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 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%.

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 Allocate required nutrients (Technote 18).

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. 

Related information

• Fibre Metabolism (Technical note)
• Do lactating cows on pasture require additional fibre? (Infosheet)
• Facts and Figures - Nutrition