Questions have been arising about the impact some of the cooler temperatures experienced over the last couple of days and those to come will influence soybean still in the field across the state. Similar to other crops, it will greatly depend on numerous factors, including: temperatures, duration, stage of the crop, and conditions surrounding the event.
As with most wintery weather, temperature is the overarching issue. Temperature is an important and somewhat dynamic aspect for soybean production. Cool temperatures, especially paired with excessive moisture, are not favorable conditions for soybean production. However, temperatures above 40F will rarely result in detrimental impacts on soybean production, especially if paired with more optimum conditions before and following. These cooler temperatures will slow or even halt soybean growth for a period but should not result in large amounts of permanent damage.
Below 40F, more permanent damage would be expected. A yield decrease would be expected with prolong periods below this critical threshold, mainly from decreased seed size due to premature seed shrinkage. Colder temperatures will continue to be detrimental to the crop. The next critical threshold for soybean plants is 28F. Prolong periods with these cooler temperatures will result in whole plant death.
The impact of these temperatures and what this will actually mean for crop yield will depend on the stage of growth.
Growth stage is one of the largest determining factors in determining the extent of damage that could occur during cool or near freezing conditions. The first thing that needs to be determined is if the soybean plant has reached maturity. At maturity, the individual seeds have separated from the plant itself and anything that will negatively impact the plant will not little to no impact on the seed itself. Maturity begins at growth stage R7 (one mature colored pod anywhere on the plant). At this stage, plant and seed dry weight have been fully accumulated and seed moisture is around 60-70%. Past this stage, seed and plant dry-down are the primary activities. However, it should be noted that in soybean plants, especially in indeterminate soybean varieties, a plant will not reach maturity at the same stage throughout the entire plant. Therefore, this will not be universal throughout a plant let alone an entire field.
Soybean plants beyond maturity:
Once the soybean plants have reached maturity and plant/seed dry weight have been mostly accumulated, frost to freezing conditions will have only a minor impact on yield. Research has shown that a freeze around R7 growth stage has most often resulted in a <10% yield loss. In fact, a freeze event can help to defoliate some leaves still remaining on the plant. However, depending on the condition of the plant, a frost or light freeze can actually result in the plant remaining green for long or could result in high amounts of greenbean syndrome. Fields should be evaluated once conditions have warmed following the freeze event to determine potential harvest activities needed.
The freeze event around Oklahoma will require action much faster as it has been paired with widespread precipitation. If pods were at or near harvestable prior to recent rain or freeze events, they will need to be harvested quickly as wetting and dry of the pods will decrease the integrity of the pods potentially resulting in increased shattering. Therefore, growers should place priority on harvesting these soybean fields prior to any other activities.
Soybean plants not yet at maturity:
Some later-season double-crop soybean field might not have reached maturity. Recent and potential freeze events will have a larger impact on yield of these plants. If soybean have reached full seed (R6; where the seeds have begun to touch in a fully seeded pod), yield declines associated with a major freeze could reach as high as 30%. This will be primarily due to early and rapid seed shrinkage without accumulating total seed weight. Plants less mature will suffer increased damage. Frost or freezes on plants that have only reached beginning seed filling (R5) could result in as high as 60% yield decline.
Other factors that could influence freezes on the soybean crop:
While this may sound ominous for late-planted double-crop soybean fields, especially the crop planted following late wheat harvest, some positive effects may help. First, the temperatures do not appear to be getting cold enough to do substantial damage to the crop. Initial forecasts suggested that temperatures may get close to the below freezing mark. Additionally, the statewide rainfall received in the last several days can help stabilize the temperatures at or near the soybean canopy.
Any freeze event on a soybean crop prior to harvest is not ideal, as soybean are sensitive to late-season environmental stress. However, the positive thing for most soybean fields throughout the state is they have reached, at minimum, the R6 growth stages with, even a majority of our double-crop fields, having reached R6.5 to R7. As mentioned previously, beyond this stage minimal yield effects would be expected. However, on these less mature soybean crops, growers need to evaluate how they will harvest their soybean crop following these frost to freeze events, as some form of desiccation may be needed.