The influence of row spacing and plant population on the effect of weed competition in soybeans

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Abstract
An experiment was done to determine the reduction in yield attributable to weed competition when soybeans were grown in 25,50,75 and 100 cm rows and within-row densities of 10, 20 and 40 plants m-1. There was no effect of weeds on yield when soybeans were grown in 25 cm rows but a 20 per cent decrease with 50 cm rows, 26 per cent with 75 cm rows and 37 per cent with 100 cm rows. Within-row density had no effect but there was a trend towards an interaction between within-row density and the presence or absence of weeds. Narrow rows and higher within-row densities increased the height of the bottom pod, lodging and the number of pods m-2 but decreased stem diameter, number of pods per plant and the yield per plant. Narrow rows decreased yield per pod and seeds per pod. There was a trend, which was not significant (P < 0.05), towards reduced yield per pod and seeds per pod with higher within-row densities. Weed competition reduced stem diameter, pods per plant, pods m-2 and yield per plant. Weeds had no effect on the height of the bottom pod, yield per pod and seeds per pod. Soybean grain weight was not changed by row spacing, row density or weed competition.