Corn plants appreciate a stress-free summer

Over the next several weeks the upper canopy in Ohio corn fields will look much different, as tassels begin to emerge and extend beyond the top leaves of the plant. This visual change signifies that the corn crop will soon be shifting from vegetative into reproductive growth. The first reproductive growth stage (R1) begins when silks extends outside of the husk leaves and typically occurs two to three days after tassel emergence.

The two basic processes that occur during corn reproduction are pollination (transfer of pollen grains from the tassel to the silks) and fertilization (joining of the pollen grain and ovary to create an embryo). Though these processes seem very simple, there is a lot riding on their success. Approximately 85% of the variability in grain yield is related to the number of kernels produced per acre while the remaining 15% of the variability in grain yield is related to the weight of these kernels. Thus, failure in either of these reproductive steps can have a detrimental effect on yield.

The most stress-susceptible period in corn is about one week prior to silking and continues for two weeks after silking. The timing of the stress can result in dramatic differences from one field to the next. A very small variation of even just a day or two difference in flowering date or planting date can cause a substantial effect in the success of pollination and kernel set. Water stress is the most common stress factor in corn, although high temperatures and loss of leaf area — from hail, diseases, insects, etc. — can also reduce yield during this critical time especially when coupled with drought stress.

Severe drought any time during plant development can have a negative effect on yield potential, but the greatest impact is when drought stress occurs during the pollination and early grain-fill stages. Research at Iowa State University has shown that yields can be reduced by as much as 40 to 50% when corn undergoes four consecutive days of severe stress during the pollination phase. Moisture stress that occurs during the vegetative growth stages or during later grain fill has much less effect on yield. Stress effects from drought that takes place from one week prior to tasseling through the milk stage causes lack of ovule fertilization or kernel abortion. Moisture stress that occurs from the dough stage to black layer will lead to reduced kernel weight.

The greatest risk of kernel reduction at pollination is the asynchrony of pollen shed and silking. Drought stress tends to accelerate pollen shed while delaying silk growth, which can result in poor “nick” of the pollen and silks. A single plant will shed pollen for only about five days but because of natural field variability in plant development a whole field may take up to 14 days to complete pollen shed. Silks begin elongating from the ovules of the ear shoot about seven days prior to emerging from the ear shoot. Silk growth occurs by cell expansion, so if a plant lacks the necessary water than silk development will be slowed or halted. Silks that emerge after most of the pollen is shed will not likely be pollinated. Moderate silk delay can cause poorly-filled ear tips, whereas more severe stress can result in ears that are nearly or completely barren.

Corn maximizes its growth rate at 86 degrees F during the day and 60 to 65 degrees F at night. When daytime temperatures reach into the 90s, then plant stress results. High daytime temperatures decrease photosynthesis efficiency, which causes the plant to make less sugar that will in turn be used to produce plant biomass and grain. High night temperatures on the other hand can be even more detrimental to corn than high daytime temperatures. Elevated nighttime temperatures increase the respiration rate, causing the plant to spend more energy for the maintenance — energy that could have otherwise been used in dry matter accumulation. The respiration rate of plants increases rapidly as the temperature increases, approximately doubling for each 13 degree F increase. With high night temperatures more of the sugars produced by photosynthesis during the day are lost, which leave less available to fill developing kernels. High nighttime temperatures also result in a shortened grain fill period that will limit the time the crop has to add additional starch to the kernels.

Curious to know how successful your corn fields were at pollinating? Follow the steps below and find out.

1. Remove the ear from the plant.

2. Gently peel the husks from the ear so that the silks are disturbed as little as possible.

3. Grab the ear by the base and hold the ear so that the tip is pointing toward the ground.

4. Gently shake the ear. Silks will fall from fertilized ovules, while silks of ovules not fertilized will remain attached to the developing ovules.

5. Percent pollination is equal to the percent of ovules with no silks attached.

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