A model to optimise the requirements of lactating dairy cows for physically effective neutral detergent fibre

This study modelled multiple physiological responses of dairy cows to physical and chemical characteristics of a diet aiming to optimise their requirements for physically effective neutral detergent fibre, expressed inclusive of particles-dry matter > 8 mm (peNDF >8 ). Extensive research data, comprising a wide range of feeding conditions (n = 64 studies and 257 different dietary treatments), were used to parameterise the model, while statistical modelling was used to account for the inter- and intra-experiment variation as well as to derive the model estimates. Physiological thresholds and 'safety limits' of peNDF >8 for maintaining different physiological variables were derived using non-linear statistical modelling. Results showed that peNDF >8 content in the diet is a key factor stimulating rumination activity, maintaining optimal ruminal pH and promoting fibre digestion. Modelling data with regard to the association of fibre digestion and time duration of ruminal pH 8 suggests that feeding of less than 13.7% peNDF >8 (the lower 'safety limit') is critical to prevent depression of fibre digestion in dairy cows. The study also indicated that the beneficial effects of peNDF >8 on ruminal pH and fibre digestion can be at the expense of the dry matter intake (DMI) level of high-producing cows when the peNDF >8 threshold of 14.9% in the diet is exceeded. In terms of the optimisation of peNDF >8 requirements, the modelling data suggest that feeding of 17-18.5% peNDF >8 can be beneficial in maintaining ruminal pH, while allowing a relatively high DMI (22.3-22.7 kg · d −1 ) for average high-producing dairy cows.