Field experiments were conducted at Oakes, Absaraka, and Tappen, ND, in 2006 and repeated at Oakes and Absaraka, ND, in 2007 to evaluate early season weed control of common lambsquarters and redroot pigweed in onion with POST herbicides applied at multiple reduced rates (microrates) and to determine whether microrate herbicide treatments effectively reduced early season broadleaf weed competition, caused crop injury, or affected yield. Application rates of bromoxynil, oxyfluorfen, metribuzin, and acifluorfen were reduced to 0.25, 0.13, and 0.06× of their lowest labeled rate and applied in sequential applications (every 7 d) either two or three times. The 0.25× rate of bromoxynil (70.1 g ae/ha) provided the greatest control of common lambsquarters (95%). The 0.25× rates of bromoxynil and oxyfluorfen (70.1 g ai/ha) provided the greatest control of redroot pigweed (93 and 85%, respectively). Microrate applications of metribuzin or acifluorfen did not effectively control common lambsquarters or redroot pigweed. In 2006, no onion injury was observed. However, in 2007, applications of oxyfluorfen resulted in approximately 15% injury, regardless of the herbicide rate or the number of applications. Plants outgrew symptoms by 4 wk after treatment and were similar to the untreated plants. Onion treated with oxyfluorfen had the greatest total yield, followed by onion treated with bromoxynil. Onion treated with acifluorfen had a greater total marketable bulb yield than onion treated with metribuzin, but yield was considered poor compared to the other herbicide treatments. Three microrate applications provided greater weed control and increased yield compared with two applications across herbicides and rates. Results suggest that microrate applications of bromoxynil and oxyfluorfen will provide early season broadleaf weed control in onion.

Early-season weed competition may cause substantial yield losses in onion. Oxyfluorfen and bromoxynil are POST herbicide options for weed control once onion has developed two leaves, which often takes 4 to 6 wk. Multiple applications of oxyfluorfen at 35 and 18 g ai ha−1 and bromoxynil at 35 and 18 g ae ha−1 with adjuvants were evaluated for onion safety and weed control under controlled greenhouse conditions. Oxyfluorfen at 35 g ha−1 plus organosilicone surfactant caused 42% onion injury at 12 d after three sequential applications. Onion treated with bromoxynil at 18 g ha−1 plus high surfactant oil concentrate had lower fresh weight (0.7 g) compared to methylated seed oil (MSO) (1.2 g) or petroleum oil concentrate (POC) (1.3 g) at the same bromoxynil rate. The addition of nonionic surfactant to bromoxynil, averaged across bromoxynil rates, provided 17 and 39% control of redroot pigweed and common lambsquarters, respectively. Redroot pigweed control with oxyfluorfen at 35 or 18 g ha−1 plus any tested adjuvant was excellent (≥ 93%). Results suggested the use of POC or MSO with either oxyfluorfen or bromoxynil for subsequent field trials because of similar common lambsquarters and redroot pigweed control and onion safety.