A biophysical dairy farm model to evaluate rotational grazing management strategies

This paper describes a biophysical dairy farm model developed as part of the SEPATOU system that simulates pasture-dominated feeding strategies in a dairy cow enterprise. It can reproduce the effects of various technical management options applied to a set of grazing fields on a daily basis over a period of several months. The model is made up of three submodels that deal with the soil (availability of water), sward and animal (cow intake and milk yield) components. It includes as driving variables the main factors that farmers can control (nitrogen fertilizer rate, defoliation frequency and intensity, composition of cows' diet) in order to attain some objectives such as the intended grazing herbage contribution in the cows' diet or the amount of milk produced per cow or per ha. The intended purpose of the SEPATOU system has led to several original developments. In order to simulate various defoliation regimes, growth and senescence processes are dissociated as there is no longer synchronization between them, and the effect of grazing intensity is expressed in terms of the ratio between the herbage mass after and before grazing. The animal intake submodel that can deal with mixed feeding combines two approaches, one based on energy requirements and the other on the relationship between herbage mass, herbage digestibility and intake. The model of herbage digestibility is based on herbage mass and takes into account its variation down the sward profile; this latter aspect plays a key role in the plant-animal interaction. The model has been validated using typical farm cases in Brittany. It provides realistic estimates of the state variables involved in the processes, such as herbage mass and daily milk yield, and it credibly predicts the timing of key events (e.g. date of turnout to grass, end of first grazing cycle).

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