By Matt Reese
The rain had to stop sometime. After a year of record setting rainfall amounts that plagued crop producers trying to get any fieldwork done, the faucet shut off in the early growing season this spring. The welcomed dry conditions allowed for a great start to the season, but have since turned into a source of growing concern.
“The season started out really well. We didn’t start as early as a lot of people in the neighborhood. We got started around April 14 and finished corn by the 24th,” said Chad Kemp, who farms in Preble County. “We started planting soybeans on April 30 and got done on May 15. Everything went in really well and we got a great start with everything. Since the planting season ended on May 15, though, we’ve only gotten about .6-inch of rain. The rains have come .2, .2, .1, and a half a tenth. We started seeing signs of stress in mid-June when the crops started rolling up in the 90-degree weather.”
The earliest planted corn on the farm has started to tassel and pollinate as dry conditions continue through the current extended temperatures near 100 degrees, and some of the later planted corn never got a good start in the already dry soils.
“If it starts raining, the guys who planted later are going to look pretty smart,” Kemp said. “If it doesn’t rain, we’re all in the same boat, and it won’t matter when you planted. If we don’t get significant rainfall in early July, I think it is going to really hurt. I think we have already really been hurt.”
Of course, the pollination period is the most critical period in the development of a corn plant from the standpoint of grain yield determination.
“Drought effects on yield potential are greatest during the reproductive stage,” said Peter Thomison, Ohio State University Extension corn specialist. “The potential for yield losses to soil moisture deficits increases dramatically when plants begin to flower. During tassel emergence, four days of moisture stress has the potential to reduce yields 10% to 25%. Silk emergence is the most critical period in terms of moisture use by the plant. During this stage, leaves and tassels are fully emerged and the cobs and silks are growing rapidly. Four days of moisture stress during silk emergence has the potential to reduce yields 40% to 50%. Keep in mind that the stress conditions we are alluding to over these four day periods are severe and involve extensive leaf rolling, characterized by plants with ‘pineapple’ like leaves throughout much of the day. Fields with scattered plants exhibiting some leaf rolling late in the afternoon are probably not experiencing severe stress.”
In a recent CORN Newsletter, Thomison pointed out the following key steps in the corn pollination process.
• Past studies indicate that pollen shed may begin up to three days prior to silk emergence and continue for five to eight days with peak shed on the third day. However, silks may actually emerge before tassels fully emerge and pollen shed starts in certain hybrids under favorable conditions. Under very dry conditions, silk emergence may be delayed, and such “asynchronization” of pollen shed and silking may result in poor kernel set and reduced grain yields. When such delays in silking are lengthy, varying degrees of barrenness will result. This year it’s very likely that silk emergence will be delayed in severely drought-stressed corn fields unless we receive some timely rain.
• On a typical midsummer day, peak pollen shed usually occurs in the morning between 9:00 and 11:00 followed by a second round of pollen shed late in the afternoon.
• Pollen grains are borne in anthers, each of which contains a large number of pollen grains. The anthers open and the pollen grains pour out in early to mid morning after dew has dried off the tassels.
• Pollen shed is not a continuous process. It stops when the tassel is too wet or too dry and begins again when temperature conditions are favorable. Pollen stands little chance of being washed off the silks during a rainstorm as little to none is shed when the tassel is wet. Also, silks are covered with fine, sticky hairs, which serve to catch and anchor pollen grains.
• Under favorable conditions, pollen grain remains viable for only 18 to 24 hours. However, the pollen grain starts growth of the pollen tube down the silk channel within minutes of coming in contact with a silk and the pollen tube grows the length of the silk and enters the female flower (ovule) in 12 to 28 hours.
• A well-developed ear shoot should have 750 to 1,000 ovules (potential kernels) each producing a silk. The silks from near the base of the ear emerge first and those from the tip appear last. Under good conditions, all silks will emerge and be ready for pollination within three to five days and this usually provides adequate time for all silks to be pollinated before pollen shed ceases.
• Pollen of a given plant rarely fertilizes all the silks of the same plant. Under field conditions, 97% or more of the kernels produced by each plant may be pollinated by other plants in the field. The amount of pollen is rarely a cause of poor kernel set. Each tassel contains from 2 million to 5 million pollen grains, which translates to 2,000 to 5,000 pollen grains produced for each silk of the ear shoot. Shortages of pollen are usually only a problem under conditions of extreme heat and drought.