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Technical Bulletins

The Economics of Bypass Amino Acids

Bypass Protein is Important…

Adequate rumen-bypass protein is critical to milk production in today’s high-producing dairy cows. The importance of formulating for bypass protein was demonstrated in a trial with fifty-six Holstein cows at Kansas State University1. The results are in Table 1. Some important observations were made in this trial.

Amino Acid (Table 1)

  • Cows fed a ration with 16% crude protein and based on Soy Best® (heat-treated screw press soybean meal) consumed more feed and produced more milk than cows fed rations with 16% or 18% crude protein and based on solvent-extracted soybean meal.
  • Compared with blood meal/meat-&-bone meal, cows fed Soy Best consumed more dry matter and produced more milk.
  • Cows fed the 16% protein ration with Soy Best were the most profitable. But metabolizable amino acids are the key.

Why did cows fed Soy Best out-perform the others? Much of the answer lies in the quality of the bypass protein, or in other words, the amino acid composition of the bypass protein supplied by the different ingredients.

Bypass protein is important, but by no means is it the whole the story. The fact of the matter is, cows have specific requirements for the various building blocks of protein. The building blocks are called amino acids. Those amino acids which a cow cannot synthesize and which she must receive in her ration are called essential amino acids. Methionine and lysine are two essential amino acids which can be limiting in many dairy rations.

When comparing protein sources, what really matters is how much methionine and lysine escape the rumen and are absorbed in the small intestine. That’s called metabolizable methionine and metabolizable lysine. Using values from CPM Dairy Ration Analyzer version 3.0.4a and market ingredient prices, it is possible to calculate the cost of metabolizable methionine and lysine from various feedstuffs. Those costs and the values used to calculate them are in the spreadsheet shown in Table 2 (see bottom of page). As the table shows, when you’re shopping for metabolizable methionine and lysine, Soy Best is a good buy.

On the face of it, that seems to be simple enough. But, as in much of life, there is more to the story. An ingredient such as Soy Best can be a palatable and economical source of metabolizable lysine and metabolizable methionine in many rations. But for cows producing ninety pounds of milk or more, even that ingredient can become limiting in terms of its metabolizable methionine concentration. That limitation has to do with the constraint of dry matter intake. In such a case, it may be necessary to incorporate a protein source with a higher concentration of metabolizable methionine, or a synthetic methionine source, even if that ingredient is more expensive per unit of methionine and possibly less palatable. In such a ration, Soy Best can be used to supply the metabolizable lysine and the bulk of the requirement for metabolizable methionine and a methionine supplement or blood meal can be used to supply the last bit of the methionine requirement while still not violating the dry matter intake maximum.

The synthetic supplements are the most concentrated sources of methionine. They offer the greatest formulation flexibility and enable maximum use of more economical vegetable bypass protein products such as Soy Best and protein sources that may suffer more extensive rumen degradation such as canola meal.

How Much Methionine is in There?

The CPM program comes with a feed ingredient library. That library includes metabolizable methionine values that have been reported by the scientists at Cornell, Penn State, Miner Research Institute and other locations.

There has been some controversy about the methionine values for soy products in CPM. Some nutritionists feel the values are overstated and reduce them in their own formulations. Cornell University has recently evaluated the methionine content of Soy Best. The Cornell laboratory reported a methionine value of 1.73% of RUP3 (rumen undegraded protein). The value for Soy Best in the CPM program is 1.59%.

In other words, the CPM program has a reasonable and conservative methionine value for Soy Best.

Reproduction Is Also Part of the Story:

With regard to reproduction, there’s a cost to getting protein quality wrong and a benefit to getting it right. Dr. Van Saun’s research is a case in point. Rations having two different levels of rumen undegraded protein (UIP) and different amino acid profiles were fed to dry cows. The levels were 0.75 lb. of UIP versus 1.3 lb. of UIP per cow daily. According to standard values in CPM Dairy Ration Analyzer, these rations delivered 18.7 grams versus 19.6 grams of metabolizable methionine and 60.2 grams versus 73.4 grams of metabolizable lysine, respectively.

Dry cows that were fed the ration with more metabolizable methionine and lysine had better body condition after calving and increased milk protein percent and yield. The researchers speculated that feeding supplemental bypass protein in the dry period improves performance in early lactation by minimizing the amount of maternal amino acids that must be mobilized to support fetal growth in late gestation.

The closer the amino acid composition of bypass protein is to matching the cows amino acid requirements, the less total dietary protein needed to meet those requirements. That means less protein is wasted by being degraded to ammonia and excreted as urea. Increased efficiency of protein utilization is associated with reduced milk urea nitrogen (MUN) and blood urea nitrogen (BUN). Lower MUN and BUN, in turn, are associated with improved reproductive performance, such as fewer services per conception.

Summing it Up

The amino acid composition of rumen undegraded protein is critical to dairy production and reproduction. Today, tools are available that allow us to value feedstuffs in terms of their contribution of metabolizable methionine and metabolizable lysine. That enables nutritionists to formulate rations for maximum profit. Soy Best can play an important role in the dairy producer’s quest for profit.


  1. Shirley, et al. Expeller soybean meal as a source of rumen undegradable protein for lactating cows. Kansas State University. Dairy Day 1997, page 28.
  2. Van Saun, et al. J. Dairy Sci. 76: 236-244. 1993
  3. Van Amburgh and Ross. Cornell University. Personal communication. December, 2003.

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