Many questions come with the first frost of fall, some being – how heavy will it be and how long will it last, will it injure my still growing double crop soybeans or will they survive? All these questions persist because there are many factors in play that can impact the severity of the frost and in turn the detriment to the final yield.
For Central Illinois, the 10 year average first 32°F freeze date ranges between October 11-20th, but the potential this year is looking to be a bit earlier than average. The National Weather Service uses 32°F to identify frost but it is not abnormal to see frost when temperatures are slightly above 32°F because ground temperatures can be a few degrees cooler than the air temperature. Meaning, when the forecast calls for a low of 35°F, there is a chance that the ground temperature is cool enough for frost to form. A damaging or killing frost occurs when the temperature drops below 30°F for 4 hours or longer.
If the temperature is lower than 30°F for a stretch of four hours or more, there will be a negative impact on still maturing double crop soybeans. The severity of damage really depends on the growth stage, the lowest temperature reached and the duration of it. Light frosts that only stick around for a short period of time will most likely only harm the leaves in the upper canopy in the plant and will allow the pods and seeds to continue to develop. As the intensity of the frost increases, the greater risk there is to damage stems, pods, and seeds, and reducing the quality of seed and yield potential. The good news is soybean fields with thick plant canopies hold soil heat better than bare ground which helps protect the middle and lower canopies from the cold temperatures to an extent.
Following a frost, it is best to wait a couple of days before determining the amount of damage. Any leaves that were affected will be wilted and dried but will remain attached to the plant. Young pods at the top of a plant may wither and abort off following a freeze prior to maturity. With light frosts, the upper canopy should be the only part damaged but as the intensity increases, the further into the canopy damaged tissue will be found. Seeds that are still maturing and damaged by the frost will shrivel, reducing yields through smaller seed size and lower test weight. For beans that are in the R6 or full bean growth stage, yields could be reduced up to 50% with a killing freeze but only up to 20% with a light frost. Those seeds impacted will shrivel up and remain green in color and will also take longer to dry down. The more mature a plant is, the less of a hit that the final yield will take. Beans that are halfway through seed fill (R6.5), have the potential to decrease yields up to 15% with a killing freeze or have only a 5% loss with a light frost. Any beans that reached R7 or R8, physiological maturity prior to the frost will have little to no damaged or detrimental effects on the final yield.
Remember, there are too many variables at play to determine how much damage a double crop soybean field will sustain based on a forecasted low temperature.
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