By Peter Thomison
According to the National Agricultural Statistics Service for the week ending July 28, 2019, 32% of the state’s corn was silking compared to 75% for the 5-year average. Given the wide range in corn planting dates this year, most corn will not achieve tasselling and silking until we are well into August. 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. Stress conditions (such as hail damage and drought) have the greatest impact on yield potential during the reproductive stage. The following are key steps in the corn pollination process.
Most corn hybrids tassel and silk about the same time although some variability exists among hybrids and environments. On a typical midsummer day, peak pollen shed occurs in the morning between 9:00 and 11:00 a.m. followed by a second round of pollen shed late in the afternoon. Pollen may be shed before the tassel fully emerges. Pollen shed 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 (see figure). Pollen is light and is often carried considerable distances by the wind. However, most of it settles within 20 to 50 feet.
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. In addition, 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 as many as 2 million or more pollen grains, which translates to at least 2,000 pollen grains produced for each silk of the ear shoot. Shortages of pollen are a problem under conditions of extreme heat and drought; they may also occur in fields characterized by uneven emergence in later emerging plants. 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, modern hybrids rarely exhibit this problem unless they experience extreme drought stress. Some of the new drought tolerant hybrids have shorter “anthesis silking intervals”, i.e. pollen shedding and silk emergence are more closely synchronized than hybrids more susceptible to drought. This shorter anthesis silking interval mitigates the impact of drought stress during pollination.