Malheur Experiment Station Information for Sustainable Agriculture

2,4-DB Ester Alternatives and Setback Herbicides for Alfalfa Seed Production

Corey V. Ransom and Joey Ishida
Malheur Experiment Station
Oregon State University
Ontario, OR, 1998

Introduction

Weed control during alfalfa establishment is critical for the production of weed-free alfalfa seed because weeds reduce yields from competition and contamination by weed seeds. The recent loss of 2,4-DB ester has limited the herbicide options available for alfalfa-seed producers. Additionally, high temperatures during establishment can further restrict the herbicides that can be used without injury to the alfalfa crop. New herbicides offer potential to provide effective weed control during alfalfa establishment with minimal injury to the crop.

In addition to being used for weed control, contact herbicides like paraquat are sometimes used to delay early season plant development by burning back alfalfa plants. We will term herbicides used for this purpose as "setback" herbicides. Growers intensively delay early plant development so that flowering coincides with warm weather that is conducive for alfalfa leaf-cutting bees to pollinate the crop. Setback herbicides are applied instead of cultivation. Reducing the use of cultivation to delay plant development may reduce the occurrence of diseases introduced into injured alfalfa crowns. With the introduction of several new contact herbicides, it is likely that some of these herbicides may be useful for delaying crop development to synchronize the onset of bloom with the optimum time of leaf-cutting bee emergence and pollination activity. The use of setback herbicide products may also provide some incidental weed control.

Purpose

This study evaluated registered and non-registered herbicides for weed control in alfalfa seed production. A second trial compared alfalfa setback with herbicides and mechanical methods.

Procedures

The weed control trial and setback trial were both conducted on cooperators' fields, and general management practices were carried out by the cooperators. Herbicide treatments were applied with a CO₂-pressurized backpack sprayer. Alfalfa seed yields were obtained by harvesting the center 3.75 ft of each plot with a small plot combine.

Data were analyzed using ANOVA, and treatment means were separated using a protected least significant difference at the 5 percent level, LSD (0.05).

2,4-DB Ester Alternatives

Raptor and other herbicides offer the potential to control weeds during alfalfa seed crop establishment with minimal crop injury. A trial was initiated to evaluate Raptor, Pursuit, 2,4-DB amine, Basagran, Buctril, and Tough alone and in combinations for weed control and crop tolerance in alfalfa seed establishment. All treatments were compared to 2,4-DB ester. The predominant weed species was hairy nightshade with low populations of common lambsquarters and kochia. The study was a randomized complete block design with three replications. Herbicide treatments were applied May 6. Temperatures were greater than 80 °F for several days following the applications, resulting in severe alfalfa injury by some treatments.

Alfalfa Setback

A trial was established to evaluate herbicides for potential as setback agents and to determine alfalfa seed yields with different setback techniques. Alfalfa maturity was delayed by mowing, cultivation, and applications of paraquat, Liberty, and Blazer herbicides on April 21. Paraquat and Liberty were also applied at 20 and 40 gpa. Treatments were arranged in a randomized complete block design with 4 replications. Evaluations included alfalfa setback, height, flowering, and seed yield. A severe hail storm July 4 damaged alfalfa plants at this location.

Results

2,4-DB Ester Alternatives

Buctril alone caused significant alfalfa injury and increased injury when applied in a tank mixture with other herbicides (Table 1). Raptor combinations with Buctril or 2,4-DB generally increased alfalfa injury compared to Raptor applied alone. Raptor at 0.063 lb ai/acre caused more alfalfa injury than any other rate of Raptor applied alone. Combinations of Raptor (0.032 lb ai/acre) with Tough or Basagran also resulted in increased injury. While Basagran alone was not very injurious, combinations of any other herbicide with Basagran resulted in severe injury. Raptor alone and tank mixtures containing Raptor provided better control of hairy nightshade than Basagran, Buctril, 2,4-DB, and 2,4-DB ester. A few treatments containing Tough provided hairy nightshade control similar to treatments containing Raptor. Common lambsquarters control was similar among most treatments except Basagran alone provided almost no control. Kochia control was poorest with Basagran, 2,4-DB, or 2,4-DB ester applied alone. Alfalfa seed yields were variable, and Basagran applied alone was the only treatment that did not increase yields in comparison to the untreated check.

Alfalfa Setback

In the "setback" trial, all the herbicides examined effectively delayed alfalfa maturity, but the activity was different for each herbicide (Table 2). Alfalfa height was significantly reduced by all treatments compared to no setback 17 days after treatment (DAT). By 31 DAT alfalfa heights were not different. Mowing and paraquat had the greatest alfalfa suppression 3 DAT, and by 7 DAT Liberty provided similar alfalfa suppression. Blazer was among the slowest herbicide to show symptoms on the alfalfa. All mechanical or chemical setback treatments reduced flowering 31 and 58 DAT. By 71 DAT there were no differences in flowering among treatments. Treatments including no setback, cultivation, mowing, and Liberty (40 gpa) had among the highest alfalfa seed yields.

