Source: Purdue University

By Amanda Gee

WEST LAFAYETTE, Ind., Sept. 23, 2013 — Soybean producers can easily get an idea of the yield potential of their crops with a Purdue Extension soybean specialist's calculation method.

Soybean yield potential is built on many factors, including the genetics selected, management decisions during the season and the weather. Yield components of soybeans are pods, seed size and number of seeds per pod.

"Individual plant production varies, and every field will vary based on pests, soils, fertility and other factors," said Shaun Casteel. "But I've simplified the process of estimating soybean yields so that producers can scout multiple areas quickly while maintaining representative estimates."

Casteel's system is based on estimated yield in one ten-thousandth of an acre. The basic formula involves multiplying the number of pods by the number of seeds per pod, then dividing that result by the seed size factor. That calculation will show the estimated bushels per acre.

To calculate, producers first need to count the number of pods in one ten-thousandth of an acre, an area determined by a 21-inch length of a row of plants and how far apart the rows were planted.

"Nearly 90 percent of our Indiana soybean acres are planted in 30-, 15- or 7.5-inch rows, so just remember that each count needs to be 21 inches long," Casteel said. "You will count the number of pods in one row for 30-inch width, two rows for 15-inch and four rows for 7.5-inch."

Producers should count the number of pods that are at stage R5 or higher - when they can see seeds. 

Next, they must determine the number of seeds per pod. Casteel said using the average of 2.5 seeds per pod is best because there can be a range of 1-4 seeds per pod.

"This value is conservative since we don't know exactly how the rest of the season will finish," Casteel said.

Changing this one value can increase or decrease yield estimates.

The third step is to calculate seed size factor. Casteel said the starting point is seed size factor 18, equaling about 3,000 seeds per pound.

"If you expect larger seeds from late-season rains, you would divide by a lower seed size factor such as 15, which equals about 2,500 seeds per pound," he said. "If the field has late-season stress, such as a lack of water, you would divide by a higher seed size factor like 21, or 3,500 seeds per pound."

For most Indiana fields in 2013, the seed size factor will be between 18 and 21, Casteel said.

The three values – number of pods in stage R5 or higher, number of seeds per pod and seed size factor - go into Casteel's equation. For example, 250 pods times 2.5 seeds per pod divided by a seed size factor 15 equals 41.7 bushels per acre.

Fair soybean growth with limited pod retention but with good late-season moisture will result in a fair crop.  

Although producers can start estimating yields as soybeans enter the R5 stage, the estimates will be more accurate as soybeans develop and enter R6, or full seed. If soybeans are just coming into R6, Casteel said the yield potential still depends on pod retention and seed size.

The weather is an important contributor to yield potential, and dry conditions over the past 4-5 weeks have lowered yield potential in some fields.

"Reductions in excessive heat and the return of rain helped yield potentials more in seed size than pod retention assuming the soybeans are into R6 and beginning to drop leaves," Casteel said. "If fields are green, soybean yield potentials could improve. If fields are losing foliage, yield gains will be very limited."