Italian ryegrass (Lolium multiflorum Lam.) is one of the most important temperate forage grasses on a global basis. The present study estimated trait variance, inter-trait correlation and broad-sense heritability (H²) for herbage yield, nutritive values and morphological characteristics of Italian ryegrass, and analysed associations between these traits and single nucleotide polymorphism (SNP) markers. A field-based, spaced-plant nursery experiment was conducted to assess 960 genotypes from seven cultivars and breeding lines, in a randomised complete block design with four replicates. Significant correlations, broad ranges of variation and significant genotypic variance components were identified for 18 measured phenotypic traits. Moderate H² estimates (0.36–0.52) were obtained for nutritive values, compared with low to high H² estimates (0.20–0.69) for yield over five cuttings, and moderate to high H² estimates (0.40–0.78) for morphological traits. Within- and among-cultivar variation ranged from 40.0% to 95.1% and 4.9% to 60.0%, respectively, depending on different traits. Association analysis identified 65 significant SNP–trait gene associations involving multiple genomic regions, which warrant further validation. The implications of trait correlation and heritability for selection and improvement are discussed.