By Sydney Moore, BSc. MSc., University of British Columbia and Kaitlyn Dancy, BSc. MSc., University of Guelph
Milk Fat Synthesis
Milk protein and fat content will vary across dairy cow breeds. While Holstein dairy cows typically have the lowest milk fat percentage, due to their high milk yield, they often still produce the greatest amount of butterfat per day. The table below provides average 305-day milk yield per cow (kg), % fat, and % protein of dairy cow breeds within the Canadian dairy industry, recorded across 2019.
|Breed||305d milk yield/ cow (kg)||Fat||Protein|
*table adapted from CDIC 2019
Milk fat or “lipids” from the milk consist primarily of triglycerides. Triglycerides are made of three fatty acids attached to a glycerol backbone. Milk fat from dairy cows is estimated to consist of over 400 different fatty acids, all of which vary in chain length and number of unsaturated bonds. However, research has shown that approximately only 20 of these fatty acids make up the majority of the fatty acid profile in milk, with the highest portions being myristic, palmitic, stearic and oleic fatty acids. All fatty acids can be broadly divided into two main groups: short-chain vs long-chain fatty acids. Short-chain fatty acids are created in the udder of the cow, from short-chain volatile fatty acids (VFAs) like acetate and butyrate, which are produced from the fermentation of fiber in the rumen. Long-chain fatty acids are produced from the digestion of rumen microbes, from body fat of the cow and from dietary fatty acids fed to the cow. These long and short-chain fatty acids are combined (~50/50) in the mammary cells, and are secreted into the milk in the form of milk fat globules.
Perspectives of Milk Fat
Now more than ever, consumers are concerned about the quality and sustainability of feeds being fed to cows. What we feed our cows has a direct effect on the quantity and quality of milk that they produce, as well as their health and welfare. While milk composition will vary depending on the breed of the dairy cow, milk in Canada generally consists of approximately 87% water, 4% fat, 3.25% protein, 4.6% lactose, with the remainder consisting mainly of vitamins and minerals. With respect to the human population, milk fat (and the fatty acids that make up this milk fat) are considered an important nutrient in the diets of many humans globally. While, some past research has reported milk fat as having negative impacts on human health, research from the last decade has provided substantial evidence of its benefits. Multiple studies have surfaced more recently demonstrating the positive effects milk and subsequent milk fat can have on human health. According to the Dairy Research Institute and National Dairy Council in the U.S, consumption of milk, regardless of fat level, has been associated with lower blood pressure and a reduced risk of diabetes and cardiovascular disease. From the perspective of the dairy industry, maintaining milk fat and protein are essential from an economic standpoint, as these components are needed for the manufacturing of dairy products. Additionally, these components are crucial with respect to herd management, as deviations in milk fat or protein can indicate possible nutrition or health problems within the herd.
Feeding To Support Milk Fat
When feeding dairy cows, it is crucial to keep in mind that what we feed them impacts the stability of the rumen and the production of volatile VFAs within the rumen. Feeding high amounts of starch can rapidly decrease the pH of the rumen, possibly leading to ruminal acidosis and a reduction in neutral detergent fiber (NDF) digestibility. This can lead to a condition known as milk fat depression, where the animal’s milk fat drops by ~0.2% or more. Not only is this not ideal from a production perspective, this state of the rumen is also an animal health and welfare concern as it can cause distress to the animal and increase her risks of a variety of unwanted metabolic disorders.
Avoiding overfeeding grains and high starch feeds is crucial to supporting rumen health, production and the overall health and well-being of our dairy cows. Many studies have investigated the effects of supplementing sugars to the diets of dairy cows as an alternative to increasing starch content to support the energy and production demands of the animal. Cows are naturally adapted to high sugar diets, as sugars are water soluble carbohydrates that are rapidly fermented and absorbed through the rumen wall. Sugars also have no known negative impact on rumen pH. Additionally, feeding sugars supports production of the VFAs acetate and butyrate, key fatty acids that contribute to milk fat synthesis. This was supported by research from McGill University which showed that butyrate concentration in the rumen increased greatly when dairy cows were fed diets containing ~7-10% total sugar.
Research from Kansas State University in 2011, demonstrated that by replacing 5% of dry matter from corn with dietary molasses (55% sugar/DM basis), rumen pH and de novo fatty acid synthesis improved, increasing the yield of short chain fatty acids during a diet-induced state of milk fat depression. De novo fatty acids are created within the mammary gland consisting of short-chain fatty acids, which are produced by the fermentation of fiber in the rumen. Recent research has reported that de novo fatty acid production has a strong correlation with the production of protein and milk fat by the mammary glands. Additional research from Kansas State University also demonstrated that molasses supplementation to dairy cows may even have positive impacts on the human population consuming milk from these supplemented cows. Molasses supplementation has been shown to increase trans-11 C18:1 (also known as vaccenic acid) in milk fat. Vaccenic acid has been shown to supress malignant lesions and cancer cells in human and rat mammary glands as well as the human colon.
Furthermore, feeding sufficient amounts of high quality forages can also have a drastic impact on the yield of milk fat. Researchers from Penn State University recommend feeding lactating cows a ration where ~40-45% of the DM of the diet comes from forage. Forage intake is critical to supporting fiber-digesting bacteria that increase butyrate and acetate production in the rumen. Overall, feeding to support rumen health is essential for milk fat synthesis as decreases in rumen pH will reduce production of butyrate, limiting de novo fatty acid synthesis and subsequent milk fat production.
How we feed cows, and their feeding environment, can also impact feed intake and, consequently, milk production and component yields. Factors such as stocking density, space at the feed bunk, frequency of feeding, and characteristics of the diet (mixed ration or component feeding, particle length, quality of forage, palatability, etc.) can all influence milk components by affecting dry matter intake (DMI) and feeding behaviour. Ensuring cows are not over-stocked, and have equal access to fresh, palatable feed that is offered multiple times per day, can drastically reduce feed sorting and slug feeding (both undesirable behaviours that can greatly impact rumen pH and rumen health). Avoid feeding bulky forages with long particle sizes. Research from the University of Guelph has shown that longer forage size in the diet will result in increased sorting behaviour against that fraction/component of the diet and encourage feelings of gut fill, reducing DMI overall. Forage should ideally be cut to ~ 1 inch within a TMR to decrease sorting behaviour and maximize DMI. Adding liquid molasses to the TMR has also been shown to reduce sorting behaviour and increase DMI by 2-4 kg/d. Furthermore, when ration changes are needed, making gradual changes to the ration can allow rumen microbes time to adapt to ensure VFA production and microbial protein production (a contributor to milk protein yield) is not depressed.
To conclude, it is important to not only think about what we are feeding our cows (i.e the ingredients and chemical composition of the diet), but also how we are feeding them (are we feeding them more than once? Do all cows have access to feed? Are we discouraging feed sorting? etc.). The what and how of feeding cows are equally as important and impactful on the resultant milk yield and quality. Feeding high sugar ingredients, such as a molasses-based liquid feed, caters to both of these factors. Molasses supports fiber-digesting bacteria, improving NDF digestibility and increasing butyrate production in the rumen. This in turn, supports rumen health and pH and has been shown in multiple studies to increase milk fat percentage and yield. Additionally, molasses is a sustainable, highly palatable feed, with “sticky” characteristics that allow for binding of all fractions and components of the diet, limiting feed sorting and improving DMI. Maybe the answer to supporting milk fat production in our cows is sweeter than we think!
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