How to retire glyphosate tolerant alfalfa fields

OMAFRA weed management specialist Mike Cowbrough.

Retiring alfalfa fields used to be a simple process. Conventional tillage farmers would just drop the mouldboard plow in the fall, while those under a minimum tillage system would rely on a glyphosate burndown before planting a new crop the following spring.

With the introduction of glyphosate tolerant (GT) alfalfa, however, glyphosate is no longer a retirement option and farmers are looking to determine the best management choice to transition these fields to other crops.

That’s where Ontario Ministry of Agriculture, Food and Rural Affairs weed management specialist Mike Cowbrough comes in. He notes that labels of many glyphosate products now offer suggestions on companion products that can give growers good control of GT alfalfa and provide an easy transition.

During fall 2018 and again this spring, Cowbrough tested three different companion products ( 2,4-D, dicamba, or clopyralid) in six different glyphosate tank mixes in a test plot at the Elora Research Station, near Elora, Ont. In this video, Cowbrough explains the goal of the trial was to assess cost, application timing (early fall, late fall, and spring) and re-cropping considerations. (Story continues after the video.)

Overall, the six treatments range in cost from $7 to $40 per acre. In the trial, Cowbrough found that all the tank mixes, when applied early fall (early October) did a good job controlling GT alfalfa. The story was much different when the same six treatments were applied late fall (late October). None of the late applications delivered acceptable control. Cowbrough concludes that colder late fall days, with daytime air high temperatures ranging from 0 to 6 degrees C and lows of -2, don’t allow the products to get into the plants and do the job.

When the warmer temperatures returned this spring, Cowbrough tested the same six tank mixes to find that they were again successful in controlling GT alfalfa. There are re-cropping concerns, however, for growers planting soybeans or dry beans after a spring application of some of the products. As illustrated in the video, the trial indicates it is best for growers planting these legumes to avoid dicamba or clopyralid, the active ingredient in Lontrel herbicide. In this case, Cowbrough says it’s best to stick with a product like 2,4-D Ester 700.

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