09 Oct Winter is Coming, Beware of the Fall Frost!
As the seasons start to change and harvest starts wrapping up, there is one thing prairie farmers dread: overnight frost. In this week’s edition of growing possibilities, we are going to look at how early fall frost can affect different crops at the end of the growing season and the soil microbes that help them grow.
One of the most frost sensitive crops on the prairies is canola. Canola is most sensitive to frost during immature stages, when moisture content is higher, so frost during these stages will affect yield the most. If you have an overnight frost event on in your canola field, it is recommended to wait 24-48 hours before checking for discoloration and soft mushy seeds in green pods or for pods that have fallen right off the plant (1).
Like canola, frost damage to cereal crops largely depends on the moisture content in grain kernels. Wheat and barley crops in earlier stages of maturity will have more moisture and therefore be more susceptible to frost damage than those in later dough stages or fully mature (2). The same is true for corn, but kernels may be unaffected if frost only damages the top tissues of the plant. If only tissues above the ear are damaged by frost; kernels should be unaffected and will continue to mature normally. If frost damages the whole stalk, then kernels will shrivel and stop maturing. Physiologically mature plants will be unaffected by frost and kernels will still develop normally if frost hits after maturity (3).
Here in Manitoba, many farmers still have soybeans out in the field. As long as soybeans are at the R7 stage (pods are yellow with at least 1 pod per plant turning brown) frost damage will be minimal (5% yield loss or less). When soybeans have reached the R8 stage (95% of pods are brown and dried seeds are rattling in pods) then frost damage will be negligible (4).
For other dry beans and pulses, frost events can be far more damaging to yield. Any temperature below freezing can hurt pulse crops, with shrivelling and discoloration of pods being an indicator of damage. Like in corn, frosty weather affects the pods and plant tissues at the top of the plants first, where exposure is the greatest. If frost hits the top of your canopy, pods lower down the canopy may still be ok (5).
Fall frost events not only damage your crops, but the hard-working soil microbes that help them grow as well. The numerous freeze-thaw cycles that come with fall frost damage soil microbes more than the sustained frozen ground during the winter months (6). The depletion of nitrogen (N)-fixing rhizobia populations during these cycles is the reason we suggest applying an inoculant to your pulses and soybeans yearly. Rhizobia that survive the winter are often in reduced numbers and not in optimum health. Rejuvenating these populations with fresh, healthy rhizobia will help your crop reach its full nodulation and N-fixation potentials.
6) Liu, M., Feng, F., Cai, T., & Tang, S. (2020). Soil Microbial Community Response Differently to the Frequency and Strength of Freeze–Thaw Events in a Larix gmelinii Forest in the Daxing’an Mountains, China. Frontiers in Microbiology, 11(June), 1–12. https://doi.org/10.3389/fmicb.2020.01164