Six experiments were done in northern New South Wales over three seasons to study the effect of weeds on the yield of dry-land grain sorghum and methods of weed control. The relation between crop row spacing and weed control by inter-row cultivation or atrazine (2-chloro-4-ethylamino- 6-isopropylamino-l,3,5-triazine), or both was studied. Where the site yield was high because of favourable growing conditions, an increase in the crop row spacing reduced yield. At low yielding sites, an increase in the row spacing increased yields. At most sites, weed growth was greater with wider row spacings, which resulted in a larger reduction in crop yield where weeds were not removed. Inter-row cultivation reduced weed growth to less than half that of the unweeded controls. Pre-emergent atrazine gave good weed suppression, generally reducing weed growth to less than 10% of the unweeded control when applied at 2.5 kg/ha active ingredient. Post-emergent atrazine was much less effective. Inter-row cultivation combined with a band of pre-emergent atrazine over the crop row was as effective in weed control as an overall spray of pre-emergent atrazine.
