Genetic merit for milk yield has moved faster than most feeding programmes have kept up with. Sire breeding values for milk have climbed steadily for decades, and the same is true for fat and protein. The practical result is that a lot of cows now have more genetic capacity for components than the diet in front of them is set up to deliver. Get the nutrition right and you unlock what’s already sat there in the genetics; get it wrong and you’re capping a cow’s output regardless of what she’s bred for.
Where most farms leave components on the table
Volume is easy to chase and easy to see on the tank readout. Components are won or lost on precision: energy and metabolisable protein supply first, then the amino acid balance on top of that, particularly methionine and lysine relative to energy intake. Modern requirement models put a lysine to methionine ratio close to 2.7-3:1 (NASEM, 2021). Hit those targets and cows can lift milk fat and protein yield without needing more feed in front of them. The energy is simply being partitioned more efficiently rather than wasted or excreted as urinary nitrogen. Get the balance wrong and you’re paying for feed that isn’t converting into a component you’re actually paid for.
Forage quality sits underneath all of this. Diets need enough effective fibre, digested well, alongside starch and sugar levels that support rumen fermentation without tipping it over. Sugar in the 5-8% of dry matter region and well-digested starch both help microbial growth and fibre digestion, which is what ultimately supports component yield rather than just intake.
The other side of the coin: fat depression
Push too hard the other way, with diets low in effective fibre and high in rapidly fermentable starch or unsaturated fat, and you get the opposite problem: milk fat depression. The mechanism is well established. Altered rumen biohydrogenation produces a specific fatty acid which travels to the udder and directly suppresses the enzymes and gene regulators responsible for fat synthesis in the mammary gland (Bauman and Griinari, 2003; Dewanckele et al., 2020). It’s a genuine risk on TMRs that are pushed hard for energy without enough attention to fibre and fat inclusion, and it’s worth checking for whenever fat percentage drops without an obvious health explanation.
There’s a commercial angle here too, because the same biological pathway that causes unwanted fat depression can, in a controlled way, redirect energy away from fat synthesis and toward yield or body condition recovery. Some rumen-protected supplement products are formulated specifically to exploit it during early lactation, when energy balance is tightest. Whether that trade-off is worth it on a given farm depends on the milk contract and what’s actually paid for fat versus litres. It is worth a proper conversation with whoever’s advising on the ration before reaching for anything branded.
What it means for the milk cheque
This isn’t just a production question in the UK, it’s a pricing one. The AHDB standard litre for the 2026/27 milk year is based on 4.29% butterfat and 3.43% protein, and most GB contracts pay a bonus or penalty against a standard like that rather than paying flat per litre (AHDB, 2026). A herd sitting meaningfully below that on either constituent is leaving money on the table every month regardless of yield, and it’s worth running your own numbers through the AHDB milk price calculator against your actual contract rather than assuming litres alone tell the story. The Vacca milk solids calculator does the same job against your specific contract terms and current ration, and is worth running before assuming a nutrition change is the fix rather than a contract or pricing one.
Bottom line
Components are won on precision, not volume. Get the energy, protein and amino acid balance right and the genetics you’re already paying for through breeding can actually show up in the tank. Push the diet too hard for fermentable energy without the fibre to match, and you’ll lose fat percentage instead of gaining it. Either way, it’s worth checking where your own diet sits against these targets before assuming yield is the only number that matters.
References
- Bauman, D.E. and Griinari, J.M. (2003). Nutritional regulation of milk fat synthesis. Annual Review of Nutrition, 23, 203-227.
- Dewanckele, L., Toral, P.G., Vlaeminck, B. and Fievez, V. (2020). Invited review: Role of rumen biohydrogenation intermediates and rumen microbes in diet-induced milk fat depression: An update. Journal of Dairy Science, 103, 7655-7681.
- National Academies of Sciences, Engineering, and Medicine (NASEM) (2021). Nutrient Requirements of Dairy Cattle: Eighth Revised Edition. Washington, DC: The National Academies Press.
- AHDB (2026). Milk Price Calculator and UK farmgate milk prices. ahdb.org.uk/dairy/milk-price-calculator (accessed July 2026).