We are rapidly approaching the end of the traditional time for weed control in 2010. When should you begin thinking about controlling weeds for next year?

Palmer amaranth

Depending on the weed problem, some burndown applications for 2011 will be made this fall, but that is a discussion for another day. Spring burndown has been moving up into early February and even January over the last few years.

But, depending on the weather, most growers will not be concerned about summer annual weeds like Palmer amaranth or barnyardgrass until next April or May. Weed scientists in the Mid-South will tell you that you need to consider weed control for 2011 right now.

Whether it is just university researchers discussing a new topic or a reaction to the current glyphosate resistance crisis, there has been considerable talk over the last year about weed seed production and managing the weed soil seed bank.

I had been thinking about this concept for a while when I heard (Delta Farm Press contributor) Ford Baldwin use the term “whole farm weed management” at a meeting last February. Ford suggested that you have to consider next year’s crop when controlling weeds this year. I think his idea best describes the direction we need to be headed for weed control in the future.

The weeds that are not controlled in the 2010 crop will produce the seeds that will germinate and grow into the weed problems encountered in the 2011 crop. We have all walked through relatively clean fields in the spring and observed small, dense patches of weeds like the pitted morningglory shown in the accompanying picture.

I took the picture of pitted morningglory in a corn field at Stoneville, Miss. Very few morningglories were present in other areas of the field except for this one patch that was about 10 feet square. These morningglory seedlings likely emerged where a single morningglory went uncontrolled last year.

The picture of Palmer amaranth was taken this spring in a cotton plot that received only one glyphosate application in 2009. There were very few Palmer amaranth plants in the plot in 2009, but in 2010 it’s a train wreck because the surviving Palmer amaranth plants from last year were allowed to produce seed.

A single female Palmer amaranth (Palmer amaranth produces male and female plants, but only the female plants produce seed) plant can produce from 200,000 to 600,000 seed.

If you make a conservative assumption that one Palmer amaranth female only produces 100,000 seed one year, 50 percent of those seed are viable and germinate the following year, and you control 99 percent of those new plants, there are still 500 Palmer amaranth plants to deal with as descendents of that single surviving plant from the previous year.

Now amplify that over an acre while assuming the same seed production per plant, seed germination rate, and weed control percentage. On 40-inch cotton rows, if one female Palmer amaranth plant survives every 150 feet of row, then there is potential to have one Palmer amaranth per square foot the following year.

One plant every 150 feet may not look too bad driving down the highway, but one Palmer amaranth per square foot is catastrophic. Even with this being a conservative scenario, it is obvious how quickly we can get behind on weed control.

Pitted morningglories

If the Palmer amaranth in the previous example is glyphosate-resistant, then the repercussions of allowing a few plants to survive are disastrous.

Barnyardgrass is another weed (although not yet identified as resistant to glyphosate) that should be a top priority in whole farm weed management. In a soybean-rice rotation, barnyardgrass left uncontrolled in the soybean year will produce seed to cause serious problems in the following year’s rice crop.

Up to this point, glyphosate has been an excellent grass herbicide, so grass control in Roundup Ready crops has not been a major concern. However, barnyardgrass is the most troublesome weed in rice, just like glyphosate-resistant Palmer amaranth is the most troublesome weed in cotton or soybeans.

Barnyardgrass may survive a glyphosate application in soybeans because it was too big at the time of the glyphosate application, because the soybeans never fully covered the row middles and barnyardgrass emerged late in the season, or it may emerge in late summer following harvest of early soybeans. Regardless, barnyardgrass allowed to survive and produce seed in the soybean rotation year can be crippling to a rice weed control program.

So, weed control in next year’s rice crop should begin during this year’s soybean crop.

In 2008, we collected data on late-season barnyardgrass emergence. In plots that received three glyphosate-only applications, barnyardgrass density was seven plants per square foot in August. Where Dual Magnum was added to the second glyphosate application, barnyardgrass density was less than one plant per square foot in August.

Although these numbers were collected in a cotton study, the data speaks to the value of a residual herbicide with glyphosate. Fewer surviving or late-emerging barnyardgrass plants means less seed production, which in turn could impact herbicide applications in next year’s rice crop.

Although it is July, there are still many fields throughout the Delta where the crop is small enough to make one more trip. Even if you are not in an area with glyphosate-resistant weeds, give careful consideration to what may be planted in your fields next year, and decide if a few more dollars this year could seriously benefit your bottom line next year.

A layby application of a residual herbicide just before canopy closure could go a long ways toward reducing weed seed production.

Weed control does not exist in a vacuum. Weeds have tremendous capacity to survive and produce seed. That is part of what makes them weeds. If we get lax in our management for just one year, we may not ever be able to catch up.

e-mail: [email protected]