Process-based modelling to understand which ryegrass characteristics can increase production and decrease leaching in grazed grass–legume pastures

Abstract

A significant challenge for the pastoral farming systems is to maintain or increase production while reducing leaching of nitrogen, and for pastoral systems, this means reducing leaching from urine patches. Here we explore the potential impact of four ryegrass characteristics to increase pasture production and reduce leaching from ryegrass–white clover pastures. We focus on understanding which characteristics are desirable, the stage before investigating the achievability of those characteristics in a breeding program. Those characteristics were: the winter- or summer-dominance of growth (GP); the ability of the plant to intercept radiation at low pasture mass (LI); rooting depth (RD); and resistance to moderate water stress (WF). The impact of these ryegrass characteristics, both singly and combined within a ryegrass–clover pasture, was explored across a range of soils, climates, irrigation management, and urine patch concentrations using the process-based model APSIM. Of the four characteristics tested, LI was the most effective in increasing production and reducing leaching in all environments. The characteristics RD and WF were moderately effective, with RD having a greater impact on reducing leaching whereas WF had a greater effect on increasing production. The characteristic with the least impact was GP and it seems that ryegrass is currently well adapted for typical temperatures in New Zealand. The production and environmental effects of the characteristics were additive. The characteristics should be investigated further in the typically more diverse mixtures normally found in pastures but show promise for achieving improved production while reducing leaching provided they can be achieved in a breeding program.