Government of New Brunswick

Nutritional Effects on Milk Composition

December, 1992 - Issue 92.2
Nutrition Newsletter
Murray Snowdon, Livestock Nutritionist


The shift toward consumption of lower fat dairy products will result in milk pricing schemes that put greater emphasis on milk protein. Substantial increases in cheese consumption have also helped in shifting emphasis to protein production. Fortunately, milk fat and milk protein production are genetically correlated, so our emphasis on milk fat production has ensured genetic progress in milk protein production.

Milk protein and fat levels have often been reported by researchers and marketing boards on a percentage basis, thus emphasizing level rather than yield. One must be careful however, to distinguish between changes in protein content (i.e. percentage) and changes in protein yield since these two factors are not always affected in the same way.


Breed differences in milk protein level are smaller than the differences in fat content, but are large enough to make a substantial difference in returns under multi-component payment schemes. Typical milk composition values for the common dairy breeds are given in Table 1.

FAT, % 3.6 4.8 3.9 4.6
PROTEIN, % 3.2 3.8 3.3 3.6
LACTOSE, % 4.7 4.9 4.9 4.9


Non-nutritional and non-genetic factors affect milk fat and protein in the same manner. The factors that lower fat test lower protein test and those that raise fat test raise protein test. A summary of these factors is given in Table 2.

Stage of lactation protein test decreases 1st two months of lactation, increases thereafter
Cow age protein test declines with age
Season protein test usually lowest in summer, highest in winter
Milking Interval no effect
Mastitis protein test decreased with mastitis infection


Milk protein is less variable than milk fat and more difficult to influence with diet. Unlike environmental effects which tend to move protein and fat test in the same direction, most nutritional factors which increase milk protein decrease milk fat and vice versa.

Energy Intake and Forage Level in the Diet

Dietary forage level, dietary grain level and dietary energy level are usually confounded; changing any one of these constituents in a diet usually affects the other two, making it difficult to view these items separately. Generally though, a diet low in forage and high in energy will increase milk protein output.

On the other hand, a low energy, high forage diet will tend to lower milk protein test and milk protein production. Some researchers have observed a drop in protein production from high forage diets even when compared to low forage diets of similar energy content.

Dietary Protein Level

Milk protein level increases with increased dietary protein level but the changes are usually quite small - estimated at .02 percentage unit increase in milk protein for each percentage unit increase in dietary protein.

Increased dietary protein levels often increase ration digestibility and feed intake, thus increasing a cow's energy status and milk yield. In this situation, protein yield may be increased through increased milk production while milk protein percentage may not be affected.

Practically speaking, cows fed a ration well balanced in terms of meeting milk production needs are unlikely to show large responses in milk protein level increases in ration protein level.

Dietary Protein Rumen Degradability

Increasing the amount of dietary protein that escapes rumen degradation, i.e. "by-pass" protein will sometimes cause an increase in milk protein output. When responses are noted, the increase in milk protein yield can be due to an increase in milk volume and/or an increase in milk protein percentage. Unfortunately, results are not consistent and specific farm recommendations are difficult to make. Supplying a source of rumen by-pass protein which has a good amino acid balance will improve the chances of increasing protein production. Fish meal is one protein source worth considering.

Amino Acid Supply

Viewing protein requirements in terms of its constituent amino acids is a more detailed and sophisticated way of looking at a cow's protein needs. Only in recent times have nutritionists given serious consideration to this approach. The production goals set for today's cows have forced researchers to look much closer at this subject in an attempt to understand the requirements of high producing animals.

Since amino acid supply beyond the rumen is crucial to high levels of milk production, there has been considerable research with amino acids that have been protected against rumen degradation.

Although milk protein output sometimes increases when rumen-protected amino acids (usually methionine or lysine) are fed, the results have been variable and inconclusive. However, there is research underway that may lead to ration balancing programs and feed analysis systems that will enable us to predict when to expect a response to supplementary amino acids.

Some feed companies already use protected amino acids especially methionine, in topdress supplements and premium dairy feeds.

Dietary Fat

Milk protein level is often depressed when fat or oil is fed. This depression in milk protein level can occur with any fat source. Fat from oilseeds, free oil or protected fat sources have all been seen to decrease protein level in milk. However, since milk production often increases with fat feeding, protein yield may stay the same or increase despite a decrease in protein percentage.


In the future, the milk constituent levels most important to the farmer will depend upon milk pricing formulas and on market demands for breeding stock. Although milk component yield expressed as a percentage may be important under some quota systems, the actual kilogram yield of components needs to be considered when evaluating the response to diet change.

Research in the past did not focus on milk protein, and in many feeding trials protein test was not even reported. For this reason, less is known about milk protein than milk fat production. Adding to the challenge of future research is the possibility that we may wish to raise milk protein while lowering milk fat production.

Most ration balancing systems for dairy cattle ignore milk protein when predicting dietary requirements. As we focus more on protein and learn more about how to control it, protein level of milk will be routinely incorporated into our ration formulation programs.

Although the most certain way to increase milk protein test and yield is to feed a diet which depresses milk fat, this approach is not recommended. A milk fat depressing diet is usually accompanied by an acidic rumen, depressed forage digestion, off-feed problems, and feet and leg problems.

Although dietary manipulations tend to move protein and fat levels and yields in opposite directions, we often encounter herds with exceptionally high BCAs for milk, fat and protein production. These herds demonstrate that with the right combination of genetics, management and nutrition we can attain high levels of all of these production parameters.


When all is said, there are not many practical steps that can be taken to ensure improved protein yield without sacrificing fat yield. However, the following steps offer some chance of success and are worthy of consideration.

(1) Choose bulls in your breeding program that will improve your herd's protein production potential; the heritability of protein production is similar to that for fat.

(2) Feed high (17-19%) protein diets to cows in early lactation.

(3) Ensure that diets for early lactation cows provide adequate good quality "by-pass" protein. Products like fishmeal, meat and bonemeal and bloodmeal are products often considered.

(4) Feeding the protected amino acids lysine and methionine may be beneficial, especially if high production cows can be targeted.

(5) Most high production herds want to include some fat in their diets. They should be aware however, that fat feeding, at high levels in particular, presents the risk of lowered protein test and perhaps protein yield. Limited evidence suggests that supplementary niacin or supplementary fishmeal will alleviate this depression.


The nutritional effects on milk protein production is a complex subject. This is especially true when one tries to sort out the differences in responses in milk protein percentage versus milk protein yield. Those who wish to do more reading should refer to the following articles:

  1. DePeters, E.J. and Cant, P.J. 1992. Nutritional Factors Influencing the Nitrogen Composition of Bovine Milk: A Review. J. Dairy Sci. 75:2043-2070
  2. Emery, R. S. 1978. Feeding for increased milk protein. J. Dairy Sci. 61:825.
  3. Linn, James G. 1989. Altering the composition of milk through management practice. Feedstuffs. July 17, 1989
  4. Sutton, J. D. 1989. Altering milk composition by feeding. J. Dairy Sci. 72:2801-2814