Controlling Russian Thistle and Knotweed in Wheat

Dr. Dan Ball and I conducted an herbicide trial in the Holdman area last spring looking at controlling Russian thistle and knotweed in winter wheat. We focused on alternatives to phenoxy-type herbicides such as 2,4-D, Banvel and Clarity, and possible herbicide resistance management strategies. The study looked at the cost of eliminating phenoxy-type herbicides to minimize potential for long-distance transport of herbicides.

Russian Thistle Photo Source: University of California IMP

Twenty-four different treatments were evaluated, 6 without phenoxys and 18 mixtures with 2,4-D or dicamba. (See Table 1). A randomized complete block design was used. Three replications of each treatment were made.

All treatments effectively controlled the targeted weeds although a dry season limited pressure from late germinating weeds. Control percentages on Russian thistle ranged from 94% to 100% with phenoxys and 88% to 100% without phenoxys at 6 weeks post application. Only 2 treatments resulted in significantly higher yields over the weedy check.

The herbicide, Aim, was successful in controlling Russian thistle and knotweed at both rates used. Aim, a Group 14 herbicide, is not a phenoxy-type herbicide, nor is it a sulfonylurea. Given the very low vapor pressure of Aim, carfentrazone-ethyl (7.2 x 10-6 Pa @ 20°C), the compound is not expected to volatilize to a significant extent. In addition, soil surface and leaf surface volatility studies showed no significant losses of Aim after 24 hours.

Prostrate Knotweed Photo Source: University of Illinois

Costs of the different treatments were estimated. The numbers should only be used for comparative purposes. Costs will vary depending on individual circumstances. Economic analysis of treatment costs shows a median cost of treatments with phenoxys of $7.00 while treatments without phenoxy increase to a median cost of $13.00. Selecting the lowest cost treatment from each category shows $4.50 with phenoxy versus $6.75 without phenoxy. Costs for the different treatments ranged from $4.50 to $19.25 per acre.

Aim is a contact herbicide. The weeds must be up and actively growing. Coverage is essential for good control. The label calls for a minimum of 10 gallons of spray volume per acre for ground applications and 3 gallon minimum for air. Dan did remark that the Aim consistently spots the wheat leaves following treatment. He says that it will clear up in about 2 weeks, and in our study we did not see any negative impact to yields. Read and follow the label directions.

1. Froelech, L. & Hullebroeck, M. "Environmental Fate of Carfentrazone-ethyl Cereal Grain Herbicide." [On-line]. Available: http://www.chemsoc.org/chempest/html/6D-0038.html