Objective: Field research was conducted at Oakes, ND, in 2005 to determine the effect of bromoxynil and oxyfluorfen rate and spray volume on early postemergence weed control in onion (Allium cepa L.). Bromoxynil and oxyfluorfen are two postemergence herbicides commonly used in onion. Bromoxynil and oxyfluorfen may cause leaf injury, so a high application volume (50 gal/acre) is recommended to provide safety to onion. Onion must also be in the two-leaf stage before herbicide application, but at this stage, many broadleaf weeds are already beyond the growth stage for control by either herbicide. Our objective was to determine if lower herbicide rates applied at the one leaf stage and lower spray volumes could be used to control weeds without injuring onions.

Materials and Methods: The study was conducted at the Oakes Irrigation Research Site, Oakes, ND. Onion variety ‘Varsity’ pelleted seed was planted on April 18. To evaluate bromoxynil and oxyfluorfen use rates, bromoxynil was applied at 0.075 and 0.3 lb ai/acre and oxyfluorfen was applied at 0.025 and 0.1 lb ai/acre. To evaluate the effect of volume, herbicides were sprayed with 10, 25 or 50 gal/acre of water. Treatments were applied at the first true leaf stage (May 30) with a CO₂-pressurized backpack sprayer at 30 psi. Spray nozzle sizes were 8001, 8002 and 8004 for 10, 25 and 50 gal/acre, respectively. Plots were 6 foot wide and 17 foot long. Treatments were arranged in a randomized complete block design with four replications. Plots were evaluated for weed coverage and control, and onion height and injury one and three weeks after application. At the fifth leaf stage, a standard application of bromoxynil and oxyfluorfen (0.375 lb ai/acre and 0.125 lb ai/acre) was made to all treatments, except checks. Spray volume was 18.5 gal/acre. Pendimethalin at a rate of 0.62 lb ai/acre at a volume of 18.5 gal/acre was applied after the fifth leaf stage as a final late season weed control measure. A final weed control evaluation was taken one week before harvest. On October 4, 10 feet of the middle two rows of each plot were harvested for grade and yield analysis. After harvest, onions were allowed to cure and then were graded. Split, diseased and double bulbs were graded as culls regardless of diameter.

Results: Onion height varied depending on treatment early in the growing season. However, one week before harvest, height was the same for all treatments except the weedy check. Plant stand was higher for oxyfluorfen at 0.1 lb/acre and 25 gal/acre (14 plants/m), all other treatments were statistically the same. Oxyfluorfen applied in 50 gal/acre showed the best control 1 and 3 WAT. All other treatments, except the 0.075 lb/acre rate of bromoxynil applied at 50 gal/acre, had statistically the same weed control. Treatments containing oxyfluorfen had the best control 3WAT. There was a severe decline in weed control late in the season due to intense weed pressure. No treatment yielded as well as the hand-weeded check (339.5 cwt/acre). Of the chemical treatments, those with oxyfluorfen resulted in the highest yield. Oxyfluorfen at 0.025 lb ai/acre at 25 gal/acre had the best yield (206.1 cwt/acre). As for bromoxynil, 0.3 lb ai/acre at 25 and 50 gal/acre had a lower yield than 10 gal/acre and 0.075 lb ai/acre yield was the same with any volume.

Herbicide	Rate	Spray volume	Onion Injury 1WAT¹	Weed Control²
Herbicide	Rate	Spray volume	Onion Injury 1WAT¹	1WAT	3WAT	1WBH
	lb ai/acre	gal/acre	---------------------------------------- % ---------------------------------------

Weedy check	-	-	0 c³	0 d	0 e	0 c
Hand-weeded check	-	-	0 c	100 a	100 a	100 a
Bromoxynil	0.075	10	4 c	85 abc	58 cd	18 bc
Bromoxynil	0.075	25	1 c	74 bc	48 d	10 bc
Bromoxynil	0.075	50	1 c	65 c	45 d	0 c
Bromoxynil	0.3	10	5 bc	89 ab	74 bc	5 bc
Bromoxynil	0.3	25	5 bc	76 bc	35 d	0 c
Bromoxynil	0.3	50	1 c	84 abc	58 cd	8 bc
Oxyfluorfen	0.025	10	26 a	89 ab	91 ab	25 b
Oxyfluorfen	0.025	25	26 a	71 bc	88 ab	5 bc
Oxyfluorfen	0.025	50	19 abc	73 bc	86 ab	18 bc
Oxyfluorfen	0.1	10	25 ab	84 abc	78 abc	0 c
Oxyfluorfen	0.1	25	38 a	90 ab	79 abc	0 c
Oxyfluorfen	0.1	50	31 a	91 ab	96 ab	13 bc

²Average control of common lambsquarters, redroot pigweed, hairy nightshade and purslane.

³Means followed by the same letter within each column are not significantly different using Fisher’s protected LSD test at P≤ 0.05.

Herbicide	Rate	Spray volume	Yield (by onion diameter in inches)
Herbicide	Rate	Spray volume	1 to 2 1/4	2 1/4 to 3	>3	Total
	lb ai/acre	gal/acre	-------------------------- cwt/acre -------------------------

Weedy check	-	-	0.0 a¹	0.0 d	0.0 c	0.0 d
Hand-weeded check	-	-	40.0 a	127.1 a	172.5 a	339.5 a
Bromoxynil	0.075	10	56.6 a	45.9 bcd	12.2 c	114.8 bcd
Bromoxynil	0.075	25	45.4 a	37.9 bcd	21.9 bc	105.2 bcd
Bromoxynil	0.075	50	59.8 a	34.7 bcd	9.1 c	103.5 bcd
Bromoxynil	0.3	10	47.1 a	31.5 bcd	7.5 c	86.0 bcd
Bromoxynil	0.3	25	33.6 a	14.5 cd	4.2 c	52.4 cd
Bromoxynil	0.3	50	43.3 a	20.4 cd	0.0 c	64.1 cd
Oxyfluorfen	0.025	10	47.5 a	89.6 ab	68.8 b	206.1 b
Oxyfluorfen	0.025	25	41.6 a	53.9 bcd	26.7 bc	122.2 bcd
Oxyfluorfen	0.025	50	50.7 a	56.5 bcd	29.9 bc	137.2 bc
Oxyfluorfen	0.1	10	28.4 a	43.3 bcd	45.9 bc	117.4 bcd
Oxyfluorfen	0.1	25	38.9 a	71.5 abc	44.8 bc	155.4 bc
Oxyfluorfen	0.1	50	50.7 a	67.8 abc	44.8 bc	158.6 bc

¹Means followed by the same letter within each column are not significantly different using Fisher’s protected LSD test at P≤ 0.05.