How Protein Source Affects Protein Metabolism in Ruminants

Warren Rusche, Assistant Professor & SDSU Extension Feedlot Management Specialist

Written by Madeliene Nichols under the direction and review by Warren Rusche.

(Courtesy: K-State Research and Extension, CC BY 2.0)


 

Distillers grains have become the standard supplemental protein source for growing and finishing beef cattle. They are readily available in the Midwest, usually priced attractively, and contain appreciable amounts of crude protein. While it is common to balance beef cattle diets based on the crude protein system, it does not paint an accurate picture of what is occurring inside the animal, leading to gaps in nutrition and economic losses. To overcome these issues, many producers and nutritionists have adopted the metabolizable protein system, which partitions the protein requirements into those of the ruminal microorganisms and the host.

Rumen microorganisms require peptides, amino acids, and ammonia to grow and to break down feed nutrients. These inputs can be supplied by ruminally degradable protein (RDP) from feedstuffs or ammonia from non-protein nitrogen (NPN) can also be used by the rumen microbes. Most cattle diets meet or exceed microbial RDP requirements, but in instances where RDP is low, the rumen microorganisms will recycle urea to meet their requirements.

On the other hand, cattle require amino acids that are either supplied by ruminally undegradable protein (RUP) or microbial crude protein (MCP). In many feeding scenarios, as much as two-thirds to three-fourths of amino acids absorbed by beef cattle are provided by MCP. However, it is difficult to determine what amino acids are absorbed from MCP, so it is important to supply adequate RUP with a balanced amino acid profile to ensure the animal’s needs are met.

When using the metabolizable protein system, it is easy to see that not all proteins are created equal. Distillers grains may contain appreciable levels of both RDP and RUP, but corn protein is relatively low in lysine, an essential amino acid. It is possible to overcome minor discrepancies in limiting amino acids by increasing dietary crude protein, but the increased nitrogen recycling and excretion associated with high-protein diets also come with greater energetic costs. Increasing dietary crude protein also has negative environmental effects. Because feedlot manure tends to have a lower nitrogen: phosphorus ratio than most crops require, applying manure to cropland or pasture to meet nitrogen requirements results in a quantity of manure phosphorus that is 5- to 10-fold greater than the phosphorus needs of the crop and potentially leads to excess runoff of both soluble and insoluble phosphorus. As a result, producers may be required to build filter strips and buffer zones around waterways.

We do have opportunities to improve nitrogen use efficiency by feeding protein sources that better meet the requirements of both the microorganisms and the animal without feeding excess crude protein. Soybean protein may be more expensive than corn protein, but soybeans contain adequate amounts of RDP and RUP and more appropriate amounts of lysine. Soybeans and soybean meal can also be heat treated, increasing the ratio of RUP: RDP and better meeting the requirements of the animal while reducing the total amount of crude protein needed in the diet.

Soy protein will be more readily available as new crushing plants open across the Midwest. While the current price point is less appealing compared to distillers grains, providing some or all the supplemental protein needs from higher quality protein sources such as soy protein is certainly worth considering especially as producers and nutritionists try to meet the needs of cattle with greater genetic potential.


Source: South Dakota State University