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Early weed pressure costs yieldEarly weed pressure costs yield

Soybean Watch: Side-by-side comparison shows that plants growing among weeds produce less yield.

Tom J. Bechman

July 21, 2023

2 Min Read
A man holding soybean plants in each hand while standing in a young soybean field
NO COMPARISON: Note the extra nodes on the plant on the left, which grew without weeds nearby. More nodes mean more beans and more yield, agronomist Steve Gauck says. Photos by Tom J. Bechman

Nobody likes thistle patches. Yet getting rid of them is easier said than done. And the longer thistles grow before they’re knocked back with herbicide, the more yield you stand to lose. That’s why Steve Gauck believes in controlling all weeds early in soybeans, not just thistles.

Studies have been done on how much early competition from various weeds can impact soybean yield, says Gauck, a regional agronomy manager for Beck’s, based near Greensburg, Ind. Beck’s sponsors Soybean Watch ’23. The more weeds per foot of row, the greater the expected yield loss, based on these studies.

“You can see it by examining plants that grow with weed competition, even for the first few weeks,” Gauck says. “Plants tend to be spindly, because they’re trying to outcompete the weed next to it for sunlight. Weeds like thistles, which grow in dense patches, make it even tougher for small soybean seedlings to compete.”

Plant-to-plant comparison

Gauck puts two soybean plants side by side. One grew in a thistle patch for the first five weeks after emergence, while the other grew in a weed-free area. The differences are striking.

Growth stage. The plant not hindered by weed competition got off to a better start and was further along in development than the plant growing with weeds around it. Plants that see a faster start can usually make fuller use of the growing season, Gauck says.

Related:Plant comparison shows what soil compaction can do

Number of nodes. The plant with only soybeans for neighbors had several more nodes up and down the stem than the plant growing in a thistle patch. “More nodes mean there are more fruiting structures and more places for pods,” Gauck explains. “High yields are based upon maximizing number of nodes and pods per acre.”

man bending over, checking young soybean plants growing in a field next to a patch of thistles

Closeness of nodes. “Perhaps the most striking difference was that there was a node about every inch of stem on the normal plant,” Gauck says. “On the plant stretching up for sunlight, nodes were 2 or more inches apart. The net results will be fewer nodes overall when the plant finishes growing. Fewer nodes mean fewer pods, fewer beans and less yield.”

Limited chance for recovery. There is no way to get the nodes back that “might have been” in the lower part of the plant growing with weeds, Gauck says. “Once a plant becomes spindly and only puts on three or four nodes on the bottom 6 to 8 inches of the plant, there is no mechanism for adding back those nodes later,” he explains. “The plant may recover somewhat if weeds are removed, but the damage to yield potential is already done.”

About the Author(s)

Tom J. Bechman

Editor, Indiana Prairie Farmer, Farm Progress

Tom J. Bechman is editor of Indiana Prairie Farmer. 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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