Relating Silk Emergence
to Kernel Set
What Factors Affect Corn Pollination?
This is a critical time of year for the corn plant. A number of factors can affect corn pollination.
Learn more about:
Facts About Corn Pollination
Corn pollination is one of the most critical times for the corn plant. A number of factors such as drought and heat stress can significantly affect pollination.
Since pollination takes place during a 5 to 8 day period, stress during a prolonged period during pollination can have a greater impact on pollination.
Keys to successful pollination
- Each tassel can produce between 2 and 5 million pollen grains. Considering there are 750 to 1,000 potential kernels on each ear having enough pollen in the field is not usually a problem.
- Most of pollination takes place during the midmorning hours or late afternoon, when daytime conditions are typically dry, but not during the peak temperature periods of the day.
How to tell if an ear is pollinated?
- When the tassel emerges and starts to shed pollen, typically silks emerge anywhere from about the same time to within a couple of days.
- The silk will detach from the kernel within 2 to 3 days after successful fertilization.
- Gently strip back the husk and shake the ear. Silks will fall off kernels that that have been pollinated. Silks that remain attached indicate those ovules have not yet been pollinated.
Corn Pollination Success
Pollination Success is Critical to Final Yield
- The number of kernels set is largely determined near the time of pollination.
- Yield losses due to reduced kernel set at pollination cannot be fully regained.
Kernel set requires the successful completion of several plant processes.
- Production of viable pollen by the tassel.
- Interception of pollen by receptive silks.
- Embryo and endosperm development.
- Pollen shed or anthesis is controlled by a combination of genetic and environmental factors.
- Once pollen grains have matured inside corn anthers, these anthers begin to dry or dehisce.
- Anthers typically shed pollen around midmorning as anthers dry in the heat and sunlight.
- As anthers dehisce, they split apart to allow pollen grains to fall into the open air.
- Pollen grains are viable for only a few minutes after they are shed until they desiccate.
- A tassel normally sheds pollen for about 5 days.
- Pollen shed in a field can last up to 2 weeks.
- Each silk that emerges from an ear shoot connects to a single ovule, or potential kernel.
- A silk must be pollinated for the ovule to develop into a kernel.
- Silk emergence proceeds from the base to the tip of the ear over the course of 4 to 8 days.
- Silks will continue to elongate for up to 10 days after emergence or until they are pollinated.
- Silk receptivity decreases over time following emergence due to the senescence of silk tissue.
Stress at Pollination Can Reduce Yield
- Stress susceptible period extends from 1 week prior to silking to approximately 2 weeks after silking.
- Yield losses during this period result from reduction in kernel number and are therefore irreversible.
Drought Effects on Silk Growth
- Reduction in kernel number may result from asynchrony of pollen shed and silking.
- Silk elongation requires high water potential ― drought stress can delay silking and increase the anthesis-silking interval (ASI) ― the time between the start of pollen shed and silk emergence.
- Silks that emerge after most of the pollen is shed may not 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.
Heat Effects on Pollen Shed
- The location of the tassel exposes it to high radiation and potential temperature extremes.
- Extreme heat stress (over 100 F) can reduce pollen production and viability.
- Severe losses in pollen production or viability are necessary to affect kernel set, which would require an extended period of extremely high temperatures.
- Drought stress can prevent pollination, as well as cause successfully pollinated kernels to abort.
- Drought stress causes kernel abortion by reducing photosynthesis and carbohydrate availability following pollination.
- Aborted kernels will appear white and shriveled. The yellow embryo may also be visible.
- Insects such as corn rootworm beetles and Japanese beetles can interfere with pollination by clipping silks.
- Clipped silks can still elongate and receive pollen; however continuous intense insect activity can result in reduced seed set.
Nielsen, R. L. 2007. Silk Emergence. Purdue Univ.
Nielsen, R.L. 2007. Tassel Emergence & Pollen Shed. Purdue Univ.
