To remove old Roundup Ready alfalfa stands in no-till situations, apply herbicide in fall, then hit any surviving plants with a postemergent weed killer in the rotation crop the following spring.
That’s the recommendation Mark Renz, University of Wisconsin Extension weed scientist, is sticking with despite the fact that fall 2012 herbicide applications were mostly ineffective in his research plots. Stressed by drought, the aging Roundup Ready alfalfa was growing slowly and didn’t adequately absorb the chemicals, says Renz.
The poor results “emphasize the importance of actively growing alfalfa plants for successful removal with herbicides,” he says.
Herbicides in combination with tillage are the most effective option for terminating Roundup Ready alfalfa. Aggressive tillage alone can work well, too. But removing the transgenic crop is more challenging for no-till farmers, who must rely solely on herbicides other than glyphosate, Renz points out.
He uses 2,4-D and/or dicamba (the active ingredient in Banvel, Clarity and other products) because they’re effective and less expensive than other growth-regulator herbicides. Neither chemical is registered for use on alfalfa but can be applied if the crop won’t be harvested, he says.
In a 2006-07 trial, fall applications of 2,4-D and dicamba alone or together controlled more than 95% of alfalfa plants. Spring applications of the two products together suppressed the crop through May, but some plants recovered and were problematic in the following crop.
The experiment Renz started last fall was aimed at evaluating different herbicide application timings and needed follow-up treatments in the ensuing corn crop. He applied either 2 pints/acre of 2,4-D or 1 pint/acre each of 2,4-D and dicamba to plots when the alfalfa was 3-5” tall. The applications, made Nov. 5, 2012, and May 1, 2013, were late because fall regrowth was slow and the alfalfa greened up late in spring.
Fall treatments were initially effective, but most of the plants recovered in spring. Fall-applied 2,4-D-only plots showed 52% alfalfa control on May 28 and had regressed to 0% control by June 26. Fall 2,4-D-plus-dicamba treatments had 83% control on May 28, but only 50% control in late June.
Results from spring 2,4-D applications were disappointing, too, resulting in 23% alfalfa control on May 28 and 2% on June 26. The spring-applied herbicide combination worked much better, killing 97% of the alfalfa plants.
Renz believes fall-applied herbicides will outperform spring-applied chemicals most years, and fall application avoids the plant-back restrictions of many products. Most 2,4-D labels recommend applying it seven to 14 days before planting corn to avoid crop injury.
The fall application window can be narrow, too, because treatment has to be delayed until the crop regrows to 3-5” after the last cutting. Alfalfa growth ends when the nighttime temperature drops to the mid-20s. Mid-October is the ideal treatment time in southern Wisconsin, but it can be made as late as early November some years, says Renz.
Check the weather forecast before spraying, he advises. For best results, daytime high temperatures should reach the mid-50s within a couple days after the treatment.
Don’t forget about other weeds in alfalfa fields to be terminated, he adds. If quackgrass is prevalent, for example, you might want to add glyphosate to the tankmix.
Then scout next year’s corn for volunteer Roundup Ready alfalfa plants, and make a post application if needed. Products containing dicamba, such as Distinct, Marksman and Status, are good choices, as are Hornet, Python and Stinger, says Renz.
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