Influence of velvetleaf (Abutilon theophrasti) and common sunflower (Helianthus annuus) density variation on weed management outcomes

Interactions between initial weed seedling density and postemergence herbic ide and mechanical weed control were studied in two field experiments condu cted between 1994 and 1996. Increasing seedbank densities of velvetleaf (0 to 500 seed m(-2)) in soybean or common sunflower (250 to 2,500 seed 1.3 m( -2)) in corn or soybean were established at Lincoln and Mead, NE, respectiv ely. Emerged seedlings were treated with increasing intensities of weed con trol from none to bentazon alone or with interrow cultivation. A positive l inear relationship between initial seedling density and density of survivin g seedlings was consistently observed. As initial seedling density increase d, more survivors were present after treatment. As intensity of weed contro l increased, the number of seeding survivors decreased. Resulting reproduct ive fitness decreased with increasing management intensity but remained pos itive when regressed against surviving seedling densities. Weed management outcomes were dependent on initial seedling density, such that the absolute number of survivors increased, while proportion of survivors appeared cons tant within the density ranges studied. These research findings emphasize t he need to account for weed infestation level when assessing efficacy of we ed management systems and provide evidence that patchy weed distributions m ay persist in part because of the need for considerably higher management i ntensities in high density patch centers.