Richard Taylor, Extension Agronomist; rtaylor@udel.edu
Our new Plant Pathologist for the University of Delaware, Dr. Nathan Kleczewski, asked me to put together an article on the impact of wet or humid weather on corn pollination for this issue of the Weekly Crop Update. I’ve spent some time reviewing the literature and not surprisingly mostly it covers the impact of high temperatures and/or drought on corn pollination.
We should all begin on the same base for the discussion that follows so I’ll begin with a quick review of corn reproduction. Corn is a monoecious plant which means that each plant has both male and female reproductive structures present. Unlike many other monoecious grasses and broadleaf plants, corn has the male and female flowers separated on the plant. In addition, corn has self-incompatibility factors that reduce the number of kernels that are self-pollinated to about 3 to 5 percent. Along with the plant’s own self incompatibility, pollen shed (the male’s contribution) and silking (the female conduit to the ovule or ovary) are separate both temporally to some degree, depending mostly on soil moisture, and spatially for each plant.
The corn tassel, or male portion of a corn flower, begins to form very early deep in the plant whorl at about the V5 growth stage. Severe stress, including nutrient stress and, in particular, chilling stress at tassel initiation, can reduce tassel branching and flower formation. Keep this in mind for our later discussion of April through mid-June temperatures and rainfall.
The female flower is the ear which begins formation at about the V5 growth stage. Potential ears form at each node, up to about leaf node 12 to 14. Normally, only the upper-most ear shoot develops at the plant density we use, although several ear shoots can develop along the edge rows where there is more sunlight or in areas where the stand is very thin. The female flowers, containing the ovule that if successfully fertilized by a pollen grain will become a kernel, are in paired rows running the length of the cob. The maximum number of these paired rows is determined soon after ear initiation or from the V5 to V7 growth stage. Ear length or the total number of kernels per row is not determined until just before tassel emergence. Severe stress from all factors (environmental, cultural, and biotic) can interfere with ear formation or row length beginning at the V5 growth stage.
Silks develop and elongate from the surface of each ovary (containing one ovule or egg) on the ear. Silk growth is from the base of the cob and proceeds up to the ear tip. The first silks to emerge are from the base or butt of the ear and usually emerge a few days after pollen shed begins. All silks emerge within 2 to 3 days of first silking. With enough soil moisture, silks can grow from 1 to 1.5 inches in length per day usually reaching a maximum growth rate on the third or fourth day after first silks emerge. Silks continue to grow although more slowly until they die about 10 days after emergence and are receptive the entire time. Silks have small hairs on the surface that capture the pollen and hold it for pollen tube initiation and growth. Silks can desiccate prematurely during heat or moisture stress, creating an erratic pattern of fertilization along the ear although most fertilized ovules (kernels) will be located near the butt end of the ear.
How about pollen shed? This begins around the VT or tassel emergence stage although with older hybrids pollen shed usually did not begin until after the tassel was fully emerged from the last leaf whorl. The male flowers near the main tassel axis open first and then pollen shed proceeds upward and downward and then throughout the lateral tassel branches. In a corn field, some pollen shed can occur for up to 2 weeks but more typically pollen shed is over a 5 to 8 day period with a peak pollen drop around the third day. Pollen shed usually takes place in the morning when temperatures are cooler but not until the tassel and anthers dry from rain, high humidity, or dew. Pollen shed and viability are minimally affected by environmental stress although hot, dry conditions can reduce both pollen viability and the length of pollen shed.
How did the prolonged cold spring and wet June and July weather impact pollination in corn on Delmarva? To start off, let’s go back to April when we had about one third of the days in that month when rain fell. The monthly average temperature was in the low to mid-50s and rainfall totals were generally slightly above average. Many corn growers try to begin to plant as early as possible in April although the rainfall events did keep some corn from being planted until June.
