27 April 2022
Dr Emma Davies: IBERS, Aberystwyth University.
- The fatty acid composition of milk can be manipulated by making changes to a cow’s diet.
- The milk of cows grazing fresh grass have higher levels of unsaturated fatty acids including omega-3 and omega-6 fatty acids than the milk of cows consuming conserved forages and mixed rations.
- Increasing polyunsaturated fatty acid levels in milk has the potential to increase the human health benefits associated with milk, further increasing the marketing potential of grass-based dairy production systems.
Introduction
The milk produced by dairy cow breeds is composed of approximately 87% water and a variety of dissolved, emulsified, and dispersed compounds. These compounds include lipids, proteins, carbohydrates, salts, vitamins, and minerals. Of the different types of lipids contained within milk, fatty acids are of particular interest as these are known to have significant impacts on human health.
Cow’s milk contains over 400 fatty acids. These are classified according to their molecular structure and are termed saturated, mono-unsaturated, or poly-unsaturated fatty acids. Saturated fatty acids have a carbon backbone connected by single bonds only, meaning that they are fully “saturated” with attached hydrogen atoms. In contrast, the carbon backbone of unsaturated fatty acids has one (mono) or more (poly) double bonds present, meaning it has fewer hydrogen atoms bonded to its carbon chain. These differences in molecular structure are responsible for the different characteristics of saturated and unsaturated fats. For example, saturated fatty acids tend to be solid at room temperature, while unsaturated fatty acids tend to be liquid at room temperature.
Unsaturated fatty acids can be further sub-classified according to the position of their first double bond. In omega-3 fatty acids, the double bond occurs at the third carbon atom, whereas in omega-6 fatty acids, the double bond occurs at the sixth carbon atom (using Greek nomenclature). Alpha-linolenic acid (ALA), an omega-3 fatty acid, and linoleic acid, an omega-6 fatty acid are two essential polyunsaturated fatty acids for humans. They are essential because they cannot be synthesised by the human body and must be consumed. Other examples of omega-3 fatty acids include docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), while another omega-6 fatty acid is arachidonic acid. Meat, milk, eggs, fish, and plant oils are the primary sources of these fatty acids in the human diet.
Polyunsaturated fatty acids are classified according to the position of their first double bond.
Generally, polyunsaturated fats are considered to be more beneficial to human health than saturated fats, as the latter have been associated with conditions such as increased low-density lipoprotein (LDL) cholesterol, heart disease and stroke. However, unsaturated fatty acids such as omega-3 and omega-6 fatty acids have protective biological functions. One omega-6 fatty acid present in milk is conjugated linoleic acid. This has been identified as having potent anticarcinogenic properties and is one of the most extensively studied fatty acids in milk. However, the dietary balance between omega-6 and omega-3 fatty acids strongly affects their function. Diets with a high ratio of omega-6 to omega-3 fatty acids, such as the modern western diet, are associated with an increased risk of cardiovascular disease, cancer, and inflammatory and autoimmune diseases. However, diets with a low ratio of omega-6 to omega-3 fatty acids (1-4:1), are thought to reduce the risk of many chronic diseases. Therefore, substituting saturated fats for polyunsaturated fats, especially conjugated linoleic acid and ALA, and consuming a diet with a small omega-6 to omega-3 ratio is important to optimise health.
As the saturated and polyunsaturated fatty acid profile of milk is influenced by the cow’s diet, this can be manipulated to produce milk with a fatty acid composition considered beneficial to human health. One European Innovation Partnership project involving 20 dairy farms from Southwest Wales is aiming to investigate which of their pasture management practices result in the highest levels of polyunsaturated fatty acids in their milk.
What factors affect milk fatty acid composition?
The fatty acid concentration of milk is dependent on a number of factors, including the composition of fatty acids within the cow’s diet, the microbial activity of the rumen, and fatty acid synthesis within the mammary gland.
