By David Karki

This year, we had a slow soybean planting due to high rainfall in June. This was followed by below normal temperature for the whole growing season, which delayed soybean growth and development of this major summer crop. Meteorologists are predicting temperatures in the thirties for most of the northern part of South Dakota (starting the second week of September). This brings up a question about what will this low temperature do to soybeans that are on the field. 

Temperature regarding crop maturity can be broadly divided into two categories: above freezing (> 32F) and below freezing (< 32F) and its effect on the crop can vary according to the growth and development stage. Soybean vegetative growth and early reproductive development occur before July and August, therefore, low temperature is not always an issue. However, crop at later maturity stages always bear a chance of being exposed to low temperature or frost damage. 

Temperatures above freezing will simply delay the crop maturity and harvesting due to the seed moisture content. Pods that are at R6 stage have seed moisture content of about 70% and are ideal moisture content for harvest is below 15%. If the pods are on earlier than R6 stage, low temperatures will hamper the development of the seed and will have significant impact on yield. 

Soybean Reproductive Growth Stages

Stage Identification
R1 First flower: One flower at any node on the plant
R2 Open flower at one of the two uppermost nodes
R3 Pod initiation: Pod 3/16” long at one of the four uppermost nodes
R4 Full Pod: Pod 2 cm (3/4”) long at one of the four uppermost nodes
R5 Seed Initiation: Seed within one of the pods at the four uppermost nodes
R6 Full seed stage: Pod at one of the four uppermost main stem nodes has one seed that has extended to the length and width of the pod
R7 Physiological maturity: Presence of one pod anywhere on the plant having the mature brown color; 50% or more of leaves are yellow
R8 Full maturity: 95% of pods have the mature brown color

The influence of below-freezing temperatures on soybean harvest and yield can depend on growth stage, amount and duration of the frost, and cultivars planted. Light frost for short duration may only affect the top leaves where plant will keep developing seeds. Severe freeze will damage leaves, pods, and stems. Saliba et al. (1982) reported significant yield loss due to freezing temperature on all soybean cultivars at or before R6 stage, however, the effect varied among cultivars beyond R6 stage. One can expect up to 5% to 15% yield loss if the growth stage is between R6 and full maturity. Protein content is not generally affected by freeze but oil content can be slightly reduced if the freeze occurs before R6.