2 November 2001


Failing to optimise milk fat

and protein to suit buyers

contracts can mean missing

out on valuable income.

Jessica Buss finds out about

how to modify milk fat % in

high forage diets and using


IT IS more difficult to manipulate milk composition when only feeding cows small amounts of concentrates in a high forage diet. But alternative forages and combinations of different forages can be used to raise or depress butterfat %.

There will be some cases when reducing milk fat can be economic, such as in years when quota prices are increased. It is possible to depress milk fat using polyunsaturated fat supplements and high starch diets, explains Richard Dewhurst of the Institute of Grassland and Environmental Research, Aberystwyth.

Polyunsaturated fats (vegetable oils) are believed to increase levels of trans fatty acids produced in the rumen. This effect is thought to be responsible for depression in milk fat when brewers grains, which contain unsaturated fat, are fed, he adds.

High starch diets are also thought to increase production of trans fatty acids by depressing rumen pH.

When cows are fed too little fibre, inadequate fibre fermentation can depress milk fat, but in most high forage diets this is unlikely to be a problem, says Dr Dewhurst. "However, maize silage which is finely chopped is the least effective fibre source, so maize silage is most likely to be associated with milk fat depression."

Feeding diets of 100% maize can also lower milk fat because it lowers rumen pH. One study shows that when fed as the sole forage, fat can be reduced by as much as 0.4% compared with feeding no maize.

Legumes can also depress milk fat in some cases, when fed as the sole forage – otherwise milk fat is likely to increase. This lower fat % has been recorded when feeding red clover silage and when grazing pure white clover, he adds.

Increasing fat

When the aim is to increase milk fat % to suit a contract which pays on a constituent basis, small improvements are possible in a high forage diet.

"One way to increase milk fat is by improving forage intakes. The first step in ensuring good intakes is to provide fresh, high quality forage which is freely available," he advises.

Mixed forage diets also promote higher intakes. "This is the case even with forages which have a lower fibre content than grass silage, such as maize, which lead to lower milk fat % when fed alone."

In studies at CEDAR, Reading University, including fodder beet to replace a third of the grass silage increased milk fat % the most at 0.1%, compared with grass silage alone. Feeding maize silage and urea-treated whole-crop as a third of the forage also increased milk fat between 0.01% and 0.03%.

Mixing grass and clover silages in a study at IGER also increased milk fat % above feeding grass silage. Including red clover silage with grass silage increased fat by 0.12% and with white clover silage the increase was 0.2%. But feeding either of these clover silages alone did not result in the same response, he adds.

Body condition score management can also be useful in managing milk fat. "Fat cows have a low appetite for forage and this can depress milk yield and fat %."

Dry cow feeding

Increasing the energy supply to cows in the last few weeks before calving can dramatically increase milk fat % in the subsequent lactation, says Dr Dewhurst.

"We have seen marked increases in response to improving the quality of silage fed in the late dry period. But this is not advisable on farms where milk fever is a problem without careful attention to mineral balances."

Care is also needed to avoid cows becoming over-fat, which will occur when a higher energy diet is fed for a prolonged period. "Cows which are too fat at calving eat less forage, lose condition and tend to have lower milk protein % in the next lactation."

Even in high forage diets, its possible to alter butterfat % produced by altering ingredients and increasing high cow intakes, says Richard Dewhurst.


&#8226 Mix forages.

&#8226 Promote high cow intakes.

&#8226 Care with dry cows.

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