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To gain information about how widening of the row distance influences seed yields in first-year perennial ryegrass, experiments with four row distances in three types of perennial ryegrass were conducted at the University of Aarhus, Faculty of Agricultural Sciences. Perennial ryegrass was undersown at 12-, 24-, 36-, or 48-cm row distance in a cover crop of spring barley. The seeding rate in perennial ryegrass was 6 kg seeds ha-1 regardless of row distance. Although increasing the row distance from 12 to 48 cm had a negative effect on the yield component number of reproductive tillers, the yield was not affected in the first-year seed production in three perennial ryegrass cultivars. Regardless of row distance the seed rate was 6 kg ha-1 and hence in-row plant density in autumn and spring was higher at 48 compared with 12 cm; however, in all three cultivars the highest number of reproductive tillers was recorded at 12-cm row distance. Row distance affected seed yields of only the diploid amenity cultivar 'Allegro', where a row distance of 48 cm reduced the seed yield compared with 12- and 24-cm row distance. When data from the three cultivars were merged there was a positive correlation between the seed yield and seed weight (r=0.72***), whereas the correlation between seed yield and the number of reproductive tillers was negatively correlated (r= - 0.49***). This may reflect choice of cultivars in the experiment with the tetraploid forage cultivar 'Tivoli' having the lowest number of reproductive tillers, highest seed weight, and highest seed yield, and the diploid amenity cultivar 'Allegro' having the highest number of reproductive tillers, lowest seed weight, and the lowest seed yield. When the three cultivars were merged, there was a positive and highly significant correlation between seed weight and seed yield (r=0.72***). In contrast, there was no correlation between seed weight and seed yield when data were analysed for the individual cultivars. This suggests a cultivar-dependent relationship between seed weight and seed yield and furthermore between number of reproductive tillers and seed yield. [ABSTRACT FROM AUTHOR]