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

Dairy cow systems based on grassland utilization are char acterized by a variation of harvested forage quality from 1 year to another and a fluctuation of grass availability an d 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 m odelling approach enables a represent ation of the complex relationships between the cow and pasture as they interact. A dynamic m odel of intake and milk production, focused on grassland utilization by the dairy cow, has been developed. The model o perates in a deterministic fa shion with a daily time step and is capable of dealing with a day-to -day variation in grass availability an d quality at grazing as well as a constant feeding regime during the winter. The model has been bu ilt based on the theory that milk production is a result of (i) the energy requirements d efined by the potential milk production and ph ysiological 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 ex perimental data (two groups of dairy cows in the winter time and one group at grazing). The model demonstrates a sa tisfactory range of accuracy (root-mean-square devi- ation equal to 1·8, 2·1 and 1·4 kg/cow/day). Model validatio ns indicate that milk production predictions are sensitive to the diet offered (forage, grass and con centrate supply) and depend on dairy cow characteristics and their require- ments (pMP). This model can be connected to a grass growth model in order to develop whole farm simulations