As soybean (Glycine max) plants reach full seed (R6) growth stage, flowering ceases, pods have developed, and seeds are filling pods throughout the plant. Favorable growing conditions during seed fill will not increase the number of seeds in advanced pods; however, stressful growing conditions can affect seed development and reduce the number of seeds in a pod. As seeds and pods begin to mature at R7 growth stage, stress has little effect on yield potential; however, if pods drop to the ground or shattering occurs, fewer bushels are likely to be harvested.
Four essential factors (plants per acre, pods per plant, seeds per pod, and seeds per pound) are used to calculate estimated potential yield. Determining plants/acre can be accomplished by a couple of methods, the 1000th acre and the hoop (Figure 1). These methods, particularly the hoop method, are best conducted in the spring when plants are smaller and a record kept for use in the fall. Regardless of method, several counts should be obtained to determine a final average plants per acre count.
The 1/1000th acre method determines plants/acre by counting the number of plants in 1/1000th of an acre (Table 1) and multiplying that number by 1000. The hoop method, best utilized with drilled soybean fields involves determining the diameter of a hoop, tossing it randomly in the field, and counting the number of plants inside the hoop (difficult if done on nearly mature plants). Multiply the number of plants within the hoop by the appropriate factor in Table 2 to determine the number of plants/acre. If making a hoop, an appropriate diameter is 28.25 inches which allows for multiplying by 10,000. The hoop can be made by cutting a tube such as anhydrous tubing to a length of 88.75 inches and adjoining the ends.
The average number of pods per plant can be obtained by counting the number of pods with at least one seed on 10 consecutive plants. (don’t skip small plants).1 Divide by 10 to get the average pod count for that location. Example: At one location, a total of 240 pods were counted on 10 consecutive plants for an average of 24 pods per plant.
The average seeds per pod can be calculated by selecting 10 random pods and counting the seeds in each. Dividing the total number of seeds provides the average per pod for that location. Healthy plants can average about 2.5 seeds per pod while those under stress may decrease to 2.0, 1.5, or fewer per pod.
Example: 5 pods with 4 and 5 pods with 3 seeds, respectively, would be 20 + 15 ÷ 10 or 3.5 seeds per pod.
The seeds per pound calculation can be challenging. Some research indicates a value of 2,500 seeds per pound is a good average; however, other locations report an average of 3,400 seeds per pound should be used.1 Stressed soybean plants may have smaller seed; therefore, a higher seeds per pound number should be used when calculating yield potential. Original seed size from a seed bag may provide a reasonable indication of seed size to use for the calculation. When the seed tag is not available, 2,500 seeds per pound should be used.
To estimate average yield in bu/acre, the following formula, which uses a standard factor of 1 bushel of soybean seed weighing 60 lb/bu, can be used:
((plants/acre x pods/plant x seeds/pod) ÷ seeds/lb)) ÷ lb/bu = average bu/acre
Example: (134,000 plants/acre x 24 pods/plant x 2.8 seeds/lb) ÷ 2,500 seeds/lb) ÷ 60 lb/bu = 60 bu/acre average.
Adding the average yields from each field location and dividing by the number of locations provides the overall estimated average yield/acre for the entire field.
Lee, C. and Herbek, J. 2005. Estimating soybean yield. AGR-188. University of Kentucky. http://www2.ca.uky.edu/ 22013.
Corn Field Guide, 2nd edition. Iowa State University. 32014.
Corn & Soybean Field Guide. ID-179. Purdue University Extension. Additional
Casteel, S. 2012. Estimating soybean yields—simplified. Purdue University. https://www.agry.purdue.edu/
Ciampitti, I. 2014. Estimating soybean yield potential: There is a web-based app for this also! K-State Agronomy eUpdates. Kansas State University. https://webapp.agron.ksu.edu.
Lindsey, L. 2013. Estimating soybean yield. C.O.R.N. Newsletter 2013-28. The Ohio State University. https://agcrops.osu.edu/newsletter/corn-newsletter Flanary, W. 2012.
Estimating soybean yields prior to harvest. University of Missouri Extension. http://extension.missouri.edu/ Pedersen, P. 2014. Soybean growth and development. PM 1945. Iowa State University.
Web sources verified 08/08/18. 130906033002