Relating Silk Emergence at Pollination to Kernel Set at Harvest
Crop Insights written by Stephen D. Strachan, Ph.D., DuPont Research Scientist
- Maximum kernel set and grain yield occur when pollen shed coincides with rapid silk growth.
- Silks attached near the base of the ear emerge first, and silk emergence progresses toward the tip of the ear. Kernel set also progresses from the base to the tip of the ear.
- Under the environmental conditions of this study, silks remained receptive to pollination for about 5 to 6 days. Maximum silk growth was about 1.6 inches per day.
- For this study, the number of kernels per ear correlated highly (r2 = 0.95) with grain yield per ear.
- Grain yield was constant if the total number of kernels/ear was constant. The corn ear may compensate for poor kernel set at the base by producing more kernels at the tip.
Successful harvest of corn seed or grain requires adequate pollen when silks are receptive during pollination. Pollen shed, or anthesis, is controlled by a combination of genetic and environmental factors. The genetic background of the corn plant provides a general baseline for the necessary number of growing degree units until anthesis.
Once pollen grains have matured inside corn anthers, these anthers begin to dry and dehisce. As anthers dehisce, they allow pollen grains to fall into the open air and possibly land on receptive corn silks. The process of pollen growth and maturation inside corn anthers with the subsequent drying and splitting of corn anthers to allow pollen release is similar to the process of growth and maturity in soybean, with the drying and splitting of the pods to release the soybean seeds.
Environmental conditions, such as relative humidity and temperature, are contributing factors for anther dehiscence. During summer nights, temperatures are relatively cooler and relative humidities higher, so anthers maintain a more hydrated state. As the morning progresses, temperatures tend to increase and relative humidities decrease, thus drying corn anthers. Anthers containing mature pollen typically shed pollen around mid-morning. A tassel will normally shed pollen for about 5 days.
Silk emergence is also controlled by a combination of genetic and environmental factors. The genetic background of the corn plant provides a general baseline for the necessary number of growing degree units until silks emerge. Temperature and available moisture also play extremely critical roles for silk emergence, elongation, and exsertion past the tip of the husk. Under ideal temperature and moisture conditions for corn growth, silk exsertion is very rapid and consistent throughout similar areas of the field. However, if corn plants are subjected to dry conditions or drought stress, silk emergence may be delayed and rates of silk growth reduced. Changes in silk growth may affect the number of kernels set on the harvested ear.
Field Study Conducted
DuPont conducted a field study relating silk growth at pollination to kernel set at harvest. Ear shoots were covered before any silks were exposed. On the day before silks were to be exposed to pollen, researchers selected 20 ear shoots, clipped the silks back to the tip of the husk, and covered them with ear shoot bags for 1 day. The next day, the shoot bags were removed from 10 of the shoots, exposing the silks to open pollination for 1 day. These ear shoots were then covered with bags to eliminate additional exposure to pollen.
The remaining 10 ears were hand-harvested. New silk growth was measured from the tip of the husk for silks attached to developing ovules in kernel positions 5, 15, 25, and 35 from the base of the ear. Cob length and width were also measured. Silk lengths were measured by covering the exserted silks with red “carpenter’s chalk.” The ears were carefully extracted from the husks with silks still attached, and the lengths of red silks at the various kernel positions were measured. This procedure was repeated daily during the entire pollination window. Environmental and soil moisture conditions were nearly ideal for corn growth and pollination. After the crop matured, the selected ears were harvested, kernels per ear were counted, and grain yield per ear was measured.
Relating Daily Silk Growth to Grain Yield
The following series of pictures illustrates daily silk growth and kernel set of matched ears during pollination. Data in the graphs are averages of 10 replications. Pictures for a particular day represent average responses.
Pictures for the 12 days during pollination follow the same format: a picture of the developing ear and a picture of a corresponding ear at harvest.
Silks first emerge from near the base of the ear. Silks must intercept pollen for kernel set to occur.
Initially, silks grow most rapidly at the base of the ear. Kernel set matches silk emergence.
