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Figure 1. Soybean pod and seeds during the full seed (R6) growth stage. Photo courtesy of Palle Pedersen, Iowa State University.
Once soybean plants start the R6 growth stage (green pods with seeds that fill the cavity) or beyond, soybean yield potential can be estimated (Figure 1).1 Yield estimates should be taken at five to ten random representative locations across each field.
Step 1. Count the number of plants in 1/1,000th of an acre (see Table 3), and multiply this number by 1,000 to get plants per acre.
Step 2. Count the pods on each plant for 10 successive plants in a row; do not skip plants that are smaller in size. Calculate the average number of pods per plant.
Step 3. Estimate seeds per pod. Healthy plants average 2.5 seeds per pod. For soybeans under stress, this value may decrease to 2.0, 1.5, or fewer seeds per pod.
Step 4. Estimate seeds per pound, which may be the most challenging part of the yield estimate process. Original seed size from a seed bag can provide a reasonable indication of soybean seed size. When the seed tag is not available, 2,500 seeds per pound should be used.
Step 5. Estimate bushels per acre. One bushel of soybeans weighs 60 pounds. Estimate bushels per acre using the following formula:
(Plants per acre x Pods per plant x Seeds per pod) ÷ Seeds per pound ÷ Pound per bushel = Bushels per acre
Yield estimates made closer to harvest are more likely to represent final yield potential.
Late Soybean Stages of Growth. R5 begins when a seed in a pod at one of the four uppermost nodes is 1/8-inch long. Rapid seed fill during this growth stage requires large amounts of water and nutrients. One half of the nitrogen (N), phosphorous (P), and potassium (K) requirement is redistributed from stored nutrients in the plant’s vegetation and the other half comes from N fixation and nutrient uptake by the roots.
R6 is the full seed growth stage. R6 begins when one of the four uppermost nodes on the main stem has a pod containing a green seed that fills the pod cavity (Figure 1). Seeds have a rapid growth rate through R6.5, peaking at R7. Root growth is complete around R6.5.2
Beginning maturity (R7) is when one pod on the main stem has achieved the brown or tan mature color. The R8 growth stage is when 95% of the pods have achieved their mature color. A general guideline is that 5 to 10 days of good drying weather after R8 is needed for soybean to reach harvest moisture of less than 15%.
The average number of days for each soybean reproductive growth stage is shown in Table 3.
Effects of Stress. Stress during the R4 to R6 growth stages can cause more potential yield reduction than at any other time of the season.2 The most critical period is from late pod formation through early seed fill. Yield reduction at this time is mainly from a decrease in the number of pods per plant and seeds per pod. Young pods and seeds are more prone to abort under stress than older pods and seeds. By the time the seeds and pods begin to mature at R7, stress has little to no effect on yield, unless pods fall to the ground or seeds are lost to shattering.
1 Lee, C. and Herbek, J. 2005. Estimating soybean yield. University of Kentucky Cooperative Extension. 2 Endres, G. and Kandel, H. 2015. Soybean growth and management quick guide. North Dakota State University Extension. Publication Number A1174 http://www.ag.ndsu.edu. Web sources verified 08/05/15. 150806095031