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Corn pollination and fertilization are among the most important phases of crop development. Although much of the yield potential of the corn plant (particularly ear length and row number) is established earlier in the season, successful pollination can help determine the extent to which yield potential is met.
Figure 1. A corn plant at silking stage. A strand of silk can grow 1 to 1.5 inches per day until pollination occurs.
Pollination is the process by which pollen grains are transferred from the tassel to the silks. Fertilization does not occur until the male reproductive cells from the pollen actually unite with female reproductive cells from the ovule. Therefore, successful pollination does not always result in fertilization.
Pollen shed (anthesis) begins shortly after the corn tassel is fully emerged from the whorl (VT growth stage). Pollen shed may occur for up to 2 weeks, but usually lasts for 5 to 8 days, with peak shed by the 3rd day.2 Flowering typically occurs in the morning and may be delayed during rain or excessive humidity. Very hot, dry conditions can reduce pollen viability and decrease length of pollen shed.
Silks from the base of the ear are the first to emerge from the husk, followed by those progressively closer 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 In addition to natural senescence, heat or moisture stress can desiccate the silks prematurely.
Highly variable flowering dates in a given field can reduce total pollen available to receptive silks. Also, severe heat or moisture stress may delay silking and hasten pollen shed to the extent that little pollen remains when silks become receptive. Poor pollination resulting from asynchronous pollen shed and silking can result in barren ears or unfertilized ovules occurring mainly toward the tips of the ears.
Pollen that lands on a silk is captured by small hairs called trichomes present on the surface of the silk. 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. Although many pollen grains may germinate along the surface of the silk, only one grain will generate a pollen tube resulting in fertilization.
Normally, pollination is a continuous process with fertilization occurring gradually along the ear as silks emerge. A mass of long, green silks is an indication that pollination has not occurred. This could be the result of silk emergence after most pollen has shed, or delayed pollen shed due to extended rainy, cloudy conditions. The latter should be of little consequence if flowering resumes prior to silk senescence. However, anything that interferes with the optimum window for pollination could potentially reduce fertilization and kernel set.
Figure 2. After gently removing the husks, observe to see if silks are attached or drop away. Fertilization has not yet occurred in ovules that have attached silks.
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. Slowly and carefully unwrap the husk leaves being careful 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 (Figure 2). When most of the silks easily drop away from the cob, you can be assured of successful pollination. Pollination progress can be determined by estimating the percentage of silks that fall away from the ear and random sampling can help indicate progress for an entire field.
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 in the youngest kernels that are located at the tip of the ear.
1 Anderson, 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. 2 Emberlin, J., Adams-Groom, B., and Tidmarsh, J. 1999. A report on the dispersal of maize pollen. Soil Association. 3 Nielsen, 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