In order to study the effects of redroot pigweed and lambsquarter densities on potato yield, an additive experiment were conducted in 2004 and 2005. This experiment was done in split plot based on randomized complete block design with four replications. Treatments were two weed species in main plots and weed density in sub plots (0, 2, 4 and 8 plant per meter of row). Hyperbolic and exponential models were used to model the effect of redroot pigweed andlambsquarter densities on potato yield. The best model was the hyperbolic model. However, the differences between the two models were not significant. 2, 4 and 8 plant of redroot pigweed per meter of row reduced potato yield 15.72, 28.55 and 36.04% in 2004 and 13.18, 23.05 and 31.85% in 2005. These values for lambsquarter were 9.73, 18.84 and 25.09% for 2004 and 8.28, 15.17 and 21.59% for 2005.

Key words: Competition, Amaranthus retroflexus, Chenopodium album, Solanum tuberosum.