Estimating Soybean Yield Potential

Harvest will be quickly upon us, and yield estimates will be occurring across the countryside. Crop yield potential is normally estimated by determining the number of plants in an area, the number of reproductive units, and the weight of those reproductive units.

Potential Yield = plants/acre x seeds/plant x seed weight

Yield potential estimates for most row crops have three yield components, but how those yield component numbers are determined are different for each crop. Before estimating soybean yield potential, a word of caution; estimating soybean yield potential can be tricky. Soybean yield components can vary greatly by soybean product, different parts of the field, and from plant to plant. It is not advised to conduct a yield estimate during early reproductive stages as the plant has not fully determined the number of seeds that it may produce. Soybean yield potential estimates should only occur after the R6 (full seed) growth stage. Yield estimates should be conducted in at least 5 to 10 random places in the field and these random places should be representative of the plants in that area. By sampling in multiple places, a more uniform yield estimate for the entire field can be obtained.

Step 1: Determine number of plants per acre.

• Take time to calculate the number of harvestable plants per acre, as this number could have changed from when stand counts were determined earlier in the season.
• Determine the row width and measure the corresponding row length to achieve 1/1000^th of an acre.
• Count the number of plants in the row for 1/1000^th of an acre and multiple by 1000, to estimate the number of plants per acre.
  • For example, in 30-inch rows, measure 17 feet 5 inches in row length and count the number of plants in that row. If 114 plants were counted in the row, multiply by 1000 to have an estimated 114,000 harvestable plants per acre.

Step 2: Estimate the number of pods per plant.

• To estimate pods per plant, count the number of pods on each plant for 10 consecutive plants in one row.
• Divide the total number of pods by 10 to determine the average number of pods per plant.
  • For example, if a total of 350 pods were counted, divide by 10 to reach an average of 35 pods per plant.

Step 3: Estimate number of seeds per pod.

• When determining the number of pods per plant, look at those pods from the 10 consecutive plants from Step 2.
• If there is about an even mix of three and two seeded pods, use the general 2.5 seeds per pod value.
  • Typically, a value of 2.5 seeds per pod is used with healthy soybean plants when there is an average growing season. However, in stressful growing conditions, this number may be lower. This is when it would be advised to count the number of three seeded, two seeded, and one seeded pods to determine if a lower value of 2 or 1.5 seeds per pod should be used.

Step 4: Estimate the weight of the seed

• Seed weight is the most difficult yield component to estimate correctly.
• Seed weight can vary by seed product and growing conditions.
  • When there is ample rain in August, larger seeds are typically produced which correspond to larger seed weights.
  • Stressful conditions of hot and dry usually produce plants with small seeds and lower seed weight.
  • A general ‘rule of thumb’ factor for seed weight is 2500 seeds per pound. However, a better indicator is typically what the original seed size was from the seed bag tag at planting. The seed weight factor can be adjusted if the final yield estimate is unusually too high or low.

Step 5: Estimate the final yield (bushel/acre)

Make sure to set up the equation correctly to have the correct units for each portion of the equation. For the equation, the weight of one bushel of soybean (60 pounds) is needed. Using the numbers in the examples above, the hypothetical soybean potential yield estimate from one spot in the field would be:

Potential yield (bu/acre) = (plants/acre) x (pods/plant) x (seeds/pod) ÷ (seeds/lb) ÷ (lb/bu)

Example:
bu/acre = 114,000 plants/acre x 35 pods/plant x 2.5 seeds/pod ÷ 2500 seeds/lb ÷ 60 lbs/bu = 66.5 bu/acre

These steps should be repeated at multiple locations across the field to get a representative average of estimated yield potential for the field. A reminder that yield potential estimates are only as good as the numbers used in the equation. Yield estimates made closer to harvest are more reliable than yield estimates taken earlier in the season when pod and seed development are occurring. Contact your local Channel Seedsmen concerning questions and assistance in estimating soybean yield potential.

Katherine Rod
Agronomist

Source:

Knott, C. and Lee, C. 2018. Cultural practices. Chapter 4. A Comprehensive Guide to Soybean Management in Kentucky. ID-249. University of Kentucky College of Agriculture Food and Environment Cooperative Extension Service.

Source verified 6/22/23. 1110_274488