Physically effective fibre on rumen function, performance and health of UK dairy cows (PhD)



To sustain the increase in milk production an increase in the level of concentrate supplementation and the production of high quality forages, with a trend towards lower dietary fibre levels. The particle size (PS) of the diet has been proposed as a key factor, along with its fibre (measured as neutral detergent fibre; NDF) and non-forage carbohydrate concentration to ensure a healthy rumen function and maintain animal performance. When forage PS and peNDF (physically effective fibre) are too low, they adversely affect rumination, decrease ruminal pH and the level and activity of cellulolytic bacteria in the rumen. As a consequence, there is an increased risk of subacute ruminal acidosis (SARA), and milk fat depression.

Key Findings:

  • It is important to feed a diet with the correct particle size distribution: too long a particle length can reduce milk yield and increase diet selection, whereas too short a particle length can reduce milk fat content and possibly lead to sub-acute acidosis. 
  • Monitoring ration mixing consistency and degree of sorting are key to promptly detect and correct any mistakes. A well-formulated diet can result in poor cow performance and a higher risk of SARA if it is not well mixed or if it can be easily sorted.
  • Make sure to have a certain amount of feed refusals (around 3-5% of the mixed ration fed) to avoid underfeeding and ensure that these refusals have a composition similar to the ration originally fed out.
  • In diets properly formulated, mixed and fed out, a short cut grass silage does not not on its own increase the risk of SARA. Cows fed short cut grass silage will eat more dry matter and produce more milk; milk fat content (but not yield) may however be lower
  • Feeding a mixture of grass and maize silage increases intake, milk production, milk protein content and live weight gain in comparison with grass silage alone.
  • Inclusion of maize silage does not increase the risk of SARA regardless of the grass silage chop length.
  • Under UK conditions, feeding high levels of maize silage with short chop length grass silage and high dietary starch can be achieved without a negative impact on rumen pH, intake or milk performance if the diet is appropriately formulated, mixed and fed so that there is no diet selection.​

For further information the report is available.

Project code:
01 July 2015 - 30 June 2018
AHDB sector cost:
Project leader:
Harper Adams University


41110026 final report 2018

About this project

Aims and Objectives

  • To determine the article size (PS) distribution of grass and maize silages and total/partial mixed rations on UK dairy farms
  • To evaluate the consistency of ration mixing and extent of diet selection
  • To evaluate the effect of chop length of grass silage when fed alone or in combination with maize silage on dairy cow performance, reticulo-rumen pH, milk quality, diet digestibility, metabolism and feeding behaviour
  • To determine the effect of different concentrate starch levels when fed at different ratios of a short chop grass silage to maize silage on rumen pH, digestibility, rumen function and passage kinetics, eating behaviour, immune response, and performance in dairy cows.