Spring time is here for most areas of ND. Time to starting thinking of how to carry out the vast plans that were made throughout winter. When it comes to the soybean crop, getting the PRE applied may be one of those logistical challenges we struggle with. However with soybeans your PRE program is the most important component to managing glyphosate resistant weeds. The problem is predicting when an activating rainfall will occur is as difficult as ever even though we know that the amount of rain can impact the success of PRE herbicides. The question then becomes, “how will the herbicide be affected if we don’t get enough rain to activate the product?” We conducted a study in 2016 to answer that question. For those that are interested, here are the materials and methods, otherwise you can skip to the results below:

Three herbicides are of particular interest, Spartan (sulfentrazone), Fierce (flumioxazin + pyroxasulfone), and metribuzin (Sencor), which represent the chemistries of a large portion of the rain activated PRE herbicides for soybeans. Each herbicide was compared in corn stubble that was either tilled or left un-tilled. A center pivot irrigation system was used to simulate rainfall (0.5” of water) for the herbicide activation treatments. The herbicides were activated within 1 day of application, 7 days after application, rotary hoed 7 days after application, or received no activation treatment. The first natural activating rainfall occurred 16 days after treatment so the non-activated treatment was actually activated a little over 2 weeks following herbicide applications. The weeds in the study area were common lambsquarters and redroot pigweed. All existing weeds received glyphosate treatment immediately after planting so that only the pre-emerge component of the herbicides was evaluated.

See Table 1 for a complete list of results. There were a couple key findings from this study. 1) metribuzin did not perform well when activation was delayed by more than a week, and even after 1 week without activation, control of lambsquarters and especially redroot pigweed was less. 2) Rotary hoeing 1 week after applicatoin did not improve the performance of metribuzin. In fact, rotary hoeing reduced the effectiveness of metribuzin in our conventional till treatments (the assumption is that we actually planted weed seeds but did not activate the product. Rotary hoeing however did improve the activity of Spartan under conventional till. 3) Spartan and Fierce were still effective without activation for 7 days, but after that point, Spartan was started to lose effectiveness somewhere between 7 and 16 days after application. The activity of Fierce did not decline in this study, even after 16 days without activation.

Table 1. Comparison of herbicide activation strategies 1 and 7 days after herbicide treatment (DAT) with different herbicide and tillage strategies.

Mike Ostlie, Ph. D.
Mike.Ostlie@ndsu.edu
Research Agronomist