RUP increased due to a greater inclusion of fishmeal (1.5, 3.0 or 4.5%), ADG increased during the 56-day growing period.

In a study investigating increasing RDP concentrations of finishing diets offered to heavy crossbred yearling steers (870 lb), Wagner and others from Colorado State University and Five Rivers Cattle Feeding, kept RUP constant at 5.1% of the diet so that the RDP concentration ranged from 51.4 to 64.8% RDP as a percentage of dietary crude protein. The ADG for these finishing steers increased in a linear fashion as the percentage of RDP increased, while dry matter feed intake tended to increase, while feed efficiency did not change.

Overall, smaller calves (<660 lb) require a greater percentage of crude protein, with a greater percentage of the crude protein in the diet being RUP. As these calves grow to weights >660 lb, they require a lesser percentage of dietary crude protein. Additionally, as cattle get larger and deposit more fat, they require less RUP, so a greater percentage of the crude protein can be supplied as RDP. As a result, these protein needs could be met with cheaper feed ingredients, such as urea, during the finishing period.