Lentil (Lens culinaris) is an important pulse crop in the southern and western cropping zones. Weed management can be difficult in lentil because of its poor early growth and the limited range of safe selective post-emergent herbicides available. Experiments were conducted at Minlaton, South Australia, and Horsham, Victoria, to examine the effects of early vigour on the ability of lentil to compete against a broadleaf weed. Early growth in lentil was manipulated by selecting genotypes with different levels of early vigour and by using a range of sowing rates. Canola (Brassica napus cv. Beacon) was used to mimic the growth of a cruciferous weed and it was sown at 0, 0.25 (Minlaton only), 0.5, 1 or 2 kg/ha. Lentil genotypes were selected that represented the range in early vigour currently available within the breeding program. Another experiment examined the effect of plant density of lentils with different degrees of early vigour on the yield of canola. Grain yield of lentil declined as the density of canola increased. The initial reduction in canola yield over sites was about 4%/plant.m2, but was as high as 12%/plant.m2 in one treatment. This yield loss is similar to that reported for other grain legume crops, but is much higher than the initial yield loss reported for wheat. The differences in early vigour between genotypes were insufficient to affect the competitive ability of lentil. In contrast, increasing the sowing rate of lentil significantly reduced canola grain yield and increased lentil grain yield. When the density of canola was low (10 plants/m2), doubling the lentil plant density to 200 plants/m2 limited the yield loss to 10%. The results suggest the level of variation in early vigour among the present genotypes is insufficient to increase the competitive ability of the crop. Increasing the plant population of lentil is a more effective means of suppressing weed growth and increasing grain yield.