New tiller growth in Lolium perenne seed crops does not significantly reduce seed yields. Two greenhouse experiments showed that seed yield per ear was not affected by the number of tillers or regrowth of cut tillers after anthesis. Seed yield increased in one genotype under specific conditions, with no new tillers produced. Tiller removal increased nitrogen concentration in remaining tillers, regardless of regrowth. Competition for carbohydrates or nitrogen between seeds and new tillers is not a major factor in low seed yields. The limiting factors are likely processes within the ear itself.