CORN
Crop Observation and Recommendation Network

July 16 to July 22, 2001

C.O.R.N. 2001-22

In This Issue:

A) Will Dry Weather Affect Corn Yield Potential?
B) Soybean Defoliation Assessment
C) Wheat Yield, Grain Quality And Seed Treatments
D) Soybean Aphid
E) Weed Control Notes

A) Will Dry Weather Affect Corn Yield Potential? (Peter Thomison) CORN Questions

As dry weather continues across much of Ohio, more corn fields are showing signs of moisture stress with leaf rolling evident during midday hours. Continuing high temperatures and limited rainfall are increasing the possibility of yield losses to moisture stress. Will drought stress during late vegetative growth stages and silking result in significant yield loss? It will depend on how soon we receive sufficient rainfall to alleviate moisture stress. Many corn fields are silking and most corn will probably be pollinating within the next 7 to 10 days.

To estimate the impact of this dry 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 Classen 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 Classen 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 (about the 17th collared leaf stage).

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.

B) Soybean Defoliation Assessment - (Bruce Eisley, and Ron Hammond) CORN Questions

Many soybean fields have reached the early bloom stage of development, which is stage of growth that is most susceptible to defoliation by foliar feeding insects. During the pre-bloom stages of growth, soybeans were able to tolerate up to 30% levels of defoliation. However, from bloom to pod-fill, defoliation by insect pests should not be allowed to exceed 15%.

At the present time, adults of the 1st generation of bean leaf beetles are becoming active, and Japanese beetles feeding on soybeans have been reported. The combined effect of these insect defoliators may cause levels of defoliation that exceed the 15% action threshold and may warrant application of a rescue treatment.

Two-spotted spider mite populations are starting to occur in soybean fields because of dry and warm conditions. Most of these problems will probably occur in field edges, but can proceed into the field if not controlled.

Where one or more pest populations are causing economic levels of defoliation, the bottom line for deciding whether a rescue treatment is warranted or not depends on an accurate assessment of defoliation. It is relatively easy to overestimate insect defoliation. Thus, it is important that defoliation assessment be based on a random sampling procedure that reduces the possibility for error.

A minimum foliar sample should include the collection of trifoliate leafs from the top, middle and lower portions of 10 randomly selected plants. From each trifoliate one should discard the most and least damaged leaflets. At this point one should have 30 leaflets upon which the defoliation estimate should be based. The process may be repeated in various locations of a large field if defoliation appears to be significantly different from one area of a field to another. It is important that one not sample from the perimeter of a field where Japanese beetle and grasshopper activity tend to be high.
Illustrations of insect defoliation levels are presented in various
Extension publications such as the OSUE Field Crops Pest Management Circular
#22 on Soybean Insect Defoliation Assessment, which can be retrieved from the Ohioline website at http://ohioline.ag.ohio-state.edu/icm-fact/fc-22.html or from the OSUE Corn, Soybean, Wheat and Alfalfa Field Guide (Bulletin 827) also on the web at http://www.ag.ohio-state.edu/~ohioline/b827/b827_57.html.

C) Wheat Yield, Grain Quality And Seed Treatments (Pat Lipps) CORN Questions

Reports of wheat yields from around the state indicate that yields were generally good, but not quite as good as last year. Most reports indicate grain yields in the 60 to 80 bu/A range with a few fields reaching 100 bu/A and some in the mid 50 bu/A range. Test weights are very good with most fields having grain tested at the 58-62 lb/bu range. Armyworm affected fields also had relatively good yields which indicates that the armyworm ate the flag leaves after sufficient carbohydrates were developed by the flag leaves and the glumes of the head. Flag leaves are very important to carbohydrate accumulation to be transported to the developing seed during the early stages of grain fill (heading through flowering). After pollination the glumes on the heads take over the major responsibility for carbohydrate production for filling the grain. This transition of carbohydrate synthesis and transport to the grain from the flag leaves to the glumes is gradual and is possibly affected by damage to the leaves or the glumes. It appears we have much to learn about the timing of armyworm damage and its effects on yield. Thresholds for insecticide application to control armyworm should probably be tied to wheat growth stage because once the grain reaches 40% moisture (soft dough stage) translocation of nutrients from the leaves and glumes ceases. Damage to leaves and glumes after this time has little impact on yield.

