The science of dairy cattle protein intakes and the subsequent milk protein production has evolved significantly since amino acid balancing became an important nutrition strategy on dairy farms. While the industry has come a long way, current and future research in this field will further advance its effectiveness in putting high-value components and pounds of milk in the bulk tank.

What have we learned?

Amino acids are the building blocks of protein. The term “limiting amino acid” is applied when a particular amino acid is supplied in an amount that is less than the body demands. When this happens, a cow is limited in the amount of milk protein she can synthesize.

Many years ago, the National Research Council (NRC) determined that lysine and methionine are the first two limiting amino acids for milk protein production in lactating cows. Past and ongoing research clearly shows the positive effects of amino acid balancing on milk protein production.

University research and on-farm application have consistently shown a milk protein increase of about 0.1 to 0.2 percentage units. Milkfat content often increases as well.

In addition, amino acid balancing can reduce the amount of total dietary protein fed. This lowers feed costs, reduces nitrogen excretion and increases protein efficiency.


Unlike monogastrics, ruminants present a unique challenge in predicting metabolizable amino acid supply available for absorption due to the contribution of lysine and methionine from rumen microbes plus the contribution from the rumen-undegradable protein fraction (bypass protein).

When balancing for amino acids, it is important to have ruminal-protected sources of methionine and post-ruminal digestible sources of lysine. Use research-proven, high-quality methionine and lysine sources to give the best chance for an optimal response. Remember, not all sources are created equal.

Amino acid balancing comes down to the economics. Adding ruminal-protected methionine and lysine costs money, but pounds of protein and fat ideally should cover the cost and give you a good return on investment when done correctly.

What is the current state?

Current research in amino acid balancing has focused on transition cows with supplementation both pre- and post-calving. The results show that amino acid balancing has positive effects relating to both health events and lactation performance.

As we all know, the transition period is a highly stressful time for an animal. By feeding a high-quality methionine source, research shows that methionine stimulates the secretion of the very low-density lipoprotein, which helps transport triglycerides (fat) away from the liver. This, in turn, helps prevent the cow from experiencing ketosis.

In addition to reduced liver fat accumulation, non-esterified fatty acids decrease more rapidly in transition cows supplemented with methionine. Methionine also has an antioxidant-like effect during the transition period. Cows supplemented prior to calving showed signs of decreased systemic inflammatory state accompanied by greater liver antioxidant capabilities.

Some of the observed responses of feeding supplemental rumen-protected methionine through the transition period are:

  • Increased dry matter intake(+4.6 pounds)
  • Increased energy-corrected milk (+8.6 pounds)
  • Increased milk true protein content (+0.19 percentage units)
  • Increased milkfat yield(+0.41 pounds)
  • Increased milk protein yield(+0.21 pounds)
  • Increased liver function
  • Decreased inflammation
  • Decreased oxidative stress

A more recent study investigated the interaction between amino acid supplementation and feeding Rumensin. Rumensin has been available since 1977 in the beef industry and was approved for lactating dairy cows in 2004.

Rumensin is well-recognized for its positive effect on feed efficiency – lower dry matter intake (DMI) with similar or better milk production. Previous work and experience taught us that very high inclusion rates of Rumensin may result in fat-test depression and, worse yet, protein-test depression.

As stated previously, amino acid balancing has been shown to increase component production. The plan for this research trial was to feed a high level of Rumensin (540 mg per head per day) and amino acid balance the diet to compensate for the previously shown depression in fat and protein production.

Four different treatments on the 12-week trial were as follows:

  • Control (no amino acid balancing, no Rumensin)
  • Amino acid balancing (diets were only amino acid balanced)
  • Rumensin (only Rumensin, no amino acid balancing)
  • Amino acid balancing and Rumensin combination

Feeding Rumensin significantly reduced dry matter intake by 3.3 pounds per day when compared to the control treatment. Milk production was similar across all four treatments; cows produced approximately 106 pounds per day on 2X milking with the use of rBST.

The trial showed no effect of treatment on milkfat content. Milk protein content tended to be reduced by Rumensin, while amino acid balancing significantly increased milk true protein content by 0.07 percentage units. Regardless of Rumensin feeding, amino acid balancing increased milk protein content and yield.

What does the future hold?

What’s next for amino acid balancing and supplementation?

Some of the most recent work with amino acid balancing is on its impact on reproduction and calf nutrition. Amino acid balancing calf milk replacers (similar to the process of balancing for cows) is being evaluated to better use amino acids to optimize growth and body composition.

A recent study presented the effects of amino acid balancing on reproductive performance in dairy cows. The study evaluated the effect of supplementing rumen-protected methionine on early embryo development in super-ovulated cows.

No differences were found in embryo fertilization or development; however, methionine had a dramatic effect on gene expression. The implications of this on pregnancy and calf physiology are not fully understood.

While the practical benefits of amino acid balancing on lactation performance and transition cow health have been clearly defined, further research is needed to address the impact on reproductive physiology.

As with all nutrition strategies, the science is constantly evolving. New insight and solutions will continue to develop, which will help nutritionists and dairy producers more effectively and efficiently feed dairy cows to profitably produce milk and high-value components. PD

Augusta Hagen