22 March 1996

Try a new diet to manipulate fat for profit

Whats the best way to manipulate milk fat and protein to suit the liquid market? Genus senior nutritionist Diana Allen reports

MOST liquid milk contracts specify a minimum butterfat of 3.5-3.7% and minimum protein of 3%. Provided milk is above the stated minimum, there is no price advantage in selling higher quality milk. Indeed there may be a considerable quota advantage to producing milk of a lower butterfat content than your quota butterfat base.

Feeding for very high milk quality usually means good quality and expensive rations. So there should be some price advantage in not having to chase this milk quality. But some of the suggested ways for depressing milk fat involve feeding costly raw materials and the cost-benefit ratio must be calculated on an individual farm basis.

Breeding for milk quality is a long-term process and there is always the risk that market demand will have changed by the time your target milk quality has been met. Physiology and environmental effects on milk quality are well known but are of limited value when we want to routinely manipulate milk quality in response to changes in the market (see Table 1).

Manipulating milk quality through nutritional changes is the best option. Depressing milk protein can be achieved by under feeding cows. But this is obviously not a good idea since milk yield, health and fertility will also be affected adversely. The aim should be to keep milk proteins at a good level by encouraging high feed intakes while avoiding feeding expensive raw materials to achieve very high protein which is not being paid for.

Diets that depress milk fat tend to alter rumen fermentation. As higher levels of concentrate are fed, rumen fermentation changes. The rumen becomes more acid and the acetate to propionate ratio decreases. Milk fat drops.

Fat depression generally occurs when forage makes up under 45% of the ration dry matter but this depends on the fibre level of the forage. High D-value grass silages and clover silages will tend to reduce milk fat and increase protein. The effect of lush spring pasture on reducing milk fat is well known.

It is tricky to predict exactly when a drop in milk fat will occur when altering the forage to concentrate ratio. The rumen is quite effective at adapting to dietary changes. But when taken too far the results can have dramatic effect on the cow. Sub-clinical acidosis causes reduced appetite and laminitis. Severe acidosis can be fatal.

Adding some fat to the diet (particularly saturated fat) will tend to increase milk fat content. But feeding too much fat (particularly unsaturated fat) can effect fibre digestion. This slows down the rate that food passes out of the rumen and reduces feed intake by the cow. Rumen-protected fats were developed to overcome this difficulty. Natural examples are whole oilseeds, such as whole cotton seed and whole soybeans (heat treated). These do not reduce intakes because the oil is released very slowly as the seeds are digested.

Before adding fat to the diet to try to depress milk fat, work out the fat content of the ration. Some fermented by-products such as brewers grains and distillery grains contain high levels and will depress milk fat if fed in high enough quantities. Most rations can safely contain around 6% fat without affecting rumen digestibility and intakes. Over this level and the source of fat will need to be monitored carefully. Within the 6% guidance certain fat sources can be successful at reducing milk fat. These include heat treated whole soybeans, partially extracted rapeseed, ground full fat rapeseed, naked oats and maize distillers grains (see Table 2).

It is best to give cows a well balanced ration in terms of energy and protein and types of energy and protein sources and not have to worry about trying to lift milk fat and protein. The danger with actively trying to depress milk fat is in going too far and upsetting rumen function with consequent effects on production, health and fertility. Most of the liquid milk contracts have severe penalty clauses for going below the minimum quality standards. If you want to go down this route, careful ration formulation and analysis are necessary and advice should be sought. &#42

&#8226 Keep to specified minimum of 3.5-3.7% fat and 3% protein.

&#8226 Maintain proteins at level sufficient to encourage intake.

&#8226 Fermented by-products and fat sources will depress milk fat.


Table 1: Factors affecting fat and protein content of milk

Milk fatMilk protein



Age of cow/lactation number.Age of cow/lactation number.

Stage of lactation.Stage of lactation.



Rumen fermentation.Energy intake.

Effective fibre.Protein intake.

Lush pasture.Starch content of ration.

Rumen active fats.Mixed forage diets.

Feed additives.

Table 2: Practical ways to manipulate milk composition

Milk fatMilk protein

Alter forage to concentrate ratio

High level of cake (>9kg)-+

Low level of cake (<3kg)–

High D-value grass silage-+

Well chopped maize silage (>50% forage)-+

Brewers grains (>15kg)-+

Rolled/ground wheat (4kg in TMR)-+

Naked oats (2-3kg)-+

Unsaturated fats–

Complete diet feedersMany options.

Silage + cake systemsAdd maize silage or brewersgrains to the ration.

Use a high starch usaturatedoil cake.

Manipulating milk quality by changing diet ingredients is the best option.