June 15, 2018
- The processes of corn pollination and subsequent fertilization are among the most important phases of crop development.
- Although much of the corn yield potential is established earlier in the season, successful pollination can help determine the extent that yield potential is met.
- Understanding the pollination process is the first step in learning how to manage for successful kernel set and grain fill.
Parts of the Corn Flower
Corn is monoecious, which means that male and female reproductive structures are present on each plant. However, unlike many other monoecious grasses and dicots, male and female flowers are in separate locations on the plant. Given the separation of the ear and tassel and considering the vast amounts of pollen transported within a field, it is understandable why corn is primarily cross-pollinated. Only a very small percentage (<5%) of kernels may be fertilized by pollen from the same plant.2
The tassel is the male flower of corn. Each tassel is comprised of a central tassel stalk and lateral branches. Around 1,000 spikelets are formed on each tassel bearing 2 small florets. Contained within each floret are 3 anthers, producing thousands of pollen grains. The anther and attached filament comprise the stamen or male reproductive structure.
Figure 1. Tassel with anthers visible on central spike.
The ear is the female flower of corn. Potential ears are initiated at each node up to about the 12th to 14th leaf node, but typically only the uppermost ear fully develops. The female florets are located in paired rows along the surface of the ear. The florets contain the ovules that will become kernels upon successful fertilization. A primary ear may develop up to 1,000 ovules, and around 400-700 are usually harvested. Row number is determined shortly after ear initiation, but ear length is not set until just before tasseling.
Silks develop and elongate from the surface of each ovary on the ear. The silk functions as the stigma and style of the female flower providing a pathway for the male reproductive cells to reach the ovule. Silks begin growing from ovaries at the base of the ear, then progress toward the tip.
Figure 2. A corn plant at silking stage. A strand of silk can grow 1 to 1.5 inches per day until pollination occurs.
Pollination and Fertilization
Pollination occurs when pollen grains are transferred from the tassel to the silks. Fertilization does not occur until the male reproductive cells (pollen) unite with female reproductive cells from the ovule. Therefore, successful pollination does not always result in fertilization. Pollen shed begins shortly after the corn tassel is fully emerged from the whorl (VT stage). Pollen shed usually lasts for 5 to 8 days with peak shed by the 3rd day.2 Flowering typically occurs in the morning with delays during rain or excessive humidity. Hot, dry conditions can reduce pollen viability and decrease length of shed.
Silks from the base of the ear are the first to emerge from the husk, and continue up to the ear tip. Silk longevity is around 10 days under typical growing conditions, but because not all silks are exposed simultaneously, viable silks may be present for around 14 days.1 Heat or moisture stress can desiccate the silks prematurely.
Variable flowering dates in a given field may reduce total pollen available to receptive silks. Severe heat or moisture stress may delay silking and hasten pollen shed reducing fertilization. Poor pollination can cause barren ears or unfertilized ovules near the tips of the ears.
Pollen that lands on a silk is captured by small hairs called trichomes. The pollen grain germinates immediately, producing a pollen tube that grows down the length of the silk, resulting in fertilization of the ovule within 12 to 28 hours.
Normally, pollination is a continuous process with fertilization occurring gradually along the ear as silks emerge. A mass of long, green silks indicates that pollination has not occurred. Anything that interferes with pollination may reduce fertilization and kernel set.
Within a few days of successful fertilization, silks will detach from fertilized ovules. This can be observed by performing the “ear shake test."3 Make one long cut through the husk leaves from the base of the ear to the tip. Carefully unwrap the husk leaves making sure not to pull any silks away. When holding the ear from the base end, you will notice that silks either drop away from the cob or remain attached. If silks drop away successful pollination has occurred.
Successful fertilization does not always result in a harvestable kernel. For several weeks following fertilization, reduced photosynthate caused by cloudy conditions, moisture stress, heat stress, or any factor reducing photosynthetic rate can cause fertilized ovules to abort. This normally occurs to kernels located near the tip of the ear.
Management Considerations
- Water use requirements for corn are highest during pollination. Where available, irrigation can mitigate pollination problems and enhance grain fill.
- Nitrogen and phosphorous uptake are rapid during pollination and grain fill; therefore, proper fertility is necessary for optimum kernel set and reducing mobilization of nutrients from the stalk.
- Always select corn products that exhibit proper heat and drought tolerance for a geography. Spread risk by diversifying product selection with different maturities.
- If considering an insecticide application for silk-clipping insects, make sure the pollination window is not missed. Checked tassels early to mid-morning for pollen shed. When tassels are no longer shedding pollen, silks do not need protection.
Sources:
1Anderson, S.R., Lauer, M.J., Schoper, J.B., and Shibles, R.M. 2004. Pollination timing effects on kernel set and silk receptivity in four maize hybrids. Crop Sci. 44:464-473
2Emberlin, J., Adams-Groom, B., and Tidmarsh, J. 1999. A report on the dispersal of maize pollen. Soil Association
3Nielsen, R.L. 2012. A fast & accurate pregnancy test for corn. Corny News Network. Purdue University. http://www.agry.purdue.edu/
Cárcova, J., Uribelarrea, M, Borrás, L., Otegui, M.E., and Westgate, M.E. 2000. Synchronous pollination within and between ear improves kernel set in maize. Crop Sci. 40:1056-1061
Corn production: A growing problem? 2007. Integrated Crop Management. Iowa State University Extension
Nielsen, R.L. 2007. Ear size determination in corn. Corny News Network, Purdue University. http://www.agry.purdue.edu/
Nielsen, R.L. 2010. Silk Development and Emergence in Corn. Corny News Network, Purdue University. http://www.agry.purdue.edu/
Nielsen, R.L. 2010. Tassel emergence and pollen shed. Corny News Network, Purdue University. http://www.agry.purdue.edu/
Purseglove, J.W. 1972. Tropical crops. Monocotyledons 1. Longman Group, London
Ritchie, S.W., Hanway, J.J., and Benson, G.O. 1993. How a corn plant develops. Iowa State University. Sp. Rpt. No. 48.
Web sources verified 04/27/16. 140510060646