The transfer of nitrogen (N) from legumes to grasses is an important process in low-input forage production systems, and may be improved by selecting compatible species and cultivars. This study sought to examine what effect species and cultivar have on plant growth and N accumulation in temperate grass-legume mixtures under a range of nitrogen fertility levels. A pot study using two cultivars each of alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), perennial ryegrass (Lolium perenne L.), and timothy (Phleum pratense L.) in all grass-legume combinations was devised. Compatibility indices, based on plant performance grown in combination versus alone, were used to quantify the net impact legumes and grasses had on each other. The presence of legumes had an overall negative effect on the growth of grasses (87% compared with growing alone), but did improve tissue N content by weight and total accumulated N. Improvements in total N were highest in a single timothy cultivar (Champ; 169%), but highest net total N was achieved in a ryegrass cultivar (Bastion; 1.92 mg N). Results indicate that grass N accumulation in legume-grass mixtures may be influenced more by grass N demand than legume supply, which suggests that competition between grasses and legumes may be a major determinant of N transfer efficiency.