Productivity of grazed perennial ryegrass dairy pastures from different ecotypes under nitrogen and irrigation treatments

A two-year experiment (1996-1998) was conducted at Hamilton, New Zealand, t o compare the productivity of perennial ryegrasses from different ecotypes, tinder rotational grazing by dairy cows. 'Ellett' ryegrass, representing t he Mangere ecotype, a major source of parent material for current perennial ryegrass cultivars, was compared with 'Grasslands Ruanui' representing an older Hawke's Bay ecotype. Nitrogen (N) fertiliser and irrigation were subp lot and sub-subplot treatments, respectively. Total herbage accumulation ov er two years was 13% greater for 'Ellett' ryegrass than for 'Grasslands Rua nui', with higher herbage production occurring mainly in spring and summer. 'Ellett' ryegrass had a consistently higher tiller weight and lower tiller density than 'Grasslands Ruanui'. Tiller density response to N was much gr eater for 'Grasslands Ruanui' (+8698 tillers m(-2)) than for 'Ellett' (+969 tillers m(-2)) over the second year (spring 1997 to autumn 1998). Both rye grass cultivars showed increased herbage accumulation and tiller weight whe n treated with N. Cultivar herbage accumulation was increased on average by 30% over the whole trial in response to N, but by only 19% in summer in re sponse to irrigation. 'Ellett' ryegrass showed more pulling than did 'Grass lands Ruanui' during grazings over summer and autumn, especially when treat ed with N, but this had only minor effects on yield. Data on the reproducti ve behaviour of perennial ryegrass cultivars and tillering responses to N a pplied in spring-summer, could provide useful selection criteria for persis tence under intensive dairying. Ryegrass cultivars with different morpholog ies and tillering responses to N may require different grazing managements to optimise their performance. Pulling losses in summer are likely to be gr eater for cultivars with large upright tillers and when N fertiliser is app lied.