The low temperatures experienced in the early morning hours of September 12 might have some impact on the summer row crops, primarily in northwest and west central Kansas. The main question is: How will the low temperatures affect each crop? The answer won’t be immediately known, but symptoms of low temperature injury might be seen in the next coming weeks.
In most of the state corn is at the dent stage or beyond. Corn will mature when the black layer is formed at the lower section of the kernel. Depending on the relative maturity of the hybrid, corn requires 200-240 growing degree units from dent stage until maturity (black layer). In terms of days, this growing degree requirement will be related to the air temperatures in the coming days. This may be about 20 days (plus or minus 10), but it will depend on the temperature until maturity. This assumes 20-24 growing degrees per day.
Corn is affected when temperatures are below or at 32 F. The lower the temperature, the less exposure time it will require to cause damage. Clear skies, low humidity, and minimum or no wind conditions can promote damaging frost even when temperatures are above 32 F. The temperatures of about 35 degrees F experienced during the early morning hours of September 12, especially in northwest and west central Kansas, can cause variable freeze damage, depending on the growth stage and position in the field among other factors. A proper assessment is recommended a week after the frost event. Green leaf canopy may be affected. Seed size and quality can also be impacted if the corn is in an early reproductive stage (dough vs. dent growth stages).
Most of the Kansas soybean crop has already set pods and is entering into the final reproductive stages (grain filling and senescence - dropping leaves). Low temperatures below 32 F can interrupt the seed filling in soybeans. Yield impacts can be expected when soybean are not yet at the R6 growth stage (full seed size). Similarly to corn, temperatures below 30 F can kill the soybean plants. With soybeans, the absolute temperature value is more important than the duration of the cold stress. The greatest damage is at temperatures less than 28 degrees F. If frost damage occurs when soybeans are at the beginning seed stage (R5, seeds are 1/8 inch long in the pod of one of the four uppermost nodes), yield can be dramatically impacted by the effect of the freeze on seed number and size. As the crop approaches maturity, the impact of a freeze event on soybean yields declines.
Kansas’s sorghum has already headed and almost half of the crop is coloring. Still, low temperatures might impact the crop through reductions in seed weight. Lower temperatures will decrease the growth rate of the seed, impacting seed size and making the harvesting process more difficult. Small and lightweight grain will be difficult to be thresh. The temperatures experienced last night may kill leaves, but if the conditions were not below 30 F, the plant will continue the grain filling until maturity (black layer). A freeze will kill the sorghum plant if the stalks are frozen, which would create an impediment for the flow of nutrients from the plant to the grain, stopping seed growth and impacting final yields. Freeze damage lowers the test weight of grain sorghum. In general, the less developed the sorghum is at the time of the killing freeze, the lower its test weight will be.
More information about the effect of these low temperatures will be provided in the next coming weeks. Stay tuned.
Ignacio A. Ciampitti, Crop Production and Cropping Systems Specialist
Mary Knapp, Weather Data Library