Not all fiber behaves the same in dairy rations

The equations used to estimate forage fiber digestibility do not always apply to co-products such as canola meal.

For decades, dairy nutritionists have relied on a simple rule: as lignin increases, fiber digestibility decreases. This relationship holds for traditional forages such as alfalfa and corn silage, where lignification limits microbial access to structural carbohydrates. Applying the same assumption to non-forage fiber sources, however, can undervalue co-product fiber because it ferments differently. Fiber fermentability should therefore be evaluated by source and its effects on the rumen.

The forage bias

Many older ration-balancing equations were developed from forage fiber data.  In a plant stem, lignin acts like rebar in concrete, anchoring and shielding cellulose and hemicellulose from microbial enzymes.

The non-forage reality

In many oilseeds and co-products, fiber is found primarily in the seed hull or protective coatings rather than in a rigid stalk. In these ingredients, lignin may not bind to the remaining cell wall as it does in forages. As a result, non-lignin fiber can remain available to rumen microbes.

Canola meal as a case study

Canola meal has often delivered more energy in practice than published values would suggest. Although it contains more neutral detergent fiber (NDF, about 28%) and lignin than some alternative vegetable proteins, traditional forage equations overstated its indigestible fraction. Using the conventional approach of multiplying lignin by 2.4 predicted that more than half of its fiber would remain undigested. To investigate, researchers evaluated samples from 12 Canadian meal-processing plants. The resulting Journal of Animal Science study tracked the fate of the fiber components during extended rumen incubation.

The long-term incubation results showed that nearly all non-lignin fiber in canola meal is highly digestible. The measured indigestible fraction was essentially equal to the lignin content, indicating that virtually all remaining fiber is available to rumen microbes.

Advanced models such as NASEM (2021) and CNCPS (2026) account for the distinct behavior of non-forage fiber rather than applying fixed forage equations. As a result, they can substantially revise the estimated energy value of co-products, as illustrated for several ingredients in an earlier DairyBusiness magazine article.

What this means for ration formulation

High-quality forage supplies the physically effective fiber needed for rumination and rumen-mat formation. Digestible non-forage fiber can complement that forage base by providing energy without adding starch, helping reduce the risk of subacute ruminal acidosis (SARA). A high fiber value on a feed tag does not necessarily indicate filler; used appropriately, co-products can support milk yield and fat synthesis while controlling ration costs.

Although the two leading ration-formulation systems use different laboratory inputs, both have moved away from the outdated “lignin discount” that historically undervalued non-forage fiber sources.

NASEM uses 48-hour in vitro or in situ NDF digestibility. For a rapidly passing co-product such as canola meal, this measure closely reflects the fiber a high-producing cow can digest during rumen retention.

CNCPS instead estimates digestion rates from a multipoint curve. Because non-forage fiber differs from forage fiber, commercial laboratories supporting CNCPS typically measure fermentation at 12, 72 and 120 hours.

In simple terms, NASEM asks, “How much fiber is digested at a critical time point?” CNCPS asks, “How quickly do the fiber pools disappear?” Despite these different approaches, both reach the same practical conclusion for canola meal: once lignin is treated as indigestible, nearly all remaining structural carbohydrates are fermented and supply energy to rumen microbes.

Whether energy is estimated from NASEM’s 48-hour digestibility value or CNCPS’s multipool kinetic curve, the latest models recognize canola meal fiber as an efficient, highly digestible energy source that supports microbial protein production and milk-component synthesis.

Further reading

AMTS. 2026. CNCPS v 6.5 changes: uNDF. https://agmodelsystems.com/cncps-v-6-5-changes-undf/

Arce-Cordero, J.A., Paula, E.M., Daniel, J.L., Silva, L.G., Broderick, G.A. and Faciola, A.P., 2021. Effects of neutral detergent fiber digestibility estimation method on calculated energy concentration of canola meals from 12 Canadian processing plants. Journal of Animal Science, 99(11), p.skab309.

NASEM. 2021. Nutrient Requirements of Dairy Cattle: Eighth Revised Edition. The National Academies Press, Washington, DC.

Wood, B. 2022. Getting energy values correct. https://dairybusiness.com/getting-energy-values-correct/