Row spacing is more important than seeding rate for increasing Rhodes grass (Chloris gayana) control and grain yield in soybean (Glycine max)

In Australia, soybean (Glycine max (L.) Merr.) is planted at a low density in wide rows, and weeds substantially reduce yield because of opportunities for their growth in the wide rows. Field studies were conducted over 2 years at the University of Queensland farm, Gatton, Australia, to assess the effect of row spacing and seeding rate on the competitiveness of soybeans with a model weed, Rhodes grass (Chloris gayana Kunth). The experiment was conducted in a split-split plot design, replicated three times. Main plots comprised two seeding rates (40 and 80kg ha-1), subplots two row spacings (25 and 75cm), and sub-subplots four Rhodes grass infestation periods (weedy from planting to maturity, weedy from 3 weeks after planting (WAP) to maturity, weedy from 6 WAP to maturity, and weed-free from planting to maturity). The results showed that seed rate did not influence Rhodes grass biomass or soybean yield. Soybean yield was greater and Rhodes grass biomass was less in the 25-cm rows than the 75-cm rows. For the 25-cm rows, Rhodes grass biomass in the plots infested beyond 3 WAP was 81-89% less than in the season-long weedy plots, whereas for the wider row crop, this reduction was only 60-75%. For the 25-cm rows, soybean yield in the plots infested with Rhodes grass beyond 3 WAP was 30-36% less than under weed-free condition. However, for the 75-cm rows, this reduction was 56-65%. The results suggest that planting soybean in wider rows caused greater reduction in yield and required an earlier weed management program than planting in narrow rows. The study also suggested that narrowing row spacing was more important than increasing seeding rates for improving weed control and soybean grain yield