Introgression of reproductive traits from the annual, profuse flowering, ball clover (Trifolium nigrescens Viv.) into white clover (Trifolium repens L.) is one breeding strategy to improve seed yields of T. repens that must be achieved without sacrificing agronomic performance and persistency under grazing. The yield and persistency of hybrids between white clover and T. nigrescens were compared under rotational sheep grazing over three harvest years. The hybrids included the backcross (BC) 2 and 3 generations produced using white clover as the recurrent parent. The large-leaved T. repens variety Olwen, medium-leaved varieties AberDai and Menna and the small-leaved variety S184 were sown as controls. Hybrids and control varieties were sown with a perennial ryegrass companion; between April and the end of October in each harvest year the plots were rotationally grazed with sheep with clover and perennial ryegrass (DM) yield and the proportion of clover present measured over the growing season. The clover and total DM yields of the BC2 and BC3 were generally comparable with the small- and medium-leaved varieties within the experiment and significantly greater than the yields of the large-leaved variety Olwen. Throughout the 3 years of the experiment the BC2 maintained a clover content above 0·30 and comparable with the small-leaved varieties, while the clover content of the BC3 was comparable with the small- and medium-leaved varieties in the first and third harvest years. No significant difference in perennial ryegrass production was observed when grown with the backcrosses or the control varieties. Differences in stolon and growing-point density were observed at the end of the experiment with the density of the BC2 and BC3 less than the small-leaved variety S184 but, in common with the medium-leaved varieties, greater than the large-leaved variety Olwen. The implication of these results for the use of this material in future experiments and in the white clover breeding programme is discussed.