Modelling the impact of changing the decision to graze from rotation length to ryegrass leaf-stage

Abstract

We used modelling to test the hypothesis that rotation length based on the emergence of three live leaves per ryegrass tiller (3-leaf principle) would result in greater pasture production, and increased milk production and profit per hectare than rotations based on standardised decision rules (best management practice; BMP). This hypothesis was tested using a computer program designed to model the interactions between climate, paddocks, animals and management policies. The model was set up for a farmlet (Strain Trial, Herd 1, 04/05 season) and simulated over 10 different climate years (93/94-02/03), two different sets of rotation lengths (3- leaf and BMP), and over a range of comparative stocking rates (85 - 100 kg Lwt/t DM). Average return on assets (ROA) was measured with random price sets drawn for milk, the variable cost of purchasing silage, and land appreciation rate. For the systems and climate years tested in this exercise the model predicted a greater (P<0.05) average ROA for BMP rotations compared to 3-leaf rotations (9.85 and 9.73%/year, respectively). The 3- leaf rotations resulted in a small (P<0.001) increase in pasture grown (0.17±0.02 t DM/ha). However, BMP scenarios were more profitable because of greater annual milksolids production per hectare (1%), mainly achieved during peak lactation. The 3-leaf principle showed benefits in the second half of the season, but this could not compensate for the earlier loss in production. Keywords: dairy farm system, grazing management, rotation length, whole farm model.