Fall Frost in Soybeans – Assessing the Risk

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.