Experiments evaluated the effect of glyphosate rate and Anoda cristata density, on crop and weed biomass and weed seed production in wide (70 cm) and narrow rows (35 cm) glyphosate-resistant soyabean (Glycine max). Soyabean density was higher at 35 cm row spacing as an increase in planting rate in narrow-row soyabean is recommended for producers in Argentina. Soyabean biomass at growth stage V4 (four nodes on the main stem with fully developed leaves beginning with the unifoliate leaves) was higher when grown on narrow than in wide-rows but was not affected by the presence of A. cristata. At growth stage R5 (seed initiation – seed 3 mm long in a pod at one of the four uppermost nodes on the main stem, with a fully developed leaf and full canopy development), crop biomass was greater in narrow rows compared with wide rows with 12 plants m−2 of A. cristata. In narrow-row soyabean, a single application of a reduced rate of glyphosate maintained soyabean biomass at R5 and provided excellent weed control regardless of weed density. In wide-row soyabean control was reduced at the high weed density. Regardless of row spacing, A. cristata biomass and seed production were severely reduced by half of the recommended dose rate of glyphosate but the relationship between biomass and seed production was not altered. Glyphosate rates as low as 67.5 g a.e. ha−1 in narrow rows or 540 g a.e. ha−1 in wide rows provided excellent control of A. cristata. To minimize glyphosate use, planting narrow-row soyabean are effective where A. cristata density is low.