It is not unusual in southern Manitoba, as we move from summer to fall, for the first significant dip in overnight temperatures to occur in late August to early September. Sometimes it is of no consequence to our soybean crop. The overnight lows forecast for the next few days suggest that we need to be on our toes this time around.
Here are some points to consider:
Severity of Frost
Every frost event in every field happens within its own unique microclimate and the amount of damage to the soybean crop depends on the interaction of several factors. It is not simply a matter of how cold it ultimately got overnight unless temperatures drop dramatically. This is not usually the case with the first frost event of the fall.
Further, temperatures reported at weather stations are usually done at around chest height. At this time of year, the air temperature at ground level versus four or more feet above ground level can be very different in either a way that helps or hurts us. Ultimately you have to go out and examine each of your fields to determine any potential impacts on yield and quality. And remember that observations made the morning after the frost event will likely be misleading. Usually, about 72 hours has to pass for the full effects of frost to be seen and a realistic assessment of yield and quality loss to be made. This is very frustrating but also very necessary.
Besides how cold it ultimately got overnight, other factors that influence frost severity include:
- Duration of cool temperatures – longer duration of freezing temperatures can cause more damage.
- Soil moisture – moisture in the soil will retain heat.
- Canopy thickness – narrow, thick rows maintain heat longer than wide rows. Good canopy closure helps.
- Wind speed – stronger wind can reduce the severity of frost.
- Cloud cover – more nighttime cloud cover can reduce plant damage.
- The pattern of overnight lows before the frost event – Did we step into the cold temperatures gradually? Although much more limited than something like canola, soybeans can acclimate to gradually dropping temperatures to a limited extent and tolerate frost a bit better than if we go from warm summer conditions to frost in a sudden overnight change.
- Crop Stage – the crop is the most vulnerable while it is still filling seed and that seed has a high moisture content:
- R5-R6 (Worst case scenario) – Green and immature beans will shrivel and remain green.
- R6.5 – Green-yellow beans will have a mixture of green seed that will not mature and yellow seed that will mature.
- R7 – Mature, yellow beans will continue to dry down slowly with minimal yield and quality loss.
A light frost (0 to –1°C) may only kill top leaf growth and not penetrate the canopy very deeply. Plants should continue to mature but may take longer to ripen and there may be some green seed within pods where leaves were killed. Depending on the severity, a hard frost (< –1°C) will cause damage to green stems, pods and seeds, reducing yield and quality and may kill the entire plant. When entire plants are killed, seed fill stops.
Impact on Soybean Yield and Quality
The impact of fall frost on soybean yield and quality is influenced by the development stage or maturity of the crop. Later-seeded soybeans and longer-season varieties are at greater risk of damage. When frost is in the forecast, check your soybeans before and after the potential frost event to assess whether any damage was incurred.
For images and full details on the percentage of yield loss expected from frost at different development stages, refer to the MPSG bulletin on Soybean Maturity and Low Temperatures.