All carbohydrates are not created equal, especially when it comes to how cows digest and respond to them. “Sugars” like glucose, fructose, and sucrose behave differently from starch in the rumen and those differences matter when formulating diets.
Sugars are NOT Fast Starch
Sugars differ from starch in two key ways: how they can impact rumen pH and milk fat. Unlike starch, sugars are not typically associated with low rumen pH and ruminal acidosis, so you shouldn’t think of sugars as “fast starch” and the rumen problems that brings to mind. While rumen bugs do ferment some sugar quickly, they also store some of it in their cells and ferment that more slowly. As a result, some, but not all the sugar a cow eats is immediately converted to acetate, propionate, and butyrate, the volatile fatty acids (VFA) produced during fermentation. This more gradual fermentation pattern can help support a healthy rumen pH. An added bonus is that sugars don’t tend to depress ruminal fiber digestion as compared to starch and may even improve it, if degradable protein is not limiting. Compared to fiber and starch, sugars ferment to give a greater proportion of butyrate, and that may matter for their impact on butterfat.
Sugars and Milk Fat
A common production response to feeding sugars is an increase in milk fat yield. Why? While we don’t have all the answers yet, the effect seems to involve how sugars influence rumen fermentation, specifically the rate and production of butyrate and acetate and how the mammary gland uses these to make milk fat. Fatty acids in milk fat come from three sources: dietary fat, cow’s body fat, and “de novo” where the mammary gland makes them “from scratch.” De novo production is important for increasing milk fat yield beyond what the diet and body support. Rapidly fermented sugars in the rumen may boost milk fat by giving the cow a quick, pulse dose of VFA. Her body then decides what to do with these: Burn them for energy or convert them into something else…. like fat. When more butyrate and acetate is available than needed for immediate energy demands, the excess could be used to make fat. That fat can contribute to milk fat or to body condition, likely depending on a cow’s production capabilities and stage of lactation.
Sugars in Ration Formulation: An Overlooked Variable
The effects of sugars that affect cow productivity and health are not currently considered in our ration balancing programs. For example, in CNCPS, sugars as part of water-soluble carbohydrates are accounted the same as starch, but with a different rate of fermentation. But CNCPS doesn’t capture other impacts, like sugar-driven fat production and fiber digestibility. So, it’s up to the nutritionist to keep in mind other effects related to sugars when creating diets.
Measuring “Sugars”: One Term, Many Definitions
“Sugars” and soluble carbohydrates are measured a variety of ways. For molasses, total sugars as invert is the official way of reporting the sucrose content on the tag. Some labs now measure sugars by chromatography, reporting simple sugars, sucrose, and also lactose, which is found in milk-products like whey permeate. Water-soluble carbohydrates (WSC) is a broad category, which covers all carbohydrates soluble in water, including sugars, short chain carbohydrates, soluble starch, and also fructans in cool-season grasses. WSC values are typically higher than invert sugars and chromatography sugars because they include a wider variety of carbohydrates. Programs such as CNCPS have been calibrated to use WSC: make sure to use the carbohydrate metric a program calls for.
Mary Beth’s Best Guesses
As we learn more about sugars and WSC, it may be useful to measure both if they influence cow and rumen performance differently. Sugars like sucrose and glucose do ferment more rapidly than fructan, lactose, or starch, potentially changing how cows respond. We already work with different protein sources to meet the cows’ needs. Providing carbohydrates with different fermentation profiles could hold a key to optimizing lactation performance in a way that relying on starch alone may not. What are your goals for your herd, and do your rations support them?
