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The objective of this study was to quantify the effect of perennial ryegrass (Lolium perenne L., PRG) sward density on seasonal and total DM yield under simulated grazing and animal grazing by cattle, and to assess the effectiveness of visually estimated ground scores (GSs) for predicting sward PRG density. The study incorporated five different seeding rates of PRG, each replicated three times, to simulate swards ranging in PRG density typical of different ages and conditions. There was no significant difference between defoliation managements for total DM yield, but sward PRG density had a significant effect on both the seasonal and total DM herbage yield under both systems. Under simulated grazing, total DM yield ranged from 10 to 12 t DM / ha with increasing sward PRG density at a GS range of 1 - 4 (mean of 2 years data, P 0.01). Under animal grazing, the yield range was from 10 to 12 t DM / ha for a GS range of 1 - 3 (mean of 2 years, P0.01). The largest differences in DM yield occurred during the spring period. The relationship between sward DM yield and GS was significant (P 0.001) for both simulated and animal-grazed swards. Each unit increase in midseason GS (June) related to an average yield increase of 350kg DM / ha under simulated grazing and a 721 kg DM/ha increase under cattle grazing. Every unit increase in the GS at the end of the grazing season (December) was associated with a 460 or 1194 kg DM/ha increase under simulated and animal grazing, respectively. These results show that visual estimates of density were an effective tool in describing PRG density and that this could be related to DM yield potential. Further investigations may provide a threshold value below which the renewal of swards could be advised based on a visual GS of PRG.