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How do you manage a weed that is such a prolific seed producer?

I’ve received quite a few phone calls regarding this question lately. In fact, the question of seed production and the weed seed bank raised enough interest that I created a table to examine how to manage the amount of seed a waterhemp produces. The table is included below, and you are welcome to download the spreadsheet and test out the numbers yourself. The numbers discussed below are all derived from the “Waterhemp By the Numbers” table.

The scenario: Let’s say we have a 40-acre soybean field that has one waterhemp per acre that goes to seed. That field would look clean wouldn’t it? With only forty waterhemp plants spread out over 40 acres, that field would be something to be proud of. Let’s say that each one of those waterhemp produces only 200,000 seeds. I say “ONLY” because waterhemp have been documented to produce as many as 300,000 to 500,000 seeds per plant, especially when they have no competition from other waterhemp plants. In fact, Purdue University says without competition, waterhemp plants may produce more than 1 million seeds per plant (https://www.extension.purdue.edu/extmedia/BP/gwc-13.pdf).

We know that not all of that those waterhemp seeds will grow the next year. For easy figuring, let’s say that 25% of them will germinate each year for 4 years after those 40 waterhemp go to seed. The good news on waterhemp (if there is such a thing) is that although waterhemp produces a lot of seed, it only survives in the soil for about 4 years. In contrast, seed from velvetleaf and cocklebur can survive for decades. This means that if we eradicate seed production of waterhemp in a field for 4 years, we can drastically reduce the waterhemp pressure in that field.

The herbicide program: Now let’s decide what kind of herbicide program we are going to run. It has been well documented that waterhemp germinates over a longer part of the growing season than other weeds.

Bob Hartzler, Iowa State University

As you can see, by June 1 nearly all the velvetleaf and woolly cupgrass had germinated, but only about 20 percent of the waterhemp. This means that when waterhemp is the driver weed in a field, it will take at least a 2-pass herbicide program with overlapping residual control to manage this weed.

Let’s say we run a recommended rate of a pre-emergence herbicide program and we put it on two weeks before planting. Because of the rate and the timing, we get 60 percent control of waterhemp. Now, you may think this is low, but remember the chart above, depending on the rate used AND the timing it was applied, 60 percent control of a weed that continues to germinate through June and July is pretty good! In fact, just like my 200,000 seeds produced, I may be too generous with this level of control. Let’s also say we get our post treatment in a timely manner and use another residual and we can achieve 99 percent control of the 40 percent of waterhemp that survive the pre-emergence herbicide treatment.

Crunching waterhemp seed numbers: The numbers above are what I put into the “Waterhemp By the Numbers” table. Here is what it calculates for waterhemp seed the first year after the 40 waterhemp go to seed:

Waterhemp Escapes 40 plants
Seeds Per Escape 200,000
Total Seeds Produced 8,000,000
% Germinating Year 1 25%
# of Waterhemp to Control 2,000,000
% Pre-Emergent Control 60%
# of Waterhemp to Control 800,000
% Post-Emergent Control 99%
# of Waterhemp Escapes 8,000
# of Waterhemp Per Acre 200

As you can see, our waterhemp pressure has risen from just one waterhemp per acre to 200 per acre, even with 60 percent control from the pre-emergence and 99 percent from the post treatment!

What about year 2? We start with the 200 waterhemp plants PLUS we must add back in 25 percent that are germinating from year one and on we go. Feel free to open the attached spreadsheet to see the details.

Bottom line is that even with 60 percent control from the pre-emergence herbicide treatment and 99 percent from the post emergence treatment, at the end of three years from when the original one waterhemp per acre went to seed, we end up with over eight million waterhemp seeds per acre! From one to eight million in just four years. Incredible!
Summary and best solutions: You may be thinking that all hope is lost. Waterhemp is going to take over the world and there is nothing we can do about it! Well, it’s not quite that bad. Now that we have examined these numbers, let’s talk about the best solutions. Remember that the amount of seed produced is waterhemp’ s survival strength, but those seeds only survive about four years in the soil, so here are my thoughts (based on the results from the spreadsheet) on the best ways to manage this weed.

1. First, those 40 waterhemp in year one should have never been allowed to go to seed. If you see scattered waterhemp going to seed in your soybeans, go pull them! With just a couple hours’ worth of hand-rogueing work, those 8,000,000 seeds produced by those 40 waterhemp could have been eliminated. Remember, it is NOT just one waterhemp plant you are pulling, it is 200,000+ potential waterhemp you are pulling!
2. We should be running higher rates of pre-emergence products to get that 60 percent control up to +80 percent control. This way there are significantly less waterhemp to control post-emergence. A 99 percent control rate of a smaller number of escapes is a smaller number! I encourage you go back to the attached spreadsheet and raise the percentage control in the pre-emergence treatment up to 80 percent. In the scenario given, raising the pre-emergence control from 60 percent to 80 percent lowers the potential waterhemp population from 8M per acre to 1M per acre. This is still a lot of waterhemp, but it represents an 87.5 percent reduction. It makes a huge difference.
3. Growers need to utilize multiple modes of action and overlapping residuals to make sure we don’t lose any more to resistance and to control those waterhemp that might contain multiple resistance mechanisms.