Fat contains 2.25 times more energy than carbohydrate. After maximizing carbohydrate in the diet, fat is often added to meet the remainder of the energy needs. High levels of rumen available fats (above 5% of the ration DM) can decrease growth of the fiber-digesting microbes, decreasing fiber digestibility and intake. If more energy is needed beyond that supplied by carbohydrates and rumen available fats, rumen inert fats can be added. The maximum level of total fat should be 7% of the ration DM. Fats can improve reproductive performance.

As we feed for higher levels of milk production, energy often becomes the first-limiting nutrient. The most economical way to increase the amount of energy supplied to the cow is to increase the amount of forage dry matter eaten and to increase the digestibility of forage fiber. Much progress has been made in this area through improved cow comfort, feeding management, forage selection, as well as forage management. But, for high milk production, more energy is needed than can be supplied by forage alone. A significant portion of the cow’s energy requirements can be provided with non-fiber carbohydrate (NFC) in the form of grain. But, in early lactation rations, the limit on the amount of NFC which can be fed without adversely affecting rumen function by creating rumen acidosis is often reached prior to meeting the level of energy required by the cow. For this reason fat is often used to increase the energy density of a ration and to meet the remainder of the cow's energy requirement. One pound of fat contains 2.25 times more energy than a pound of carbohydrate. It important to remember that the cow’s requirement for NFC must be met, otherwise, the growth of the rumen microbes will be reduced.

Fats are absorbed from the small intestine and used for energy by the cow’s body tissues or used for production of body fat and milk fat. Fats are not used for energy by the rumen microbes. High levels of rumen available fats (above 5% of the ration dry matter) can decrease the growth of the fiber-digesting microbes and, ultimately, decrease fiber digestibility and feed intake.

Fats are classified as either saturated (containing no double bonds in their chemical structure) or unsaturated (containing at least one double bond). Saturated fats are less detrimental to the rumen microbes. Unsaturated fats are hydrogenated and turned into saturated fats by the rumen microbes. The fiber microbes are especially susceptible to inhibition by unsaturated fats. We are able to get away with high levels of unsaturated fats in feed ingredients like roasted soybeans and whole cottonseed because these fats are slowly available in the rumen. Liquid fats are quickly available to the microbes and need to be in a saturated form to be least detrimental to the microbes.

Tallow is often a cheaper source of fat than commodity fats such as whole cottonseed. When available, it can be effectively used to supply fat. For dairy cows, the highest grade of tallow, having a high level of saturated fat (or a high titer) is desirable.

Grades of Liquid Fat

  1. Animal Fat – Includes rendered fat from beef and pork by-products. It can be called Tallow if the titer is 40 or higher. It is called Grease if the titer is under 40. Lower titer indicates higher unsaturated fat levels. Pork fat is more unsaturated than beef fat.
  2. Poultry Fat – Includes fats from 100% poultry offal. Poultry fat is fairly unsaturated.
  3. Blended Feed Grade Animal Fat – Includes blends of tallow, grease, poultry fat, and restaurant grease.
  4. Blended Animal and Vegetable Fats – Includes blends of feed grade animal, poultry, vegetable fats and/or restaurant grease.
  5. Feed Grade Vegetable Fat – Includes vegetable oil, acidulated vegetable soapstocks and other refinery by-products.

Of the five fat sources above, only fat sources with an iodine value between 35 and 50 should be fed to dairy cows.

A fat of this type can be added up to 3% of the diet dry matter. It should be stabilized with an antioxidant to prevent rancidity. Oxidative rancidity produces an unstable product that is unpalatable. Rancidity can also destroy Vitamins A, D, and E. Tallow should also be low in moisture and have few impurities. Refined beef tallow should be almost pure white in color. If more energy is needed beyond the amount supplied from the rumen fermentation and rumen available fats, rumen inert fat can and should be used. The maximum level of fat in the total ration should be 7% (5% from rumen available sources and 2% from rumen inert fats).

Types of Rumen Inert Fats

Calcium Salts of Fatty Acids (examples, Megalac and Energi II):

- 84% fat

- Calcium salt of fatty acids derived from palm oil

- broken down once it hits the acid of the lower gut

Hydrogenated Fatty Acid Products (examples, Energy Booster 100; Biopass):

- 99% fat

- 100% free fatty acids that are hydrogenated (saturated)

Hydrogenated Tallow Products (examples, Carolac, Alifet, Dairy "80"):

- 95-99% fat

- hydrogenated triglycerides

Rumen inert fats are also called rumen “bypass”, “protected” or “escape” fats.