Silks along the ear grow at different rates. Silks grow very rapidly for a short time and then begin to slow down in rate of growth. (Field is at 50% silk.)
Under these environmental conditions, maximum silk growth was ~1.6 in/day. Silk growth is slower under drier conditions.
All silks are visible and rapidly growing. Pollination of the entire ear can be completed in 1 day.
All silks are visible and growing. This is the second of 2 days in which the entire ear was pollinated.
Silks toward the base of the ear are growing much more slowly and losing receptivity. Kernel set is poor at the base of the ear.
Under these environmental conditions, silks remained viable for 5 to 6 days. The first silks to emerge are the first silks to lose pollen receptivity.
Silk growth slows while cob growth increases substantially. The ear produces more kernels at the tip to compensate for fewer kernels at the base.
Silks along the entire ear remain receptive to pollen for 5 to 6 days. The sequence in loss of silk receptivity follows the sequence of silk emergence.
As silk growth slows, the developing ear converts more energy into cob growth to provide space, water, and nutrients for developing kernels.
Cumulative Growth During Pollination
- Silks continue to grow until either the developing ovule to which the silk is attached is fertilized, or until the silk completes its life cycle.
- Total silk growth approaches approximately 9 inches past the tip of the husk.
- Based on daily silk and cob measurements during pollination, the developing ear focuses the majority of its resources on silk growth in order to better catch viable pollen.
- After a few days, silk growth slows and the developing ear focuses its resources on cob growth in order to provide space and support for developing kernels.
- The probability of successful pollination decreases as silks enter the latter phases of their life cycles.
Estimating Percent of Pollination
Silks remain attached to the developing ovule until the ovule is fertilized. After the male and female gametes have fused, the developing ovule creates an abscission layer at the base of the silk. The silk no longer receives water and nutrients, causing the silk to turn brown and die. Usually a silk will start to turn brown about 1 day after successful fertilization. A more accurate and timely method to estimate the percent of successful pollination in a field is the following method:
- Remove the ear from the plant.
- Gently peel the husks from the ear so that the silks are disturbed as little as possible.
- Grab the ear by the base and hold the ear so that the tip is pointing toward the ground.
- Gently shake the ear. Silks will fall from fertilized ovules, while silks of ovules not fertilized will remain attached to the developing ovules.
- Percent pollination is equal to the percent of ovules with no silks attached.
Covered ear (silks not exposed to pollen)
Normal ear (silks exposed to pollen daily)
A few silks may remain attached to ovules after pollination is complete. In this study, the silks that were still attached did not grow past the tip of the husk (see picture above). When this occurs, the end result is a mature ear with a few missing kernels. Under certain environmental conditions, some hybrids may produce ears with a few missing kernels at the base of the ear even when moisture and pollination conditions appear to be ideal. In these studies, total growth of silks at the very base of the ear was less than total silk growth along the rest of the ear (see "Cumulative Growth During Pollination"). The combination of slightly less total silk growth at the base of the ear and the tendency for silks to occasionally not emerge from the husk may at least partially explain this incomplete kernel fill.
The crown of the mature corn kernel has a silk scar where the silk was attached to the developing ovule. This silk scar can be difficult to see on kernels of some corn hybrids. The photo below is of a corn line that shows a prominent silk scar at maturity.
Estimating Grain Yield When Incomplete Pollination Occurs
Grain yield per ear is a function of the total number of kernels produced times the weights of these individual kernels. In these studies, grain yield per ear correlated very highly with the number of kernels produced per ear. A small loss in kernel number does not substantially reduce grain yield because the kernels surrounding the area of the missing kernel will often compensate by becoming a bit larger. However, as the loss in kernel count becomes excessive, the remaining kernels on the ear cannot grow sufficiently to compensate for the loss in kernel numbers. Ears produced on "Day 5" and "Day 8" have different appearances but similar grain yields and kernel counts. Corn ears will often compensate for poor kernel fill at the base of the ear by increasing the kernel fill at the tip. The corn plant has a specific amount of resources that it will devote to grain fill. During the grain-filling interval, the corn plant will adapt to the best of its ability with the resources available to produce maximum yield of the viable kernels attached to the corn ear.
