With a mild winter coming to an end, an early and warm spring has arrived, giving everything an early start, including weeds. Resistance has spread quickly through southern Ontario and according to Peter Sikkema, Ph.D., field crop weed management professor, University of Guelph, studies found that glyphosate-resistant giant ragweed, common ragweed and Canada fleabane in soybean have resulted in yield losses of greater than 90 percent in fields with the greatest weed pressure. With the introduction of dicamba-tolerant (DT) soybean, growers have another option to aid in the fight against herbicide resistance.

When it comes to resistance management in soybean, growers should be using the right tools for the job. It is important that growers determine the weed species present in each field and the size of the weeds at the time of application. In addition, weed density and future crops in the rotation will influence the appropriate weed management strategy.

According to Sikkema, the impact of glyphosate-resistant weeds in soybean is very much correlated to relative time of weed and crop emergence and the weed density in each individual field. "There will be a number of benefits of Roundup Ready Xtend or DT soybean," said Sikkema. "Having said that, it is very important that you understand the biology of each individual weed species to determine what the optimal application timing for dicamba in DT soybean will be."

The optimal soil-applied herbicide in soybeans will be determined by the weed spectrum in each individual field, the soil type in that field and what crops are planned for the field in the future.

Growers should consider a two-pass system with multiple modes of action at each pass to control tough glyphosate-resistant weeds.

"Based on our research, I think that it is important that farmers have a diverse crop rotation as part of an overall weed management strategy," continued Sikkema. "Within each one of those crops, growers should have multiple modes of action on every acre, every year."

There are a number of options available as soil-applied herbicides for two-pass weed control in DT soybean.