Irie Moussiaux and Andie Majewski, Graduate Research Associates and Dr. Kirby Krogstad, Assistant Professor, Department of Animal Sciences, The Ohio State University
Introduction
Dietary carbohydrates comprise more than half of the diet fed to dairy cattle. Getting the carbohydrate profile perfected to maximize milk production and profitability is the goal of every practicing dairy farmer and nutritionist. One of the choices that dairy nutritionists face is whether to include sugar or additional starch to provide digestible energy to the cows. This is an important choice because starch and sugar behave differently in the rumen and may lead to different production responses. The choice between starch and sugar is not a "this or that” proposition. The optimum carbohydrate profile for efficient milk production may include both starch and sugar, but finding the right balance is the secret sauce to feeding lactating dairy cattle.
Chemistry and Digestibility of Starch vs. Sugar
Starches and sugars are both classified as non-structural carbohydrates. Starch is a complex carbohydrate, or a polysaccharide. The average total tract digestibility of starch is about 94%, varying with grain type and grain processing methods (Ferraretto et al., 2013). Sugars are simple carbohydrates and can be further classified as either monosaccharides or disaccharides. Monosaccharaides are the simplest form of sugars and include glucose, fructose, and galactose. Disaccharides include sucrose and lactose. Sugar is rapidly and completely digested in the rumen (Weisbjerg et al., 1998). Regardless of the chemical structure, both starch and sugar are highly digestible sources of energy for the cow.
Figure 1. Comparison of water soluble carbohydrates (WSC) and starch contents in common feedstuffs on a DM basis (NASEM 2021).
Sugars are generally fermented more rapidly in the rumen than starch, although precise degradation rates are difficult to quantify (Gao and Oba, 2016; Allen, 2000). Sugars are fermented at a rate of about 300%/h, while starch fermentation rate is about 40%/h, depending on the grain type and particle size (Oba et al., 2015). Both sugar and starch can increase microbial protein synthesis, increasing the supply of metabolizable protein and volatile fatty acids to the cow (Broderick et al., 2008). While starch fermentation can produce propionate in the rumen (Firkins and Mitchell, 2023), sugar fermentation can increase butyrate production which may promote rumen papillae growth and fiber digestion (Gao and Oba, 2016).
Acidosis Risk
As mentioned previously, both starch and sugar are rumen degradable with sugar being degraded more rapidly than starch. Although it is counterintuitive, supplying sugar in place of starch may reduce the acidosis risk by increasing ruminal pH. Unlike starch, the inclusion of sugar in diets has not been shown to reduce ruminal pH when incorporated at moderate levels (Oba et al., 2015). Perhaps rumen pH is better modulated by enhanced rumen papillae growth and ruminal VFA absorption when sugar is fed. If true, this process creates a rumen environment more capable of modulating rumen pH (de Ondarza et al., 2017; Gao and Oba, 2016). Thus, including sugar in a ration, when balanced with fiber and total fermentable carbohydrates, poses minimal acidosis risk.
Non-Nutrient Considerations
Starch is commonly provided through ground or flaked corn, barley, wheat, or sorghum, while sugar sources include molasses, whey, bakery waste, and citrus pulp; (Figure 1; NASEM, 2021). Beyond their nutritional values, starch and sugar sources differ in physical form and palatability, which can influence dairy cow feeding behavior and total mixed ration (TMR) consistency. Liquid sugar sources can reduce sorting behavior when feeding a TMR. For example, liquid molasses can bind fine particles together, reducing ingredient separation in the TMR and promoting more consistent intake of the formulated diet. In some cases, inclusion of molasses has increased DMI (Broderick and Rodloff, 2004); the research suggests that the increased DMI may be related to improved palatability. Despite these potential benefits, practical challenges of feeding sugar must be considered. In colder climates, molasses and whey are prone to freezing, which can complicate storage and handling (NRC, 2001). Therefore, selection of carbohydrate sources for dairy diets should consider nutrient value, non-nutrient characteristics to support consistent intake, and cost.
Production Effects
Sugar and starch affect milk and component yields in different ways due to how they are fermented in the rumen. Increasing sugar in the diet can increase butyrate production, a precursor of de novo fatty acid synthesis in the mammary gland (Gao and Oba, 2016). This can contribute to increased milk fat concentration. Conversely, increasing starch in the diet may increase milk protein production, though it can also cause milk fat depression (Boerman et al., 2015) due to increased production of trans-10, cis-12 conjugated linoleic acid (CLA) in the rumen, inhibiting milk fat synthesis (Peterson et al., 2003). The contrasting effects that sugar and starch have on milk fat production indicate the need to identify an appropriate ratio of sugar and starch to optimize milk fat and protein production without sacrificing milk production. The addition of 5 to 7% dietary sugar, resulting in 6 to 8% sugar of the total diet DM, paired with 22 to 27% dietary starch, may be optimal to increase fat-corrected milk without limiting overall milk production (de Ondarza et al., 2017).
Increasing sugar in the diet at the expense of starch can reduce the overall energy of the diet, limiting milk yield and protein production (de Ondarza et al., 2017), despite potentially increasing milk fat concentration (Hall and Zanton, 2022). In recent research shared by Norato et al. (2025) at the American Dairy Science Association annual meeting, increasing sugar while reducing starch (from 0% sugar and 32% starch up to 12.3% sugar and 20% starch) linearly increased milk fat concentration but decreased yields of milk, energy-corrected milk, protein, and lactose. Therefore, drastically increasing the sugar content of a diet while dramatically reducing its starch content will hinder dairy cow productivity. Perhaps slightly increasing the amount of sugar in a diet, while moderately decreasing the amount of starch may be an advantageous way to provide enough energy to maintain milk and milk protein yields while increasing milk fat yield. A proper mix of sugar and starch may also mitigate ruminal acidosis risk.
Conclusion
Incorporating both starch and sugar into dairy cow diets is essential for optimizing cow health and milk production. Rather than prioritizing one carbohydrate source over the other, achieving the right balance of starch and sugar is key. Ultimately, sugars may support rumen health and increase milk fat concentration as they can promote butyrate production and rumen papillae growth. Starch serves as a primary energy source that supports microbial protein synthesis and yields of milk and milk protein. Starch cannot completely replace sugar in the diet because excessive starch can increase the risk of ruminal acidosis and lead to milk fat depression. Conversely, sugar cannot completely replace starch as milk production may decline, and the cost of sugar is typically higher than the cost of starch. Therefore, strategic inclusion of both sugar and starch is critical when appropriately feeding high-producing dairy cows and optimizing income over feed costs.
References
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