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13;10;9;9;9;9;9;Corn pollination is dependent on the health and viability of the reproductive structures on a plant, the silk and ear (female) and tassel (male). Early in the life of the plant, about V5 growth stage, both structures are initiated. If a plant is dissected at V5, miniature ears can be found at many of the leaf nodes and a small tassel can also be found. Shortly after ear initiation, the number of kernel rows is determined; however, ear length is determined just before tasseling. Early environmental conditions and other stresses can influence the number of kernel rows that are developed.
Each individual kernel is an ovule and will have a silk associated with it. The miniature tassel consists of the many spikelets that line each tassel branch. The spikelets contain two flowers or florets that contain the male reproductive structures or stamen. Three anthers that produce thousands of pollen grains are associated with the stamen of each floret. Multiplying the number of spikelets on a tassel by the pollen grains produced within each one, shows that the number of total pollen grains for each tassel can be several million. Even with all this pollen, less than five percent of the ovules on an individual ear are typically fertilized with pollen originating from the plant’s own tassel. Severe stress at V5 can potentially reduce tassel branching and spikelet formation.1
13;10;9;9;9;9;9;Shortly after the tassel is fully emerged from the whorl (VT growth stage), pollen shed or anthesis begins. Tassel spikelets along the main tassel stem open first and release the pollen within. Spikelet opening continues up and down the main axis and throughout the branches for up to two weeks; however, five to eight days is normal with a peak release around day three.2 Pollen shed is heaviest in the early morning and can be delayed during rain or extremely high humidity. Generally, pollen shed and viability are minimally affected by average environmental conditions; however, extremely hot and dry conditions can reduce pollen viability and decrease the length of time pollen is shed.
About two days after pollen starts to drop, silks begin emerging from the ear husk. Silks originating from the base of the ear appear first, with those from the ear tip emerging last. All silks typically emerge within two to three days (Figure 1). Silk elongation can continue to some extent until one pollen grain of the many that lands on it initiates the growth of a pollen tube to the ovule or potential kernel. Silk viability is around ten days but may be as long as 14 days.3 An abundance of long green silks is an indication that fertilization has not taken place.
Fertilization of the ovule occurs within 12 to 28 hours after the pollen tube initiates. Once the ovule is fertilized, the silk detaches from the ovule and begins to die and turn brown. Fertilization success can be determined by removing an ear and carefully removing the husks to view the potential kernels and silks. Shake the ear and observe for undetached silks; if attached, the ovule is not fertilized.
Several situations can cause unfertilized ovules including: 1) silk emergence after pollen has become non-viable, 2) delayed pollen shed because of extended rainy and cloudy conditions, 3) silks clipped by insects, and 4) other injuries to the silks such as hail. In rare occurrences, a condition called “silk balling�? may occur. In this situation, silks become wadded up under the husk and as a result are unavailable for pollen grains. 13;10;The condition likely develops when the husks are so tight around the developing ear that the silks are prevented from growing out normally.13;10;13;10;9;9;9;9;9;
13;10;9;9;9;9;9;Several conditions can cause kernel set to be less than desirable. The reduction in kernels can occur early in the plant’s life, at pollination, or kernels may abort because of other conditions after fertilization.13;10;13;10;9;9;9;9;9;
13;10;9;9;9;9;9;Drought Stress. Plants undergoing drought stress usually push the tassel out of the whorl. However, silks require moisture to grow and extend beyond the husk; therefore, plants under severe drought stress can have a reduction in kernel number because silks are not available (Figure 2).
Farmers with access to irrigation can apply water during this critical time to help reduce the effect of drought and silk retardation.
13;10;9;9;9;9;9;Insect Feeding. Corn rootworm beetles and Japanese beetles prefer to feed on corn silks. Clipped silks may not be able to re-grow and become receptive before pollen becomes non-viable with high insect populations and associated silk feeding. Crop scouting can help identify potential insect problems. If thresholds are surpassed, appropriate insecticides may be applied to reduce their effect and help preserve or protect ovule fertilization.
Pollen Desiccation. It is unusual for pollen to desiccate or die prior to successful fertilization of the silks. However, if the plant is under severe heat stress, pollen can dry out and become non-viable.
Extended Rainy or Cloudy Weather. The photosynthesis process is interrupted during rainfall and significant cloud cover. Should this occur for an extended period during pollination, pollen may not be released when the silks are receptive. This could be evident at different areas on the ear depending on when the stress occurred and which silks were available.
Temperature Variances. Temperature variances just before and during pollination may delay tassel emergence or cause silks to become non-receptive.4 As an example, temperatures could be relatively warm just prior to tassel emergence and then become very cool when tasseling and silking start.
Kernel Abortion. Aborted kernels appear different than unfertilized ovules. Aborted kernels have a white to yellowish color because starch was beginning to develop in the kernel. Unfertilized ovules have no coloration or development. Aborted kernels can be caused by plant stresses occurring just after pollination has occurred. Heat, drought, saturated soils, extreme temperature variances, extended rainy and cloudy days, high night-time temperatures, and fertility deficiencies are among the stresses that can cause tip back or unfilled ear tips (Figure 3). In general, all of these have an effect on photosynthesis and energy production. When energy is reduced, the plant responds by reducing the number of kernels.
13;10;9;9;9;9;9;When reduced kernel set is found, regardless of the reason, there is nothing that can be done to change the situation for the current crop. However, scouting and determining the cause(s) may lead to management practices that can help reduce the potential for losing ovules in future crops. Temperature and drought conditions cannot be controlled but tillage, water and fertility management, plant population and spacing, seed product selection, insect control, and timely herbicide application are among the many factors that may help reduce plant stress, improve pollination success, and ultimately increase the number of kernels set. 13;10;13;10;9;9;9;9;9;