By William J. Wiebold
One and one quarter inch.
That's the amount of rainfall in July and August that corn plants need each week to maintain rapid growth and produce the best possible yields. Mother Nature has not been kind to Missouri's corn crop. One indication is the rapid expansion of trouble areas in the Drought Monitor. As of July 12, about 60% of Missouri's land area is abnormally dry or in one of the categories of drought. Most of the affected area is in southern and central Missouri, but the near-term weather forecast throughout the state is not encouraging. Some parts of Missouri recently received rain. Water use by corn plants can reach 0.25 to 0.33 inch per day when air temperature is near 100°, so the small rainfalls that were common may not have been very beneficial.
A reason for concern is that much of Missouri's corn crop is in its most vulnerable stage of development (R1). Table 1 summarizes the effects from moderate drought on corn plants and the resulting impact on yield. Some effects are reversible with adequate precipitation. Unfortunately, most effects, especially those that occur at R1 will not improve even with plentiful rain.
|Drought effect on plant
|Full expansion of first 6 leaves and partial expansion of 2 to 4 additional leaves
|Slightly smaller early leaf area; first 6 leaves are a minor part of the total leaf area
|Elongation of primary root system
|Reduced uptake of some mineral nutrient, e.g. potassium
|Initiation of secondary root system
|Rootless corn syndrome; roots will not grow into dry soil; aggravated by soil compaction
|Ear buds initiated in leaf axils
|All leaves and nodes initiated
|Little or no effect
|Rows of potential kernels per ear established
|Fewer rows of kernels result in smaller ear diameter; minor effect, mostly under genetic control
|Growing point becomes embryonic tassel
|Rapid elongation of cells in stem internodes
|Shorter stem internodes result in shorter plants
|Minor to medium
|Full expansion of leaves 7 through 14 and partial expansion of next 4 to 6 leaves
|Smaller leaf area; less sunlight capture results in less photosynthesis during grain fill
|Potential kernels (female flowers) added to ear rows
|Fewer kernels per ear row results in shorter, smaller ears
|Internode elongation in upper stem including tassel peduncle
|Shorter stem internodes result in shorter plants; tassel may not fully emerge
|Medium to major
|A few additional potential kernels (female flowers) added to ear rows
|May not occur during drought
|Silk growth initiated and continues
|Slower silk growth; may affect timing of silk emergence
|Pollen shed from tassels
|Little effect; pollen shed from tassel for 3 to 7 days; all tassels in a field may require 14 days to complete pollen shed
|Silks emerge from husks
|Poor nick, silks may emerge after some to all of pollen grains shed
|Pollen tube growth
|Failed fertilization; requires 24 hours
|Aborted kernels, smaller ear size
|Medium to major
|Smaller kernels, reduced test weight
|Medium to minor
Because of this year's wet spring, corn planting was delayed and this delayed development in some fields. Some of these fields experienced drought conditions during some or all of what should be rapid stem elongation. Stem elongation occurs from V6 through VT. Water pressure is the driving force for expanding stem cells. With reduced water availability individual cells will not elongate to their maximum size and plants will be shorter than normal. If a few nodes are affected the change in height may not be noticeable or it occurs in small areas of the plant. But, if most internodes are affected, then plants may be quite short – perhaps one-half normal height. Of course, leaf cells are also affected so leaves will be smaller than normal. This effect on plant height and leaf area is not reversable. Once plant cells mature, cell walls harden and additional expansion is not possible. The effect on yield from shorter plants and smaller leaves is difficult to predict. It depends on the degree of the effect and if stress is relieved during silking and early grain fill. With adequate rain, yield reduction from shorter plants may not be major.
Drought that occurs during elongation of the final five internodes through R2 (blister) is most harmful to corn yield. As the tassel prepares to emerge, silks elongate and extend outside the husks. Timing of tassel and silk appearance is crucial to corn yield. Yield depends on silks being fully visible when pollen sheds. Unfortunately, drought slows silk elongation but has little effect of tassel emergence. If stress is great enough most of the pollen sheds before silks. The result is poor kernel formation, especially on the tip of the ear. This reduction in kernel number is not reversible (Figure 1).
Assuming silks were able to intercept pollen, the next 24 hours are critical to corn yield. It takes this long for the pollen tube to grow from the captured pollen grain to the area on the ear that will become the fertilized kernel. Silks must remain fully turgid during pollen tube growth. Even an infinitesimal drop in water content can interrupt tube growth. If pollen tube growth is stopped no kernel will form. This may be the most vulnerable aspect of corn yield to drought stress and why even a short period of stress can cause considerable damage to yield if it occurs at this time. In severe weather, only a few kernels will form (Figure 2). This reduction in kernel number is not reversible.
If kernel fertilization was successful, kernel number can still be reduced. Kernel growth requires a constant supply of water and nutrients from the mother plant. Drought can decrease the growth of young kernels to the point that their development is stopped. These aborted kernels remain small even if their neighbors continue to grow.
When drought prevents kernel fertilization or stops kernel growth the results cannot be reversed. A tiny ear remains a tiny ear. Silk and pollen tubes are highly vulnerable to lack of available water. For these reasons weather conditions surrounding R1 can be quite damaging to corn yield. With extreme weather, yield can be reduced more than 60%. Missouri farmers are facing weather challenges not seen for a decade.