Published 2001 (Rev July 2007)
URL: http://www.kingcorn.org/news/timeless/EarShake.html

A Fast & Accurate Pregnancy Test for Corn

R.L. (Bob) Nielsen
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
Email address:
 

• Silk clipping by insects can interfere with pollination success.
• Silks normally detach from fertilized ovules within days of successful pollination and thus can be used as an early indicator of pollination progress and/or success.

Silk clipping by corn rootworm beetles, Japanese beetles, and other obnoxious critters during pollination can be severe enough to impede pollen capture and germination. While you may be tempted to apply insecticides at the first sign of these insects, Purdue entomologists tell us that treatment is not necessary unless the silks are being continuously clipped back to less than 1/2 inch long before pollination is 50 percent complete (Krupke et al., 2007). Silk length is easy to measure, but how do you determine the progress of pollination?

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Obviously, one could wait impatiently until kernel development was visibly apparent. Within about 10 to 14 days after pollen shed, developing kernels will resemble white blisters on the cob (Nielsen, 2007a). Unfortunately, by then any insect interference with the pollination process will have already taken its toll on kernel set. Luckily for us, the corn plant exhibits an earlier indicator of pollination progress.

Silk Detachment From Fertilized Ovules

Remember that each ovule (potential kernel) on the ear develops a silk (the functional “style” of the female flower) that elongates and eventually emerges through the ear’s husk leaves the tip of the ear shoot (Nielsen, 2007b). The silks represent the "pathway" for the male gametes in the pollen to fertilize the female gametes in the ovules.

Once a pollen grain is "captured" by a trichome or "hair" of a silk, the pollen grain germinates and develops a pollen tube that contains the male genetic material. The pollen tube penetrates the silk and, with adequate moisture and temperature, elongates down the length of the silk within 24 hours and fertilizes the ovule.

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Silks (styles) attached to ovules on ear shoot.	Silks emerging from husk leaves of ear shoot.
Pollen grains captured on trichomes or "hairs" of silks.	Schematic drawing of pollen tube development within a silk.

Within 2 to 3 days after an ovule has been successfully fertilized, the base of the silk will collapse and detach from the immature kernel. The kernel itself will usually not be recognizable to the naked eye at this stage. Silks of nonfertilized ovules remain attached, however, and will continue to lengthen and be receptive to pollen for up to 10 days after emergence from the ear shoot. Even if never fertilized, silks will remain attached to the ovules. Within days of full silk emergence, therefore, pollination progress may be estimated on individual ears by estimating percent silk detachment.

Closeup of attached vs. detached silks.

Closeup of attached vs. detached silks.

The Ear Shake Technique

For each ear, make a single lengthwise cut from the base of the ear shoot to the tip with a sharp knife, through the husk leaves to the cob. Slowly unwrap the husk leaves, taking care not to rip any silks from the ovules yourself. Then gently shake the ear. Silks of fertilized ovules will drop away; silks from unfertilized ovules will remain attached.

With practice, pollination progress can be easily determined by estimating the percentage of silks that fall away from the cob. Sampling several ears at random throughout a field will provide an indication of the progress of pollination for the whole field.

Ear with few detached silks, indicating little ovule fertilization.

Ear with majority of silks detached, indicating nearly complete ovule fertilization.

One Last Comment

While the "ear shake" technique helps you estimate pollination progress with respect to ovule fertilization, remember that pollination progress includes pollen shed progress. Check the tassels in early to mid-morning hours to determine whether anther exsertion and pollen shed are still occurring.

Remember, that anther exsertion and pollen shed typically begin on the central tassel stalk, then spreads progressively throughout the tassel branches (Nielsen, 2007c). If no further pollen shed is likely to occur, it doesn't matter how badly those nasty insects are clipping silks. Unfertilized ovules will remain unfertilized ovules if there is no pollen left in the field. Spraying the bejeebers out of a field at that point is simply a costly form of revenge!

Related References

Krupke, Christian H., Larry W. Bledsoe, and John L. Obermeyer. 2007. Managing Corn Rootworms - 2007. Purdue Univ. Coop. Ext. Service Pub. No. E-49-W. [On-Line]. Available at http://www.entm.purdue.edu/Entomology/ext/targets/e-series/EseriesPDF/E-49.pdf. (URL verified 7/2/07).

Nielsen, R.L. (Bob). 2007a. Grain Fill Stages in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/GrainFill.html (URL verified 7/2/07).

Nielsen, R.L. (Bob). 2007b. Silk Emergence. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/Silks.html (URL verified 7/2/07).

Nielsen, R.L. (Bob). 2007c. Tassel Emergence & Pollen Shed. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/Tassels.html (URL verified 7/2/07).