Study Soybean Growth To Build Up Yield

These basic factors come into play as you adjust planting date and other management practices. ( Darrell Smith )

From the coffee shop to the elevator, the talk these days is about high soybean yields. And for good reason: “We are seeing higher yields in fields where we have never seen them before,” says Farm Journal Field Agronomist Ken Ferrie. “They are not coming from every field, but overall farm yields have bumped upward.”

Some of the reasons for higher soybean yields, Ferrie says, include climate change, technology, earlier planting and plant genetics.

Grant Noland has noticed the trend toward higher soybean yields on his Blue Mound, Ill., farm. “In the past four years, we have taken a much more in-depth approach to soybean production, and our farm’s yields have continued to rise,” he says.

Drawing upon advice from BASF Technical Marketing Agronomist AJ Woodyard, “who opened our eyes to high-yield soybean production,” Noland now plants soybeans earlier and pays close attention to variety selection, plant population and fungicide use.

New practices must make sense economically, Noland points out: “Higher yields are important, but we remain focused on revenue.”

Because many aspects of management might change as you transition into high-yield soybean production, some basic knowledge about soybean culture is required.  

1. Environmental Response

The way soybeans grow and develop is a function of each variety’s response to its environment. If you change the environment, by planting earlier for example, you will change the way the plant develops.

2. Growth Habit

Soybeans are classified by their growth habit. With indeterminate varieties, vegetative growth continues after flowering. Most varieties grown in the Corn Belt are indeterminate. Determinate varieties, grown mainly in the South, cease vegetative growth when flowering begins.  

3. Maturity Group

Maturity ranges are based on a variety’s ability to use the growing season in a given climatic zone. Maturity groups span from Group 00 in northern Minnesota to Group 9 in Florida. Groups 00 to 4 are indeterminate. Groups 5 to 8 are determinate. Maturity ranges are east-west belts about 100 to 150 miles deep.

“What places a soybean in a maturity group is when it initiates the reproductive stage,” Ferrie explains. “This is triggered by night length. There’s quite a difference in night length from Minnesota to Florida, so we need maturities for each zone.”

4. Internal Clock

Soybean plants have an internal clock that tells them when to begin flowering. The plant’s time-keeping system is triggered by sunlight and darkness, which causes the phytochrome protein in the leaves to switch from active to inactive status.  

“In a fully mature trifoliate leaf set, the phytochrome protein is converted to the active form when sunlight hits the leaves,” Ferrie says. “The longer sunlight hits the leaves, the more protein is converted from the inactive to the active form. After the sun goes down at night, the protein begins to convert back to the inactive form. When the level of inactive protein in the trifoliate gets high enough, flowering begins.

“This is true for both indeterminate and determinate soybeans. The level of protein that has to convert from active to inactive varies by maturity group,” he adds.

The amount of active protein that needs to be converted to inactive is much higher for Groups 5 through 8. As a result, they need longer nights to get enough active protein converted to inactive.

“Northern (shorter-season) soybeans trigger flowering with a lower level of inactive protein in the leaf — in other words, flowering is triggered by shorter nights,” Ferrie explains. “They don’t have to convert as much protein from active to inactive. So flowering is triggered by the hours of darkness in each zone.”

There’s a big difference in the hours of sunlight and darkness as you move from north to south. “In St. Paul, Minn., on June 20, there are 8 hours and 23 minutes of darkness. But in New Orleans, there are 9 hours and 54 minutes of darkness.”

5. Growth Stages

Soybean development is described as growth stages. “As plants add nodes, they add vegetative stages, described as V1, V2, etc.,” Ferrie says “When they begin flowering, determinate soybeans stop adding vegetative stages and enter the reproductive stage — R1, R2 and so on. If a farmer says his determinate soybeans are at V10, I know those plants have 10 nodes but no flowers. But indeterminate varieties can add both stages at the same time — so an indeterminate soybean might be at V6/R2 or V10/R2.”  

For 10 days after germination, food stored in the cotyledons feeds the plant. Then photosynthesis takes over. Unlike corn, the growing point is above ground, so the plant starts responding to air temperature shortly after emergence. But the cotyledons usually don’t supply nitrogen until the V3 or V4 stage.

Axillary buds develop at each leaf axis, including the cotyledon axis. “If axillary buds survive, the plant continues to grow,” Ferrie says. Each node has axillary buds, which are the growing points; the more nodes, the more growing points. Axillary buds might develop into a branch or a flowering cluster and pods, or they might stay dormant.”

Two node stages prior to R1, axillary buds in the top of the stem develop a flower cluster called the raceme. Pods develop from the bottom of the raceme first.

From that point, the reproductive process begins:

 The first flowers signal R1.

“In indeterminate varieties, this begins at the third through sixth node from the top growing point,” Ferrie says, “but only if the night is long enough to trigger flowering.”

At R2, flowers appear on the two uppermost nodes. “An indeterminate plant typically is at 50% of its weight and total nodes,” Ferrie says. “For indeterminate plants, R2 is the start of the rapid growth stage.”

At R5, seeds of the upper four nodes will be about ⅛" in size. “At R5, an indeterminate plant has attained maximum height, node number and leaf area index,” Ferrie says. “Nitrogen fixation drops off rapidly and seed development enters a rapid pace. At R5.5 the nutrients stored throughout the plant start to translocate to the pod.

 At R6, green soybeans fill the pod cavity at one of the uppermost nodes, and root growth stops.

At R7, the pod reaches a mature color, and the seed is 60% moisture but physically mature.
 
 At R8, 95% of pods have mature color. At this point, the plant usually is 10 days from harvest.”

6. Branching

Branching is a function of environment and genetics. “The main growing point on top of the plant controls axillary buds on the stems below,” Ferrie says. “If the main growing point exerts a lot of control, you get a straight-line bean. If it has weak control, you get a bushy bean. When sunlight hits the lower leaves, it puts on pressure to branch. When the rows close, branching slows. Wider rows and lower densities trigger more branching.

“All soybeans will branch when planted in low densities,” Ferrie says. “But bushy varieties do it faster and have more branches than straight-line varieties. In a typical growing season new growth stages appear every five days from emergence to V5 and every three days from V5 up to R5.”

7. More Pods, More Bushels

Soybean yield has three components: total number of pods per acre, number of beans per pod and weight per bean.

“Although beans per pod and bean size can make a significance in yield, the maximum limit for both is genetically determined,” Ferrie says. “So large yield increases result from more pods per acre,” Ferrie says. 

Yield Above Trend

Soybean yields have been above trend every year by an average of 3.7 bu. per acre. To read ag economist Gary Schnitkey’s analysis of yield trends in various states, go to AgWeb.com/soybean-growth.


High-Yield Soybeans Series
Adjusting practices to grow higher soybean yields isn’t simple. There are technology and management practices to master. To help you put everything together, visit www.AgWeb.com/high-yield-soybeans

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