Dakota Farmer hasn’t had many articles about using tillage to control weeds, and that is probably a mistake. Recently, University of Minnesota educators Lizabeth Stahl, Jared Goplen and Lisa Behnken wrote on the topic.

“Herbicide-resistant weed populations are limiting herbicide options and effectiveness in many fields,” they wrote. “Implementing nonchemical options, such as cultural and mechanical control tactics, can help make weed management systems more effective and durable.”

Flushes of early-emerging weeds such as giant ragweed, common lambsquarters and winter annuals can be taken out with preplant tillage. The tillage has to be aggressive enough to destroy the weeds, not just uproot and transplant them.

U of M trials at Waseca, Minn., in 2016 showed that delaying soybean planting until May 19 resulted in preplant tillage removing nearly 49% of the giant ragweed that emerged over the season. Soybean yield potential was still around 94% of optimal at the May 19 planting date, based on long-term U of M research results. In contrast, soybean yield potential of the early planting date averaged 99% of optimal; however, preplant tillage removed less than 8% of the giant ragweed that emerged over the season.

Different tool for waterhemp
Preplant tillage doesn’t work as well to control waterhemp, though. Waterhemp emerges later in the season, typically emerging over an eight- to 10-week time period. That’s why residual herbicides or the layering of residual herbicides is more effective than preplant tillage. Layering is making an application at planting and then 30 days later

Cultivating, though, can help control waterhemp, the Extension specialists say.

Cultivating allows you to remove weeds without setting back the canopy as some postemergence herbicides can, leading to faster canopy closure and a more competitive environment for weeds.

Cultivation was evaluated in U of M research trials in 2015 and 2016. A preemergence application of Boundary (1.95 pints per acre) was followed by either Liberty (29 fluid ounces per acre) or mechanical cultivation. In 2016, final waterhemp control was significantly better with the Boundary/Cultivation treatment (98%) compared to the Boundary/Liberty program at 89%.

The soybean canopy also closed sooner where cultivation occurred, so waterhemp that emerged under the canopy in July after cultivation did not survive.

Both treatments resulted in similar yields to the top-yielding treatment. Similar results for weed control, canopy closure and soybean yield were found in 2015.