Narrow Rows are a Weed Control Win, New Study Says

If you can’t beat them, crowd them out! That’s the wisdom from a new meta-analysis on the effect of narrow rows in corn and soybeans on weed control. 

Narrow-row soybeans can canopy faster, squeezing out light and resources for weedy invaders like this barnyardgrass plant. (Photo credit: Muthu Bagavathiannan, TAMU)

The study, authored by a multi-state team of researchers led by University of Nebraska weed scientist Dr. Amit Jhala and graduate student Mandeep Singh, examined 35 studies from 12 states between 1961 and 2018. 

The researchers came to a pretty clear conclusion: Narrow rows (fewer than 30-inches wide) suppressed weed density, size (biomass), and weed seed production, and bumped yields up, but only in soybeans. 

“Overall results suggest that narrow row spacing can potentially be used as an integrated weed management tool in combination with herbicides in soybean for the management of herbicide-resistant weeds,” the researchers concluded. 

Small Row Space, Big Weed Suppression

The study’s results are largely focused on soybeans, which accounted for nearly 80% of the studies the researchers examined. 

In narrow rows, soybeans canopy faster, stealing precious sunlight from their weedy interlopers between the rows. So while it’s not terribly surprising that the practice would suppress weeds overall, the numbers the researchers uncovered were eye-opening. 

Within narrow-row soybean fields, on average, the density of weed populations thinned out by up to 42%, and weed size (or biomass) shrank up to 71%. Weed seed production also fell by 45% on average. Fields with multiple applications of herbicides and 7.5-inch rows showed the most statistically significant drops in weed seed production, the researchers noted. 

Soybean yield also fared well in narrow rows, with the researchers finding an average increase of 12% across the studies they examined.  

Narrow rows can also help buy time in the spring to manage emerged weeds. Past research has found that the “critical time for weed control,” meaning the time when weeds can be removed before they affect yield, comes earlier in wide-row bean fields, around the V1 growth stage. 

In contrast, in narrow-row fields, farmers had until the V2-to-V3 growth stage to control weeds with herbicide passes before the yield loss became permanent.

(Worried about soybean rows increasing your risk of white mold? See why these Wisconsin researchers think you can avoid that obstacle.) 

Narrow rows pair well with other IWM tactics, such as cover crop residue. (Photo credit: Teala Ficks)

Fewer solid conclusions could be drawn for corn, in large part because the researchers only found six studies that examined the effect of narrow row spacing in cornfields. The difference between row spacing in those selected corn studies was also less pronounced than in the soybean studies, where row space options ranged from 30 inches to as low as 7.5 inches. As a result, meaningful differences in light interception between the smaller range of corn row spacing likely wasn’t observed, the researchers noted. 

You can read their full study here.

 For more information on using row spacing for weed management, see this GROW webpage and these GROW news articles.

Article by Emily Unglesbee, GROW; header photo by Bruce Dupree, Alabama Extension