Weed control in onions (Allium cepa L.) grown under three tillage systems [Philippines]

Tolerance of three locally grown onion cultivars namely: Red Creole, Batanes and Lasona, to treatments by oxyflourfen and butachlor was studied under the laboratory and field conditions. Oxyflourfen at 0.75 kg/ha applied as pre-emergence, considerably reduced the percentage of cv. Red Creole and cv. Batanes both grown from true seeds. Effect on cv. Lasona, however, was minimal. Likewise, post-emergence application of oxyflourfen at 10 days after emergence (DAE) or at the first true leaf stage of onion plants produced scorching and tip-curling symptoms but with no mortality of seedlings. At two weeks after post-emergence treatments, remarkable recovery of the seedlings was observed. The major weed species associated with mulched onion, grown after lowland rice were: Oryza sativa L. (volunteer rice) with an average relative density of 58.1%; Cyperus rotundus L. with 31.1%; Echinochloa colona (L.) Link. with 5.7% and Trianthoma protulacastrum L. with 5.2% relative density, respectively. Under zero tillage, C. rotundus became the problem weed. However, it was effectively suppressed in minimum and in conventionally cultivated plots. With the growing conditions in the study, yield reduction due to weed competition was proportionate with the yield potential of the cultivar hence cv. Red Creole, a high yielding cultivar, had recorded the highest yield reduction of 47.4% as compared with 29.6 and 25.5% for Batanes and Lasona, respectively, both of which are low yielding cultivars. The weed control treatments that provided adequate suppression of the weeds associated with onions compared with the handweeded treatments were oxyflourfen (0.75 kg/ha) at 3 days after weed emergence (DAWE), oxyflourfen (0.75 kg/ha) at 7 DAWE and oxyflourfen (0.5 kg/ha) at 3 DAWE. These weed control treatments produced also onion bulb yields comparable to weed-free check. Cultivation of the soil, then allowing flushes of weeds to grow for sometime before re-cultivation gave complementary effect especially in the control of C. rotundus in mulched onions.