Early Season Damage
Temperatures as low as -2 °C can be tolerated for short periods of time when soybeans are just emerging or at the VC growth stages. Young plants at the VC stage are slightly more frost tolerant compared to soybeans at the V1 or V2 growth stage. Once trifoliate leaves are present plants become more susceptible to temperatures below zero. Plants that are damaged below the cotyledons by early-season frost or hail cannot recover. If frost or hail damages the growing point (apical meristem) of the plant, but not the stem portion below the cotyledons, the plant will send out new shoots. New growth will come from the point where leaves or cotyledons attach to the stem (nodes), see figure 1. Be patient when assessing the extent of damage to a field. It can take up to a week before new growth emerges under cool conditions. Research trials shows that leaf loss at early growth stages has little impact on final yield or maturity, see table 1, Percent Yield Loss of Soybean at Various Levels of Leaf Area Loss and Growth Stages.
Figure 1. New growth emerging on a node of a hail damaged plant.
Table 1. Percent Yield Loss of Soybean at Various Levels of Leaf Area Loss and Growth Stages
| LEGEND: = = no data available | ||||||||||
| Growth Stage | Percent Leaf Area Destroyed | |||||||||
| 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 | |
| VC-Vn | – | – | – | – | – | – | – | – | – | – |
| R1 | – | 1 | 2 | 3 | 3 | 4 | 5 | 6 | 8 | 12 |
| R2 | – | 2 | 3 | 5 | 6 | 7 | 9 | 12 | 16 | 23 |
| R2.5 | 1 | 2 | 3 | 5 | 7 | 9 | 11 | 15 | 20 | 28 |
| R3 | 2 | 3 | 4 | 6 | 8 | 11 | 14 | 18 | 24 | 33 |
| R3.5 | 3 | 4 | 5 | 7 | 10 | 13 | 18 | 24 | 31 | 45 |
| R4 | 3 | 5 | 7 | 9 | 12 | 16 | 22 | 30 | 39 | 56 |
| R4.5 | 4 | 6 | 9 | 11 | 15 | 20 | 27 | 37 | 49 | 65 |
| R5 | 4 | 7 | 10 | 13 | 17 | 23 | 31 | 43 | 58 | 75 |
| R5.5 | 4 | 7 | 10 | 13 | 17 | 23 | 31 | 43 | 58 | 75 |
| R6 | 1 | 6 | 9 | 11 | 14 | 18 | 23 | 31 | 41 | 53 |
| R6.5 | 0 | 1 | 1 | 3 | 4 | 5 | 7 | 13 | 18 | 23 |
| Printed with permission from National Crop Insurance Services (NCIS). | ||||||||||
Mechanism of Frost Damage
Freezing temperatures injure plants primarily by forming ice between or within cells. The water that surrounds the plant cells freezes first (at about 0 °C), while the water within the cell contains dissolved substances that depress the freezing point by several degrees. When the water around the cells becomes ice, water vapour moves out of the cells and into the spaces around the cell, where it becomes ice. The reduced water content of the cells depresses further the freezing point of the intracellular water. This process can continue to a point without damaging the cell, but eventually ice crystals form within the cell and disrupt the cell membrane, destroying it.
Stem Damage
Broken or cut-off stems have a greater impact on yield and maturity compared to leaf loss. If stem loss is under 50% prior to flowering, yield loss will usually be less than 10%. When evaluating hail damage, check for bruising on the plant stem. Plants have a difficult time recovering from severe stem damage. Stem damage also increases susceptibility to disease. Slight bruising, which does not cause stem breakage, results in minimal yield loss. In terms of yield reduction, soybeans are most vulnerable during the flowering and seed fill period. If stems are broken there will be a decrease in the number of pods per plant impacting final yield. Delays in maturity and seed size also occur.
Late Season Cold Temperature and Frost Damage
Soybeans are a warm season-crop making them more susceptible to cool temperatures, especially during flowering. If there are prolonged periods below 10º C during flowering, pollen formation can be affected. Sustained cold temperatures can result in poorly developed pods, called parthenocarpic pods (also known as “monkey pods”). There are significant cold tolerance differences between varieties. Varieties that have tawny pubescence (i.e., yellowish-brown hair) are generally more tolerant of cold than those with grey pubescence.
Until soybeans reach physiological maturity (R7 stage), they are easily injured by frost. Physiological maturity is attainted when one pod has changed to brown or grey on the main stem. Frost after physiological maturity does not damage the soybean plants if the pods remain intact. Prior to physiological maturity, seed quality will be affected. Yield can decrease by up to 80% if there is a severe frost during flowering or pod fill. Freezing during pod fill will result in severely damaged seed with a greenish, “candied” appearance. Even moderately frosted beans with a greenish colour and wrinkled seed coat are considered damaged and can be discounted. Frost-injured plants may reach maturity earlier but will have seed moisture levels equal to non-frosted plants. Germination of seed from frost damaged plants will be severely reduced. The Canadian Food Inspection Agency classifies frost-damaged soybeans as those “whose cotyledons, when cut, are green or greenish brown in colour with a glassy, wax-like appearance.”
Yield reductions from late-season frost injury are smaller as the crop matures. Frost during the R5 stage reduces yield by 50%-70%. Frost at the R6 stage reduces yield by 20%-30%. Once the crop reaches the R7 stage, only a relatively small loss can be expected. When full maturity is reached by the seed, no yield reductions occur.