Chemical control of weeds in onion (Allium cepa L.)

The effect of pre- or post-planting application of selected herbicides on growth and yield of transplanted onion and weeds was investigated in two field experiments conducted in Jordan during the 1997/1998 growing season. Weed competition throughout the season reduced onion yield by 94% and lowered onion quality. All herbicides [except metribuzin and diphenamid under rain-fed conditions, and linuron + terbutryn (Tempo (R)) under irrigation] increased onion growth and yield over the weed-infested control. Post-planting application of oxyfluorfen (at 2 1 ha(-1)) at the three-to-four leaf stage resulted in better onion bulb yield than in the hand-weeded, weed-free control at both locations. Pre-planting treatment with oxyfluorfen at 2.5 1 ha-(1) gave higher yields than the weed-free control in irrigated onion, but not under rain-fed conditions. Oxadiazon (at 4 1 ha(-1)) gave higher yields than in weed-free onion as a post-planting treatment under rain-fed conditions, and was second to oxyfluorfen for bulb production. Pre-planting application of pendimethalin (at 4 1 ha(-1)) was also effective, and increased onion yields compared to weed-infested onion plots or other herbicides used under irrigation. Of all herbicides tested under Jordanian conditions, oxyfluorfen and oxadiazon were the best for yield and weed control in onion, followed by pendimethalin, methabenzthiazuron and linuron as pre-planting treatments, and pendimethalin, linuron and paraquat as post-planting applications.