How to estimate the grain yield potential of drought affected soybeans?
By Roger Borges, Professor of Agronomy, University of Wisconsin-Madison, Extension Soybean and Small Grain Specialist , 608-262-7975 rborges@wisc.edu
Estimating yields of soybean fields severely affected by drought stress is a difficult task. The spotty nature of drought-related damage dramatically increases the number of observations needed to obtain reasonably reliable estimates. To estimate soybean yield, we must measure number of plants per acre, measure number of pods per plant, measure number of seeds per pod, and assume seed weight.
1. Count the number of pod-bearing plants in 1/1,000 acre. For 7-inch rows count plants in 75 feet of row, 15-inch rows count 35 feet and 30-inch count 17.5 feet of row. For example: 120,000 pl/ac.
2. Determine pods per plant by counting number of pods containing one or more seeds from 10 plants selected at random. Divide total pod number by 10 to get pods/plant. For example: 20 pods/plant
3. Determine the number of seeds per pod by counting the number of seeds at the previous ten plants and dividing it by the number of pods on those ten plants. For example: 2.0 seeds/pod
4. Assume seed weight. For example: 3,000 seeds/pound or 180,000 seeds/bushel.
Under normal conditions, a pound of soybean equals 2,300 to 2,900 seeds. Drought stress may significantly reduce seed size. Smaller seed size means more seeds per pound or bushel.
Calculate the estimated yield using the formula:
(plants/acre) x (pods/plant) x (seeds/pod) ÷ (seeds/bushel) = bu/acre.
Example: If the number of pod-bearing plants in 1/1,000 acre was 135,000, the average number of pods per plant was 20, the average number of seeds per pod was 2.2, and the number of seeds per bushel is assumed to be 180,000, the estimated yield would be:
135,000 x 20 x 2.2 ÷ 180,000 = 33 bu/acre.
Repeat steps 1-3 for several places in the field to improve the accuracy of the yield estimate.
Reference
http://soybean.agronomy.wisc.edu/publications/98_estimate_yield_aa.htm
