The miracle of corn pollination still amazes Bob Nielsen. “It’s a wonder pollen finds its way to each of these silks the way they mat together.
“Can you see all the little hairs on each silk?” asked the Purdue University agronomist, lifting the fragile filaments toward the sun. “They are one of the secrets to what is happening out here.”
Much of the corn in the Midwest has gone through the critical stage of flowering known as pollination over the past week. Success or failure during this period of the corn plant’s life greatly influences the potential grain yield at harvest. Balmy days and cool night temperatures are resulting in good news for many corn farmers in 2014.
“We’ve had some wet feet in some regions, but fields that didn’t suffer an early weather event or late start look really good,” Nielsen told DTN during tour of the university’s 1,100-acre research farm.
Nielsen is a corn guy. Followers of his King Corn and Chat n’ Chew Cafe websites know his fervor for understanding corn as a crop.
There’s a lot going on under the husks and around them too. Each potential kernel (ovules) on the ear develops a silk that elongates and eventually emerges at the tip of the ear. Silks represent the pathway for fertilization and pollen must find each individual silk to result in a kernel of grain.
The yellow or white “dust-like” pollen that falls from a tassel represents millions of nearly microscopic, spherical, pollen grains. Estimates of the total number of pollen grains produced per tassel range from 2 million to 25 million. Each pollen grain contains the male genetic material necessary for fertilizing the ovary of one potential kernel. Once dispersed into the atmosphere, pollen grains remain viable for only a few minutes.
“Pollen is captured by the silk hairs [trichomes],” Nielsen explained. “The pollen germinates and develops a tube that penetrates the silk and elongates down the length of the silk, carrying the male genetic material. All that happens within 24 hours. It really is something,” he said.
Pollen grains can land and germinate anywhere along the length of an exposed receptive silk. Many pollen grains may germinate on a receptive silk, but typically only one will successfully fertilize the ovule.
Within two to three days after the ovules have been successfully fertilized, the base of the silk will collapse and detach from the immature kernel. Those kernels that have not been fertilized remain attached and continue to lengthen and be receptive to pollen for up to 10 days after emergence from the ear shoot.
SHAKE IT BABY
Nielsen uses the ear shake technique to test to see if pollination has occurred and to what extent. 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 ovules. Then gently shake the ear. Silks of fertilized ovules will drop away; silks from unfertilized ovules will remain attached.
“These immature ovules may still make it,” he said, pointing to a few attached silks along the ear. “Maybe they were at the bottom of that mat of silk and took a little longer to get fertilized?”
Nielsen said as silks first emerge from the husk, they lengthen as much as 1.5 inches per day for the first day or two, but gradually slow over the next several days. Elongation of an individual silk stops shortly after pollen is captured, germinates and penetrates the silk.
If not pollinated, silk elongation stops about 10 days after silk emergence as the silk tissue starts to die back. Unusually long silks can be a diagnostic symptom that the ear was not successfully pollinated.
Silks remain receptive to pollen grain germination up to 10 days after silk emergence, but to an ever-decreasing degree. The majority of successful ovule fertilization occurs during the first four to five days after silk emergence.
With practice, pollination progress can be 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.