CFS Growing Insights 7.24.2019

Weather Update

With rainfall over the past week that continues to push 2019 3”-5” above-average since May 1st, this week will also usher in optimum pollination weather as tassels emerge over the next few weeks.  Pollination will occur over 4-8 days over normal conditions, but stress from heat or moisture speed up the process to 2-3 days.

Fortunately, temperatures are forecasted to be in the low 80’s, ideal for pollen drop. As silks emerge, it will also be important to scout corn fields that have a history of corn rootworm pressure to ensure beetles aren’t “clipping silks,” which interferes with successful pollination.

Corn Foliar Diseases

With extreme heat, moisture, and humidity from last week, it set up a perfect environment for corn diseases to thrive. One common challenge is identifying different diseases. Gray Leaf Spot and Bacterial Leaf Streak are often misdiagnosed due to their similar appearances.

Gray Leaf Spot (GLS) is more rectangular with smooth margins and less “yellow” in color. Bacterial Leaf Streak is more of a “yellow” color, and more irregularly shaped with wavy margins. Gray Leaf Spot flourishes following wet/humid and warm weather conditions.

Warm conditions with high relative humidity also favor Bacterial Leaf Streak development, but since it’s caused by bacteria, fungicides aren’t effective on controlling the disease.


Corn Reproductive Stages

Emerging tassels signal the beginning of the corn plant switching from vegetative to reproductive growth. On average, it takes about 55-60 days between from VT/R1 to get to physiological maturity (32-33% moisture). With all of the variables in the 2019 growing season final corn yield, tipback, and abnormal pollination will often tell the story of the weather and stresses the crop experiences over the next 55-60 days.


Iowa State University ICM article offers these thoughts important insights to consider on how stresses affect corn yield in the reproductive stage:

  • R1 or the silking stage: The silk is visible outside the husk and pollination occurs when pollen grains fall on silks. Under normal conditions, silks will grow 1-1.5 inches each day until fertilized.
  • R2 or the blister stage occurs about 10-14 days after silking. The kernel is visible and resembles a blister on the cob at this stage. The kernel is filled now with clear fluid. If you dissect the kernel you will be able to see an embryo, this is the portion that sprouts the next year when the seed is planted. The kernels are approximately at 85% moisture content and this will decrease as they near maturity. If severe stress occurs now or during R3, kernels may be aborted from the tip-down to lessen the load on the plant.
  • R3 or the milk stage will occur approximately 18-22 days after silking. The kernel is now yellow on the outside with the inside containing milky white fluid. Starch is rapidly accumulating in the kernel. By R3 cell division in the endosperm is complete and kernel growth that occurs now is due to cell expansion and starch-fill in the individual kernels.
  • R4 or the dough stage will occur approximately 24-28 days after silking. The interior of the kernel has now thickened to a dough or paste-like substance. The kernels have now accumulated about half of their mature dry weight. Stresses will not likely cause kernel abortion by this stage. Prior to R5 the kernels at the tip of the ear will begin to dent (beginning dent).
  • R5 or the dent stage will occur approximately 35-42 days after silking. Kernels are dented in at the top and are drying down. Kernels have 55% moisture content at the beginning of R5. You will be able to see a line separating yellow from white on the kernel; this will progress downward as the kernel matures and the starch hardens. Stress is only able to reduce kernel weight at this time by hindering dry matter accumulation.
  • R6 or physiological maturity occurs approximately 55-60 days after silking. All kernels have reached their maximum dry matter accumulation now since the starch layer has moved completely to the cob. A black or brown layer will be visible at the base of each kernel. Tip kernels will first reach this black layer stage followed by basal kernels. Kernel moisture is now between 32-33% with much variability due to hybrid and environment. Moisture moves out of the kernel easily if the plant is still green. Stress that occurs now will have little effect on yield except if plant lodging or insect feeding on the ear occurs.
The recent heat has helped 2019 move closer to “normal,” and with many fields tasseling in the countryside, we can begin to predict when black layer (physiological maturity or R6 with 32-33% moisture) will occur.

The table below outlines the estimated black layer dates using WinField United’s Field Forecasting Tool with different relative maturity hybrids and planting dates. This field is based in Truman, MN, and assumes “average” heat for the next 60 days. Certainly, large shifts in the weather can have a dramatic impact on when the finally see physiological maturity!