Controlling Alfalfa in Minimum Till Cropping Systems

Retiring an established alfalfa crop in minimum tillage cropping systems can be challenging. Often the biggest reason for poor control of an old alfalfa crop is that the appropriate herbicide rate is not used and the timing of application is not optimal. 

Treatment Options and Rates

Glyphosate (eg. Roundup Weathermax, Touchdown Total) is the most common active ingredient used for control of alfalfa. However, producers have observed that glyphosate applied alone will sometimes offer inconsistent alfalfa control and the addition of 2,4-D Ester improves control. 

Demonstration trials at Kemptville College (University of Guelph) in 2008 provided a comparison of different glyphosate tank-mixes compared with glyphosate alone. Over two hundred participants at the Eastern Ontario Crop Diagnostic Day were asked to select the treatment which they felt provided the best control of alfalfa. The tank-mix of glyphosate + 2,4-D Ester proved to be the winning treatment (Table 1 and Figures 1,2 & 3). Glyphosate applied alone will often result in re-growth the next spring, particularly when low rates (i.e. 0.67 L/ac) are used. 

Source: Quesnel, Banks and Cowbrough, 2008

Figure 1. Visual control of alfalfa 4 weeks after an application of glyphosate (540 g/L) at 1 L/ac.

Figure 2. Visual control of alfalfa 4 weeks after an application of glyphosate (540 g/L) at 2 L/ac.

Figure 3. Visual control of alfalfa 4 weeks after an application of glyphosate (540 g/L) at 1 L/ac + 2,4-D Ester (564 g/L) at 0.5 L/ac.

Figure 4. Untreated Control

Application Timing - Fall or Spring?

Fall glyphosate applications provide significantly better control of alfalfa than spring applications (Table 2). If a fall herbicide application cannot be accommodated, then the addition of 2,4-D Ester to glyphosate in the spring will improve control (Table 2). The limitation of applying 2,4-D Ester with glyphosate in the spring is that on the current glyphosate label, you are limited to planting only spring cereals, not underseeded to legumes and at a 14 day pre-plant interval.

Source: J.E. Shaw and R.H. Brown, 1989

The University of Guelph recently published the results of a 3 year study which evaluated pre-plant 2,4-D Ester applications to soybeans. They found that applications of 0.5 L/ac of 2,4-D Ester (564 g/L) applied 7-14 days pre-plant caused no crop injury and grain yields equivalent to the weed-free control plots¹. However, this application timing is currently not listed on the glyphosate label.

Summary

• Fall herbicide applications provide the best control of alfalfa in minimum tillage cropping systems.
• The tank-mix of 2,4-D Ester + glyphosate applied to alfalfa in the fall often improves control and reduces the likelihood of re-growth in the spring.

¹Soltani et al. 2008. Effect of amitrol and 2,4-D applied at the preplanting and pre-emergence of soybean. Weed Biology and Management. Vol 8. pp 139-144.

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