Are transition cows getting enough metabolisable protein?

Dairy producers could be limiting early lactation performance by underfeeding metabolisable protein, as well as the amino acids methionine and lysine, to transition cows.

Prof Jim Drackley of the University of Illinois warns that widely used US nutrition models may underestimate metabolisable protein (MP) requirements, particularly during late gestation when demands extend well beyond maintenance and foetal growth.

See also: ‘Little wins’ improve milk and protein yield on Cornish farm

In a 2021 update, the US National Academies of Sciences, Engineering and Medicine recommended increasing crude protein levels to 12% for far-off dry cows and heifers, and to 13% for close-up cows.

This equates to just over 1kg of MP/day. UK feed recommendations are similar.

However, Jim believes this figure remains too low. “The factorial requirements don’t include things we can’t yet model properly, such as mammary development, immune function and colostrum formation,” he explains.

He adds that an activated immune system demands a high level of protein.

Estimates from other species suggest immune activation can increase MP requirements by 7-10%, which is not currently factored into most rationing systems.

Evidence from research

Research at the University of Illinois has shown that first-calving heifers fed about 1.1kg daily of MP pre-calving produced more milk post-calving than those fed lower levels.

Jim says there is a clear correlation between MP supply and milk production and protein yield.

Further studies are now under way to better quantify optimal prepartum MP supply and its effects on performance.

Critical role of amino acids

Meeting MP requirements is not just about crude protein, says Jim.

The balance of essential amino acids – particularly lysine and methionine – is critical, as these are often limiting the supply of MP in dairy rations.

“Methionine is needed for milk protein production, but it does far more than that,” he explains.

Methionine is a precursor for cysteine, another sulphur-containing amino acid essential for milk protein.

It is also involved in choline synthesis, DNA methylation (which regulates gene expression) and the production of glutathione, a key antioxidant.

Studies dating from the late 1990s through to recent work show consistent benefits from supplementing rumen-protected methionine during the dry period.

Supplementation of rumen-protected methionine to bring total daily intake to 30g pre-calving has been shown to increase dry matter intake (DMI) by 2kg before calving and up to 4kg post-calving.

These cows also produced more energy-corrected milk.

Similar benefits have been seen with lysine supplementation.

Research at the University of Illinois found that supplementing pre-calving cows with lysine increased fresh cow DMI and boosted milk fat, milk protein and casein yields.

Muscle mobilisation and MP balance

Cows typically experience a negative MP balance during the first week after calving, forcing them to mobilise skeletal muscle to meet amino acid demand for milk production, explains Jim.

If dietary protein supply is inadequate before calving, cows may begin mobilising muscle earlier, increasing the risk of ketosis and reducing milk protein output.

“There is evidence that thinner cows mobilise more body protein than cows in better condition,” he says.

“Research also shows that cows with greater muscle thickness mobilise more protein from about six weeks pre-calving until four weeks post-calving, fuelling higher milk production.

Across a full lactation, muscle thickness drops sharply until about 60 days in milk, plateaus until about 240 days, then recovers in late lactation and during the dry period.

“Cows appear to rebuild muscle reserves in late lactation and the dry period, so they start the next lactation in the same place,” he explains.

This suggests MP requirements may be greater than current recommendations, as the amount of protein cows can mobilise depends on the size of muscle reserves rather than dietary MP alone, he adds.

Practical implications

Feeding higher levels of MP pre-calving increases muscle protein reserves, supports higher milk production and can reduce the risk of post-calving disease, says Jim.

The optimal lysine-to-methionine ratio in MP is about 3:1, equivalent to about 90g/day of lysine and 30g/day of methionine.

© Adobe Stock

Common UK feeds such as maize, barley, brewers’ grains and grass silage tend to be relatively low in lysine, but soyabean meal is a good source.

Meanwhile, maize gluten meal and sunflower meal are among the best natural sources of methionine.

As many milk contracts move away from feeding soya, it is worth checking that the correct balance of amino acids is being supplied, warns Chris Bartram, head of nutrition at Mole Valley Feed Solutions.

“If you’ve removed soya from the diet, lysine supply could easily be lower than required.”

He says protected rape is a good source of lysine and farmers can also feed rumen-protected forms of amino acids.

Chris Bartram and Jim Drackley were speaking at a recent Mole Valley nutrition masterclass at the Gables Hotel near Bristol.