Efficacy of Nitrogen Uptake and Water Use Efficiency in Perennial Ryegrass Forage Production: Criteria for Evaluating Varietal Diversity

In breeding programs for perennial ryegrass, this study examined whether variation among cultivars in the nitrogen nutrition index (NNI) and in carbon isotope discrimination (δ13C) correlates with forage yield under dense micro-sward conditions. Results showed that NNI variability can predict yield differences when environmental constraints are minimal and plants compete for light. No direct correlation between δ13C and yield was found under the experimental conditions, but δ13C variation aligned with the geographic origin of the genetic material, indicating potential value for genetic resource management.