How feed viscosity effect performance ?

In animal nutrition, particularly for monogastric species like swine and poultry, feed formulation goes far beyond meeting basic nutritional needs. One critical yet often underestimated factor influencing performance, gut health, and nutrient absorption is feed viscosity. This is especially relevant in Southeast Asia, where the widespread use of fibrous by-products such as rice bran, cassava and copra meal presents new challenges in formulation. As local feed ingredients and strategies evolve, understanding and managing viscosity becomes essential for every nutritionist seeking to improve efficiency and profitability.

What is Feed Viscosity?

Feed viscosity refers to the thickness or resistance to flow of the digesta in the gastrointestinal tract. When feed is mixed with digestive secretions, certain components, particularly soluble fibers, can form viscous gels. This gel-like environment slows down nutrient absorption, impairs enzyme activity, and creates favorable conditions for harmful microbial growth.

But not all fibers behave the same way. Only soluble fibers increase viscosity because they dissolve in water and form molecular networks that bind water and thicken the intestinal contents. Common examples in the Southeast Asian context include pectins in cassava, arabinoxylans in rice bran, and mannans in copra meals.

Both pigs and poultry are affected by viscous diets, but poultry are more sensitive due to their shorter gastrointestinal tract and faster transit time. In broilers, increased viscosity is directly linked to reduced nutrient absorption and wet litter problems. In piglets, the impact is milder but still significant, particularly in the post-weaning phase when the digestive system is immature. With growing use of non-traditional fiber sources in Asian piglet diets, attention to viscosity is more important than ever.

What Causes Feed Viscosity?

Viscosity mainly arises from soluble non-starch polysaccharides (NSPs) complex carbohydrates found in many plant-based feed ingredients. In Southeast Asia, key sources include:

• Arabinoxylans in rice bran and corn DDGS

• β-glucans in certain rice bran fractions and cereal by-products

• Pectins in soybean hulls, citrus pulp, and cassava

• Mannans in copra meal

These soluble NSPs absorb water and swell, forming gel-like solutions that trap nutrients and limit enzyme access. Viscosity is not determined by any single ingredient but is the result of interactions among ingredient types, fiber solubility, processing methods, and even storage conditions.

For example, using cassava pulp together with rice bran may result in a higher-than-expected viscosity due to cumulative soluble fiber content, even when total dietary fiber appears acceptable on paper. This makes viscosity prediction difficult in practical field formulations.

Several strategies can help mitigate high feed viscosity:

1. NSP-Degrading Enzymes (NSPases)

Enzymes such as xylanase, β-glucanase, and pectinase are effective tools to break down the molecular structure of soluble NSPs:

• Xylanase targets arabinoxylans (common in rice bran), cleaving linkages in the xylan backbone. This reduces the water-holding capacity and breaks down the gel matrix, thereby lowering viscosity and releasing entrapped nutrients.

• β-Glucanase hydrolyzes glucans, especially present in some rice bran and cereal residues. It disrupts the long-chain polymers that cause gelling and thickening in the intestinal tract.

• Pectinase degrades pectins, which are abundant in cassava and soybean hulls. Pectin’s branched galacturonic acid chains absorb large amounts of water; pectinase depolymerizes these chains, improving flow and nutrient diffusion.

The combined use of NSPases improves nutrient accessibility, reduces fermentation of undigested carbohydrates, and stabilizes gut microbiota.

2. Insoluble Fiber Supplementation

Adding purified insoluble fibers like lignocellulose improves gut motility and supports mucosal function. This reduces the retention of viscous digesta and supports healthy bacterial populations, especially in weaned piglets and slow-growing poultry lines.

3. Ingredient Selection and Formulation Control

Limiting high soluble fiber ingredients (or reducing their inclusion rates) and choosing fiber sources with low water solubility can help manage viscosity. For example:

• Replacing part of cassava pulp or soybean hulls with corn

• Using defatted rice bran instead of full-fat bran when NSP levels are too high

• Avoiding finely ground copra in high doses unless enzyme support is provided

  
  

How to Measure Feed Viscosity In Vitro

Feed viscosity can be measured using in-vitro laboratory protocols that simulate gastric and intestinal digestion. After enzymatic treatment, the fluid portion of the digesta is tested using a rotational viscometer to quantify resistance to flow.

This is an effective method to measure the effect of xylanase, glucanase or pectinase enzymes on the viscosity of each ingredient to develop optimal mix of enzymes. But this method has limitations in consistency and reproducibility on complete feed and are not used in routine by feed manufacturers

Conclusion

Feed viscosity is a crucial but often overlooked parameter that directly impacts digestive physiology and feed efficiency, especially in Southeast Asian conditions where fibrous by-products are common and cost pressures are high. Soluble fibers are the primary contributors to viscosity, and their impact is more severe in poultry than in swine.

Predicting viscosity remains challenging due to ingredient variability, but simple in-vitro tests and targeted enzyme strategies can help nutritionists take back control.

David SERÈNE _ NutriSPICES Director