Grain yield per acre is also a function of the total number of kernels per acre times the individual weights of these kernels. 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 weights of these kernels (Otegui et al., 1995). Corn growing on an acre of soil under specific environmental conditions is capable of devoting a certain amount of resources to grain yield. Two cornfields with similar grain yields will most likely have similar amounts of kernels produced per acre. The cornfield with the higher population will likely have more but smaller, harvested ears than a cornfield planted at a lower population. The challenge before agronomists and farmers is to manage an ever-changing supply of resources so that the corn properly partitions these resources between vegetative and grain yield demands.
Otegui, M. E., F. H. Andrade, and E. E. Suero. 1995. Growth, water use, and kernel abortion of maize subjected to drought at silking. Field Crops Research 40: 87-94.
Conditions at Pollination Determine Kernel Set in Corn
Because of its impact on kernel number and final yield, the pollination process is one of the most critical periods in the development of the corn plant. While number of kernel positions is determined earlier in the corn plant's development, number of kernels actually set is largely determined near the time of pollination. Losses due to reduced kernel set at pollination cannot be fully regained, even if favorable conditions persist the rest of the season. This article will review the process of kernel set in corn, and recent research studies which evaluate the role of stress in early reproductive development.
Kernel Set Is Often Reduced By Stress
Kernel set in corn requires the successful completion of several plant processes, including pollination, fertilization, and embryo and endosperm development. Pollination includes production of viable pollen by the tassel, and interception by functional silks. The pollen grain adheres to the silk, germinates and sends a structure called the pollen tube down the length of the silk. The pollen tube penetrates the ovary and the male gamete unites with the egg cell to complete fertilization. If embryo and endosperm development is maintained, a kernel will be set.
Under good growing conditions, the sequence of events ending in kernel set usually progresses satisfactorily, but under stress conditions, the process may be interrupted at 1 or more junctures. Tassels may produce less pollen under heat stress, and pollen may lose its viability. When drought stress occurs, silks may delay emergence until pollen shed is nearly over. Drought may also induce ovary dysfunction, or abortion of the newly formed zygote, embryo, or kernel. High temperatures, drought, reduced sunlight, and loss of leaf area have all been identified as stresses which can affect corn during the early reproductive stage.
Timing Of Stress Investigated
Many studies have shown that stress occurring near the pollination stage has a detrimental effect on yield. Most researchers place the beginning of the stress-susceptible period at about 1 week prior to silking. Various studies suggest that this critically sensitive period continues for 1 to 2 weeks after silking, and some extend the period even more.
Reduced kernel set due to various levels of drought stress.
Yield losses during this period result from reduction in kernel number, and are therefore irreversible. Reduction in kernel number may result from incomplete pollination due to asynchrony of pollen shed and silking ("silk delay"), ovary dysfunction due to low water potential, or abortion of the newly formed zygote or embryo due to insufficient carbohydrate availability. The older the kernel, the less chance that abortion will occur.
Pollen Shed Affected By Heat
The location of the tassel exposes it to both high radiation and potential temperature extremes. Researchers have found that extreme high temperatures, rather than drought per se, have the greatest effect on pollen production and viability. Several studies have shown that in vitro pollen viability decreased as tassels were exposed to high temperature treatments, but viability was not affected by drought conditions, even when visible wilt and lower leaf senescence were induced by the low water potential. It can be concluded from these studies that high temperatures are more detrimental than drought stress to pollen development.
Extreme losses in pollen production or viability may be necessary to affect kernel set. Field studies in which pollen amount was limited using male sterile plants showed that a reduction in pollen amount of at least 80% over the course of the pollen shedding period was required to reduce kernel set.