Dry weather during grain fill prevented Stagonospora glume blotch for causing extensive damage in most areas of the state. This head disease was detected at low levels in many fields, but the dry conditions prevented the fungus from colonizing most of the glumes and invading the developing seed. Head scab was at low levels in most areas of the state, but a few fields in cental and the far northwest counties reported significant levels of scab. The relatively high test weights reported from most fields indicate that there are likely few grain quality issues at this time. Growers that have decided to save seed of this years wheat crop for planting this fall should have the seed tested for germination. Low germinations would indicate quality issues that would most likely be caused by head scab. Regardless of the germination level, all wheat saved for seed should be thoroughly cleaned to remove small and lightweight kernels. A gravity table is most efficient at removing diseased seed from seed lots. All seed should be treated with one of the many highly effective fungicide seed treatments now available: Dividend XL, Raxil-Thiram, Raxil MD, Raxil XT). We would recommend LSP Flowable Fungicide to be tank mixed at a low rate with another wheat seed treatment to improve efficacy for scab damaged seed. This would be recommended for seed lots that have relatively low germinations (80% or lower) due to seed borne scab. Hopefully we will not see seed lots with this low of a germination level this year since better quality seed is likely to be widely available. More information on wheat seed treatments can be obtained in OSU Extension Bulletin 639 and 693A, both of which can be viewed on the web site Ohioline (http://www.ag.ohio-state.edu/~ohioline/b639/b639_17.html).

D) Soybean Aphid (Ron Hammond, and BruceEisley) CORN Questions

The latest update on the soybean aphid is that we are finding it in most counties we look in. This past week, we found them in the following additional counties: Delaware, Franklin, Henry, Defiance, Williams, Fulton, Madison, Clark, Miami, Preble, Darke, Hamilton and Butler. This is in addition to Wood, Wayne, and Clark counties where they were previously recorded. The aphids are very easy to find in many parts of the state. Although we are only finding a few aphids on the plants in most fields, there are instances where we are seeing over 100 aphids per plant on a few plants and a single instance in Henry County where we saw 1000+ aphids. This latter area had plants covered with aphids, along with a heavy coating of honeydew. In areas where numbers are higher than a few per plant, we are seeing a lot of ladybugs feeding on the aphids and ants harvesting the honeydew. The presence of both can be used to help locate aphids. A web site having more information on the aphid in the Midwest is
http://www.pmcenters.org/Northcentral/Saphid/aphidindex.htm

E) Weed Control Notes (Jeff Stachler) CORN Questions

1. It is our recommendation to not apply Flexstar or Reflex for the remainder of this season. The rotation restriction to corn is 10 months. If we stay dry or if Flexstar/Reflex are applied late in the season, there is a possibility for carryover injury to occur next year in corn.
   
   
2. Much of Ohio is very dry. Those individuals with late planted soybeans should consider spraying very soon before the weeds get larger and more water-stressed at which time they will be harder to control. Herbicide injury to soybeans at this time of the season and later can cause significant yield reductions on small soybeans, so choose products and adjuvants wisely. When weeds are water-stressed, spray before the hottest part of the day and use the appropriate adjuvants to maximize weed control. If dry conditions persist only a single herbicide application should be necessary in late-planted soybeans.
   
   
3. Early-planted soybeans are still stressed and are not providing adequate canopy closure in some fields. Scout these fields for new weed emergence, if the weeds are thick they may compete with the soybeans more than normal and an herbicide application may be necessary. Research indicates that weeds that come through large soybeans (about 18 inches or taller) usually do not cause a yield reduction. However, if enough weeds are present, they can get in the way of harvesting, especially grasses, eastern black nightshade, burcucumber, and morning-glories. Weeds that are nearly the same size as the soybeans at this time will reduce soybean yield. It is clearly a judgement call as to whether late emerging weeds should be controlled.

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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.

Editor: David Jones        Web Editor: Tom Rosati

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