As I write this, air temperatures are climbing into the 90's for the next several days. Such temperatures back in June caused corn leaves to roll, especially in fields where soil compaction was severe (P&C Newsletter , 6/23/95). Now that Indiana's corn crop is entering its critical pollination and fertilization period, what effects can drought and heat stress have on corn grain yield?
There is no doubt that successful pollination (see accompanying article, SEX in the Corn Field: How's It Done? ) goes a long way towards guaranteeing grain in the bin this fall. Stress during pollen shed and silking can cause more yield loss than almost any other period in the crop's development. Conversely, optimum weather during pollination can set the stage for good yields this fall.
Heat Stress. High-temperature damage to pollination in Indiana almost always occurs in conjunction with drought stress, rarely by itself. Thus, separating heat stress from drought stress effects on pollination is usually difficult.
Temperatures in excess of 95 degrees, especially when accompanied by low relative humidity, can dessicate exposed silks, but affect silk elongation very little. Pollen is likely damaged or killed by mid-90's or greater temperatures, especially when accompanied by low relative humidity.
Luckily, pollen shed typically occurs during early to mid-morning hours before temperatures climb to such dangerous heights. Furthermore, pollen maturation for a given tassel occurs over time and 'fresh' pollen is available every morning until pollen shed is complete.
Successful pollination can therefore occur even during lengthy periods of high temperatures if soil moisture reserves are adequate to meet the plants' demands. Bottom Line: Where soil moisture is adequate, high temperature by itself will not severely impact the yield of a given field.
Drought Stress. Severe drought stress, as indicated by continual or nearly continual wilting of the plant, affects the pollination process primarily by slowing down silk elongation. Silks begin elongating from the ovules of the ear shoot about 7 days prior to silking. The silks from the butt of the ear elongate first, followed by those from the central part of the ear, then the tip of the ear.
Inadequate plant water potentials can slow down silk elongation, resulting in delay or failure of the silks to emerge from the ear shoot. Silks that do emerge may desiccate rapidly under severe moisture deficits and become non-receptive to pollen. Ironically, drought stress tends to accelerate pollen shed, often resulting in a poor timing 'nick' between pollen shed and silk emergence.
Beginning about 2 weeks before silk emergence, corn enters the period of grain yield determination most sensitive to drought stress. Nearly continual wilting of the plant due to drought stress at this stage can decrease yield 3 to 4 percent per day. During the actual silking and pollen shed period, severe stress may reduce yield up to 8 percent per day. During the 2 weeks following silking, severe stress may reduce yield up to 6 percent per day.
Fields that will be most susceptible to heat and drought stress during pollination will be those where severe soil compaction has restricted the corn root system from penetrating deeply this year. Such shallow root systems will 'run out' of available soil moisture sooner than more deeply developed root systems. Similarly, fields where significant root injury occurred from corn rootworm larvae feeding will also be more susceptible to hot, dry conditions.