Weed Control and Economic Comparisons in Soybean Planting Systems

A 3-yr field study was conducted in 1997, 1998, and 1999 at Stoneville, Mississippi, to study the effects of soybean planted in 19-cm (narrow) rows/high density (NR-HD) as compared to 57-cm (medium) rows/medium density (MR-MD) and 95-cm (wide) rows/low density (WR-LD) on weed control, yield, and net return under conventional tillage (CT) and no-tillage (NT) systems with preemergence (PRE) only, postemergence (POST) only, or PRE + POST herbicide programs. Control of eight major weeds was at least 82% regardless of planting system. Soybean planted in NR-HD system resulted in 65% and 78% less total weed biomass as compared to soybean in MR-MD and WR-LD system, respectively, 10 wk after planting. Soybean planted in NR-HD system produced 8% and 27% higher yield compared to soybean planted in MR-MD and WR-LD system, respectively. Net returns were similar between NR-HD and MR-MD systems ($151 to 159/ha), but were higher than in WR-LD system ($106/ha). With the exception of yellow nutsedge, control of all weed species studied was 90% or better regardless of herbicide program. Control of yellow nutsedge ranged from 85% to 91% among herbicide programs. Overall, total weed biomass was lower with PRE + POST (520 kg/ha) as compared to a PRE (1460 kg/ha) or POST (1450 kg/ha) only program. The PRE + POST program gave higher soybean yield (2100 kg/ha) and the yield reduction was 9% to 10% with either a PRE only or POST only program. Net returns were lowest from the PRE + POST ($112/ha) program as compared to PRE only ($153/ha) or POST only ($138/ha) program. Weed control, total weed biomass, and soybean yield were similar for CT and NT systems. This research showed that yield advantage of NR-HD system was partly due to improved weed control.