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July 9 July 15, 2002
C.O.R.N. 2002-21
In This Issue:
A) Reminder On Osu Weed Science Field Day
B) Drought And Heat Stress Effects On Corn Yield Potential
C) Key Steps In Corn Pollination
D) Making Lemonade Out Of A Lemon Of A Season
E) Soybean Aphid
The OSU Weed Science field day is this Wednesday, July 10, at the OARDC Western Branch in South Charleston. This informal tour of OSU research plots provides the opportunity to compare many different herbicide programs and approaches to weed control in corn and soybeans. The field day starts at 9 am and concludes by noon. There is no fee to attend. The OARDC Western Branch is on State Route 41 approximately 5 miles south of I-70. Once at the farm, drive around to the right of the barns; go past the pond, and park near the pine trees.
To estimate the impact of dry hot weather on corn yield potential, let's review the effects of moisture deficits on corn growth and development from the late vegetative stages, prior to pollination, to the dent stage of kernel development. Yield losses to moisture stress can be directly related to the number of days that the crop shows stress symptoms during different growth periods. The following summarizes findings of Iowa research by Claassen and Shaw on effects of drought on grain yields in corn. This Iowa data is widely used in estimating the potential impact of water stress on yield potential.
Vegetative Stages: Drought stress during early vegetative growth usually has a negligible impact on grain yield. However, during later vegetative stages, when kernel numbers per ear are determined, plants are more sensitive to stress. According to Claassen and Shaw's findings, 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 5-10 percent. Kernel row numbers on the ear are determined by the 12th collared leaf stage and the potential number of kernels per row is complete about one week before silking.
Tassel Emergence: As the tip of the tassel begins to emerge from the whorl, the upper stalk internodes rapidly elongate and the ears begin to expand. Silks from the base of the ears are also rapidly elongating. Four days of moisture stress at this stage has the potential to reduce yields 10 to 25%.
Silk Emergence to Pollen Shed: At this stage, leaves and tassels are fully emerged and the cobs and silks are growing rapidly. This is the most critical period in terms of moisture use by the plant. Four days of moisture stress at this stage has the potential to reduce yields 40-50%.
Blister Through Dent Stage of Kernel Development: About 12 to 36 days after silking, the cobs, husks and shanks are fully developed and the kernels are increasing in dry weight. Moisture stress will reduce kernel fill from the ear tip down. Four days of drought at the blister stage has the potential of reducing yields 30-40%, and at dough stage, 20 to 30%.
Where dry weather has contributed to corn stands with uneven emergence, and development, yield loss may range from 5 to 20% depending on various factors such as the length of emergence delays and the percentage and distribution of later emerging plants. Where there is considerable variability in plant size, smaller, stunted plants will be at a competitive disadvantage with larger plants for nutrients, water and sunlight.
The flowering stage in corn is the most critical period in the development of a corn plant from the standpoint of grain yield determination. The recent hot dry weather conditions are cause for concern because drought, high temperature stress, as well as hail damage and insect feeding, have the greatest impact on yield potential during the reproductive stage. The following are some of the key steps and phases in the corn pollination process.
Pollen shed usually begins two to three days prior to silk emergence and continues
for five to eight days with peak shed on the third day. On a typical midsummer
day, the shedding of pollen is in the morning between 9:00 and 11:00 a.m.
The tassel is usually fully emerged and "stretched out" before any
pollen is shed. Pollen shed begins at 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. 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. 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 the silks of the same plant. Under field conditions 97% or more of the kernels produced by each plant are 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. Poor seed set is more often associated with poor timing of pollen shed with silk emergence (silks emerging after pollen shed). However, modern hybrids seldom exhibit this problem in Ohio unless they experience extreme drought stress.
