STRAIGHT FROM THE HORSES MOUTH

Duane A. Lienemann,
UNL Extension Educator,
Webster County
July 6, 2012 Edition
I would bet it is not a surprise to anyone that we have been experiencing an unusual span of hot and dry weather and it looks like more is on its way. While it is a good time to go to the lake or the local swimming hole, or just vegetate in front of an air conditioner, our crops may be paying the price. With the heat hovering near the 100-degree mark about the same time as much of the corn was entering the typical 8-10 day pollination window, we can expect some problems, and here’s why.
According to UNL research on effects of drought, just 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, those 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 I am alluding to over that “four day period” are severe and involve extensive leaf rolling (pineapple) throughout much of the day. Fields with scattered plants exhibiting some leaf rolling late in the afternoon are probably not experiencing severe stress.
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, high temperature stress, as well as hail damage and insect feeding have the greatest impact on yield potential during this reproductive stage. In years when we get high day and nighttime temperatures coinciding with the peak pollination period, we can expect problems. Continual heat exposure before and during pollination worsens the response. Daytime temperatures have consistently stayed in the upper 90s to low 100s. With high daytime temperatures, the efficiency of photosynthesis decreases, so the plant makes less sugar to use or store. High nighttime temperatures increase the respiration rate of the plant, causing it to use up or waste sugars for growth and development.
Heat, especially combined with lack of water, has devastating effects on silking. If plants are slow to silk, the bulk of the pollen may already be shed and gone. Unfortunately in some dryland fields we see seed set problems because of timing problems between pollen and silking and even in some stressed areas within irrigated fields we can see stress-induced slow silking and resulting seed set issues. But historically, one of the most important problems leading to yield reduction, particularly in stressful years is the desiccation of the silk. Once silks begin to desiccate (dry up), they lose their capacity for pollen tube growth and fertilization. Even with adequate moisture and timely silking, heat alone can desiccate silks so that they become non-receptive to pollen. While this is a bigger problem when humidity is low, it is apparent that it is happening this year, especially on hybrids that silk quite early relative to pollen shed. Even with higher dew points, when temperatures reach the high 90s to the100s, the heat can still desiccate silks and reduce silk fertility.
Heat also affects pollen production and viability. First, heat over 95°F depresses pollen production. Continuous heat, over several days before and during pollen-shed, results in only a fraction of normal pollen being formed, probably because of the reduced sugar available. In addition, heat reduces the period of pollen viability to a couple hours (or even less). While there is normally a surplus of pollen, heat can reduce the fertility and amount available for fertilization of silks and prolonged exposure to temperatures typically reduce the volume of pollen shed and dramatically reduced its viability.
From a scientific point of view, for each kernel of grain to be produced, one silk needs to be fertilized by one pollen grain. 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. Now here comes the potential enigma. Shortages of pollen are usually only a problem under conditions of extreme heat and drought. Poor seed set is more often associated with poor timing of pollen shed with silk emergence (silks emerging after pollen shed). I would say that we likely have faced or soon will face both of these factors and there is no doubt in my mind that the heat and climatic conditions will have an effect on pollination and potential yield.
Now the question is, how can we determine if our corn was affected? I might suggest that there are two techniques commonly used to assess the success or failure of pollination. One involves simply waiting until the developing ovules (kernels) appear as watery blisters (the "blister" stage of kernel development). This usually occurs about 10 days after fertilization of the ovules. Another more rapid means can be used to determine pollination success. Each potential kernel on the ear has a silk attached to it. Once a pollen grain "lands" on an individual silk, it quickly germinates and produces a pollen tube that grows the length of the silk to fertilize the ovule in 12 to 28 hours.
Within 1 to 3 days after a silk is pollinated and fertilization of the ovule is successful, the silk will detach from the developing kernel. Unfertilized ovules will still have attached silks. Silks turn brown and dry up after the fertilization process occurs. By carefully unwrapping the husk leaves from an ear and then gently shaking the ear, the silks from the fertilized ovules will readily drop off. Keep in mind that silks can remain receptive to pollen up to 10 days after emergence. The proportion of silks dropping off the ear indicates the proportion of fertilized ovules (future kernels) on an ear. Sampling several ears at random throughout a field will provide an indication of the progress of pollination. Good Luck!

The preceding information comes from the research and personal observations of the writer which may or may not reflect the views of UNL or UNL Extension. For more further information on these or other topics contact D. A. Lienemann, UNL Extension Educator for Webster County in Red Cloud, (402) 746-3417 or email to: dlienemann2@unl.edu or go to the website at: www.webster.unl.edu/home