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July 20, 2011

How Heat and Moisture Stress Impact Corn Pollination and Yields

Farmers in the Midwestern U.S. are concerned about the potential impact of hot and dry conditions on corn pollination and the subsequent corn yields. High temperature damage to pollination is almost always in conjunction with drought stress thus separating the potential damage from heat stress to that of moisture stress is usually very difficult. Below are some examples of how these stresses can impact the pollination process and potential corn yields.

Impact of severe stress on ear formation two weeks prior to pollination

The two weeks prior to pollination is very important for the corn plant because that is the time when the size of the ear is determined. The number of rows per ear is determined by the time the corn plant is in the 12th leaf stage or when the plant is waist-high to chest-high. In a normal year, there are approximately 16 to 18 rows per ear, but if the plant is under severe stress prior to pollination, the number of rows may decline to 14 or even 12 rows per ear. The number of kernels per row (ear length) is determined between the 12th leaf stage and one week prior to pollination. Likewise, stress prior to pollination will result in a shorter ear length. Once the ear size has been determined, it cannot get any larger. If there is good weather during pollination and grain fill, then the pre-determined size of the ear can be achieved. If there is poor weather during pollination and grain fill, then pre-determined ear size will not be achieved and yields will be disappointing.

Impact of severe stress prior to silking and pollination

A corn plant is very sensitive to moisture stress two weeks prior to silking. A lack of moisture can result in the delayed emergence of the silk or prevent the silk from even emerging. If the moisture stress is severe enough, the silks may dry out too quickly and become non-receptive to pollen. Additionally, moisture stress may shorten the period during which the pollen is shed resulting in a poor nick, or the synchronization between pollen shedding and silking. In other words, if there is a poor nick, the plant may stop shedding pollen before all the silks have emerged. Since the silks near the tip of the ear emerge last, a poor nick can result in tip-back where the last few inches of the ear is devoid of kernels.

In general, moisture stress is more detrimental to pollination than heat stress. Temperatures above 95 degrees, especially if they are accompanied by low humidity, may desiccate the silks and result in poor pollination. The pollen's viability can start to be impacted if temperatures hit the mid-90's, especially if it is accompanied by moisture stress. If there is ample soil moisture, the pollen's viability is generally not impacted until the temperatures reach the upper 90's or low 100's.

What constitutes severe moisture stress?

If a corn plant rolls its leaves just during the heat of the day that does not necessarily mean that the plant is under severe moisture stress. Severe moisture stress is occurring if the corn leaves start to roll early in the morning and extend until in the evening, 12 to 18 hours a day. If the leaves remain rolled over night, the plant is under exceptional moisture stress.

What fields are most susceptible to stress?

The fields most susceptible to heat and drought stress during pollination are those fields with severe soil compaction or extended periods of soil saturation earlier in the season that resulted in restricted root growth. Shallow rooted plants will run out of moisture sooner than plants with a deeply developed root system.

Impact of severe stress during pollination on potential corn yields

If a corn plant wilts continually for two weeks prior to silking, yield losses can be as high as 3% to 4% per day. If severe stress occurs during silking and pollination, yield losses can be as high as 8% per day. If severe stress occurs during the two weeks after pollination, yields losses may decline up to 6% per day. The potential yield losses due to heat and mositrue stress depends on: the severity of the stress, the duration of the stress, the stage of development of the corn plant, the depth of root development, and the general health of the plant when the stress began.