Farm Progress

Narrow-row beans can help shade out light-competitive weeds like Palmer, but don’t skimp on herbicide program.

Tyler Harris, Editor

July 27, 2018

3 Min Read
GOING NARROW: In Carleton, Neb., soybeans are planted in 15-inch rows.

The fight against glyphosate-resistant Palmer amaranth has many growers rethinking their herbicide programs, but recent research in south-central Nebraska suggests row spacing in soybeans might also be worth a second look.

Parminder Chahal, postdoctoral weed science student at the University of Nebraska-Lincoln, began a study comparing 15-inch rows and 30-inch rows in rainfed soybeans this year. Both row spacings were planted with the same population, 140,000 seeds per acre, using multiple passes of a 30-inch planter to plant 15-inch rows.

“Compared to corn, you will see more Palmer emergence in soybean. The No. 1 reason is light, because the corn crop can close the canopy really fast compared to soybean. Palmer amaranth, in particular, can trap a lot of sunlight compared to other weeds. When you have narrow-row spacing, there is earlier canopy closure, and Palmer will remain shorter compared to 30-inch spacing where it can catch more sunlight,” Chahal says. “With earlier canopy closure, there’s also less chance for germination and emergence of Palmer amaranth [later in the season].”

That’s especially true for Palmer amaranth, which can germinate later into the season than many weeds. One female Palmer plant can produce as many as 600,000 seeds, and intercepting light by closing the canopy earlier can keep these Palmer plants from competing for light and reproducing.

This is only the first year of the study. But at a recent field day at the plots near Carleton, where glyphosate and atrazine-resistant Palmer amaranth dominate, Chahal discussed his observations so far.

It’s not just about row spacing. Chahal also compares different herbicide programs, including some with a preemergent application, postemergent application and overlapping residuals, as well as those that only have several post-applications. While 15-inch rows played a role in knocking back Palmer amaranth at the site, herbicide programs made a big difference, too.

“For example, one of our herbicide programs had a preemergence herbicide and multiple modes of action in post-herbicides. There, our herbicide program was really strong, and you could see almost no weeds in that plot,” Chahal says. “In that case, it might be difficult to compare whether narrow rows are working. In most programs, however, we had no pre.”

In plots with no herbicide applications and in plots with only a single post application, Chahal says, the benefits of narrow rows were more apparent, helping to close the canopy quicker, shading the ground, keeping sunlight from hitting the weed seed bank and helping with weed control earlier in the season. That’s especially the case for a weed like Palmer amaranth.

“In this field, the main weed is Palmer amaranth, and it can emerge throughout the season. So, in plots with only a postemergence application, narrow-row spacing helped quite a bit, especially when you apply a post-herbicide, and that herbicide has only burndown activity,” he says.

“When growers apply postemergence herbicides, sometimes they tank-mix a residual with a burndown herbicide so they can get better control later in the growing season,” he says. “The problem is the soil-residual herbicide needs some rain for activation. Suppose you apply and there isn’t enough moisture to activate them. In that case, if you have narrower rows, it would help.”

However, there’s still the challenge of being properly equipped for narrow rows, and not all farmers are able to make the investment in a narrow-row planter. Others may be afraid of running over beans with a pull-type sprayer in narrower rows, Chahal notes.

The bottom line is narrow rows can be part of an effective weed management program. By themselves, narrow rows won’t win the fight against resistant weeds like Palmer amaranth; it takes an integrated approach of “many little hammers,” including a preemergence and postemergence program, overlapping residuals and multiple modes of action. “The message is you can’t just depend on one technology for weed control,” Chahal says.


About the Author(s)

Tyler Harris

Editor, Wallaces Farmer

Tyler Harris is the editor for Wallaces Farmer. He started at Farm Progress as a field editor, covering Missouri, Kansas and Iowa. Before joining Farm Progress, Tyler got his feet wet covering agriculture and rural issues while attending the University of Iowa, taking any chance he could to get outside the city limits and get on to the farm. This included working for Kalona News, south of Iowa City in the town of Kalona, followed by an internship at Wallaces Farmer in Des Moines after graduation.

Coming from a farm family in southwest Iowa, Tyler is largely interested in how issues impact people at the producer level. True to the reason he started reporting, he loves getting out of town and meeting with producers on the farm, which also gives him a firsthand look at how agriculture and urban interact.

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