Milk components

About 50% of milkfat is made from short-chain fatty acids, specifically acetate and butyrate. These are produced in the rumen. Long-chain fatty acids from dietary fat, microbial bodies, and back fat, are used to produce the remaining milkfat. If high levels of rumen available unsaturated fatty acids are fed and leave the rumen as partially saturated trans fatty acids, they can inhibit milkfat synthesis. More of these trans fatty acids escape the rumen when it is acidic. Over 90% of the proteins in milk are made in the udder from amino acids, glucose, and acetate. Adequate amounts of each must be provided so that the udder cells can fully express their protein producing ability and maximize milk protein production. Changes in the ration which increase the supply of glucose to the udder will increase the amount of lactose made, increase milk production, but not increase the lactose concentration of the milk produced.

Greater quantities of processed milk products like cheese and butter can be made from milk containing higher percentages of milk components. So, usually processors pay a premium for them. Particular milk markets will vary in the actual value they place on the fat and protein in milk. Often a nutritional change will increase milk, fat, and protein production but percent milk protein or percent milkfat will remain the same or decrease.

Especially when processors pay for pounds (or amounts) of fat, protein, or solids, it may be more profitable and easier to produce a greater volume of milk without changing or by slightly reducing the percentage of milk components. It is important to do the math.


Nutritional Influences on Milkfat

Short-chain fatty acids are made in the udder from short-chain volatile fatty acids, primarily acetate and butyrate, produced from the fermentation of fiber in the rumen. Long-chain fatty acids are not made in the udder but, instead, come from dietary fatty acids, the bodies of the rumen microbes, and the fat from the cow’s back. The short-chain and long-chain fatty acids are combined (about 50/50) to form milk fat.

The rumen microbes saturate 60-90% of the unsaturated fatty acid bonds of fats coming into the rumen. Fully saturated fatty acids or monounsaturated fatty acids with trans configuration (hydrogen atoms on either side of the double-bond rather than on the same side of the double-bond as in cis configuration) may escape from the rumen. Increased amounts of these trans fatty acids at the intestine are correlated with low milk fat syndrome. It is speculated that the mammary synthesis of fat from short-chain VFA’s is inhibited by the trans fatty acids.

There are a number of reasons for an increase in the amount of trans fatty acids arriving at the cow’s intestine. When cows are fed diets containing large amounts of rumen available unsaturated fatty acids, more trans fatty acids will escape the rumen. The rumen escape of trans fatty acids increases when cows experience rumen acidosis. The decrease in the rumen acetate:propionate ratio seen with rumen acidosis is a sign of a change in rumen fermentation which also increases the rumen escape of trans fatty acids and decreases milkfat concentration.

Supplying the Products Needed for Making Milkfat

About 50% of milkfat is made from short-chain fatty acids, specifically acetate and butyrate. hese are primarily made in the rumen from the fermentation of fiber. Good fiber fermentation is the result of feeding highly digestible forages and byproduct feeds, controlling rumen pH, controlling the levels of rumen available fats in the ration (<5%), and providing adequate amounts of rumen available nitrogen and amino acids.

About 50% of milkfat is made from long-chain fatty acids. These come from fat on the cow’s back, dietary fat, and rumen microbial fat. It is difficult for the cow to take fat from her back and put it into milk when in positive energy balance (after 30-60 days in milk). So when the cow enters positive energy balance, most of these long-chain fatty acids must come from the diet. Usually, dietary fats are supplemented to meet the cow’s general energy requirements. Often when supplemented, they increase milkfat synthesis but also increase milk yield, causing no change in milkfat content. There are no nutritional requirements specifically for fat. Perhaps in the future, there will be such recommendations. But, in general, if the cow’s acidosis is controlled and energy requirements are supplied with a blend of fiber, non-fiber carbohydrate, and fat, no deficiency in long-chain fatty acids will occur to depress milkfat production.

Limiting the Products Which Inhibit Milkfat Synthesis

Rumen Available Unsaturated Fatty Acids

Rumen available unsaturated fatty acids primarily come from plant or fish sources. Fats in whole seeds, like whole soybeans and whole cottonseed, are slowly available in the rumen. These slowly available unsaturated fatty acids will usually be completely changed to saturated fatty acids before they leave the rumen. But, if the rumen environment is compromised or if large amounts of the whole seeds are fed, the fats may leave the rumen as partially saturated trans fatty acids that cause milkfat depression. If free vegetable oils are fed, expect them to leave the rumen as trans fatty acids and depress milkfat.

