UNSATURATED liquid vegetable oils, fish oils and whole oilseeds can reduce milk fat%, but the results may be dramatic and unpredictable.
Jonathan Blake explains that unsaturated liquid oils, such as soya and rape oil, are extremely rumen aggressive increasing the production of trans fatty acids.
This can dramatically reduce butterfat %. When 400g of soya oil was fed in one study, milk fat reduced by 0.3%.
But feeding too much of these oils can also reduce dry matter intake and milk yield. Feeding any fat or oil is also likely to decrease protein %. "However, saturated oils and blends of oils and fats often make little difference to milk fat production."
Whole oilseeds fed at the equivalent of 500g/oil a day, or 1.25kg whole rape and 2.5kg of whole soya, will not increase milk fat production. But at higher levels, such as 2kg of whole rape or 4kg of whole soya, milk fat can be reduced by about 0.5%, adds Dr Blake. Milk composition is, however, unaffected by feeding protected whole oilseeds.
Fish oil can also reduce milk fat by up to 0.5% when cows are fed 200g/day, but unlike other oils it does not reduce dry matter intake. "Feeding less will reduce the amount by which milk fat % falls," he says. This strategy has been successfully used to control milk fat % for contracts which are volume based. It can also increase milk yield.
"But overfeeding fish oil can result in a dramatic fall in butterfat % and overall performance," warns Dr Blake.
A more natural solution to reducing milk fat is to feed naked oats, which have a high oil content, he adds. But these need to be fed at high levels to achieve significant results. A study feeding 6kg of naked oats reduced milk fat by 2% compared with a diet including Megalac. It also increased milk protein by 0.1%.
Whole soya has reduced milk fat % when fed at high levels.