Metabolisable energy concentration in perennial ryegrass pastures: multi-site analysis of effects of cultivar, nitrogen fertiliser and white clover content

To include metabolisable energy (ME) as a nutritive value factor for ryegrasses in the Forage Value Index (FVI) requires data for New Zealand cultivars on the effects of environmental and management factors on cultivar ranking. This study tested the hypothesis that variation among cultivars, and so ranking, is not influenced by environment, nitrogen (N) fertiliser level or the presence or absence of white clover. Eight cultivars, grown with or without white clover and at two levels of N fertiliser in grazed pastures in Waikato, Manawatū, Canterbury and Southland, were sampled seasonally over 3 years and analysed for ME. There were clover, N and cultivar main effects on ME in most seasons, however, there were no interactions that affected cultivar ranking, indicating that data from field evaluation systems based on monocultures are reliable for the purposes of the FVI. However, data for absolute ME concentration are required from multiple sites to account for the observed cultivar × environment interactions.