Dairy cow`s pastures. 1. Modelling the grazing management

A decision support tool for grazing management has been developed in the form of computing calculation sheets. Its utilisation is illustrated by initiation of a rotational grazing system on the farm of l’Abbaye in Sorens in 2003. Planning was based on grass-growth references for pastures with a yield potential of 9,5 t DM/ha/year and on the hypothesis of 16 kg DM/cow/day grass-intake. Calculated surface needs were used to set up a provisional calendar of utilisation for the 20 paddocks. During the season, grass-height has been measured every 2 to 4 weeks. These measurements have been converted in grass-mass (measured farm cover) using grass-density and target grass-height values. Grass-growth was measured on small plots and grass-intake was assessed. Measured farm cover evolution has been put in relation with the balance between forage supply and nutrient demand (calculated farm cover). This comparison allowed to verify the start hypothesis and to quantify the forage resources. Due to the drought, the effective yield was assessed to 8 t DM/ha/year with a forage intake of 7,5 t DM/ha/year. Despite low forage losses (6% for the whole season), period by period analyse shows improvement possibilities to grazing management. In particular, this model brings light to the importance of practising high grazing pressure in spring and of arranging enough reserve paddocks during the summer. It also gives a frame for assessing the forage supply and for facilitating decision during the season.