The consequences of sowing seed of burr medic (Medicago polymorpha) cultivars Circle Valley and Serena that was either free of or infected to different extents with alfalfa mosaic virus (AMV) was studied in mixed species pasture swards that regenerated annually. The swards were grazed by sheep and the predominant plant species in them were burr medic and capeweed (Arctotheca calendula). Serena matured earlier and was more tolerant of AMV infection than Circle Valley. Each year, seed-infected medic plants were the source for virus spread by aphids to healthy medic plants. When the extent of infection was determined in the fourth to sixth growing seasons, the amount of virus spread in the medic component of the swards varied between seasons. Final infection in plots originally sown with infected seed ranged from 47 to 93% for Serena and from 25 to 79% for Circle Valley. Viral seed transmission rates in medic seed produced each year by these plots ranged from 19 to 31% with Serena and from 3 to 7% with Circle Valley. Final percentage infection within swards originally sown with healthy seed (control plots) was smaller regardless of cultivar (0·1–7%) as were transmission rates in their harvested seed (0–0·6%).

AMV infection of the burr medic in regenerated plots originally sown with Circle Valley seed diminished medic seed yields, thereby decreasing the proportion of medic in the seed bank. This decreased germination of medic but increased germination of capeweed. In control plots, plant densities were up to 36% greater in the medic and 52% smaller in the capeweed components than in plots originally sown with infected Circle Valley seed. In plots containing Serena there was a smaller decrease in medic seed yields due to AMV infection, so the impact on germination was less. In the fourth to sixth years from sowing, when yields were determined at different times after grazing ceased, there was either a small decrease (up to 8%) or no significant decrease in overall herbage yields due to infection with AMV. However, in plots originally sown with infected Circle Valley seed, the balance of medic to capeweed was altered in favour of capeweed, sometimes dramatically so (e.g. capeweed content increased from 19 to 45% in the fourth year from sowing). In contrast, by the end of the growing season the balance of medic to capeweed was little altered by the presence of the virus in plots containing Serena. Thus, infection with this insidious virus disease substantially diminished the ability of Circle Valley but not Serena medic to compete with other species such as capeweed in self-regenerated, mixed species pasture swards. It did this both directly by decreasing the competitive ability of the medic plants that became infected during the growing season, and indirectly via seed production and the seed bank, by altering the proportions in which the species germinated.