True fat is made up of three fatty acids connected to glycerol. Most nutritionists would agree that free fatty acids are more easily absorbed and used by the cow than intact triglyceride (the true fat made up of three fatty acids connected to glycerol). Free fatty acids are usually assigned a higher energy value. For this reason, calcium salts of fatty acids and hydrogenated fatty acid products are usually assigned a higher energy value than are hydrogenated tallow products. Remember that the energy value of rumen inert fat should also reflect the total amount of fat that it contains (for example 84% fat versus 99% fat).

Ration Formulation

1. Maximize DM Intake and Energy from Rumen Fermented Carbohydrate

  • Not more than about 40% NFC
  • The energy requirement of a high producing cow is usually higher than the amount of energy we can get from NFC's and fiber without getting into acidosis problems
  • The basal ingredients (corn, forage, protein sources etc.) usually supply 2-3% fat (% of the ration dry matter)

2. Add 2-3% Oilseeds and/or Commodity Fats (up to 5% Rumen Available Fat in the Total Ration DM)

  • Tallow (100% fat, 60% hydrogenated fat)
  • Roasted Soybean (41-43% CP, 18-20% fat, 85% of the fat is unsaturated)
  • Whole Cottonseed (23-25% CP, 20-24% fat, 71% of the fat is unsaturated)

Maintain slightly higher levels of calcium (1.0%) and magnesium (0.30%) in the total ration DM. These minerals can form soaps with rumen available fat, lowering their availability to the cow.

3. If more energy is still needed, add 2% Rumen Inert Fat (Total Fat = 7% of the Total Ration DM)

Low Milk Fat Syndrome

Short-chain fatty acids are made in the udder from short-chain volatile fatty acids, primarily acetate, 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. They form either 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). These trans fatty acids are correlated with low milk fat syndrome. It is speculated that it is the synthesis of fat from short-chain VFA’s which 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. Rumen acidosis is also known to decrease milk fat concentration. 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.

Fats and Reproduction

It is common knowledge that cows undergoing a prolonged period of weight loss following calving are usually less fertile. Many studies have been conducted to determine the effect of supplementary fat on reproductive performance. In general, fat supplements have improved fertility. Much of this response has been attributed to a reduced period of weight loss. However, in some studies, milk production was improved with supplementary fat and cows did not experience lower body weight loss yet reproductive performance was still enhanced.

Dr. Charles Staples from the University of Florida conducted extensive literature reviews as well as his own studies to determine if there is an additional effect of polyunsaturated fatty acids on reproductive performance beyond their effect on the cow’s energy status. Staples et al. (1998) suggested that polyunsaturated fatty acids may improve fertility by:

1. Sparing glucose to stimulate the release of LH from the anterior pituitary gland which stimulates the development of ovarian luteal cells,

2. Increasing circulating cholesterol concentrations and subsequently increasing progesterone levels, or

3. Inhibiting the production of prostaglandin and estradiol, thus increasing the lifespan of the corpus luteum and improving embryo survival. More research is needed in this area.


Chase, L.E., Added fat in dairy cattle rations, Cornell University Animal Science Mimeo

Emery, R.S. and T.H Herdt. 1991. Lipid Nutrition. In: The Veterinary Clinics of North America: Food Animal Practice. W.B. Saunders Company, Philadelphia, PA.

Erdman, R. 1996. Milk fat depression: some new insights. In: Proceedings of the 1996 Tri-State Dairy Nutrition Conference, May 14-15, 1996, Fort Wayne, Indiana.

McCullough, M.E. 1994. Take a close look at why you’re feeding fat. Hoard’s Dairyman, July 1994, p. 504.

Shaver, R.D. 1990. Fat sources for high producing dairy cows. In: Proceedings of the 51st Minnesota Nutrition Conference, September 18-19, 1990, Bloomington, MN

Staples, C.R., R. Mattos, C.A. Risco, and W.W. Thatcher. 1998. Feeding fish meal may improve cow fertility rates. Feedstuffs. January 12, 1998, p. 12.

Related links:

Supplemental Fat for High Producing Dairy Cows
Rick Grant and Don Kubik, University of Nebraska - Lincoln
Discusses individual oilseeds in depth

Feed Nutrients In: Feeding the Dairy Herd North Central Regional Extension Publication
J.G. Linn et al.

Should You Be Feeding Fat to Your Dairy Cows?
D.M. Amaral-Phillips, R.W. Hemken, and J.A. Jackson, University of Kentucky
Concise discussion with helpful tips for managing fat feeding on the farm

Conjugating Fatty Acids: Not Verbs
Shannon Linderoth and Clint Peck
The human health benefits of CLA (a fatty acid found in cow's milk) are dicussed


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

Paradox Nutrition

Paradox Nutrition, LLC is a nutritional consultation business for the dairy feed industry. Mary Beth de Ondarza, Ph.D. is the sole proprietor.

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