May warmed into the low to mid-60s and rainfall totals were about half to two thirds of those in April. Although the monthly average temperature was higher in May, the daily low temperature fell into the 40s the first 5 to 6 days of May, a three day period at the end of the second week of May (some areas were in the 30s and experienced a frost on winter wheat at this time), and again from May 24-27. These cold periods slowed the growth and development of corn and delayed the sidedress application of nitrogen (N) into late May or June.
I think the real impact on corn reproduction came in June when rain events occurred on half to almost two thirds of the days in the month. Rainfall totaled closed to one foot of rain during the month in some locations and on six days the rainfall totals were greater than 1 inch for many locations. The average monthly temperature was in the low 70s for June but there were still two periods, the end of the first week of June and the beginning of the third week, when daily low temperatures were in the low 50s. Many corn fields were delayed for quite some time in receiving sidedressed N and showed N deficiency symptoms. Excessive rainfall (leaching and denitrification) and the impact from the Clean Air Act that has reduced atmospheric deposition of sulfur (S) put many corn fields under severe nutrient [N, S, and even K (potash)] stress during tassel and ear formation.
Finally at the end of June and through July, we saw the return of some warm temperatures with the monthly average temperature about 78°F. From 6 to 8 days in July daily high temperatures were 90°F. or greater. Rainfall remained above average for the month with about 6 inches at many locations and from 9 to 12 days of rain events.
The nutrient stresses that corn experienced during the development of the reproductive structures will have impacted the yield potential. Another factor was the number of wet, high humidity, or dew impacted days the corn experienced during pollen shed. The tassels do not release pollen as long as they remain wet. Prolonged periods of moisture will delay pollen drop and can lead to excessive silk growth that then makes it difficult for pollen to reach each silk. This may account for an observation our county agricultural agents have made: in some fields it appears than some kernels were never pollinated on the ears.
Although we all know that poor seed set is most often associated with poor timing of pollen shed and silk emergence, we associate that condition with dry and hot weather that can lead to a lack of synchrony between pollen drop and silk emergence. Excessive growth of the silks when pollen drop is delayed by rainy weather was the only aspect I read about associated with excessive moisture. Pollen is not killed by rainfall since pollen drop does not occur until the excess moisture on the tassel dissipates and the silks remain receptive to pollen for a long period of time.
I have just a couple of final comments about corn reproduction. Successful pollination of the ovules does not ensure that they develop into full-sized kernels. We have experienced many cloudy days so far this growing season and I have some concern on whether we are getting the amount of photosythetically active radiation (sunlight) that will maximize corn’s yield potential. Early during ovule (kernel) development, the lack of enough photosynthate (sugars) from the leaves or mobilized from the stalks can lead to abortion of some of the fertilized ovules. This is often seen as poor tip fill where the tips are blank or have aborted kernels showing a slightly yellowish color. Early season soil compaction (driving equipment in a field where the soil is too wet) and saturated soil conditions also can contribute to abnormal development leading to tassel ears which we’ve seen in a number of fields this year.
Another thing I’ve observed is multiple ear syndrome or “bouquet ears”. This is characterized by multiple ears forming on the same ear shank forming a “bouquet”. The cause of this is unknown but might be due to the severe environmental and nutrient stresses the corn experienced this year.
Another type of ear abnormality to watch for is the blunt ear syndrome or “beer can” ears. This syndrome is associated with temperature stress usually thought to be due to a brief cold shock during early ear formation stages (V8-V12). This could have occurred in either the May or June periods when we had low temperatures in the upper-30s, mid-40s, or low-50s. This syndrome is characterized by ears with markedly reduce ear size and kernel numbers per row. Husk length and kernel row number often are normal. It is sometimes associated with multiple ears at each node (see above paragraph). It usually is scattered across a field and during late grain fill the development of red and purple pigments in leaves and leaf sheaths can indicate the potential for this problem.
I hope this discussion has been useful to you and helped raise your awareness of the many stresses that corn has experienced this growing season. Many fields have areas of the field where the plants have been drowned out and I expect other areas will be impacted by the nutrient stresses we’ve experienced due to the high amount of rainfall this season as well as possible pollination or reproductive limitations from the weather.