Typically, saturated fatty acids account for 70-75% of the fatty acids present in milk by weight. A significant amount of these saturated fatty acids come from the biohydrogenation of unsaturated fatty acids by micro-organisms in the rumen, and from synthesis within the mammary gland. Mono-unsaturated and poly-unsaturated fatty acids account for 20-25% and 0-5% of the fatty acids within milk respectively, with the majority of these coming directly from the cow’s diet. One particular study of the Swedish dairy herd in 2001, which has a mixed housing and grazing system similar to the predominant system type in the UK, found that poly-unsaturated fatty acids accounted for 2.3% of the total fatty acids by weight, with linoleic acid and ALA being the most abundant, with a ratio of 2.3:1.
Diet related factors that affect the fatty acid concentration and composition of milk include energy intake, fibre intake, and ruminal fermentation. However, the factors considered to be the most influential are plant specific and include the plant species, the cultivar, the plant’s growth stage, and its conservation status (fresh vs. conserved). Seasonal, regional, and weather related environmental factors also have an effect, as do the cow’s genetics (breed), and her stage of lactation.
For fresh grass and legume forages, fatty acid content is generally lower than 50g per Kg/DM. However, ALA is the most abundant fatty acid and can reach levels greater than 50g per Kg/DM during times of early vegetative growth in the spring. Total fatty acid levels within grasses are highest in spring (April), decline during the summer (June), and recover in autumn (September and November). The ratio of linoleic acid to ALA also decreases during the summer months, with ratios of approximately 4.5-6:1 during spring, 2-4:1 during summer, and 4-5:1 during the autumn.
As fatty acid levels are correlated with plant growth stage and leaf proportion, different varieties of grass produce different fatty acid profiles during different times of the year. For example, L. ×boucheanum (a hybrid of L. multiflorum x perenne) and Italian ryegrass (L. multiflorum) have been found to have higher total fatty acid and ALA levels than perennial ryegrass (L. perenne) during the spring and autumn respectively. However, perennial ryegrass has higher total fatty acid and ALA levels than Italian ryegrass and L. ×boucheanum during the summer.
Similarly, different clover species produce different fatty acid concentrations. The concentrations of linoleic acid in red and white clover are comparable to one another and to ryegrass. However, the concentration of ALA tends to be slightly higher in white clover compared to red clover, which is comparable with ryegrass. The ratio of linoleic acid to ALA in fresh white and red clover is smaller than that for grasses, at approximately 2-3:1 in both the early and late stages of growth.
The total fatty acid levels of conserved forages are lower than for fresh forages as the processes of drying, oxidation and leaf shatter reduce lipid levels, particularly linoleic and ALA levels. Therefore, lipid levels are also significantly lower in hay compared to silage. Similarly to fresh grass and clover, ALA is the most abundant fatty acid in grass silage, however, linoleic acid is the most abundant fatty acid in maize silage and whole crop wheat silage.
Dairy cow diets may also contain lipids in the form of oils or oilseeds within a mixed ration. Oils from rapeseeds and sunflowers have high levels of linoleic acid and oleic acid, an omega-9 monounsaturated fatty acid, but have relatively low levels of ALA. Safflowers, and soyabeans also have high levels of linoleic acid. However, linseed and camelina have high levels of ALA, and a high proportion of ALA when compared to linoleic acid.
Milk composition and production system
A number of studies have compared the effects of different production systems on the fatty acid composition of milk. Generally, omega-3 levels and conjugated linoleic acid levels are highest in grass-based systems, followed by organic mixed housing systems and finally housed systems. Additionally, ratios of omega-6 to omega-3 fatty acids are lowest in grass based systems, followed by organic mixed housing and housed systems.