Table 1. Alfalfa injury, weed control, and alfalfa seed-yield with postemergence herbicide combinations, Malheur Experiment Station, Oregon State University, Ontario, OR, 1998.

		Alfalfa injury					Weed control‡			Alfalfa seed yield‡
Treatment †	Rate	5-14	5-27	6-16	6-30		Hairy nightshade	Common lambsquarters	Kochia
	lb ai/acre	--------------------------------------------------------%--------------------------------------------------								lb/acre
Raptor	0.024	7	3	3	5		92	80	83	579
Raptor + Buctril	0.024 + 0.25	43	48	35	25		90	95	90	515
Raptor + 2,4-DB	0.024 + 0.5	30	62	17	27		93	92	95	454
Raptor	0.032	10	10	2	7		93	87	92	555
Raptor + Buctril	0.032 + 0.25	45	42	28	23		92	93	95	579
Raptor + 2,4-DB	0.032 + 0.5	47	77	48	43		88	90	90	518
Raptor	0.04	22	27	23	30		93	88	95	569
Raptor + Buctril	0.04 + 0.25	38	35	42	37		92	93	90	645
Raptor + 2,4-DB	0.04 + 0.5	25	38	37	30		92	87	95	476
Raptor	0.048	17	30	15	15		93	85	95	559
Raptor	0.063	25	60	62	53		95	92	95	484
2,4-DB	0.5	7	0	3	8		43	92	58	468
Buctril	0.25	28	47	13	30		63	90	95	498
2,4-DB ester	0.5	7	3	2	3		65	85	55	513
2,4-DB + Buctril	0.5 + 0.25	58	52	25	33		62	87	95	444
Tough	0.94	13	12	3	8		82	90	95	556
2,4-DB + Tough	0.5 + 0.94	23	18	7	13		88	95	95	610
Raptor + Tough	0.032 + 0.94	25	28	33	25		95	95	95	580
Basagran	1.0	10	3	5	23		60	3	30	353
Raptor + Basagran	0.032 + 1.0	53	58	48	42		93	92	95	523
Basagran + Tough	1.0 + 0.94	50	48	52	43		70	92	95	519
Basagran + 2,4-DB	1.0 + 0.5	30	23	10	27		63	75	88	515
Basagran + Buctril	1.0 + 0.25	50	52	22	43		50	90	95	418
Tough + Buctril	0.94 + 0.25	60	63	48	45		68	95	95	448
Pursuit	0.063	5	15	5	5		95	87	95	613
Pursuit + Buctril	0.063 + 0.25	23	25	18	15		95	95	95	658
Untreated	-	0	0	0	0		0	0	0	197
LSD (0.05)		15	17	21	20		14	9	25	162

^†NIS (0.25 % v/v) and 32% nitrogen solution (1% v/v) were added to all treatments containing Raptor or Pursuit. Treatments were applied May 6.

^‡Weed control ratings taken June 16. Alfalfa seed harvested September 16.

Table 2. Alfalfa setback, height, flowering, and seed yield in response to mechanical and chemical setback treatments, Malheur Experiment Station, Oregon State University, Ontario, OR, 1998.

		Spray volume	Alfalfa setback			Alfalfa height			Alfalfa flowering			Alfalfa seed-yield‡
Treatment†	Rate		4-25	4-28	5-9	4-28	5-9	5-23	5-23	6-19	7-2
	lb ai/acre	gpa	---------------%---------------			----------------in--------------			----------------%--------------			lb/acre
Mowing	-	-	53	59	39	7.3	11.0	19.7	8	40	65	523
Cultivation	-	-	43	39	31	8.4	12.3	19.6	10	51	61	667
Paraquat	0.47	20	54	56	38	9.3	12.5	17.3	1	21	40	434
Paraquat	0.47	40	53	51	35	9.2	12.8	17.9	1	33	53	432
Liberty	0.5	20	35	59	53	9.5	11.5	16.8	0	21	49	451
Liberty	0.5	40	38	56	51	9.7	11.8	17.8	0	26	46	569
Blazer	0.38	20	41	49	46	7.8	11.8	17.1	0	20	51	471
No setback	-	-	0	0	0	12.6	16.3	17.3	18	64	51	652
LSD (0.05)			8	6	10	1.2	1.8	NS	4	10	NS	151

^†NIS (0.25 % v/v) were added to treatments containing Paraquat or Blazer. AMS (3.0 lbs/acre) was added to Liberty treatments.

^‡Alfalfa seed harvested September 3.

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