Effect of Fat Form on the Fat Content of Milk

  Control Cottonseed Oil Whole Cottonseed
Milk Yield (lbs/day) 59.4 56.1 56.8
Milkfat (%) 3.54a 2.99b 3.70a

Values within a row with different superscripts (a,b) are different (P<0.05)
Adapted from Mohammed et al., 1988

Rumen Acidosis

Larger quantities of trans fatty acids escape the rumen when it is acidic. In addition, fiber digestion and acetic acid production is reduced in acidic rumens. Therefore, every effort should be made to control rumen acidosis in order to avoid low milkfat syndrome. When converting carbohydrate to energy, the rumen microbes produce acids. Rumen acids are buffered with saliva and supplemental buffers like sodium bicarbonate. Fiber stimulates saliva production. Rumen acids are absorbed from the rumen via the finger-like projections on the rumen wall, called papillae. Long fiber stimulates the movement of rumen contents to increase the absorption of acid through the rumen papillae. If acids are not sufficiently buffered or absorbed, they accumulate and result in high rumen acidity.

Feeding and Management Tips to Avoid Rumen Acidosis

  • Gradually increase grain before and after calving to increase the size of the rumen papillae prior to feeding large amounts of grain.
  • Don’t exceed 40% NFC and avoid too much rapidly digestible starch and sugar.
  • Avoid slug feeding grain (no more than 10 lbs (4.5 kg) fed at one time) and TMR sorting.
  • Feed hay prior to grain, especially first thing in the morning.
  • Make sure that at least 15% of the particles in the diet are over 1½ inches in length.
  • Forage NDF should make up more than 19% of the diet.
  • Have forage or TMR available to cows 24 hours per day.
  • Add buffers, such as sodium bicarbonate, to the ration of high-producing cows at 1% of the ration dry matter. Offer buffer and salt free-choice.
  • Limit heat stress.Cows that are heat stressed will eat less fiber. Use fans and misters. Open up the barn.

Watch for the signs of acidosis…… Daily roller-coaster intake and milk production. Inconsistent manure. Lack of cud-chewing. General cow depression.

Magnesium Oxide

Magnesium Oxide (2 oz (57 g)/day) has an additive effect on milkfat when it is fed with sodium bicarbonate. It increases milkfat either by increasing the flow rate of liquid out of the rumen and therefore, decreasing rumen acidity or by increasing the uptake of the fatty acids needed to make milkfat in the udder. The exact mechanism is unknown.

Non-Nutritional Influences on Milkfat

Genetics, season and stage of lactation can greatly impact the fat content of milk. Colored breeds produce milk with a higher milkfat content. Even within a breed, some herds and cows within herds will produce milk with higher milkfat content because of genetics. In the very beginning of lactation, cows that are using a significant amount of their body stores for energy may have an elevated fat test. Otherwise, the percent milkfat usually decreases with peak milk and increases as milk production declines after peak. Milkfat content often declines in the summer months due to heat stress.

Sampling procedure and mechanical problems can decrease the percent milkfat in the tank sample. Freezing, excessive agitation, or a malfunctioning pump can result in a drop in milkfat content. If this is the case, probably the average of individual cows (such as from a DHI test) will be higher than the milk tank analysis from the processor. Subclinical mastitis may result in milkfat depression.

Milk True Protein

The true protein content of milk affects the moisture, texture, flavor, and yield of cheese. Three major types of milk true protein exist: casein (75-85%), beta-lactoglobulin (7-12%), and alpha-lactalbumin (2-5%). In the past, many testing services and milk processors analyzed milk for “total protein”. This was simply calculated as the nitrogen content of milk multiplied by the factor 6.38. This was based on the assumption that milk protein contains about 15% nitrogen. However, non-protein nitrogen (NPN) which is of no value to the processor, was also included in this estimate of “total protein”. NPN is known to range from 2 to 10% of the “total protein”. A new method is being used to measure “true protein” in most laboratories. “True protein” does not include NPN and is a measure of the protein (chains of amino acids) valuable to the processor.

