Ryegrass genotype ‐ environment interactions in response to density, cutting height, and competition with white clover

Twelve ryegrass genotypes ranging from L. perenne to L. multiflorum X perenne , obtained from closely continuously or laxly rotationally grazed pastures, were grown as widely spaced plants, in ryegrass monoculture swards, or in mixed ryegrass‐white clover swards for 20 months. Two cutting heights were superimposed. Yield, tiller and inflorescence numbers, and tiller length were measured on individual plants. Marked genotype X environment interactions in the form of first‐ and second‐order interactions between density, cutting height, and genotype were related to the L. perenne‐L. multiflorum component in the genotype, the previous grazing history, and tiller number‐tiller weight ratios. Best indications of adaptation to previous grazing history were obtained in treatment responses of inflorescence numbers. Early size dilferences between genotypes grown widely spaced showed long‐term correlation with yields of genotypes in swards. This resulted from the relative fixing of size differences and the restriction of the expression of seasonal growth periodicity by competition among genotypes in swards. For spaced plants, the correlations of rankings of genotypes between early and later harvests and with swards at later harvests were broken down by the expression of differences of seasonal growth periodicity and equalisation of plant sizes. Interactions involving inflorescence numbers indicated how management could modify population structure during seed multiplication.