Consequences of intensive grazing by dairy cows of contrasting live weights on volcanic ash topsoil structure and pasture dynamics
- Grazing and its management can modify soil and pasture properties due to the mechanical stress applied by animals and their feeding behavior. This can cause changes on soil structure and its pore functionality. The effect of the intensive grazing by cows with contrasting size and live weight on the improvement of soil physical properties and pasture productivity was evaluated in different types of pastures. The study was conducted on an Andisol (Duric Hapludand). Three types of pastures were used: Lolium perenne and Trifolium repens mixture (BM); L. perenne, T. repens, Bromus valdivianus, Holcus lanatus and Dactylis glomerata mixture (MSM) and naturalized fertilized (NFP). Two categories of dairy cows (contrasting size and live weight) were used to graze the pastures. For a period of five years the penetration resistance (PR), soil water content (WC) and pasture productivity were measured in the field in each grazing event. Soil samplings were conducted over a 3-year period in winter, collected between 5-10 cm soil depth. The pre-compression stress (Pc), the water retention curve (WRC), the air permeability (k(a)) and pore continuity indexes (C-2 and C-3) were determined. As a consequence of grazing, tillage, pasture type as well as wetting and drying cycles, changes in the topsoil mechanical stability, porosity and pore functions were observed. The physical properties of the Andisol did not reach critical values (bulk density (d(b)), pre-compression stress (Pc), air capacity (AC), plant available water (PAW)); however, over the years an accumulative effect was observed, evidencing at the end of the experimental period a more compacted soil, which presents an increase of the bulk density (from 0.7 to 0.85 g cm(-3)), Pc (from 43 to 69 kPa) and PR (from 1.75 to 2.71 MPa) as well as a reorganization of the porous space (AC, PAW) and its functionality (k(a), C-2 and C-3). Differences in the soil physical properties and pasture production were generated by pasture type more than animal trampling. NFP showed a more stable soil and better functionality of the porous system with major pasture productivity.