Seasonal abundance and spatial pattern of Setaria faberi, Chenopodium album, and Abutilon theophrasti in reduced-tillage soybeans

A better understanding of the influence of various crop and weed management practices on spatiotemporal dynamics of weeds could improve the design of integrated weed management systems. We examined the influence of 18- and 76 -cm soybean row spacings on emergence pattern and spatial aggregation of gi ant foxtail, common lambsquarters, and velvetleaf seedling cohorts. In addi tion, we characterized the soil seedbank and determined the quantitative an d spatial relationship between the seedbank and seedling populations. Viabl e seeds of about IO weed species and twice as many species of seedlings wer e identified in the weed community. Giant foxtail and common lambsquarters were the predominant species in the seedling and seedbank population, respe ctively, each accounting for GO to 70% of the total weed species density. E mergence of giant foxtail, common lambsquarters, and velvetleaf depleted 12 to 33%, < 2% and 12 to 49% of the seedbank in the upper IO cm of the soil profile. Peak time and periodicity of weed emergence was not influenced by soybean row spacing, and peak time of emergence of giant foxtail, common la mbsquarters, and velvetleaf occurred 3 to 4, 3 to 6, and 3 to 9 weeks after soybean planting (WAP), respectively. Magnitude of giant foxtail emergence 5, 6, and 3 WAP was 98, 96, and 76% greater in 76- than in Is-cm row soybe ans only when the population of 76-cm row soybeans was 57% lower than the 1 8-cm soybeans in 1997. Giant foxtail and common lambsquarters seeds in the seedbank were aggregated in 1996 and 1997 according to the Taylor power law (TPL) and the negative binomial distribution (NBD). The TPL and the NBD we re similar in describing the spatial aggregation of giant foxtail and commo n lambsquarters but not some velvetleaf seedling cohorts. The spatial aggre gation of seedlings varied among cohorts for different weed species and was likely due to species-specific biological characteristics that influence s eed dispersal, germination, and seedling emergence. Within a 1.5-ha area, a ggregation declined with decreasing density. Within a 24-m(2) area, the lev el of aggregation of all weed species decreased as seedling densities incre ased. These results indicated that soybean row spacing influenced neither w eed emergence pattern nor weed spatial aggregation; thus, several managemen t decisions can be similar in 18- and 76-cm row soybeans.