Factors affecting milk composition

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 The yield of milk and its chemical composition strongly influence the profitability of dairy farming.

Farmers are paid for market milk by volume, provided the milk meets minimum standards of composition-not less than 3.2 per   cent fat and 11.75 per cent total solids, on a weight to weight basis. There is no minimum requirement for levels of solids-not-fat or protein. Manufacturing milk is bought on its yield of fat and protein.

 Milk composition

Milk from Friesian-Holstein cattle typically contains 87.5 per cent water and 12.5 per cent total solids. The ranges in composition of milk solids are: fat, 3.2 to 4.6 per cent; protein, 2.8 to 3.5 per cent; lactose, 4.2 to 4.8 per cent and minerals 0.6 to 0.8 per cent.

The composition of milk is influenced by non-nutritional and nutritional factors.

 Non-nutritional factors affecting milk composition


Breeding is of considerable importance, since fat and protein levels in the milk are heritable characteristics.

Gains in milk composition made from breeding are permanent and accumulate from year to year. Benefits of sire and cow selection, and of mating decisions made today, will continue to be realized in all future descendants of the herd. In this respect, selection is a very productive means of improving milk composition.

The use of breeding to improve milk composition must be clearly understood, since selection to improve one production trait may lead to a decline in another. Selection based on milk yield will result in an increase in milk, fat and protein yields, but will reduce fat and protein percentages (see Table 1).


Table 1. Predicted responses to selection.

The more +, the greater the response over time to selection criteria.
  Predicted response in -
Select for: Milk Fat % Fat yield Protein % Protein yield
Milk +++ - +++ - +++
Fat % - ++ + ++ 0
Fat yield ++ + +++ - ++
Protein % - + - ++ 0
Protein yield ++ 0 +++ + +++
Fat and protein yield +++ + +++ + +++


Similarly, selection of sires on protein or fat percentage only will result in a reduction in milk yield with minimal improvement in protein or fat yield. Because manufacturing milk is paid for on the yield of solids, you should select sires on the basis of fat PLUS protein yield. This will result in an increase in milk yield, fat percentage and yield, and protein percentage and yield.

Since the stage of lactation affects the percentage and yields of protein and fat, you need detailed herd test data to select cows for breeding or culling purposes.

 Stage of lactation

The composition of milk varies with the stage of lactation. Cows that calve in good condition produce milk with a high fat and protein content during early lactation. The percentages of both fat and protein decline during the first six to eight weeks of lactation, then progressively rise after the cow becomes pregnant to reach their highest levels in late lactation.


Although fat and protein contents decrease with increasing age, these changes are small. Since the age structure of a herd is not readily changed, the age composition of the herd is unlikely to contribute significantly to herd variation in milk composition.

 Seasonal conditions

Environmental factors that affect feed intake can be associated with pronounced variations in milk yield and composition. Temperatures consistently above 30C will reduce milk yield as well as the percentage of milk protein, because of a reduction in energy intake. Cows in early to mid-lactation and receiving little or no supplementation (that is, relying on high pasture intakes) will be affected the most by heat stress.


Clinical and subclinical mastitis decrease milk yield and so reduce fat and protein yields.

 Nutritional factors

 Level of feeding

Precalving: Increased feed intake in late pregnancy increases milk yield and the yields of fat and protein. Research has shown that for each 30 kg increase in liveweight at calving, milk yield increases by 122 kg, fat yield by 8 kg and protein yield by 4 kg during the first 20 weeks of lactation.

The effects of condition score at calving on fat and protein percentage are small.

Post-calving: The effect of feeding level on fat and protein percentage is variable. This is because the stage of lactation influences the effect of feed intake on milk composition 

If feed intake is increased during early lactation, milk yield will increase with consequent increases in fat and protein yields. As intake increases, the percentage of milk fat will decline, but protein percentage will increase slightly.

Protein production in well fed herds is rarely below 3.2 per cent, but in poorly fed herds it can fall to 2.8 per cent.

 Diet quality

Pasture species influence milk yield and composition, as shown in Table 2. The use of species associated with improved pasture quality results in increased milk, fat and protein yields.


Table 2. Effect of pasture species on milk production, as shown in two trials

  Trial I Trial 2
  Kikuyu Ryegrass Ryegrass Clover
Milk yield (L/day) 13.4 19.4 16.5 18.9
Milk Fat (%) 3.7 3.5 3.7 3.5
Milk Fat kg/day 0.51 0.70 0.59 0.69
Protein % 2.9 3.2 3.0 3.2
Protein kg/day 0.40 0.64 0.51 0.62


The increase in fat yield is caused by an increase in milk yield only, since the percentage of milk fat actually declines. increases in both milk yield and percentage protein cause the increase in protein yield.

Species differences are largely caused by inherent differences in intake. However, with ryegrass and clovers, differences still occur when they are fed at the same level.


Providing supplementary feed in the form of cereal grain usually results in increased milk, fat and protein yields. An increase in milk yield causes the increase in fat yield, since the percentage of milk fat often declines. Increases in both milk yield and protein percentage cause the increase in protein yield.

Feeding lupins also results in increases in milk fat and protein yields. Unlike the cereal grains, lupins do not reduce the fat percentage when they are fed as a supplement to cattle.

The cow generally uses protein supplements as a source of energy rather than a supply of protein to the udder. Providing there is sufficient protein in the total diet, feeding protein supplements will result in a similar increase in protein percentage as feeding a similar amount of energy from cereal grain.

If the protein content of the total diet is low, feeding protein supplements increases the energy content of the total diet by increasing the digestibility of the total diet. As a result, the protein percentage of the milk is increased.

Milk production and milk protein content will increase when an energy deficiency is corrected.

When concentrates are fed, the degree of processing can affect the fat percentage of the milk. Grains need only be cracked to allow sufficient digestion. Over-processing can reduce the fat percentage of the milk.


If the milk fat percentage has dropped, but the protein percentage has remained constant, more fibre is needed in the total diet. This is best provided by feeding hay. However, cattle need only small quantities of hay (2 to 3 kg/cow/day)when they are grazing good quality pasture.


To increase fat and protein yields by feeding, increase the energy intake of the cow by


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