Core collections are a way to improve the functioning of germplasm collections through systematic evaluation, exploitation, and improved management of the whole collection. The published sizes of cores have ranged from 10 to 30% of the collection. The first step in developing a core collection is usually stratification of the whole collection. We adopted three approaches in the process of core designation. First, the existing ecological data and newly acquired agro-morphological (evaluation) data were analysed to determine the optimal method for capturing the ecological and morphological diversity of accessions. Second, different selection strategies were compared to identify the most powerful analysis with the greatest likelihood of covering maximum diversity. Third, the correlation between the two datasets was investigated. The annual pasture legume, bladder clover (Trifolium spumosum L.), was used for this pilot study because 80% of the existing 398 accessions have near-complete ecological data. The study found that a two-step selection process using ecological data and stratified proportional strategy followed by combined datasets and maximising strategy best represents the whole collection of T. spumosum in the core.