Nutritional Influences on Milk Protein

It is much easier to change the fat content of milk than to change milk protein content. Over 90% of the proteins in milk are made in the udder from amino acids, glucose and acetate. The nutritionist’s goal is to provide the cells of the udder with adequate amounts of each so that the cells can fully express their protein producing ability and maximize milk protein production. Protein production is usually limited by the amino acid that is in shortest supply in relation to the cow’s requirement. That amino acid is called the “first-limiting amino acid”. It is the missing link of the protein chain and when it is used up, protein production will be stopped. Energy, either from glucose or acetate, can also limit milk protein synthesis.

The cow receives amino acids at the intestine from two primary sources. The rumen microbes provide 50-75% of the amino acids and rumen undegradable protein (bypass protein or UIP) provides the rest. The efficiency of converting dietary nitrogen to milk protein by the cow is fairly low (25-30%). The cow uses many amino acids for the functioning of the gut, liver, and other tissues. This makes milk protein hard to change nutritionally. The balance of amino acids, rather than simply the amounts of individual amino acids, available for protein production in the mammary gland is also important for milk protein production.

Once we understand the amino acid supply better, it might become easier for nutritionists to accurately balance rations for amino acids and to predict and improve milk protein content and yield. More research is needed to help nutritionists accurately predict how much of each amino acid will be produced each day by the rumen microbes. More research would also help to predict how much of each amino acid from feed bypasses the rumen and is absorbed at the small intestine. Fortunately, we do know something about the rumen microbes and rumen bypass amino acid supply. Studies have shown that we can make improvements based on our current level of knowledge.

Rumen Microbial Amino Acids

The first step to be taken in increasing milk protein is to take care of the rumen microbes. This means providing highly digestible forages, maximizing dry matter intake, avoiding sub-clinical acidosis, providing adequate amounts of soluble and degradable protein, and synchronizing rumen available carbohydrates and proteins on an hourly basis in the rumen. Excessive fat in the rumen (over 5% rumen available fat) can decrease the growth of the rumen microbes.

The amino acid profile of the rumen microbes is very similar to that of milk protein. Microbial amino acids are, therefore, easily and efficiently converted into milk protein by the cow.

Amino Acid Composition of the Rumen Microbes as a Percentage of Milk Amino Acids

Amino Acid
Microbial Amino Acid as % of Milk Amino Acids*
Arginine 142%
Histadine 107%
Isoleucine 93%
Leucine 94%
Lysine 99%
Methionine 93%
Phenylalanine 114%
Threonine 111%
Valine 76%

* (Rumen Microbial Amino Acid (% of total essential amino acids) ÷
Milk Protein Amino Acid (% of total essential amino acids) x 100
(Adapted from Seymour and Nocek (1994) and C.G. Schwab, Univ. of NH, 1992)

Rumen Undegradable Amino Acids

The blend of amino acids in the rumen undegradable protein will impact milk protein production. Corn and corn byproducts, such as distillers grains and corn gluten meal, are known to be low in lysine. Soy is known to be low in methionine. Animal proteins provide an amino acid package more similar to milk than corn and soy proteins do. There are also individual bypass amino acids that are now being incorporated into feeds. The blend of rumen bypass amino acids should provide a profile of amino acids that complements the microbial protein made in the rumen. The goal is to combine the two sources of amino acids to make an intestinal amino acid supply similar to that needed for milk protein production. Most studies with supplemental bypass amino acids increase milk (lbs) and milk protein yield (lbs) in early lactation but increase milk protein content (%) in late lactation.

Researchers have studied the effect of dietary amino acids on milk protein production by either infusing amino acids directly into the abomasum or intestine or feeding “rumen-protected” amino acids. A study was conducted with 259 Holstein cows among six different university herds to look at the effect of ruminally protected methionine and lysine on milk protein output. Cows were fed diets based on corn silage, corn grain, and either soybean meal or corn gluten meal. Diets were supplemented with various amounts of rumen-protected methionine and/or lysine. The cows on the corn gluten meal diets did not produce as much milk protein as on the soybean meal diets. However, lysine supplementation did significantly increase milk protein in the corn gluten meal diets. Extra methionine did not increase milk protein in cows on the corn gluten meal diet even though the methionine content of the blood was increased, indicating that methionine was not the limiting amino acid in that diet.


