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Fibre in the diet Rumen pH (acidity) Supplementary fibre recommendations

Dietary fibre is essential for a cow's healthy rumen function. This page explains that when cows eat fibre, it stimulates chewing, saliva production, and rumination. Saliva, containing sodium bicarbonate and phosphate salts, balances rumen pH and promotes the growth of rumen microbes. Dietary fibre recommendations vary based on the cow's diet and the rumen environment. Consuming a diet high in sugar or starch with inadequate fibre can lead to rumen acidosis. Yet, for cows primarily grazing on good quality pastures, adding certain types of supplementary fibre might reduce energy intake and milk production.

Dietary fibre is needed by the cow to maintain healthy rumen function. Eating fibre stimulates chewing, saliva production and rumination.

Saliva contains sodium bicarbonate (more commonly known as baking soda) and phosphate salts which help to buffer the rumen pH, and promote the growth of rumen microbes.

Fibre Saliva Ph

Fibre is the term which describes what is in the cell wall, or the material that provides structure to the plant. Dietary fibre consists of the two structural carbohydrates (cellulose and hemicellulose) and the indigestible compound lignin.  Lignin is not a carbohydrate, but binds to the structural carbohydrates, making them less accessible to rumen enzymes, therefore less digestible. The lignin content of plants increases with age, therefore the fibre in the plant becomes less digestible as it matures.

Fibre in the diet

Dietary fibre recommendations differ depending on the base diet. The rumen environment of grazing cows differs from that of cows fed a total mixed ration (TMR), or high levels of starch/sugar-based supplements. Therefore recommendations for fibre requirements based on TMR or high supplement diets need to be used with caution in pasture-based systems (and vice versa).

The common recommendations are:

  • for cows grazing high quality pastures, NDF should make up a minimum of 35% of the diet and eNDF 17%.
  • for cows eating diets high in starchy feeds such as cereal grains, NDF should be at least 27% and eNDF 20%.


Neutral Detergent Fibre (NDF) Effective Neutral Detergent Fibre (eNDF) Acid Detergent Fibre (ADF)

= hemicellulose + cellulose + lignin

 • This is a measurement of the fibre that remains after treating the plant with non-acidic, non alkaline detergent

 • This contains both the indigestible (lignin) and digestible (hemicellulose + cellulose) fibre portions and generally relates to the bulkiness of the feed.

• This is the NDF that is most effective at stimulating rumination and saliva production (e.g. long chopped roughage)

 • Feeds can be high in NDF but low in eNDF (e.g. PKE)

= cellulose + lignin

• This is a measurement of the fibre that remains after treating the plant with acidic detergent.

• The higher the ADF the lower the digestability of the fibre fraction of the plant, and generally the lower the available energy.

ADF ⇧ = Digestibility ⇩

Rumen pH (acidity)

Differences also exist in the recommended minimum rumen pH (or acidity) levels.  Cows grazing good quality pastures can tolerate a lower rumen pH (more acidic) without detrimental effects on rumen function, compared with those eating a TMR, or a diet high in starch.  This is primarily due to:

  • the different carbohydrate contents of the two types of diets,
  • the different rumen microbial populations that digest the different carbohydrates,
  • the different waste products produced from carbohydrate digestion

Supplementary fibre recommendations

If cows are eating a diet high in sugar or starch, and dietary fibre is less than recommended, rumen acidosis can occur. Adding fibre, in particular a feed high in eNDF, will improve animal performance (rumen pH, motility, VFA production).

However, when good quality pasture makes up the majority of the diet, adding a feed high in eNDF (e.g. straw) is not similarly effective in inducing rumination, or altering rumen function (pH, motility, VFA production).  In fact, the addition of a low energy, high eNDF feed to a pasture-based diet can reduce energy intake and milksolids production.

For more information on feeding fibre in early lactation see:

Understanding fibre in a cow’s diet

Watch the FeedRight video to understand:

  • The different types of fibre that are found in feed
  • What happens when a cow eats fibre
  • How fibre affects cow production and performance.

The role of fibre metabolism

Video 7:09 min

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Last updated: Sep 2023
Tags related to “Fibre”

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