Soybean cyst nematode is reeking havoc on some soybeans this season. The poor growing conditions combined with cyst is more than some of these plants can stand. Fields where there are pockets of soybeans that look especially worse for the wear may be suffering from SCN as well as the weather. You can see cyst females in some of these roots. Carefully dig plants and shake the roots gently, then look for the tiny "pearls" on the roots. This will work much better in the lighter soils. Take notes on these fields - these are the ones that should be sampled for SCN this fall following harvest. SCN fields must be managed by CROP ROTATION - by keeping the SCN populations low optimal yields can be obtained.
Surrounding states are now finding the soybean aphid in
their fields, and it is probably in fields in Ohio. Over the next 2 to 3 weeks
is when we should begin seeing aphids in increasing numbers. Thus, it is essential
that growers be checking for large populations of aphids throughout the rest
of July. Where soybean aphid occurs in large numbers, most people should have
no problem making the correct identification. If soybean aphids are located,
please send an email to hammond.5@osu.edu with the date and location.
For determining the need for treatment, sample the upper leaves from 4-5 plants
from 20 locations in the field, and rate the aphid infestation on a scale of:
0 = none
1 = 1-10 per single leaflet
2 = 10-30 per single leaflet
3 = 30-100 per single leaflet
4 = 100-300 per single leaflet
5 = >300 per single leaflet
An average rating of 3 or higher across the field suggests that soybean aphids are common and abundant, and treatment should be considered. At a rating of 3 or higher, you will probably also find aphids on the petioles and stems that suggest an increasing population. If honeydew is present in the field and noticeable, the aphid population is probably high.
Observations from other states last year suggest that the greatest benefit from an insecticide application will come in July during flowering or immediately thereafter. The later in the summer an insecticide application is made, the less the benefit from that spray. Spraying late in the season after pods have filled or following dispersal of winged aphids in mid-to-late summer will be too late to prevent economic yield loss. Good insecticide coverage is essential and will be best achieved by spraying with higher pressures and higher gallons per acre. The following insecticides are labeled for soybean aphid control, although additional ones might be labeled later. The current insecticides are Asana XL (5.8 to 9.6 fl oz per acre), Furadan (0.25 to 0.5 pt per acre), Lorsban 4E (1 to 2 pts per acre), Mustang (3.4 to 4.3 fl oz per acre), Penncap-M (1-3 pts per acre), and Warrior (1.92 to 3.2 fl oz per acre).
Care should be taken during flowering if bees are actively working a soybean
field. Although bees are not usually associated with soybeans, they do forage
in soybeans and problems could occur if hives are nearby. Label directions pertaining
to most of the insecticides and state regulations state that the insecticide
should NOT be applied to blooming or flowering crops if bees are actively working
in the target area. Thus, most materials for soybean aphid control should be
applied in early morning or late day when bees would not be active in fields.
Ohio regulations state in 901:5-11-04 of the Ohio Administrative Code "No
pesticide which is required to carry a special warning on its label indicating
that it is especially toxic to honey bees, shall be dispensed over an area of
one-half acre or more in which the crop plant is in flower unless the owner
or caretaker of any registered apiary located within one-half mile of the treatment
site shall be notified by the person who applies for himself or engages a pesticide
applicator no less than twenty-four hours in advance of such intended treatment;
provided, however, that these locations are registered and identified as required
by section 909.02 of the Revised Code of Ohio, and that such apiary locations
have been posted with the name and telephone number of the owner or a responsible
caretaker". The names and addresses of registered apiaries are available
at the Ohio Department of Agriculture at 1-800-282-1955. Growers should be advised
to follow all label directions and state regulations; it is the soybean grower's
responsibility!
Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.
Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/
C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.
State Specialists: Anne Dorrance & Dennis Mills (Plant Pathology), Peter Thomison (Corn Production), Mark Loux & Jeff Stachler (Weed Science), and Bruce Eisley & Ron Hammond (IPM). Extension Agents: Steve Bartels (Bulter), Barry Ward (Champaign), Steve Prochaska (Crawford), Greg LaBarge (Fulton), Howard Siegrist (Licking), Clark Hutson (Seneca), Roger Bender (Shelby), and Dave Jones (Allen).Editor: Dave Jones Web Editor: Tom Rosati
Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.
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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.
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