Energy is needed for maintaining milk protein production. In early lactation, increased energy seems to stimulate both milk and milk protein production with little effect on the percentage of protein in milk. Later in lactation, energy does increase the concentration of protein in milk to a certain extent. Some of this response in milk protein may be due to the extra glucose and acetate available at the udder but added energy may more importantly cause an increase in microbial protein synthesis that increases amino acid supply at the udder. Studies have shown that feeding more rumen available carbohydrate can increase milk protein production.

A European survey of 3500 cows showed that when the amount of milk protein falls below 3.0%, the number of days a cow remains open (not pregnant) increases. Low milk protein may be an indication that a cow is in negative energy balance.

Dietary Fat

Excessive amounts of dietary fat have been shown to decrease milk protein production but the reason for this is still unclear. Fat substitution for ruminally available carbohydrate may depress microbial protein synthesis and thus, decrease the amount of amino acids available at the udder. Fat may also inhibit the growth of certain microbes directly. A limited amount of evidence suggests that excessive lipids can alter the way an animal processes and uses amino acids. Some nutritionists recommend adding 1 percentage unit more UIP for each 3% added fat in a ration.

Dry Period Nutrition

A Cornell study showed increases in milk protein when cows were supplemented with a good blend of undegradable amino acids for three weeks before freshening. It was suggested that these extra amino acids decreased the use of body proteins during this period and more of that body protein was then used after the cow freshened to make milk protein.

Non-Nutritional Influences on Milk Protein

The protein content of milk is influenced by breed, genetic merit, stage of lactation, season, and udder health. Breeds other than American Holsteins generally produce milk with higher protein content. Milk protein content has a pattern similar to fat over the course of lactation. It is usually higher during the first month of lactation, declines somewhat at peak, and then increases and peaks at 250 days in milk as milk production decreases after peak. Age of the cow doesn’t have much affect on protein content of milk produced. Milk protein generally falls during the summer months as fat test does due to heat stress.

Milk Lactose

Milk lactose (milk sugar) is primarily made from glucose. There is a linear relationship between the amount of lactose produced by the cow and the amount of milk produced. Changes in the ration which increase the supply of glucose to the udder will increase the amount of lactose made, increase milk production, but not increase the lactose concentration of the milk produced. Glucose is primarily made from propionate (a volatile fatty acid produced mostly from the fermentation of grain in the rumen) and some protein.


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Polan, C.E., K.A. Cummins, C.J. Sniffen, T.V. Muscato, J.L. Vicini, B.A. Crooker, J.H. Clark, D.G. Johnson, D.E. Otterby, B. Guillaume, L.D. Muller, G.A. Varga, R.A. Murray, and S.B. Pierce-Sandner. 1991. Responses of dairy cows to supplemental rumen protected forms of methionine and lysine. J. Dairy Sci. 74:2997.

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Related Links:

Balancing Rations for Milk Components
Bill Chalupa, University of Pennsylvania, Charlie Sniffen, Miner Institute
Find in Session I of 2000 Proceedings. Provides very similar information to this article, but offers a more detialed review of research.

Manipulating Milk Protein Production and Level in Lactating Dairy Cows
Peter Robinson, University of California
Find in Session V of 2000 Proceedings. Detailed review of factors that affect milk protein levels. List is comprehensive and a good overview. Broken down into things that can be modified, and those that can't.

Nutritional and Management Factors Affecting Solids-Not-Fats, Acidity, and Freezing Point of Milk
B. Harris, Jr., Ph.D. and K.C. Bachman, University of Florida

Feeding to Maximize Milk Solids
R. Grant, University of Nebraska
Covers general feeding strategies that can help influence milk solids content.

Milk Urea Nitrogen Testing
Rick Grant, D. Drudik, and J. Keown, University of Nebraska - Lincoln
General overview of the use of MUN on the farm. Good coverage of basic information

MUN as a management tool
M.F. Hutjens, Ph.D., University of Illinois
Overview of how MUN values can be used practically on the farm. Offers scenarios when a producer may benefit from MUN testing.

Troubleshooting Problems with Milkfat Depression
V.A. Ishler and R.S. Adams, Penn State University
Comprehensive review of the many factors that can contribute to low fat test. Offers practical suggestions for control.


Mary Beth de Ondarza

Mary Beth de Ondarza
45 articles

Nutritional consultant for the dairy feed industry at Paradox Nutrition, LLC.

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Dr. de Ondarza received her Ph. D. from Michigan State University and her Masters Degree from Cornell University, both in the field of Dairy Nutrition.

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Paradox Nutrition

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