Why bypass starch deserves more attention | Dellait

Álvaro García

When dairy nutritionists discuss nutrients that escape ruminal degradation, the conversation almost always focuses on protein. Concepts such as rumen-degradable protein, rumen-undegradable protein, metabolizable protein, and amino acid balancing are now fundamental components of ration formulation. The importance of bypass protein in supporting milk production and improving nitrogen efficiency is well established.

Far less attention is given to bypass starch. This is surprising because glucose is the primary precursor for lactose synthesis, which determines milk volume. While the rumen provides propionate that can be converted into glucose by the liver, starch that escapes ruminal degradation and is digested in the small intestine supplies glucose directly to the cow.

The importance of starch digestion site has been recognized for decades. Owens and colleagues showed that starch can be digested in the rumen, small intestine, or hindgut, with important implications for nutrient utilization, rumen function, and animal performance.

Not all starch sources behave similarly. Wheat and barley are rapidly fermented in the rumen and are excellent sources of fermentable carbohydrate but contribute relatively little bypass starch. Corn starch is degraded more slowly, allowing a greater proportion to reach the small intestine, while sorghum exhibits even lower ruminal degradability. Rice and rice-derived byproducts are typically intermediate. These differences are summarized in Table 1.

Table 1. General ranking of common feed ingredients according to relative ruminal starch degradability and potential starch flow to the small intestine. Actual values vary with grain variety and processing method.

These differences illustrate a fundamental nutritional principle: dairy cows benefit from both fermentable starch and bypass starch. Fermentable starch fuels rumen microbes, support microbial protein synthesis, and promotes production of volatile fatty acids, particularly propionate. Bypass starch, on the other hand, increases the flow of starch to the small intestine where it can be digested and absorbed as glucose. Rather than competing nutritional strategies, these two fractions should be viewed as complementary components of an effective feeding program.

Several years ago, in an article titled Do Cows Have Starch Requirements?, we argued that discussions about starch should move beyond simple dietary percentages. The balance between ruminal fermentation and intestinal digestion can influence milk production, efficiency, body condition, and overall nutrient utilization. The question is not only how much starch is fed, but also where that starch is digested.

A recently published study by Anger and colleagues (2026) brings renewed attention to this topic. Their work examined whether increasing bypass starch supply could improve lactation persistency and milk component production in cows fed diets containing reduced metabolizable protein. The results suggest that bypass starch may deserve a more prominent place in dairy nutrition discussions than it has traditionally received.

What did the study show?

The researchers evaluated the effects of two dietary starch strategies in combination with different amino acid supplies. One diet contained starch that was more extensively degraded in the rumen, whereas the second was formulated to increase the supply of bypass starch reaching the small intestine. At the same time, cows received either a lower or higher supply of rumen-protected methionine and lysine.

The most striking finding was that increasing bypass starch helped cows maintain milk production later in lactation. Cows receiving the higher bypass starch diet maintained milk production more effectively as lactation progressed compared with cows receiving the more ruminally fermentable starch source.

Milk component production also benefited. Yields of milk protein and milk fat were greater in cows receiving the higher bypass starch treatment. Interestingly, these responses occurred even when metabolizable protein supply was reduced, suggesting that improving glucose availability may partially compensate for limitations in amino acid supply under some conditions.

The results highlight an important concept often overlooked in dairy nutrition. Although ruminal fermentation is essential, maximizing ruminal starch degradation is not always the optimal strategy. Once microbial requirements are met, directing a portion of dietary starch to the small intestine may provide additional metabolic advantages through greater glucose absorption.

Why might bypass starch improve persistency?

High-producing dairy cows face a continuous challenge in supplying sufficient glucose to support lactose synthesis. Early in lactation, much of this glucose is generated through gluconeogenesis from propionate. However, as lactation progresses, the demand for glucose remains substantial.

Bypass starch provides an alternative route for glucose supply. When starch reaches the small intestine, enzymatic digestion releases glucose that can be absorbed directly into the bloodstream. This may improve the efficiency of energy utilization because less energy is lost during fermentation and conversion processes.

Greater glucose availability can influence several physiological functions, including lactose synthesis, mammary gland activity, body tissue mobilization, and overall energy balance. These effects may help explain why cows receiving greater amounts of bypass starch maintained milk production more effectively throughout lactation.

The findings also reinforce the idea that nutrient supply should be evaluated in an integrated manner. Protein and starch metabolism are linked. Amino acids support milk protein synthesis, while glucose provides energy and drives milk volume through lactose production. Optimizing one without considering the other may limit overall performance.

Take-home messages

While bypass protein is firmly established in modern dairy nutrition, bypass starch often receives far less attention. The work of Owens and colleagues demonstrated decades ago that the site of starch digestion matters, and the recent study by Anger and colleagues provides new evidence that increasing bypass starch can improve lactation persistency and support milk component production.

This does not mean that more bypass starch is always better. The objective remains to provide enough fermentable starch to support rumen function and microbial growth while supplying sufficient bypass starch to enhance glucose availability in the small intestine. Achieving the proper balance is more important than maximizing either fraction alone.

As dairy nutrition continues to focus on efficiency, sustainability, and precision feeding, bypass starch may deserve a place alongside bypass protein as a key consideration in ration formulation. The question is no longer whether starch matters, but whether we are paying enough attention to where it is digested.

The full list of references used in this article is available upon request.

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