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August 6 - August 12, 2002
C.O.R.N. 2002-25
In This Issue:
A) Kernel Development and Drought Stress
B) Estimating Yield Losses In Drought Damaged Corn Fields
C) Soybean Aphid Update
D) Two-spotted Spider Mite on Soybeans
E) Bean Leaf Beetle
F) Potato Leafhoppers on Alfalfa
G) Harvesting Drought Stressed Crops for Silage and Forage
Meeting
Following pollination, kernel development (or grain fill) is the most critical period in the development of the corn plant for the determination of grain yield. Kernel development proceeds through a number of stages which are characterized by such terms as blister, milk, roasting ear, soft dough, dent, etc. Since these descriptive terms can sometimes be difficult to interpret, alternatives systems have been proposed. A staging system widely used by agronomists and crop consultants divides kernel development into six stages, designated numerically as R1, R2, through R6. The table below lists these kernel developmental stages in sequence.
Keep in mind specific number of days and heat units associated with each stage may vary from season to season, from location to location, and from hybrid to hybrid.
As was noted in a previous C.O.R.N. article, severe drought (4 consecutive days of visible leaf wilting) has the potential to reduce grain yields as much as 30 to 40% at the blister stage (R2), and 20 to 30% at the dough stage (R4).
Drought, high temperatures, nutrient deficiency, disease or insect injury, shading, hail damage, overpopulation, and other stress factors during grain fill may cause complete abortion of kernels toward the ear tip ("tip dieback"). Ear tip kernel abortion occurs when the youngest kernels resulting from the most recent pollination are cut off from nutrient flow because the supply is insufficient to fill all the kernels that have been set. Such kernel abortion is most likely to occur during the first two weeks after pollination (during the blister stage). These same stress factors may also reduce kernel size and weight. Premature plant death resulting from drought, diseases (such as stalk rots), or frost cuts off starch accumulation and results in small, light-weight (low test weight) kernels.
THE YIELD COMPONENT METHOD can be used as early as the milk stage of kernel development. The yield component method uses a numerical constant for kernel weight in an equation to calculate grain yield. Since weight per kernel will vary depending on hybrid and environment, the yield component method should be used only to estimate relative grain yields, i.e. "ballpark" grain yields.
When below normal rainfall occurs during grain fill (resulting in low kernel weights), the yield component method will OVERESTIMATE yields. In a year with good grain fill conditions (resulting in high kernel weights) the method will underestimate grain yields.
Step 1. Count the number of harvestable ears in a length of row equivalent
to 1/1000th acre.
For 30-inch rows, this would be 17 ft. 5 in.
Step 2. On every fifth ear, count the number of kernel rows per ear and determine
the average.
Step 3. On each of these ears count the number of kernels per row and determine
the average.
(Do not count kernels on either the butt or tip of the ear that are less than
half the size of normal size kernels.)
Step 4. Yield (bushels per acre) equals (ear #) x (avg. row #) x (avg. kernel
#) divided by 90.
Step 5. Repeat the procedure for at least four additional sites across the field.
Example: You are evaluating a field with 30-inch rows. You counted 24 ears (per 17' 5" = row section). Sampling every fifth ear resulted in an average row number of 16 and an average number of kernels per row of 30. The estimated yield for that site in the field would be (24 x 16 x 30) divided by 90, which equals 128 bu/acre.
THE EAR WEIGHT METHOD can only be used after the grain is physiologically mature (black layer), which occurs at about 30-35% grain moisture. Since this method is based on actual ear weight, it should be more accurate than the yield component method above. However, there still is a 'fudge factor' in the formula to account for average shellout percentage.
Sample several sites in the field. At each site, measure off a length of row equal to 1/1000th acre. Count the number of harvestable ears in the 1/1000th acre. Weigh every fifth ear and calculate the average ear weight (pounds) for the site. Hand shell the same ears, mix the grain well, and determine an average percent grain moisture with a portable moisture tester.
Calculate estimated grain yield as follows:
Step A) Multiply ear number by average ear weight.
Step B) Multiply average grain moisture by 1.411.
Step C) Add 46.2 to the result from step B.
Step D) Divide the result from step A by the result from step C.
Step E) Multiply the result from step D by 1,000.
Example: You are evaluating a field with 30-inch rows. You counted 24 ears (per 17 ft. 5 in. section). Sampling every fifth ear resulted in an average ear weight of 1/2 pound. The average grain moisture was 30 percent. Estimated yield would be [(24 x 0.5) / ((1.411 x 30) + 46.2)] x 1,000, which equals 135 bu/acre.
Since drought stress conditions this year have resulted in poorly filled small ears, there may be mechanical difficulties with sheller or picker efficiency which need to be considered. Since it will probably be cheaper to buy corn for grain than to buy hay for roughage (because of the likely forage deficit), there will be greater benefit in harvesting fields with marginal corn grain yield potential for silage.
