WEED CONTROL ALTERNATIVES FOR SPRING TRANSPLANTED ONIONS

Charles L. Webber III and James W. Shrefler

USDA-ARS South Central Agricultural Research Laboratory (SCARL) & WWAREC, Oklahoma State University

Sweet onion (Gallium cape L.) producers need additional options for controlling weeds within spring transplanted onions. As interest in sweet onion production has grown, so has the realization that uncontrolled weeds can result in a total loss of marketable onion production. Although mechanical weed control can successfully control weeds between rows, producers need reliable methods for controlling the weeds within the crop row between the onions. Two years (2002 and 2003) of field research was conducted in southeast Oklahoma (Lane, OK) to determine the efficacy of inorganic and organic herbicides for use on spring transplanted onions. Twenty-one weed control treatments, including both inorganic (pendimethalin and oxyfluorfen) and organic (Corn Gluten Meal) herbicides, were applied to spring transplanted 'Hybrid Yellow Granex PRR' onions planted in two rows, 91 cm inches apart, on raised beds. The herbicide treatments included pendimethalin applications at 0.5 kg ai/ha, 1.0 kg ai/ha, and 1.5 kg ai/ha, oxyfluorfen at 0.1 kg ai/ha, 0.2 kg ai/ha, and 0.3 kg ai/ha, and tank mixes with the herbicides pendimethalin and oxyfluorfen at each rate. Corn Gluten Meal (CGM) was applied by hand, spreading the dry material at 4 rates (100 g/m2, 200 g/m2, 300 g/m2, and 400 g/m2). The experiment also included a series of weedy-checks and weed-free treatments to serve as experimental controls. The experiment had very high weed pressure with multiple species of broadleaf and grass weeds. Treatments receiving pendimethalin (pendimethalin or pendimethalin + oxyfluorfen) resulted in a greater percent grass control and an increase in marketable onion yields compared to the weedy-check. Select weed control treatments did shift the weed composition compared to the percent grass and broadleaf cover within the weedy-check. As the grass weeds were controlled by pendimethalin, the broadleaf weeds compensated by increasing their growth. The yield advantage for pendimethalin was probably the result of early season weed control that was not apparent at harvest.

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