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What goes around, comes around with protein

Nutrition with John McKinnon

With respect to cattle nutrition, there are some topics that never seem to go away. They might drop out of the spotlight for a period of time, but they always seem to re-emerge, typically at a critical time in the production cycle. I could give numerous examples, but for the purposes of this and next month’s article, I will focus on protein nutrition. What has focused my attention on this subject was several discussions with producers this past winter. The common thread in these discussions was that these producers had switched to corn silage or standing corn as a forage source. Individuals running backgrounding operations, for example, had trouble getting the protein content of their rations to 10 per cent, even with the use of a conventional protein supplement. This is because corn silage and corn grain will typically average eight per cent to nine per cent crude protein on a dry matter (DM) basis. This issue will not be a surprise to producers in eastern Canada who have been feeding corn for many years, but as I say above, it has caught the attention of many producers who are new to this feedstuff. What I would like to accomplish with this and next month’s column, is to look at ways we can economically meet the protein needs of our cattle fed under a variety of production scenarios.

However, before we can talk about supplementation strategies, we need to understand the basic principles of protein nutrition in ruminants. You are likely aware of the term “crude protein.” For example, a barley grain sample might average 11 per cent crude protein. This value actually refers to the total nitrogen content of the sample, not its true protein content. A conventional feed analysis will measure the nitrogen content of the feed and convert it to crude protein by multiplying by a correction factor (i.e 6.25). While most of this nitrogen is related to the true protein fraction of the feed, some of it is non-protein in nature. Thus not all crude protein is true protein.

True protein, on the other hand, is comprised of amino acids (AA) that can be broken down to essential (need to be supplied to the animal) and non-essential (can be synthesized by the animal) amino acids. The protein requirement for cattle is in fact a requirement for AA, particularly the essential AA which are used for maintenance to sustain life, as well as for productive purposes (i.e. muscle and milk protein synthesis, and fetal development). During the digestive process, AA are absorbed from the small intestine. You might hear or read of the term “metabolizable protein” which refers to AA that are absorbed from the small intestine and are available for metabolic purposes. There are two primary sources of this metabolizable protein (i.e. amino acids): microbial and feed true protein.

Microbial protein primarily results from the growth and reproduction of bacteria in the rumen. These bacteria ferment the feed consumed by the animal and use the released nutrients for their own growth. For example, crude protein in silage can be broken down by the rumen bacteria to amino acids and/or ammonia. Both of these byproducts of protein breakdown in the rumen are used by the bacteria for growth (protein synthesis). In the latter case, ammonia is used by the bacteria to synthesize amino acids, which in turn are used to synthesize protein. Eventually, rumen bacteria leave the rumen and become an excellent source of essential and non-essential amino acids in the small intestine. This interrelationship between the animal and rumen bacteria in terms of protein nutrition, gives cattle and other ruminants a unique advantage in terms of survival and from a producer’s perspective gives you an advantage in terms of cost of production, a point that I will come back to in next month’s column.

As indicated above, feed protein can also serve as a source of essential and non-essential amino acids to the animal. This is because feed protein can be broken down into two fractions: degradable and undegradable protein. Rumen degradable protein is the fraction that rumen bacteria rely on for their needs, while the undegradable fraction passes out of the rumen with undigested feed and as with microbial protein, this feed protein fraction becomes available for absorption in the small intestine.

There are many factors that affect this division between rumen degradable and undegradable protein including feed processing (drying, heating), method of forage preservation (silage versus hay), plant maturity (spring versus fall pasture) and inherent differences between feeds. For example, corn typically has a higher rumen undegradable protein fraction than barley while barley silage will have a higher degradable fraction than barley greenfeed. Feeds high in rumen undegradable protein usually are byproducts of industrial processing such as corn gluten meal or corn dried distillers grains and are relatively higher priced.

While both the microbial protein and undegraded feed protein fractions are important, our primary goal with beef cattle is to maximize the microbial protein supply and, if necessary, supplement with a source of rumen undegradable protein. This strategy is the most cost-effective method of ensuring that the animals’ essential amino acids are met and in next month’s column we look at how to apply this knowledge to a variety of production scenarios.

About the author


John McKinnon is a beef cattle nutritionist at the University of Saskatchewan.


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