Estimation of economic thresholds for weed control in soybean (Glycine max (L) Merr)

Competitive ability of 10 major soybean weeds (Abutilon theophrasti Medicus (ABUTH), Amaranthus cruentus L. (AMACR), Chenopodium album L. (CHEAL), Datura stramonium L. (DATST), Panicum miliaceum L. (PANMI), Polygonum persicaria L. (POLPE), Setaria viridis (L.) Beauv. (SETVI), Solanum nigrum L. (SOLNI), Sorghum halepense (L.) Pers. (SORHA), Xanthium strumarium L. (XANST)) was studied for two years in northern Italy. The economic thresholds for soybean weed control have been determined for different herbicides and mixtures. Economic thresholds were very low in 1989, when only SOLNI and PANMI showed a threshold higher than 1 plant m-2 , while in 1990 values varied between 0.05 plants m-2 for XANST and 4.3 plants m-2 for ABUTH. The seed production - weed density relationships for 5 species (ABUTH, AMACR, CHEAL, DATST and XANST) were also determined and the long-term evolution of the potential infestations deriving from a subeconomic threshold populations were calculated. The results of the calculation demonstrate the inadequacy in the medium- long- term of weed control decisions based on single year economic thresholds.