Dynamic model of milk production responses to grass-based diet variations during grazing and indoor housing

Dairy cow systems based on grassland utilization are characterized by a variation of harvested forage quality from 1 year to another and a fluctuation of grass availability and quality during the pasture season. Consequently, the energy supplied by the diet – i.e., concentrates plus hay or silage in winter and grazing in summer, may not always match animal requirements. A modelling approach enables a representation of the complex relationships between the cow and pasture as they interact. A dynamic model of intake and milk production, focused on grassland utilization by the dairy cow, has been developed. Themodel operates in a deterministic fashionwith a daily time step and is capable of dealing with a day-to-day variation in grass availability and quality at grazing as well as a constant feeding regime during the winter. The model has been built based on the theory that milk production is a result of (i) the energy requirements defined by the potential milk production and physiological status of dairy cows; (ii) the variation of energy supply by the diet; (iii) the ability of dairy cows to mobilize or store body reserves. The model was validated by comparing milk production predictions with experimental data (two groups of dairy cows in the winter time and one group at grazing). The model demonstrates a satisfactory range of accuracy (root-mean-square deviation equal to 1·8, 2·1 and 1·4 kg/cow/day). Model validations indicate that milk production predictions are sensitive to the diet offered (forage, grass and concentrate supply) and depend on dairy cow characteristics and their requirements (pMP). This model can be connected to a grass growth model in order to develop whole farm simulations.