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Studies prove shade limits soybean yields

Shading soybeans at the wrong time hurts yields, according to various sources across the Midwest.

Tom J. Bechman, Midwest Crops Editor

October 10, 2024

4 Min Read
Jason Webster standing in a soybean field next to a cornfield
MORE LIGHT: Agronomist Jason Webster tries to lessen soybean yield decreases in corn and soybean strips by seeking ways to help soybeans best use light. That includes varying variety maturity.Photos by Tom J. Bechman

At a Glance

  • Various studies indicate that too much shading limits soybean yields.
  • Planting corn and soybeans in strips helps corn but hurts soybeans.
  • Finding ways to decrease soybean loss to shading uncovers answers.

Soybeans need sunlight to grow and produce abundantly. What happens when they don’t get as much sunlight as they need at key times?

Some inquisitive farmers in southern Indiana wanted to find out. So, they built a movable shade and positioned it over soybeans at different stages during the growing season. The result? Soybeans were impacted during the rest of the season, and yield suffered. The amount of yield loss depended upon when and how long soybeans were shaded.

University of Minnesota Extension agronomist Seth Naeve and co-workers shaded soybeans at key times as well in Minnesota. Naeve was looking for potential causes of green stem syndrome, which results in green stems with brown pods and mature soybeans inside at harvest.

Sure enough, he found that when soybeans were shaded and light was limited at certain times, more green stems showed up at harvest. In fact, shading earlier in the season than he expected produced an effect.

“We’re seeing it for shading even at R3, which is earlier than we might have anticipated,” Naeve says. “Right now, it appears that any stress which occurs from R2 through R7 could increase the percentage of green stems.”

Unlikely source

Perhaps one of the biggest indicators of just how much soybeans depend upon sunlight comes from someone who originally set out to learn more about corn, not soybeans. Jason Webster, lead commercial agronomist at the Precision Technology Institute, also known as the PTI Farm, near Pontiac, Ill., manages over 400 acres of research trials each year, attempting to determine management practices that provide farmers with maximum return on investment.

Related:Why can’t I grow 600-bushel corn?

“We became interested in planting alternating strips of corn and soybeans because we believed we could grow more corn per acre if more rows behaved like outside rows,” Webster explains. “Many people have shown that outside corn rows can yield more if treated properly because those rows receive more sunlight than inside rows. Going for maximum yield is all about harvesting more sunlight.”

After completing a three-year trial, Webster concluded convincingly that yes, growing corn in alternating strips increases corn yields — the narrower the strips, the better for corn. In the process, however, he also proved a negative. Growing soybeans in alternating strips decreases soybean yields.

“It’s all about shading of soybeans by corn,” Webster says. “But what we found was that when we put dollar signs to it, increases in income from extra corn yield more than offset decreases in soybean yield.”

So, no, Webster isn’t discontinuing his corn and soybean strip-cropping research. Instead, he is intensifying his efforts.

Chasing better bean yields

Figuring that it was all about sunlight, Webster set out to improve soybean yields in strips. Or to put it conversely, he sought ways to decrease the amount of yield loss on soybeans inflicted by taller corn strips.

“The obvious thing to try was comparing rows planted east to west to rows planted north to south,” he explains. “When the sun comes up in the east and travels toward the west all day, it makes sense that soybeans in north-to-south rows would be shaded more by shadows from corn, both in the morning and in the evening. If you plant east to west, there are far less problems with shadows and shading.”

The theory sounded good. Would it work? In fact, it did. Webster has seen significant increases in soybean yields by simply changing row direction. Yields are still well below block soybean yields without interference from corn, but they are more than if soybeans are planted north to south.

Now, Webster always makes sure to plant strip-cropping fields east to west, if possible. The trade-off is less advantage from the outside-row effect in corn when rows go east to west instead of north to south.

Jason Webster points to a graph on a sign during a presentation

The bottom line, however, is that strip planting is most profitable as a system, counting both corn and soybeans, when planted in east-to-west rows. Webster notes that net revenue in 2023 for 20-foot-wide strips in that system was $112 per acre higher compared to traditional rotations.

Seeking more sunlight

Webster isn’t done looking for ways to help soybeans capture more sunlight in strips to lessen yield decreases. This year, he has some corn and soybeans in 10-foot-wide alternating strips — or four 30-inch-row corn rows and then four 30-inch-row soybean rows.

“We tried an earlier-maturing soybean on each side of the strip next to corn, hoping they might determine yield while shading was less of a problem,” Webster says. “We went with a later-maturity variety, around a Group 4.0, in the middle two rows, hoping they could take advantage of more sunlight.”

Time will tell if these adjustments help. Webster also expects to move to 15-inch rows soon. “Whatever we can do to get more sunlight to these soybeans will help,” he concludes.

About the Author

Tom J. Bechman

Midwest Crops Editor, Farm Progress

Tom J. Bechman became the Midwest Crops editor at Farm Progress in 2024 after serving as editor of Indiana Prairie Farmer for 23 years. He joined Farm Progress in 1981 as a field editor, first writing stories to help farmers adjust to a difficult harvest after a tough weather year. His goal today is the same — writing stories that help farmers adjust to a changing environment in a profitable manner.

Bechman knows about Indiana agriculture because he grew up on a small dairy farm and worked with young farmers as a vocational agriculture teacher and FFA advisor before joining Farm Progress. He works closely with Purdue University specialists, Indiana Farm Bureau and commodity groups to cover cutting-edge issues affecting farmers. He specializes in writing crop stories with a focus on obtaining the highest and most economical yields possible.

Tom and his wife, Carla, have four children: Allison, Ashley, Daniel and Kayla, plus eight grandchildren. They raise produce for the food pantry and house 4-H animals for the grandkids on their small acreage near Franklin, Ind.

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