By Mark Glady

Last year was a wild ride for weed control.

New weed threats moving into Minnesota, dicamba herbicide challenges across the country and the continuing quest to find new ways to manage herbicide resistance were all topics that resonated in the ag world.

Mark Glady

What can you do to manage troublesome weeds in 2018? In my experience, it’s all about knocking weeds down to size. Ideally, that means not letting them emerge at all. Or, Plan B, eliminating them when they are at their smallest.

We can look back at last year’s weed story and use that information to create some strategies for the upcoming season.

1. Use residual herbicides. In my area of west-central Minnesota, common waterhemp was by far the dominant weed in 2017. This troublesome weed, as well as giant ragweed and common ragweed, continue to be very difficult to control.

Here is what I recommend for soybean fields with a history of waterhemp:

• Apply a residual herbicide at planting, which will offer control for about 30 days.

• Apply an early postemergence herbicide about 30 days after planting.

• Apply a second residual herbicide in-season on any field where you expect waterhemp pressure.

Even if a waterhemp seed does germinate, it’s easier to eliminate a quarter-inch-tall sprout in the soil with a residual herbicide than a 2-inch-tall weed. A number of farmers I work with have found that applying that second residual herbicide in-season has helped with waterhemp control.

2. Add a drift and deposition adjuvant. To eliminate any weed, it’s critical to keep herbicide applications on target. Including an adjuvant in the tank can help. Most of the adjuvants on the market are designed to do three things:

• Reduce fine particles that can drift away by putting more droplets in the ideal size range.

• Improve deposition and leaf coverage by having droplets at the correct size.

• Get more active ingredient deeper into the canopy.

Using the right adjuvant can help improve herbicide performance and protect your investment.

3. Watch for Palmer amaranth. Palmer amaranth, the “big brother” to common waterhemp, was somewhat quiet in my area last year. However, in October, the Minnesota Department of Agriculture confirmed a new discovery of Palmer amaranth in the west-central portion of the state. This means Palmer has surfaced in four Minnesota counties.

When dealing with Palmer amaranth, the basic principles of weed science don’t change:

• Control the weed with a preemergence or a layby herbicide when it’s still in the soil, has just sprouted from its seed shell and is only a quarter-inch long.

• When the sprout is treated with that herbicide, there is a much higher probability of eradicating it than if you let it emerge.

• If Palmer gets out of control, you are looking at serious yield loss and fighting that seed bank for years to come. Your vigilance will be worth it.

It’s important to note that these Minnesota infestations of Palmer amaranth have only been found in Conservation Reserve Program fields, not in row crop fields. However, this could happen — from contaminated seed mix or equipment purchased from outside the state, for example.

What to do right now
If you haven’t already done so, work with your agronomist to determine which herbicide system fits your comfort level and will help you achieve your return on investment and yield goals. Then, you’ll be able to choose the right residual herbicides, the right adjuvants and the other elements necessary to formulate a comprehensive weed-control program for 2018.

Glady is a regional agronomist with WinField United in west central Minnesota. Contact him at [email protected].