Threshold level and seed production of velvetleaf (Abutilon theophrasti Medicus)in maize.

Four tests were carried out in 1980 and 1981 to determine: (a) the economic threshold density of Abutilon theophrasti Medicus (velvetleaf) in maize, and (b) seed production with varying densities of infestation, both in the presence and in the absence of maize. The infestation was artificially created, and the density of the weed ranged from 0 to 80 plants m−2. The economic threshold, calculated using the Cousens (1987) model, varied between 0⋅3 plants m−2 and 2⋅4 plants m−2, depending on the variables considered. The presence of maize reduced the seed-rain of A. theophrasti by 50%. This seed-rain reached its maximum level at 20–30 plants m−2 in maize, and at 30–35 plants m−2 in weed monoculture. However, with only 4–5 plants m−2 in competition with maize, A. theophrasti produced 8–10 thousand seeds m−2. The usefulness of threshold density in weed management is debatable when one considers the ecological characteristics of the A. theophrasti seed, and the great capacity of seed production of this weed.