Killing off Roundup Ready Alfalfa

Six tank-mix partners are recommended for the removal of Roundup Ready Alfalfa and since they vary significantly in cost (Table 1), their effectiveness was evaluated in a trial conducted at the Elora Research Station in 2018-19. 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 and when air temperatures following herbicide application rarely exceeded 5°C, alfalfa growth was stagnant (Table 2), and control was unacceptable. 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).

Table 1. Rate and cost per acre of the six recommended tank-mix partners for the control of Roundup Ready Alfalfa

Tank-mix partner with glyphosateRateCost
2,4-D Ester 700520 mL/acre$7/acre
Engenia (dicamba)400 mL/acre$14/acre
Lontrel XC (clopyralid)135 mL/acre$22/acre
Lontrel XC (clopyralid)200 mL/acre$33/acre
2,4-D Ester 700 + Engenia360 mL/acre+ 400 mL/acre$19/acre
2,4-D Ester 700 + Lontrel XC360 mL/acre + 100 mL/acre$21/acre
Herbicide prices based on suggested retail pricing (2021). Price may vary.

Table 2. Application timing, date and growing degree day accumulation during the 14 days following herbicide application.

Application Timing

Application Date
Growing Degree Days
(14 days after application)
Late fallOctober 30, 20188.5
Early fallOctober 3, 201884
SpringMay 17, 2019118

Table 3. Tank-mix partner, rate and level of Roundup Ready alfalfa control when applied at three different timings.

Tank-mix partner

Alfalfa Control
(Applied Oct 3)
Alfalfa Control
(Applied Oct 30)
Alfalfa Control
(Applied May 17)
2,4-D Ester 700520 mL98%30%80%
Engenia (dicamba)400 mL99%34%86%
Lontrel XC (clopyralid)135 mL96%32%84%
Lontrel XC (clopyralid)200 mL96%64%95%
2,4-D Ester 700 + Engenia360 mL+ 400 mL100%71%99%
2,4-D Ester 700 + Lontrel XC360 mL + 100 mL100%75%93%

Crop plant back considerations: The application of Lontrel XC (clopyralid) increases the risk of unacceptable stand establishment of soybean or dry edible bean (Figure 4). If wanting to use Lontrel XC 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 edible beans or soybean, unless planting a dicamba tolerant (Xtend) soybean cultivar.

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

– Tank-mix partners should be applied in the fall when Roundup Ready Alfalfa is between the pre-bud to start of flowering stage, but ideally at the bud stage of growth.
– Tillage at 2-3 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).

Figure 1. The contrast in control of Roundup Ready alfalfa when a herbicide tank-mix was applied in early fall (foreground) compared to late fall (background)

Figure 2. Roundup ready alfalfa control with 2,4-D ester 700 applied in early fall 2018.

Figure 3. Roundup Ready alfalfa control with 2,4-D ester 700 applied in late fall 2018.

Figure 4. Poor soybean stand establishment due to persistence of clopyralid (Lontrel XC) applied 2 weeks prior to planting (left) compared to 2,4-D Ester 700 (right).