Amino acid nutrition in dairy cows is a critical component of optimising milk production, reproduction, and metabolic health. In New Zealand, the challenge lies in balancing amino acids within high-protein, grass-based systems, which differ significantly from the Total Mixed Ration (TMR) systems used in other parts of the world.

Understanding Amino Acids in Dairy Nutrition

Proteins are made up of amino acids (AAs), and 10 of these are essential—meaning they cannot be synthesised by the cow and must be obtained through the diet. Among these, methionine (Met) and lysine (Lys) are typically the first- and second-limiting amino acids in dairy cow diets. When deficient, they can restrict protein synthesis, even when dietary crude protein (CP) is sufficient.

Imbalanced amino acid profiles can:

• Reduce nitrogen utilisation efficiency
• Increase nitrogen excretion
• Limit productivity

Functions of Methionine and Lysine

• Methionine is involved in:
  • Building milk proteins
  • Acting as a methyl donor for metabolic functions such as DNA methylation, lipid metabolism, and liver health
  • Supporting the synthesis of antioxidants like glutathione, which help cows manage oxidative stress, especially during early lactation or transition periods
• Lysine is important for:
  • Synthesising casein and whey proteins
  • Supporting immune function
  • Contributing to hormone and enzyme production

Supplementing Amino Acids in Pasture-Based Systems

To provide methionine and lysine in New Zealand dairy systems, farmers can use rumen-protected (bypass) amino acid products. These are formulated to withstand ruminal digestion and be absorbed in the intestine.

When selecting products, it’s important to consider:

• Concentration
• Absorption
• Bioavailability

These factors help determine the appropriate supplementation levels.

Balancing the Ratio

Supplementing amino acids is not just about quantity—it’s also about the correct ratio. A lysine-to-methionine ratio of approximately 3:1 in the metabolizable protein (MP) is often considered ideal for milk protein synthesis.

In practical terms, some trials have used:

• ~2.60% methionine
• ~7.00% lysine of MP
  This results in a Lys:Met ratio of ~2.8–3.0:1, which has been associated with improvements in milk protein yield and nitrogen use efficiency.

When to Supplement

It is advisable to supply methionine and lysine during critical physiological periods, such as:

• Transition period (a few weeks before and after calving), when cows experience stress on liver function, immune response, and body reserve mobilisation
• Peak lactation (around 8–12 weeks post-calving), when milk synthesis and metabolic demand are at their highest.