Effect Of Drought On Silk Growth
Silk elongation requires high water potential. Under drought conditions, most silk elongation occurs at night, when water potential is highest. Inhibition of silk growth due to drought stress often results in asynchrony of pollen shed and silk emergence, commonly referred to as "silk delay." A study by Herrero and Johnson3 showed that drought stress can add 3 to 4 days to the normal interval between first pollen shed and first appearance of silks. Consequently, the last silks to appear, those emanating from the tip of the ear, may emerge after most of the pollen has shed. This lack of synchronization is considered to be the primary factor limiting kernel set during drought conditions. Barren or poorly filled ear tips can result from moderate to severe silk delay.
Effect Of Drought On Silk Receptivity
Silks are considered receptive to pollen if they support germination of pollen grains, growth of the pollen tube within the silk, and passage of the pollen tube to the wall of the ovary. Basetti and Westgate1 showed that silks normally remain receptive for about 7 days after emergence from the ear shoot, after which time they senesce, beginning at the base of the silk. However, silks are usually pollinated within 1 or 2 days after emergence, and fertilization occurs within 24 hours of pollination.
Basetti and Westgate2 examined silk receptivity under drought conditions. As expected, silk elongation was slowed by drought stress, and could be completely inhibited under severe stress. The pollen tube growth rate was also slowed by half or more, such that pollen tubes required over 48 hours to reach the ovary. This slower rate of pollen tube growth was only detrimental if pollination was delayed until 5 days after silks first emerged, since silks sometimes senesced at the base before pollen tube growth was completed.
In the field, it is highly unlikely that silk senescence or other loss of silk receptivity causes significant kernel loss. Silks typically are pollinated within 24 hours of exposure, and pollen tube growth is completed well before silks senesce. Other studies have shown that silks pollinated at water potentials low enough to prevent kernel formation were still able to support pollen germination and pollen tube growth. Consequently, evidence points to events occurring during or after fertilization as the cause of reproductive failure.
Effect Of Assimilate On Kernel Abortion
Recently, some researchers began to suspect that drought-induced developmental failure soon after fertilization may be due to starvation for substrate. Drought stress is known to inhibit photosynthesis, and carbohydrate reserves are already low at anthesis. This hypothesis is strongly supported by studies designed to alter assimilate supply at flowering. Shading plants during pollination and early kernel growth decreases seed set, while supplemental light increases the number of kernels. Studies with prolific hybrids known to have high carbohydrate availability to the ear showed less sensitivity to water stress. Studies that supplied additional solutes via stem infusion dramatically increased seed set in water-deficient plants. Collectively, these studies suggest that assimilate supply at flowering may regulate kernel set in water-deficient plants.
To further test this hypothesis Schussler and Westgate4 designed parallel experiments which limited photosynthesis by either drought stress or shading. Drought stress was induced by withholding water to inhibit photosynthesis by 50% or 100% during flowering and early kernel growth. Shading was accomplished by using 50% shade cloth or a dark treatment to likewise inhibit photosynthesis by 50% or 100%. Effect on seed set is shown below:
The moderate drought and 50% shade treatments reduced kernel set by a similar amount. The severe drought and dark treatments, which completely inhibited photosynthesis, also inhibited kernel development. A very high correlation was found between leaf photosynthesis at pollination and kernel set. The results of this experiment suggest that assimilate availability to the developing ear was the factor primarily responsible for kernel loss in water-deficient plants. Reduced kernel set was found to be primarily due to zygotic abortion. However, ovary dysfunction can also occur under low water potential, such that fertilization is inhibited.
1Bassetti, P., and M.E. Westgate. 1993a. Emergence, elongation, and senescence of maize silks. Crop Sci. 33:271-275.
2Bassetti, P., and M.E. Westgate. 1993. Water deficit affects receptivity of maize silks. Crop Sci. 33:279-282.
3Herrero, M.P., and R.R. Johnson. 1981. Drought stress and its effects on maize reproductive systems. Crop Sci. 21:105-110.
4Schussler, J.R., and M.E. Westgate. 1991. Kernel set of maize at low water potential: II. Sensitivity to reduced assimilates at pollination. Crop Sci. 31:1196-1203.
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