Legumes have the capacity to associate with rhizobia strains and fix atmospheric N₂. Ball clover (Trifolium nigrescens Viv.) has reseeding ability and when planted in mixtures with annual ryegrass (Lolium multiflorum Lam.) contributes to add N to the systems and to expand the grazing season. This experiment tested 3 seeding rates of ball clover (0.9, 1.8 and 2.7 kg ha⁻¹) in a mixture with annual ryegrass compared with annual ryegrass in monoculture, fertilized (50 kg N ha⁻¹) or not with N. Response variables included percentage of ball clover, dry matter yield (DMY) annual ryegrass, DMY ball clover, total DMY, percentage of plant N derived from atmosphere (%Ndfa) and N fixed contained in the shoot (N_fix). The study was performed in a complete randomized block design with 4 replicates. Ball clover proportion in the mixture increased (P = 0.02) from 32% (at 0.9 kg ha⁻¹ seeding rate) to 47% (at 1.8 kg ha⁻¹ seeding rate), with no significant differences (P > 0.05) observed between the 2 higher seeding rates (1.8 and 2.7 kg ha⁻¹). Ball clover DMY increased linearly (P = 0.02) with increasing seeding rates, with DMY of 711, 1,117, and 1,165 kg ha⁻¹ for 0.9, 1.8, and 2.7 kg ha⁻¹ seeding rates, respectively. Annual ryegrass DMY (P < 0.0001) and total DMY (P = 0.0007) were greater in the N fertilized treatment (3,810 kg DM ha⁻¹). Average %Ndfa was 95% for ball clover and there was no difference among treatments (P > 0.05). The N_fix showed linear increase with seeding rate (P = 0.02) with values ranging from 18 to 28 kg N ha⁻¹. Treatments including seed rates of 1.8 and 2.7 kg ha⁻¹ were similar in botanical composition, DMY ball clover, DMY ryegrass, total DMY, and N_fix. From the producer perspective, it is more economical to use 1.8 kg seeds of ball clover ha⁻¹. Increasing seed rates of ball clover in annual ryegrass mixtures allowed greater legume contribution in the pasture, improving forage N without decreasing yield.