The combination of inadequate rainfall and above average temperatures is creating stress conditions in many corn fields across Ohio. According to the National Agricultural Statistics Service (http://www.nass.usda.gov/oh/ ), 75 percent of the state is currently estimated to have subsoil moisture content that is rated short to very short.
During the past week, tassels began appearing in corn fields planted in April. The pollination period, the flowering stage in corn, 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.
Yield losses to moisture stress can be directly related to the number of days that the crop shows stress symptoms. According to Iowa research by Claassen and Shaw on effects of drought, four days of stress (i.e. corn wilted for four consecutive days) at the 12th-14th leaf stage has the potential of reducing yields by 5 to 10 percent. 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.
The following are 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 a.m. followed by a second round of pollen shed late in the afternoon. This pattern reduces pollen exposure to the highest temperatures of the day. Pollen may be shed before the tassel fully emerges (“stretches out"). Pollen shed usually begins in the middle of the central spike of the tassel and spreads out later over the whole tassel with the lower branches last to shed pollen.
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 is light and is often carried considerable distances by the wind.
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 3 to 5 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 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. As noted above, poor kernel set is more often associated with poor timing of pollen shed with silk emergence – with silks emerging after pollen shed (poor “nick”). However, hybrids seldom exhibit this problem unless they experience extreme drought stress.