The first step in accessing frost damage is asking how cold it was last night. A light frost of -1°C for short durations may clip off a few off the top leaves with no effect on yield. The concern begins when a killing frost at least -2°C occurs for an extended period of time. In this situation you will see frozen leaves and pods throughout the canopy. This may cause quality issues and yield reduction if the crop has not reached full maturity.
See the latest MB Ag Weather latest frost map: http://www.gov.mb.ca/agriculture/weather/pubs/minimum-air-temperature.pdf
What growth stage are your beans at, see http://www.manitobapulse.ca/soybean-staging-guide/ as a reference.
A killing frost at the R8 growth stage will see no yield or quality loss. The R8 stage is when the leaves have dropped off, all pods are brown, and seeds rattle within the pods when plants are shaken.
If however your beans are at the R7 growth stage, (which means one pod on the plant has reached its mature color), research has shown yield loss can range from 5-10 % dependent upon the severity of the frost. Quality issues in the way of green seed may also occur.
Finally, if your beans are at the R6 growth stage-(this is where pods containing a green seed that fills the pod cavity at one of the four uppermost nodes on main stem), yield losses can range from 20-30 %. You will also have green seed issues which can also lead to marketing concerns.
There are a few areas in Manitoba where the beans are at the end of this R6 growth stage. Most of the beans in Manitoba are at the R7-R8 growth stage. A light frost should not affect yield and quality for these beans. If beans were at the R6 growth stage and a hard frost occurred yield and quality losses would be noticeable.
Picture: Light frost damage on soybeans near Hamiota, 2016.
Photo from L.Grenkow, Manitoba Pulse Soybean Growers
Submitted by: Dennis Lange, Industry Development Specialist-Pulses, Manitoba Agriculture