Anthony Brereton

Using a grazing simulation model to evaluate the balance of feed demand and intake for high yielding dairy cows in a rotational grazing dairy system

A grazing system simulation model was developed that treated pasture as a series of horizontal layers with a depth of an animal bite, but varying bulk density and leaf content. It is assumed that the surface is exploited randomly in feeding stations of area determined by the animal reach. At each feeding station encounter one layer is removed during grazing so the sward surface in time becomes a mosaic of varying composition. Animals exploit or reject a feeding station on the basis of comparison with the immediately preceding encounter. Model parameters were taken from published accounts of sward properties and animal grazing behaviour, and were tested using data from field trials. Introduction of a higher yielding dairy cow to Irish grazing could change the balance between feed demand and available grass. The model was used to show that maximum mean daily intake in mid-lactation from Perennial Ryegrass may be limited to 16 kg day-1which is adequate for the current system but significantly less than the demand of high yielding dairy cows (>6000 l per year). In mid-lactation the high feed demand of highly bred cows may over-ride selectivity. When not selective the mean daily intake is increased to 20 kg, which approaches the demand but has implications for sward management.