Drought stress conditions remain a serious concern in the western region of the state, and now drought stress is gradually moving eastward in the state as the crop growing season progresses.

Recent weather has been hot, with scarce and very erratic precipitation patterns. Precipitation in many areas of Kansas has clearly been insufficient for sustaining the evapotranspiration demand of the corn, soybean, and sorghum (Fig. 1).

In some areas of the state, corn is approaching the mid-point of the growing season -- flowering time. At this stage, intense and severe drought stress can dramatically compromise the formation of grain and the final yield. The U.S. Drought Monitor from July 9, 2013 for Kansas is showing  “Exceptional” and “Extreme” drought conditions from the central to the western part of the state. In addition, more than 90% of the state is currently classified as “Abnormally Dry” or worse. Only a small area in the eastern part of the state is classified as with no drought stress (<10%).

Overall, the July 9 Drought Monitor for Kansas breaks down like this:
Exceptional drought: 24%
Extreme drought; 23%
Severe drought: 14%
Moderate drought: 15%
Abnormally dry: 14%

Specifically for corn crop, the main questions are:

How much of the yield was or will be affected by the intensity of the drought stress in the next coming weeks? The answer is not straightforward, but for dryland corn if the weather conditions persist or get worse, severe loss of grain yield potential should be expected, depending on the stage of growth and development of corn. The crop is getting, in parts of the state, close to flowering time.

Why “flowering time” is critical or important for the final grain determination? Two weeks before and after pollination is when the final grain number will be determined through a sequence of processes taking place in the female reproductive structure of the corn plants. Before this stage, ear size has already been determined. At the five-leaf to fifteen-leaf stage interval) that will play a critical role in defining the final yield, the ear size.

Why “ear size” is an important component for the yield determination in corn? The size of each individual ear is set during the 5- to 15-leaf stage of growth. This will define the maximum potential number of grains per plant (if the plant carries only one ear) through the determination of the number of ovules that can be fertilized. Thus, the “potential kernel number” is determined before the pollination occurs. The final kernel number will be physically restricted -- small ears have less space for bearing grain.

Why “pollination time” is important for corn? The success or failure of pollination will determine the number of fertile ovules that will develop into actual kernels. However, even if an ovule was fertilized that doesn’t guarantee it will develop into a kernel of grain by the end of the season. Two weeks after pollination, abortion of the fertilized ovules can take place within the corn ears. Thus, the final kernel abortion rate during that period will ultimately determine the actual kernel number to be attained at maturity. The kernel abortion rate is highly influenced by any biotic or abiotic stresses (e.g. pest pressures, hot temperatures, drought, insufficient N supply, etc.). As the stress intensifies, the abortion rate increases.

When will you be able to know if the crop’s grain yield potential has been severely or partially affected by the drought stress? Go ahead now and start looking at the corn plants and quantifying the area affected by stress. Count the number of plants per acre by taking a sample from a small area and, check the status of the plants at different parts of the day. Are those plants showing drought stress symptoms very early in the morning (i.e. leaf rolling at 9 a.m.)? Is plant height and leaf mass being severely affected by the combination of the insufficient water supply and warm temperatures?

Is the pollen being shed when silks are not yet out from the husks? Those are just “pre-mortem” observations to make, with the idea of forecasting the potential yield expected at the end of the season.

For sure, if no ear is formed (“barren plants”) within a couple of days after the flowering time, the decision is pretty straightforward. If an ear does form but drought or heat stress is severe, the decision is more difficult and depends on getting an early but reasonably accurate estimate of grain yield potential.

A very close idea of the final yield can be determined at around 30 days after silking -- even before if hot and dry conditions are severe -- just at or after the milk stage. Basically, if we can get an estimate of the number of ears per row or per unit area and some idea about the final number of grains per each unit of ear, we can forecast with a degree of uncertainty the final yield, assuming a constant seed weight, which is defined at the end of the season.

The calculation is a very simple. Start by counting the total number of kernels per ear. Consider only kernels that have similar sizes. Do not count the small kernels near the tip of the ear. Multiply that number by 0.233 (assuming a plant density of 21,000 plants per acre with a single ear per plant, and an average seed size of 70,000 kernels per bushel). For example, let’s say you sample 30 ears and the average kernel number (number of rows x total number of kernels per row) is 400 per ear. Then, 400 x 0.233 = 93 bushels/acre. Again, this is just a simple “rough” calculation you can make early in the season, and it assumes that the number of ears per acre and the seed size is constant  across the entire field.

Drought stress will undoubtedly have substantial impact on potential corn yields in Kansas.

How much will yields be reduced? That question will remain unanswered for now. The answer will be area- or site-specific, as drought stress is affecting corn differently in various parts of the state.

Stay tuned and check periodically all K-State resources for more updates about this or other issues.