Leaf area may hint at soybean yields

Traditional recommendations call for growers to put their best efforts into soils that have high potential. David Holshouser is finding, however, the parameters for potential sometimes go against traditional common sense.

Holshouser, Virginia Tech Extension soybean specialist, and his colleagues are using soybean leaf area index to measure yield potential. In the final year of a four-year study, Holshouser is finding that leaf area often plays a major role in determining yield potential.

Using GPS technology and aerial photography to document the leaf area within a soybean field, Holshouser thinks his group may be onto something that can help farmers bring consistency to soybean yields.

The research has already led him to unconventional thoughts such as increasing seed count on less-productive land and changing the way growers think about defoliation thresholds before treating for insects.

The idea for the research came in 1997 from a comment by Jim Dunphy, North Carolina State University’s soybean specialist. Dunphy noted the variability of his best yields didn’t seem to correlate to planting date, but instead had more to do with how close the plants were to each other and canopy height.

Holshouser’s research, as well as that done by other researchers, indicated it takes 3.5 feet to 4 square feet per square foot of ground at flowering to maximize soybean yield potential. “You may have 40 bushels one year and 20 bushels another year in the same area, but unless you have adequate leaf area, you’re not going to be able to maximize yield,” Holshouser says.

How to document that yield potential has been the focus of Holshouser’s research.

Early into the research, Holshouser discovered that in precision ag applications traditional soil surveys may not be useful. That caused him to look at remote sensing, yield monitors and aerial photography.

Taking off from the Franklin, Va., municipal airport recently, Holshouser was on his stomach peering through a hole in the single-engine plane’s floor, snapping infra-red photographs of soybean fields in southeastern Virginia. In four or five days, he has the film developed and ready to use ground targets to verify the information.

He uses a vegetative index to measure the plant growth and the amount of dry matter on the ground. He’ll then go into the field and compare it to what he sees in the infrared aerial photos. “It’s done a pretty good job predicting yield potential in soybeans in a double-crop situation,” Holshouser says. “The photos will show you an area of the field where the deficits are and other areas that have excessive leaf area.”

He’ll finish up the research portion of project later this summer and move toward implementing it in farmer fields.

Already, the research suggests that in more productive soils, higher plant populations aren’t needed. “That became more important when we started using Roundup Ready soybeans,” he says. “If a farmer can plant 40,000 less seed, he can save money.”

Conversely, increasing seed populations on sandier soils may give farmers the chance to increase yields. Holshouser verified the concept on small plot research.

“When we increase the leaf area and make use of the moisture, then the yield potential is there to take advantage of late-season rains,” Holshouser says. “The plant has the manufacturing ability to get the high yield. If it doesn’t have the plant area, it doesn’t have the potential for yield.”

In his studies, Holshouser has found that it takes about 3.5 to 4 square feet of leaf area per square foot of ground area at flowering to maximize yield potential. Yield flattens after about 4 square feet of leaf area. “Too much leaf area doesn’t hurt yield, but it really doesn’t help. In other words, too much leaf area tends to hurt yield potential.

This precision agriculture application could have application on seeding rates and row spacing, as well as defoliation thresholds, Holshouser says.

“Back when I started with this research, we had a significant amount of defoliation of double-crop soybeans on small plants that had been affected by the drought,” Holshouser says. “Ames Herbert and myself began to notice that defoliation thresholds were really not a good basis for estimating yield without figuring in leaf area.”

A three-year study led them to reduce the defoliation threshold for treatment from 15 percent to 10 percent.

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