Feed efficiency measures the proportion of nutrients from feed converted into milk components versus those that become waste, including manure and spoilage.
As with all areas of dairy operations, increasing efficiency – in this case, feed in the rumen – deserves attention because when it is improved, gains can be seen beyond the area of focus, not to mention in sustainability (this includes economically) as well.
Using the knowledge shared by Mary Beth Hall, a dairy research scientist from the U.S. Department of Agriculture (USDA), on May 16 at the Animal Nutrition Conference of Canada, let’s look at what contributes to feed efficiency in the rumen of dairy cows and what gives optimal results.
The first pass
In what Hall described as the “first pass,” feed enters the rumen and is processed as gas, organic acids and microbes, where the microbes become a high-quality source of protein for the cow. This first pass is important because the energy it produces can impact ruminal health performance.
The initial metabolic process is rumen fermentation of carbohydrates, which provide energy and instigate microbial protein production, which break down the carbs into short-chain fatty acids (SCFAs), crucial for energy.
SCFAs like acetate, propionate and butyrate are absorbed into the bloodstream. Acetate is crucial for milkfat production, and propionate is converted into glucose in the liver, providing essential energy.
Glycogen production ramps up
Once propionate is absorbed and transported to the liver, it becomes the focal point for glycogen production. Propionate is converted to glucose, which can then be stored as glycogen. Glycogen, a polysaccharide stored in the liver and muscles, is a critical energy reserve for dairy cows.
During periods of high energy demand, like lactation, a steady glucose supply is crucial. Farmers can enhance glycogen production by including carbohydrate-rich feeds like corn silage in the diet, ensuring a steady supply of propionate. Hall emphasized, “One of the benefits of making this much glycogen is that we reduce the pH drop by slowing the fermentation down.”
Fat action
With a high-level understanding of propionate accounted for, we can turn to milkfat production and the role of butyrate and acetate.
Butyrate production is increased with greater sugar fermentation compared to starch, and much of the butyrate produced in the rumen is made by protozoa bacteria.
Both acetate and butyrate are involved in de novo fatty acid synthesis, which can count for 20% to 40% of the fatty acids in milk, with the remainder coming from feed or the cow’s own body fat stores.
“Butyrate (four carbon) is more efficient than acetate because you've got one bond already made, and the animal didn't have to put ATP [i.e., energy] in to make that bond. So it can help energetic efficiency, and because acetate is a key energy resource for the mammary gland, if you can conserve acetate for other purposes, that's a pretty good deal,” Hall noted.
Thus, the main takeaway regarding the impact of changes to carbohydrate consumption and its link with fatty acids is, as Hall explained: “When you feed a more slowly fermenting diet – and this is what happens when you feed something that might be more rapidly fermenting like water-soluble carbohydrates – if a cow has [too] much of a load of [fatty acids] to deal with, she can't store ATP, she doesn't store acetate and butyrate, but she can turn it into fat.” Whether this fat is stored in the cow’s body or is transferred to milk components can depend on eating time and overall feed efficiency.
Protein priorities
Lastly, Hall touched on protein. Efficient microbial protein production in the rumen is crucial for supporting the cow's protein needs. The yield of microbes per unit of carbohydrate increases when more nitrogen is available, indicating the importance of adequate nitrogen supply for optimal microbial growth. These microbes are not just essential for digestion but also serve as a significant protein source for the cow.
Protein intake must be balanced though, because as Hall described, “You don't want to try this to the point where your milk urea nitrogen is up, your cow is excreting more nitrogen, and your feed efficiency for nitrogen is going down.” There is a fine balance between adequate protein and overconsumption.
In summary, Hall concluded, “The rumen has impact on overall digestion, types of nutrients available to the cow and mass of nutrients available. All of these are going to come together to affect cow performance and help to determine feed efficiency.” She recommended tweaking and monitoring diet composition and paying attention to have cows respond to these changes.
