Improving persistence of red clover (Trifolium pratense L) varieties is an important target of the IBERS red clover breeding programme. Identification of the factors contributing to poor persistence and testing improved varieties in field trials is integral to the breeding of persistent red clover varieties. Red clover varieties and selection lines bred for greater persistence were grown in mixed swards with hybrid ryegrass or a mixture of hybrid ryegrass and perennial ryegrass over four harvest years. A significant difference in the DM yield of the red clover varieties was observed with yield in harvest year 1 greater than in harvest year 4. The red clover varieties differed in the extent of this decline, due to differences in the persistence of red clover plants within swards. The implication of these results for the use of red clover in sustainable grassland systems is discussed.