Dietary protein is broken down by microbes in the rumen and by the action of intestinal digestive enzymes to amino acids. These amino acids are absorbed and then incorporated into milk and tissue proteins. The amino acid composition of any given protein – like casein in milk, for example. – is always relatively the same, so if an essential amino acid is not available in sufficient quantity to allow a cow to synthesize the casein, it is termed “limiting.” An amino acid that is available in excess cannot be substituted for the limiting amino acid.
One way to make sure cows’ amino acid needs are met is to feed excess protein. However, this is not in the best interest of the cow, the environment or the wallet.
- Providing excess crude protein becomes a burden for cows. The excess must be converted to urea, and energy is needed to produce the urea, resulting in poorer feed efficiency. The urea contributes to greater blood urea nitrogen (BUN) and milk urea nitrogen (MUN) before being excreted in the urine.
- Nitrogen excreted via urine is much more likely to contribute to pollution than nitrogen excreted in feces. Urinary urea can be converted to ammonia nitrous oxide by air and soil micro-organisms. Ammonia is a threat to air quality in the barn, and nitrous oxide is a key contributor to increases in global temperature.
- Protein is often expensive, and providing more protein than is needed in the diet increases the cost of the ration. Add to that the loss in feed efficiency caused by urea synthesis and profitability begins to suffer. By balancing diets for amino acids, we can reduce dietary protein, make rations more efficient, improve the environment and save money.
The ability of a protein ingredient to meet the amino acid needs of the cow cannot be determined simply by observing the crude protein content of the ingredient. Protein content alone is not a good indicator of economic value. First, the escape protein contribution is an important consideration. As an example, solvent-extracted soybean meal may have the same protein content as a heat-treated branded product, but the branded product commands a high price because it brings more usable amino acids to the cow.
In addition to the escape protein content of ingredients, amino acid profiles need to match requirements. Ingredients differ widely in their amino acid profiles, and feed formulation programs can refine feeding by formulating diets to meet the amino acid needs of cows. There are 10 amino acids that are termed essential, as they cannot be synthesized from other nutrients.
Rumen microbes provide a source of the essential amino acids that closely match that of milk (Figure 1). As a result, this protein is used efficiently because there is very little waste. However, when protein sources deviate greatly from the amino acids required to produce milk, efficiency declines and more protein from that source is used to meet cows’ needs.
Let’s look at some feed ingredients. The first part of Table 1 provides the crude protein and escape protein content of ingredients. It becomes obvious that escape protein bears little relationship to crude protein. For example, only about 25% of the crude protein in alfalfa silage and corn silage escapes being broken down in the rumen. The escape protein makes up 53%, 40% and 57% of the total protein for the vegetable proteins canola, soybean meal and distillers grains. Corn grain has less protein than barley, yet both provide about the same amount of escape protein.
Next, look beyond escape protein to the essential amino acids each provides. The order listed is arguably based on the amino acid most likely to be limiting, with methionine being first and tryptophan being last.
Now comparisons become a bit harder. None is a perfect match for milk protein, and none provide quite as much methionine and lysine as milk protein does. That means excess needs to be provided to meet the limiting amino acid.
This becomes a bit easier to visualize in Table 2. If each were the only available source of amino acids, then soybean meal, as an example, is only able to meet 50% of the methionine needed (from Table 1, 1.38% methionine in soybean meal versus 2.76% in milk). The feed formulator would need to provide twice as much protein from alfalfa to meet the same amount of methionine as milk. In other words, alfalfa is only 50% efficient with respect to the profile of milk. However, soybean meal is a more efficient source of lysine than the other ingredients chosen from the list.
It becomes obvious from Table 2 that corn protein (corn silage, distillers grains and corn grain) is very low in lysine, and that this amino acid will likely be limiting in many diets where corn provides the bulk of protein. Fortunately, feed formulation programs allow ingredients to be selected in quantities needed to provide the most efficient overall mixture of essential amino acids. This results in the least waste of dietary protein. 
References omitted but are available upon request. Click here to email an editor.
Brittany Dyck holds a Masters of Science degree in dairy nutrition from the University of Alberta and has been with the Canola Council of Canada since 2012.
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Essi Evans
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