Amino acids: feeding the cow's protein needs, not just crude protein

The protein that reaches the cow's small intestine comes from two sources: microbial protein made in the rumen, and dietary protein that escapes rumen breakdown. Together they form metabolisable protein — the pool of amino acids the cow can actually use.

The limiting amino acids

In typical maize- and soy-based rations, methionine and lysine are most often the first limiting amino acids for milk protein synthesis. Schwab and Broderick (2017) summarised decades of research showing that balancing these can raise milk protein yield.

Less protein, better used

Lee et al. (2012) found that supplementing rumen-protected methionine and lysine allowed metabolisable-protein-deficient diets to maintain milk yield and protein output. Feeding to amino acid targets instead of high crude protein reduces urea in milk and nitrogen excreted in urine.

• Use a ration model that predicts metabolisable amino acid supply (e.g. NASEM 2021).
• Watch milk urea nitrogen as a field check of protein efficiency.
• Rumen-protected products differ in protection and bioavailability.