Studies have shown that when in mixed housing and grazing systems, the saturated fatty acid content of milk is lowest during the summer when cows are grazing and highest in the winter when cows are housed and fed ensiled forages and concentrates. Conversely, levels of unsaturated fatty acids, including omega-3 and omega-6 fatty acids are highest during the summer and lowest during the winter. This is largely due to the differential fatty acid profiles of fresh and conserved forages and mixed rations.
One particular study conducted in Wales, compared the effects of three different pasture based systems on milk composition. The systems compared included a conventional intensive system where cows were housed indoors in winter, an organic medium-intensity system with medium pasture intake, and a conventional low-intensity system with high pasture intake based on the New Zealand system. The study found that levels of unsaturated fatty acids, considered beneficial for human health were highest in the conventional low intensity system, followed by the organic medium intensity system, and finally the conventional intensive system. The levels of saturated fatty acids were significantly higher in the conventional intensive system than the other two systems. The organic medium intensity and conventional low intensity systems also had the highest omega-3 fatty acid content, which was consistent throughout the year.
Therefore, increasing the amount of fresh forage in a cow’s diet enhances the concentration of polyunsaturated fatty acids in milk. Studies have shown that when cows are transitioned from mixed diets consisting of silage, grain, and oilseed, to a fresh forage diet containing grasses and legumes, that concentrations of polyunsaturated fatty acids, including conjugated linoleic acid increase, and can even double. Increasing the proportions of fresh forages within diets can also lead to increased omega-3 and linoleic acid levels. However, some studies have also shown that in addition to increasing unsaturated fatty acid levels, fresh grass based diets can also increase saturated fatty acid levels.
Housed cows consuming conserved forages have lower milk omega-3 and omega-6 fatty acid concentrations that cows grazing fresh forages.
The differences in fatty acid levels between grass species indicates the potential for the selection of high-lipid grass varieties. Additionally, the incorporation of different grass varieties into the cow’s diet at different times of year is likely to be beneficial when trying to maintain relatively high polyunsaturated fatty acid milk profiles. Studies have also shown that fatty acid levels, particularly linoleic acid and ALA, decrease when the regrowth interval is extended to 38 days or more. This indicates that pasture management is an important factor when managing milk fatty acid profiles, as a short grazing rotation is optimal to maintain high plant polyunsaturated fatty acid levels. Similarly, a short cutting cycle can be used to maintain maximal levels of polyunsaturated fatty acid levels in conserved forage.
The diversity of the plant species in the cow’s diet also influences milk composition. For example, switching from a monoculture maize silage to a silage cut from a diverse grass meadow can increase beneficial polyunsaturated fatty acids, including omega-3 fatty acids and conjugated linoleic acid. As clover has a higher proportion of ALA to linoleic acid than ryegrass, incorporating red or white clover into a ryegrass based forage can also increase omega-3 fatty acids, particularly ALA in milk.
The fatty acid profiles of housed cows can also be enhanced by supplementing mixed rations with seed and oils. This can enhance the unsaturated fatty acid content and decrease the saturated fatty acid content of milk. For example, feeding soybean oil or extruded soybean, which is rich in linoleic acid has been shown to increase the conjugated linoleic acid content of milk, while feeding linseed oil can increase levels of ALA and total levels of omega-3 fatty acids.
Summary
Some of the diets considered the healthiest in the world, such as the Mediterranean diet, contain relatively low levels of saturated fats and high levels of unsaturated fats. Many studies have shown that cows consuming fresh forages produce milk with a greater concentration of unsaturated fatty acids. Furthermore, the concentration of ALA and conjugated linoleic acid, two important omega-3 and omega-6 fatty acids is increased in milk produced from cows fed fresh forages. As public awareness of the health benefits of different foods and food components increases, increasing the polyunsaturated fatty acid composition of milk through dietary manipulation has the potential to increase the health status of milk. Additionally, as consumer demand for grass-fed, extensively managed milk and meat products is also increasing due to an increased awareness of animal welfare, milk produced from cows grazing fresh forages has the potential to meet changing consumer demands.
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