Objective: Field research was conducted at Oakes, ND, in 2005 to compare early weed control with bromoxynil, DCPA, dimethenamid-P and pendimethalin (aqueous capsule formulation) in onion (Allium cepa L.) and to evaluate the injury caused by these herbicides on onion.

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. Herbicides were applied at a low, medium or high rate. The middle rate was the labeled rate. The high rate was double, and the low rate was half, the middle rate. Treatments were applied directly after planting, except bromoxynil which was applied 10 days after planting. Herbicides were applied with a CO₂-pressurized backpack sprayer delivering 20 gal/acre at 30 psi through 11002 nozzles. Plots were 6 ft wide and 17 ft long. Treatments were arranged in a randomized complete block design with four replications. Weed coverage and control, and onion height and injury were evaluated two, four and eight wks after application. At the fifth leaf stage, a standard application of bromoxynil and oxyfluorfen (0.375 and 0.125 lb ai/acre) in 18.5 gal/acre was made to all plots, except checks. Pendimethalin at a rate of 0.62 lb ai/acre in 18.5 gal/acre was also applied at the fifth leaf stage as a final late season weed control measure. A final weed control evaluation was made 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: Dimethenamid-P applied at the high rate severely reduced plant stand. DCPA, dimethenamid-P and pendimethalin at the middle and high rates had good weed control. Bromoxynil at all rates had poor weed control. At the end of the season there was a severe decline in weed control due to intense weed pressure. No treatment yielded as good as the hand-weeded check (441.5 cwt/acre). Of the treatments, DCPA at the middle rate had the highest yield (231.6 cwt/acre) followed by pendimethalin at the middle rate (177.8 cwt/acre) and dimethenamid–P at the low rate (169.8 cwt/acre).

Table 59. Effect of preemergence herbicide on weed control and plant stand (6 ft of row).

Herbicide	Rate	Plant Stand 11WAT¹	Weed Control²
Herbicide	Rate	Plant Stand 11WAT¹	8WAT	1WBH
	lbs ai/acre	plants/6 ft of row	----------------------- % ----------------------

Pendimethalin	0.475	8.1 a-d⁴	68 c	15 bc
Pendimethalin	0.95	7.0 bcd	85 ab	28 b
Pendimethalin	1.9	7.0 bcd	95 a	18 bc
DCPA	3.38	10.4 a	71 bc	18 bc
DCPA	6.75	9.8 ab	84 abc	23 bc
DCPA	13.5	8.1 a-d	96 a	18 bc
Bromoxynil³	0.156	7.8 a-d	15 d	0 c
Bromoxynil	0.312	7.4 a-d	15 d	20 bc
Bromoxynil	0.625	7.9 a-d	10 d	10 bc
Dimethenamid-P	0.28	7.1 bcd	73 bc	28 b
Dimethenamid -P	0.56	5.4 cde	85 ab	25 b
Dimethenamid-P	1.13	3.5 e	93 a	28 a
Weedy check	-	6.3 cde	0 d	0 c
Hand-weeded check	-	8.8 abc	100 a	100 a

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

⁴Means followed by the same letter within each column are not significantly different using Fisher’s protected LSD test at P≤ 0.05.

Herbicide	Rate	Yield (by onion diameter in inches)
Herbicide	Rate	1 to 2 1/4	2 1/4 to3	>3	Total
	lb ai/acre	----------------------------- cwt/acre -----------------------------

Pendimethalin	0.475	49.2 ab²	75.3 bcd	30.5 bcd	155.4 bcd
Pendimethalin	0.95	45.9 ab	85.4 bc	46.5 bcd	177.8 bc
Pendimethalin	1.9	52.4 ab	45.9 cde	13.4 cd	111.5 cd
DCPA	3.38	57.6 ab	70.0 bcd	27.8 cd	156.5 bcd
DCPA	6.75	46.5 ab	103.1 b	82.2 b	231.6 b
DCPA	13.5	51.3 ab	68.4 bcd	18.1 cd	138.8 bcd
Bromoxynil¹	0.156	59.3 a	32.0 de	5.9 cd	97.2 cde
Bromoxynil	0.312	44.4 ab	66.7 bcd	13.9 cd	124.9 cd
Bromoxynil	0.625	49.6 ab	62.5 bcd	25.1 cd	137.2 bcd
Dimethenamid-P	0.28	49.6 ab	64.1 bcd	56.1 bc	169.8 bc
Dimethenamid -P	0.56	34.7 bc	49.6 cde	26.1 cd	110.5 cd
Dimethenamid-P	1.13	14.5 cd	34.7 de	18.7 cd	67.8 de
Weedy check	-	0.0 d	0.0 e	0.0 d	0.0 e
Hand-weeded check	-	36.4 abc	155.9 a	249.3 a	441.5 a

²Means followed by the same letter within each column are not significantly different using Fisher’s protected LSD test at P≤ 0.05.