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Pest Patrol: Killing off Roundup Ready alfalfa

#PestPatrol with Mike Cowbrough, OMAFRA

Contrast in control of Roundup Ready alfalfa with herbicides applied in early fall (foreground) compared to late fall (background).

Six tank-mix partners are recommended for the removal of Roundup Ready alfalfa, and since they vary significantly in cost (Table 1 below), their effectiveness was evaluated in a trial conducted at the Elora Research Station in 2018-19.

x photo: Supplied

Growing degree day accumulation after herbicide application appeared to have the greatest effect on the control of Roundup Ready alfalfa, presumably because alfalfa has a base growing temperature of 5 C. When air temperatures following herbicide application rarely exceeded 5 C, alfalfa growth was stagnant (Table 2 below), and control was unacceptable.

x photo: Supplied

Conversely, when air temperatures following herbicide applications were often above 5 C, all tank-mix partners provided similar control of Roundup Ready alfalfa (Table 3 below).

x photo: Supplied

Crop plant-back considerations:

The application of Lontrel 360 (clopyralid) increases the risk of unacceptable stand establishment of soybean or dry edible bean (Figure 4 below). If wanting to use Lontrel 360 for the control of Roundup Ready alfalfa, corn or cereals would be the safest options to plant back after application. The application of dicamba in the spring would also increase the risk of unacceptable stand establishment in dry edibles or soybean, unless planting dicamba tolerant (Xtend) soybean cultivars.

Poor soybean stand establishment due to persistence of Lontrel 360 applied three weeks prior to planting (left) compared to 2,4-D Ester 700 (right). photo: Supplied

Application tips for removal of Roundup Ready alfalfa according to Roundup Transorb and Weathermax labels:

Tank-mix partners should be applied in the fall when Roundup Ready Alfalfa is between the pre-bud and the start-of-flowering stage, but ideally at the bud stage of growth.

Tillage at two to three weeks following application can improve control and consistency under stressed conditions (drought, frost, cold temperatures, etc.).

Mix with water to achieve a total applied volume of 100 l/ha (10 U.S. gal./acre).

Roundup Ready alfalfa control with 2,4-D Ester 700 applied in early fall 2018. photo: Supplied
Roundup Ready alfalfa control with 2,4-D Ester 700 applied in late fall 2018. photo: Supplied

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].

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