CORN
Crop Observation and Recommendation Network

July 7-13, 2003
C.O.R.N. 2003-21

In This Issue:

A) Potential Wheat Harvest Problems Caused by Persistent Rain
B) Soybean Aphid
C) Assessing Corn Root System Development
D) Armyworms Move from Wheat
E) Rainfast Intervals for Postemergence Herbicides
F) Mixing Glyphosate with Manganese in Foliar Applications to Roundup
Ready Soybeans
G) Manganese Problems in Soybeans
H) Plant Analysis for Soybeans

A) Potential Wheat Harvest Problems Caused by Persistent Rain - Pat Lipps, Jim Beuerlein, Dennis Mills CORN Questions

Rain showers that have persisted through the wheat grain filling period in southern Ohio and the recent excessive rain in parts of western Ohio have contributed to several problems in wheat grain harvest this year. Additionally, the national weather service has predicted more rain for Ohio during the remainder of this week. As many of you know rain during the latter periods of grain fill and during grain ripening are not good for wheat grain quality. Potential hazards include shriveling of the grain, sprouting of the grain and the accumulation of vomitoxin in kernels affected by Fusarium head scab.

One cause of grain shriveling is repeated wetting and drying of the grain once the grain has dried to harvest moisture. As the grain dries to lower moisture levels its size shrinks slightly. Each time the grain becomes wet, the grain swells slightly and as it dries again it shrinks to a size slightly less than its previous size. This process of repeated wetting and drying is called 'puffing'. Thus, the greater the number of wetting and drying events a kernel endures the smaller the grain size will become, to a point. Puffing can account for several pounds loss of test weight of grain if the grain goes through several cycles of wetting and drying.

Sprouting is a potential problem in the Midwest wheat growing regions. Most soft red winter wheat varieties grown in Ohio are fairly resistant to sprouting and under normal conditions, are incapable of germination (dormant) for several weeks following maturity. However, persistent rainfall occurring for an extended period following maturity and before harvest can cause most varieties to exhibit some sprouting. A small acreage of soft white wheat is currently being grown in Ohio. Varieties of this class of wheat are much more susceptible to head sprouting than the soft red varieties. Test weight reductions occur with even a minimal amount of sprouting, and under severe conditions the grain may not be marketable.

Head scab has occurred at moderate level in various parts of the state. Reports from surveys in 28 Counties throughout Ohio indicate the average incidence of disease is about 9%. Incidence ranges from 0% to 73% in individual fields. The reported data indicates that the higher levels of scab are in south, central and east central counties. Additionally, the incidence of scab varies greatly by field within counties. At this point it appears the lowest levels of scab are in the west and north west counties. The persistent rain over the past few weeks has kept the heads of the wheat plants wet permitting the scab fungus to continue to grow and colonize the heads. During this growth process the fungus is capable of producing vomitoxin. Protracted periods of wet weather favor continued toxin production. We have received reports of vomitoxin in grain from southern Ohio and with current wet conditions, vomitoxin has the potential to be in grain of any field affected by scab. Grain elevators will likely be testing grain for vomitoxin levels. Most mills can process grain with levels of vomitoxin at or below 2 parts per million (ppm), and will likely reduce the grain price (dockage) if tests indicate 2 to 4 ppm in the grain. Some grain buyers may refuse grain with 5 or 6 ppm or above.

To help reduce the probability of receiving dockage, harvest the grain as soon as possible. Mature grain should be harvested as soon as the grain moisture decreases to 20 percent or less. Combines should be set to remove as much of the shriveled grain as possible. Although you are loosing some grain, the remainder of the harvested grain will have higher test weight and less vomitoxin problems. Vomitoxin is mostly associated with the smaller, shriveled grains. Following harvest, grain moisture should be lowered to below 15 percent as rapidly as possible to stop the sprouting process and to prevent the accumulation of additional vomitoxin.

B) Soybean Aphid - Hammond & Eisley CORN Questions

Soybean aphids were located in different locations of Ohio last week, suggesting that they are widespread in Ohio. We found aphids in a field south of Toledo in Wood County in northwest OH, north of Coshocton in Coshocton County in east central OH, and then a few fields east of Wooster in Wayne County. Aphids in these fields were relative easy to find, being found on at least a third to half of plants sampled in the first two counties (Wood and Coshocton). Except for a few plants, aphid numbers ranged from 1 to 5 per plant, being found on the upper 1-2 leaves of the plants. A few plants were found that had over 100 aphids per plant. These densities were larger than expected. Fields were in the flowering stage, R1-2, suggesting they were earlier planted fields. Few ladybugs were observed. Aphids were found by walking through the field and randomly picking off the upper portion of the plant containing the top two leaves and examining them for aphids.

Whether economic problems will develop in these fields or in other fields across Ohio is unclear. However, having found soybean aphids at these densities in such widespread locations in early July and knowing the tremendous capacity of the aphid to increase in population size, it is crucial that growers and consultants monitor their fields. Considering we did not see any soybean aphids last year, we are faced with a very different scenario this year compared to 2002, perhaps more similar to what we saw in northern Ohio in 2001. Whether the rainfall we have had recently and will be experiencing this week in Ohio will help reduce populations is unknown.

See the CORN newsletter for the week of June 16-22 http://corn.osu.edu/archive/2003/jun/03-18.html for information concerning management of the aphid. As aphids are found in other counties in Ohio, we would appreciate hearing about it. Please send location (county, township, and nearest roads or highways, or GPS coordinates if available), relative densities, and other information to hammond.5@osu.edu.

