Dr. Dan Talks Agronomy

Secret Soybean Revelations

Dan Davidson
By  Daniel Davidson , DTN Contributing Agronomist
Continuing to urge more production from each soybean acre is Fred Below's mission. (DTN photo by Pamela Smith)

Soybean yield has been increasing the last few years because growers finally realized the crop needs management and investment to produce more.

Fred Below's efforts to find the secret ingredients to better yields still tell the story best. The University of Illinois crop physiologist started this project in 2012 with funding from the Illinois Soybean Association and other commercial partners, and the work to identify the factors that influence yield continues. New information is being teased out of this important project each year.

Below calls the new philosophy regarding soybean management "intelligent intensification."

During the first two years, Below and his graduate students pinpointed six major factors that influence yield: Weather, fertilizer, genetics, foliar protection, seed treatment and row spacing.

Weather continues to be the defining factor. However, Below said the other prerequisites for high yields are planting early into good soil conditions, having the right soil pH, good drainage and protection against weed pressure. "Growers should always plant early, but the success of planting date is determined by weather, which is beyond growers' control," he added.

Their omission plot studies in 2012 and 2013 found that fertilizer produced a 3 to 3.5 bushel per acre (bpa) yield gain. Foliar protection (fungicide and insecticide combined) produced a gain of 3 bpa. Seed treatments and narrow rows provided 1 to 2 bpa gains.

Below has found a 3 bpa average yield increase when planting a fuller-season variety. He recognizes growers plant a range of maturities to manage weather risk and spread out harvest, but emphasizes that fuller-maturity varieties are more responsive to high management.

In 2014 and 2015, the project focused on fertility and foliar protection. The practice of fertilizing for two crops and applying it before corn has been popular. Those that fertilize before soybeans tend to focus on applying potash.

Below considers both to be poor decisions. "Soybeans require as much potash as corn and actually remove more than corn. This probably leads to the idea that potash is more limiting in soybeans than corn." Below said potassium in residual corn stover easily leaches out over the winter and supplies the needs of the following soybean plant.

Below's research shows potash isn't typically a limiting factor in soybeans. A corn crop that averages 230 bpa requires 180 pounds per acre of potash. The harvested grain removes 56 pounds and leaves 124 pounds behind in residue. A soybean crop averaging 60 bpa, requires 170 pounds per acre of potash; removes 70 pounds and leaves 100 pounds behind. In each crop, about two-thirds of what is applied is returned back in residue to be used by the next crop. Potash is soluble and leaches out of residue to go back into the soil.

A 60 bpa bean crop requires 170 pounds of potash per acre. If a grower applies 100 pounds as commercial fertilizer in a two-year rotation before corn and the corn (stover) residue leaves behind 124 pounds, there is enough potash to meet the needs of the subsequent soybean crop. Potash deficiencies are unlikely in this scenario.

"Based on the way growers fertilize, phosphate is the more limiting nutrient," Below said. Corn (230 bpa) and soybeans (60 bpa) require 101 pounds and 43 pounds per acre of phosphate, respectively. The harvested grain removes from the soil 80 pounds per acre in a corn crop and 35 pounds per acre for soybeans. Corn and soybeans leave behind only 21 and 8 pounds, respectively, in the residue. Most of the phosphate left behind is tied up soil organic matter and not readily available to the soybean crop. A grower applying only 90 pounds of phosphate on a two-year fertilizer rotation is already short 11 pounds for corn and not leaving much for the next year's soybean crop.

Below's team has also retested the secrets doing omission plots and discovered that the addition of phosphate fertilizer banded below the row gave a 5 to 6 bpa yield increase. Potash alone did not produce a yield response. Phosphate and potash combined led to a 5 bpa response. The addition of foliar protection provided a 3 bpa bushel gain as did the initial studies, and seed treatments again showed a 1 to 2 bpa boost.

Row-spacing testing showed 20-inch rows more responsive to high management than 30-inch rows. In the low-input plots with only narrow rows (20-inch) being added, there was a yield bump of 3.6 bpa. However, in the high-input system with phosphate, potash, foliar protection and seed treatments, there was a 7.6 bpa gain.

"I believe the future of corn production is 20 inches," Below said. "So soybeans will also be planted at 20 inches. And we have discovered that soybeans planted in narrow rows are more responsive to management."

Not all soybean varieties are created equal and will respond differently to stress and management. Below evaluated 18 commercial varieties and discovered a 22.6-bushel difference in yield. Moving forward, he wants to look more closely at variety and maturity groups and how they respond to intensive management.

Dan Davidson can be reached at AskDrDan@dtn.com

Follow Dan Davidson on Twitter @dandavidsondtn

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Dan Davidson