Competition and Canopy Architecture as Affected by Soybean (Glycine max) Row Width and Density of Redroot Pigweed (Amaranthus retroflexus)

Abstract

The effects of soybean row width and redroot pigweed density on growth of crop and weed were studied in field trials in 1983, 1984, and Structural relationships within the canopies of soybean and redroot pigweed in relation to row width and weed density were studied in 1984 and 1985 to assess canopy geometry in relation to intra- and interspecific competition. Early in the growing season, soybean biomass was reduced in the presence of both high and low densities of pigweed. Pigweed biomass was also reduced in the presence of soybeans, especially when grown in narrow rows (25 cm). By midseason, pigweed's contribution to total biomass had reached 43% in wide-row (76-cm) stands and 24% in narrow rows. Soybean produced two to four times more leaf area than pigweed during the first half of the growth season. Narrow-row planting favored soybean leaf area production. Soybean LAI values from weedy stands were reduced compared to those from weed-free stands. Pigweed's contribution to total leaf area averaged 29% in wide-row spacing and 15% in narrow rows. Pigweed leaf area was concentrated in the upper strata of the canopy and thus reduced light available to soybean leaves lower in the canopy. Leaf area distribution patterns suggested that soybean and pigweed were competing for light even though soybean had produced more leaf area than pigweed. These relations were consistent from year to year in spite of variable water conditions.