Corn Pollination: What does success look like?
Awww ... sex in the corn field. It's happening all around us. For the next few weeks, pollination and fertilization of corn ovules will be occurring throughout Wisconsin. The success of pollination will determine management decisions as the growing season progresses.
Pollen shed begins near VT and is essential for grain development. During this 1 to 2 week pollination period, each silk must emerge from the ear husk, and a pollen grain must land on the ovule and fertilize it for a kernel to develop.
When a pollen grain lands on a silk, a pollen tube is initiated. The pollen tube grows within the silk to the ovule where fertilization occurs and the kernel embryo is formed.
A second fertilization also takes place that results in the formation of the endosperm. Immediately following fertilization, an abscission layer forms at the base of the silk, restricting entry of genetic material from other pollen grains.
Pollen sheds from the male flowers on the tassel for 5-8 days and is dependent upon temperature, moisture and time of day (peaks around mid- to late-morning or early evenings). Pollen grain is viable for 12-18 hours (less in higher temperatures) after it drops from the tassel; most pollen falls within 20-50 feet of the plant.
Window of opportunity
"Nick" is the period when pollen shed (VT) coincides with silk receptivity (R1). Poor nick can result from hot and dry weather. Silks can be delayed and dehydrate, which hastens pollen shed and causes the plant to miss the window for pollination.
Silks will grow for 3-5 days or until pollination occurs. Silks will turn brown once outside the husk. Stresses that reduce pollination can result in an ear with a barren tip called a "nubbin." Each kernel has a noticeable point where the silk was attached; the kernel is surrounded by paleas, lemmas, and glumes.
After an ovule is fertilized, cell division occurs within the kernel for ~7-10 days. After cell division is complete, the cells fill. The outer part of the kernel is white, and the inner part is clear with very little fluid. The embryo is not yet visible. The kernel endosperm fills with photsynthate, most of which is produced by the leaf on the same node as the ear shank; this ear leaf provides up to 60% of the total grain yield.
Success or failure
There are two techniques commonly used to assess the success or failure of pollination. One involves simply waiting until the developing ovules (kernels) appear as watery blisters (the "blister" stage of kernel development). This usually occurs about 10 days after fertilization of the ovules.
Another more rapid means can be used to determine pollination success. As described above, each potential kernel on the ear has a silk attached to it. Once a pollen grain "lands" on an individual silk, it quickly germinates and produces a pollen tube that grows the length of the silk to fertilize the ovule in 12 to 28 hours.
Within 1 to 3 days after a silk is pollinated and fertilization of the ovule is successful, the silk will detach from the developing kernel. Unfertilized ovules will still have attached silks. Silks turn brown and dry up after the fertilization process occurs.
By carefully unwrapping the husk leaves from an ear and then gently shaking the ear, the silks from the fertilized ovules will readily drop off. Keep in mind that silks can remain receptive to pollen up to 10 days after emergence.
The proportion of silks dropping off the ear indicates the proportion of fertilized ovules (future kernels) on an ear. Sampling several ears at random throughout a field will provide an indication of the progress of pollination.
If pollination is poor, then harvest can occur anytime. If pollination is fair, then leave for silage harvest. If pollination is good, then normal management of the field can occur for either silage or grain uses.
University of Wisconsin CALS Agronomy professor Joe Lauer is an expert on corn production, transgenic crops and cropping systems in the Midwest.