The pollination and fertilization period is the most critical stage of development for the corn crop. Success during this stage of development goes a long way towards guaranteeing grain in the bin. 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 (remember July of 1992) can set the stage for bumper yields this fall.
Potential ear size is already determined by the time silks emerge from the ear shoots. In fact, potential kernel row number is set by the 12-leaf collar stage (about chest-high corn.) Potential kernel number per row is determined over a longer time period, from about the 12-leaf collar stage to about 1 week prior to silk emergence.
Of these two yield components, kernel number per row is more sensitive to environmental stresses. Row number, on the other hand, is fairly strongly determined by the variety's genetics and less so by the environment.
Once potential ear size is determined, good weather during pollination or grain fill will only help fulfill the potential, not make it better. Conversely, poor weather or insect damage during pollination can prevent the crop from attaining its full yield potential.
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 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.
Severe drought stress, as indicated by continual or nearly continual wilting of the plant, affects the pollination process primarily by its effect on 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 when the plant is enduring severe moisture deficits, becoming non-receptive to pollen. Ironically, drought stress tends to accelerate pollen shed, often resulting in poor "nick" of the pollen and silks.
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 has little direct effect on silk elongation. The outer membrane of a pollen grain is very thin. As a result, high temperatures and low humidity can similarly dessicate pollen grains once they are released from the anthers of the tassel.
Growers often worry about the direct effects of high temperatures on the viability of the pollen grains. Indeed, pollen is likely damaged by mid-90's or greater temperatures, especially when accompanied by low relative humidity. Temperatures over 100 degrees may literally kill pollen.
The saving grace of the matter, however, is that pollen shed typically occurs during early to mid-morning hours before temperatures climb to such dangerous heights. Furthermore, pollen maturation occurs over time and 'fresh' pollen is available every morning until maturation is complete. Successful pollination can occur even during lengthy periods of high temperatures if soil moisture reserves are adequate to meet the plants' demands.
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