30 August 1996

Reducing fibre can cut fat level but its a tricky technique

By Jessica Buss

REDUCING milk fat through feeding is possible by cutting dietary fibre levels. But the technique is hard to control and can cause ill-health, warns John Sutton of the University of Reading.

"We know dietary fibre offers one practical means of manipulation of milk fat synthesis. As the fibre level in the diet increases, milk fat content increases," he says.

The cut-off point for this is about 20% acid detergent fibre (ADF) in the diet dry matter (roughly equivalent to 35% neutral detergent fibre – NDF). Above 20% ADF there is little milk fat response to feeding extra fibre.

"Below 20% ADF you get a rapid fall in fat%. This is one way to reduce milk fat but the disadvantage is that it is a rapid drop, so is difficult to control." Low dietary fibre levels can also lead to metabolic disorders such as acidosis and lameness caused by laminitis. Another way to reduce milk fat is to feed a high level of concentrates, so reducing the proportion of forage in the diet. "When feeding 12kg of concentrates, silage intakes are low and milk fat can drop by 0.3 to 0.4%," he says. But it may be uneconomic to do so.

And over recent years higher quality silage has reduced reliance on concentrates. This is one of the reasons for the rise in milk fat.

If the aim is to produce low-fat milk for a niche market, at 3% fat, by reducing dietary fibre supply, forage intakes would need to be restricted. But that can upset the cows metabolism, he warns.

ADF and NDF differ. Be sure which one of these you are using as the proportions in crops vary widely. ADF is within the NDF in the diet but can range from 33 to 66% of the NDF.

John Sutton:"As the fibre level in the diet increases fat content rises."