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.