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Abstract
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 < 5.8 and dietary peNDF>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.
Acknowledgements
The authors would like to gratefully acknowledge the financial support received to conduct the study from the Deutsche Forschungsgemeinschaft (DFG, DR 92/12-1).