As many of you are aware, the soybean aphid has not yet become the problem that everyone had anticipated. Although some Midwest states have fields requiring treatment, the overall aphid population is much lower than we had expected. We have only found a few instances of aphid populations in Ohio, and none that we have known of have been sufficient in numbers to cause any concern. Why aphid numbers are so low is unknown, although it will make for interesting discussions this winter. Although their populations are either very low or non-existent in the state, growers should nevertheless continue to monitor their fields for the remainder of the summer in case the situation changes.
With the continued extremely hot and dry weather in regions of Ohio, the potential for outbreaks of the twospotted spider mite on soybeans still continues. Although widespread problems are not being reported as in previous drought years such as 1988 and 1999, growers should nevertheless monitor their fields to ensure mite problems are not occurring. Mite injury will first be noticed as a yellow-speckling on the top surface of the leaves, after which they begin to turn a bronze in color. If the injury continues, the plant will ultimately turn brown and die. We are beginning to see many fields with bronzing along the field edges. Most problems will occur on the field edges, although hot spots throughout the field could exist. If growers determine that a problem is occurring along the field edge, they should also check their entire field to make sure that mites are not beyond the edges.
Growers should be aware that rainfall and humid conditions help to keep the mite populations down, and also assist in stopping an outbreak. Although most regions are not getting much rainfall, higher humidity that results in plants covered with dew in the early morning will help to limit mites and their injury. These conditions are often common in August. The reduction in mites at this time is most likely because of a mite pathogen that is more common when the plants are wet. Reports from some areas of the state suggest that mites are already crashing due to this pathogen and the more humid conditions. We are seeing areas where mite injury is obvious, but yet not that many mites are being found. Before growers make the decision to treat based on the presence of mite injury, they should examine the leaves with a hand lens to ensure that mites and eggs are still present in large numbers. Spraying after the mite population has crashed will be a waste of effort and money.
Last week in this newsletter we discussed the situation with the bean leaf beetle and other defoliators. This past week, we found an exception to the general rule that bean leaf beetles will not cause sufficient defoliation by themselves to warrant treatment. A significant portion of a field in northeast Ohio was located that had defoliation well over the 15% threshold, all being caused by the bean leaf beetle. Sweep-net sampling revealed numbers at least 30-40 beetles per 10 sweeps on relative short plants. Growers are advised to scout their entire fields to ensure defoliation from the bean leaf beetle and other defoliators, including the Japanese beetle and Mexican bean beetle, is not reaching high levels.
Potato leafhoppers persist in some areas of the state and are a continued concern on alfalfa, especially under the stressful conditions that exist in some regions. Remember that Ohio's thresholds are lower when the alfalfa is under high drought stress. Growers are advised to still monitor their fields in case the populations of leafhopper remain high. However, early August is when we often begin seeing the numbers of potato leafhopper go down naturally. Because of the potential for population reduction at this time of year, growers should make sure that leafhoppers are still in their fields before spraying.
The American Legion Hall in Ottawa will be the location for a meeting on harvesting drought stressed crops for silage and forage on Thursday August 8th at 1:30 PM. The extremely dry growing season has resulted in stunted corn and soybean fields and slow growing alfalfa fields.
The meeting is geared toward dairy or beef producers and farmers who may provide
feed to dairy or beef producers. Some of the topics to be covered will include:
1) Timing the silage harvest to get the best feed possible
2) Pricing stunted crops for silage or forage
3) Adjusting rations when feeding drought stressed crops
4) Harvesting soybeans as a forage crop
5) Dealing with high nitrates
6) Care of stressed alfalfa stands to maximize their potential.
Speakers will be from The Ohio State University Extension and the Ohio Agricultural Research and Development Center. A $2.00 dollar registration fee payable at the door will be used to cover refreshments and room costs. Participants are asked the register with the Putnam County Extension office at 419-523-6294 one day in advance in advance of the meeting.
The Ottawa American Legion Hall is located at 218 West Main Street in Ottawa. Parking is available on the south side of Main Street. Entrance to the American Legion Hall meeting room is available on the west side of the building.
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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: Pat Lipps, Anne Dorrance & Dennis Mills (Plant Pathology), Peter Thomison (Corn Production), Jeff Stachler (Weed Science), and Bruce Eisley & Ron Hammond (IPM). District Specialist: Bob Fleming (Northwest) Extension Agents: Steve Prochaska (Crawford), Howard Siegrist (Licking), Gary Wilson (Hancock), Jim Lopshire (Paulding), Glen Arnold (Putnam), Ray Wells (Ross), Mark Koenig (Sandusky), Clark Hutson (Seneca) and Roger Bender (Shelby).Editor: Clark Hutson Web Editor: Tom Rosati
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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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