Modelling factors affecting reproductive development of perennial ryegrass in Waikato dairy pastures

A mechanistic model was developed to identify biological processes responsible for differences in reproductive tillering between 'Ellett' and 'Grasslands Ruanui' perennial ryegrass cultivars. In the model, a field perennial ryegrass tiller population was represented with six compartments: vegetative, emerging reproductive, mature reproductive and dead tillers (three compartments). The biological processes driving the transfer of tillers from one compartment to another were: conversion of vegetative tillers to reproductive tillers, maturation of emerging reproductive tillers, senescence of mature reproductive tillers, and death of reproductive tillers due to grazing. The model was used to analyse data from Waikato dairy pastures. This enabled some of the effects of irrigation, nitrogen, cultivar and season (weather) on the biological processes to be identified. During spring and early summer, the proportion of reproductive tillers in 'Ellett' pastures was always higher than in 'Grasslands Ruanui', regardless of treatment and year. The model explained this difference by a higher conversion rate of 'Ellett' vegetative tillers to reproductive tillers, and suggested that conversion rate is the key process determining reproductive development in perennial ryegrass. Opportunities to control reproductive tillering are discussed. Keywords: flowering, Lolium perenne, perennial ryegrass, reproductive development