Steve Gauck holds two soybean plants in a field
WEED BATTLE: Steve Gauck holds two soybean plants. He pulled the spindly one on the right from the Canada thistle patch on the far right. The plant on the left grew where there were no weeds.

Proof that weed pressure affects yield

Soybean Watch: It doesn’t take a rocket scientist to figure out which of two plants was growing with thistles as neighbors.

When Steve Gauck walks into a soybean field, he is constantly scouting, constantly looking for points to make. He soon found several of them while scouting the Soybean Watch ’17 field earlier this summer. The field has been hit by a deluge of more than 6 inches of rain in one day, pestered by slugs and affected by weeds, particularly Canada thistles.

Despite all that, Gauck says at this point, there is still good yield potential in the field. A few wet holes are empty, and stands are thinner than one might like in some spots, but soybeans have a tremendous ability to compensate, he says.

“The plants are green, and have been green from the start, despite periods of rain,” says Gauck, sales agronomist for Beck’s, Greensburg, Ind. Beck’s is the sponsor of Soybean Watch ’17.

“Whenever I dig up a plant, I find good nodulation,” he adds. “That was true even on the first visit, when plants were only 3 weeks old. Good nodulation with bacteria inside the nodules capturing nitrogen has no doubt helped the plants stay green and growing.”

What hasn’t helped is weed pressure. Several rainy periods, especially during July, made it impossible to make postemergence herbicide applications on time. Time ran out on the residual herbicide applied with the burndown to provide help on weed control.

The biggest problem was Canada thistles, which were scattered here and there, and just peeking through the rows in many places. In some places, they were already through the rows and towering over the soybeans.

Too many thistles
Gauck walked to one spot where Canada thistles were very thick. Fortunately, the entire field wasn’t infested with thistles that thick.

“I wanted to make a point,” he says. “I pulled one plant several feet away from the thistle patch which wasn’t influenced by weeds. Then I pulled a plant from inside the thistle patch.

“There was no comparison between the two plants. It’s pretty easy to tell the difference.”

The plant competing with thistles was thinner, and had fewer overall leaves and less leaf mass. The plant growing without interference from weeds was producing branches and gearing up to make the best of what has been a less-than-ideal season.

Gauck noted the difference in distance between nodes. Nodes are where each set of leaves originates, and eventually where each set of pods develops.

“The distance between nodes is longer on the plant in the thistle patch because it’s stretching out, seeking sunlight,” Gauck says. “Even if the competition was removed now, it will likely have fewer pods on the plant because there are fewer nodes. Pods develop at the nodes. To get the most pods, you need as many nodes as possible.”

Fall will be a good time to tackle Canada thistles, Gauck says. “It seems like they’ve been on the increase over the past couple of seasons,” he notes. “In a field like this one, fall would be a good time to work on them with herbicides.”

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