ABSTRACT.- Rainfed field seed yields for tall fescue and annual ryegrass in Uruguay are estimated to be close to 0.33 and 0.98 tt seed/ha, respectively. These values are far lower than those reported for other regions worldwide, what might reflect the subtropical climate, less adjusted crop management or greater losses during harvest. To better understand these differences, we quantified seed yield of 20 tall fescue and annual ryegrass commercial crops in southwestern Uruguay to assess what yield components explain yield variation, and to determine the magnitude of losses between yield estimated by manual harvest at optimal cutting time (OCT) and effectively harvested field yield.