A field study was conducted in Thailand under a rain-fed environment to determine the effect of four different cutting heights above ground level and two closing dates on dry matter (DM) production, yield components, and fodder quality of five different cultivars of Napier grass (Pennisetum purpureum Schumach), all treatments cut with the same frequency of approximately 4 weeks. Eighteen quantitative trait measurements were used to investigate the interactions between cultivar diversity and cutting regimes. Principal-component analysis (PCA) showed a clustering of two distinct cultivars and another group of cultivars in one cluster without obvious structure. There was also a clear clustering of the control (0 cm cutting height) across all cultivars whereas other cutting heights affected growth differently according to cultivar. The optimal cutting regime for obtaining high DM yield depended on the genetic background and did not relate to a reduction in the number of vegetative buds for the cultivars with a basal shooting pattern. For the cultivars with higher DM yield arising from high stubble height, the DM was differently distributed into leaf and stem material. Before cuttings, the tiller number was reduced in the control plots but not in the plots with more lenient cutting height. Average tiller yield increased with increasing cutting height to reach a maximum at 20-cm cutting height. Average harvested culm length was constant for the four cutting heights. The variation in a number of plant traits, arising from cultivar diversity, can be altered by agronomic practices, thereby causing potentially contradictory results. Defining specific interactions of cultivar-by-cutting height treatments and analysing these with PCA proved to be a useful approach for visualising clusters from multiple measurements. The proposed approach for analysing data could serve as a model for other trials with similar interactions between cultivar diversity and agronomic treatments.