• Roundup Ready is about the best thing since sliced bread for soybean weed control.
  • Tank mixes, pre-emergence programs and sequential glyphosate by itself are all excellent programs. In a normal year, they’ll be nearly perfect and indistinguishable.
  • Run the economics and- guess what – they price out extremely close.

But the magic words are “normal year”. This year, farmer fields are weedy, and our test plots are too. However, one experiment is markedly cleaner than the rest: pre-emergence herbicides followed by glyphosate. All treatments in this test include a single postemergence glyphosate treatment; but different pre-emergence herbicides are tested. We want to thank the Missouri Soybean Merchandising Council for funding this study.

In our experiment, Valor, Canopy XL, or Sencor followed by one glyphosate application beat a sequential application of glyphosate. Along the same lines, we summarized several years of work with Squadron and found that while it didn’t matter in an individual year- the long term average showed that weed control and yield were slightly better in the PRE followed by glyphosate program than in sequential glyphosate programs.

There are two important things to consider. First, our best results have come from broadleaf pre-emergence herbicides and not grass ones. Glyphosate is a good grass herbicide, and PRE herbicides bring little improvement. Pigweed may be an exception; however, when we’ve used a “grass” herbicide, we’ve needed to spray glyphosate twice and then you start losing economically. With broadleaf herbicides, one shot of glyphosate has usually done it. The other important item is to match your pre-emergence herbicide with your troublesome weeds. Squadron is a bad choice if you have lots of coffeebean. Canopy XL is not the world’s best cocklebur herbicide. Your troublesome weeds should dictate which PRE herbicide you use.

With tank mixes, the improvements are even smaller and harder to find. Again, there is nothing wrong with tank mixes; however, you should use them because you need them, not because they sound good. If morningglory is escaping – use one of the tank mixes. If it isn’t escaping – don’t. We generally get good morningglory control with glyphosate alone, but that’s in our fields and not yours.

Ford’s second theory is still valid – most years and most fields, the weed control will be indistinguishable between PRE programs, tank mixes and sequential glyphosate. Rotational restrictions, simplicity and guarantee programs are reasons that growers have shied away from PRE programs. But some years, a pre-emergence herbicide may benefit soybean weed control. If your workload is reasonable and treatments can be made on time – you may wish to stick with glyphosate alone. If you have far-away fields or have trouble getting sprayed exactly on time – use a broadleaf pre-emergence herbicide. Another good recommendation is to use a pre-emergence herbicide when you know it’s going to be a bad weed year – but if you can do that, weeds aren’t worth worrying about.

Andy Kendig is Extension weed specialist with the Commercial Agriculture Program at the Missouri University Delta Center in Portageville. Ford Baldwin is a retired University of Arkansas weed scientist and a columnist for Delta Farm Press.