Fat and fatty acid (FA) digestion and metabolism in dairy cattle is of considerable interest both to scientists and the dairy industry. The subject has received renewed interest for a number of reasons: First, we now recognize FAs can have specific and potent effects on ruminant digestion and metabolism.
Second, the addition of supplemental FA sources to diets is a common practice in dairy nutrition to increase dietary energy density and to support milk production. Our ability to understand the effects of individual FAs and different FA supplements on production parameters has direct impact on dairy industry recommendations and the usefulness of FA supplementation strategies.
The digestion and metabolism of dietary fats and FAs in dairy cows is complex. Our understanding of FA digestion and metabolism in dairy cows has advanced significantly in the last few decades. We now recognize FAs, both of dietary and rumen origin, can have different and specific effects on feed intake, rumen metabolism, small intestine digestibility, milk component synthesis in the mammary gland and energy partitioning between the mammary gland and other tissues.
Although, in general, FA supplementation has been shown to increase milk yield, milkfat yield and the efficiency of milk production, great variation has been reported in production performance for different FA supplements and, indeed, the same supplement across different diets and studies.
Recent research by us and others has focused on specific FAs and how dairy cows respond differently to combinations of FAs across different stages of lactation. Attention has been on the effects of dietary palmitic (C16:0), stearic (C18:0) and oleic (cis-9 C18:1) acids:
1. These FAs comprise a wide range of commercially available fat supplements.
2. They represent the largest proportion of FAs in milkfat and adipose tissue.
3. Due to rumen biohydrogenation, stearic acid is the predominant FA available for absorption by the dairy cow.
There is a wide range of FA supplements available for lactating dairy cattle. For example, calcium salts of free FAs and prilled saturated free FAs are common types of supplements used in the dairy industry, and they differ in FA content and FA profile.
Calcium-salt supplements typically contain 80 to 85 percent FAs, and these usually provide approximately 50 percent saturated and 50 percent unsaturated FAs. By comparison, prilled saturated free FAs contain approximately 98 percent FAs, which are approximately 90 percent saturated, 10 percent unsaturated. Typically, prilled saturated free FA supplements contain either a mixture of palmitic and stearic acids or are enriched in palmitic acid.
There is an extensive metabolism of dietary unsaturated FAs in the rumen, resulting in lipid material leaving the rumen that consists primarily of free FAs that are highly saturated. In general, the ability of ruminants to absorb saturated FAs is much higher than that of non-ruminants.
However, recent research has highlighted differences in intestinal digestibility among palmitic, stearic and oleic acids, which impacts the amount and profile of absorbed FAs available for metabolic purposes. Indeed, digestibility appears to be a good indicator of inclusion or not of a fat supplement in the diet, assuming this source of FAs does not markedly affect dry matter intake.
Milkfat synthesis in the mammary gland is dependent upon the simultaneous supply of short/medium-chain FAs and long-chain FAs. Palmitic acid has a higher preference as a substrate to start triglyceride synthesis than stearic or oleic acids.
The impact of dietary palmitic acid on milkfat synthesis has been studied extensively over the last few years, with research consistently showing improvements in milkfat yield with palmitic acid supplementation.
The impact of altering the FA profile of supplemental fat in dairy cow rations is highlighted in our recent study published in the Journal of Dairy Science. By changing the profile of the FAs supplied in a supplement, we can observe differences in nutrient digestibility, milk and milkfat yield, and bodyweight gain. A follow-up article in this magazine will provide a more in-depth review of research focusing on specific FAs and how dairy cows respond differently to combinations of FAs.
When making decisions on supplemental FA feeding, it is important to identify what you are trying to achieve and then design the nutritional program (including FA supplementation) around those objectives. The economics of the marginal return (in milk, milk components, health and reproduction) should drive the decision and be continually evaluated. Tangible factors impacted by FA supplementation not measured daily in the tank (e.g., bodyweight, energy balance, reproduction) are the most difficult parts to take into account.
In fresh cows, the high metabolic demand of lactation and reduced feed intake during the immediate postpartum period result in a state of negative energy balance. Approaches to increasing energy intake of postpartum cows include increasing starch content of the diet and supplementing FAs to increase the energy density of the diet.
Results are contradictory about the benefits of FA supplementation to early lactation dairy cows. We propose this is a result of differences in FA profile of supplements used and the time at which FA supplementation starts. Further work is required to characterize the sources of variation in response to FA supplementation.
In the future, the opportunity and challenge will be to continue to improve our understanding of how and which FAs affect nutrient digestion, energy partitioning and milk synthesis in lactating dairy cows. We will then need to effectively apply this knowledge in the feeding and management of today’s high-producing dairy cows.
Looking forward, perhaps, we will be feeding different blends of FAs for different purposes across lactation or for specific objectives.
Our knowledge of FA digestion and metabolism is rapidly advancing. Just as we recognize not all protein sources are the same, it is important to remember not all FA sources and FA supplements are the same. The key is to know what FAs are present in a feed or supplement.
Once this information is known, it is important to consider the possible effects of these FA on feed intake, rumen metabolism, small intestine digestibility, milk component synthesis in the mammary gland, energy partitioning between the mammary gland and other tissues, body condition and their effects on immune and reproductive function.
The extent of these simultaneous changes, along with the goal of the nutritional strategy employed, will ultimately determine the overall effect of FA supplementation and the associated decision regarding their inclusion in diets for lactating dairy cows. 
References available upon request. Click here to email an editor.
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Adam L. Lock
- Associate Professor
- Department of Animal Science
- Michigan State University
- Email Adam L. Lock