C) Assessing Corn Root System Development - Thomison CORN Questions

In early July, nearly all our attention is focused on corn canopy development - canopy closure, plant height, stand uniformity, etc? Unseen, but probably of greater importance to the crop's success is root development? The corn crop's access to soil moisture and nutrients during the growing season depends on establishment of an effective root system. Understanding root development can help with diagnosis of various production problems, from nutrient deficiencies early in the season to stalk rots at maturity. Estimates of root growth and penetration are complicated by a number of factors because growth is dependent upon soil moisture, aeration, texture, structure, and fertility.

Research by Dr. Stan Barber at Purdue University indicated that corn will produce similar total amounts of roots each year. The difference between years is mainly where in the soil profile the roots predominate. Warm dry soils will promote deeper root development, whereas cold, dry soils promote shallower root development.

In a year such as 2003, when the surface soil moisture after planting was above optimum moisture for an extended period, corn roots are usually confined to the surface eight or twelve inches of soil. Lateral roots are short and less abundant, very little branching, and essentially all of the root development is horizontal. In contrast, in a year where there is a period of three to four weeks with light rainfall after planting, a deeper root system is favored. Lateral roots are longer and more abundant, considerably more branching, and they likely "turn down" earlier. Such root systems generally enable the crop to tolerate late season water deficits much better than shallow root systems. Unfortunately, cool rainy weather conditions and wet soils are often the norm after planting in Ohio.

Well drained soils favor penetration of roots better than compacted soils with a hardpan and poor aeration. In one past study, a well drained soil was characterized by a general root distribution of 70% in the upper 10 inches, 19% in 10-30 inch depth, 8% in 30 to 48 inch depth and 3% in the 40 to 72 inch zone. However, for a poorly drained soil, the general root distribution was 56% in the upper 8 inches, 42% in the 8 to 28 inch depth, and 2% in the 28 to 48 inch depth.

Root development can be related to different stages of vegetative development. Under favorable growing conditions in a well drained soil the following rules of thumb would usually apply.

· Up to the 7th or 8th leaf stage (knee high) the roots will be almost parallel to the soil surface with little penetration more than a foot deep.
· Between knee-high and four feet, the roots may extend 2 to 3 feet from the base of the stalk, where they usually turn down abruptly. Penetration at this stage can range from 1 foot to over 4 feet in depth. Since roots have likely reached the row middles by about the V8 stage, they are more vulnerable to pruning by cultivation.

· As maturity approaches, roots tend to branch and penetrate deeper. Maximum root spread from the stalk is often 3.5 feet in all directions. Maximum penetration of very fine roots may be from 4 to 8 feet.

· Root mass reaches it's maximum size at silking

· Brace roots (above ground nodal roots) are initiated from the lowest 3 to 4 stalk nodes during late vegetative development and usually penetrate the soil surface by silking. Brace roots provide support to the stalk and are of considerable importance in "resurrecting" plants root lodged by strong winds.

References:
Zublena, J.P. and C.L. Parks. 1980. Nutrient and volume of water available to the corn plant are influenced by volume and depth of root development. Coop. Ext. Serv. Clemson Univ. Soils & Plant Nutrition Information Sheet.

D) Armyworms Move from Wheat - Eisley and Hammond CORN Questions

Reports last week indicated that armyworms were beginning to move from wheat fields into neighboring cornfields. The worms were from 1 to 1-1/2 inches in length and found primarily along the edge of the cornfields. This is typical movement with armyworm and the area in corn that needs to be scouted is normally the first 15 to 20 rows (although the area affected can be larger) that border the wheat field. If you have corn that borders wheat, then this would be a good time to monitor the corn for armyworms and armyworm damage. If damage is found, then a defoliation table in last weeks newsletter (http://corn.osu.edu/#linkd) might be helpful in determining whether or not it is necessary to spray. Also remember that large armyworms will be pupating before long and will not do much additional feeding.

E) Rainfast intervals for postemergence herbicides - Loux CORN Questions

The minimum period of time between herbicide application and rain can be important to know when applying during the hit-or-miss thunderstorm weather we have been subject to lately. Rainfast intervals are listed in the tables on pages 64-65 (corn) and 95 (soybeans) of the 2003 OSU Weed Control Guide. Many glyphosate products are available, and only a few are listed on these tables. The rainfast interval for most glyphosate products is one hour, while the interval for Roundup WeatherMax is 30 minutes.

F) Mixing glyphosate with manganese in foliar applications to Roundup Ready soybeans - Loux CORN Questions

Applying a tank mixture of glyphosate and chelated manganese in foliar applications to Roundup Ready soybeans is a practice approved by many glyphosate manufacturers, although not all provide specific guidelines to avoid potential problems. Manganese and other foliar fertilizer products generally have some tendency to reduce the activity of glyphosate, especially in less than ideal weed control situations (adverse weather, large annual weeds, and/or perennial weeds). We discussed this issue with several companies that sell glyphosate, and can offer the following guidelines when applying glyphosate with manganese:
- use only chelated forms of manganese.
- in situations where weeds are large or subject to weather conditions that reduce herbicide activity, make separate applications of glyphosate and manganese rather than tank-mixing.
- use labeled rates of glyphosate and ammonium sulfate in mixtures with manganese. Use spray-grade ammonium sulfate at a rate of 17 lbs per 100 gallons, rather than ammonium sulfate substitutes, since it may be impossible to determine the true ability of substitutes to reduce antagonism from manganese. Nonionic surfactant should be added where specified by the glyphosate label. Do not apply glyphosate with crop oils, soybean oils, or other additives unless specified by the glyphosate label.
- apply combinations of glyphosate and manganese immediately after mixing. Allowing mixtures to stand for long periods of time may result in settling and compatibility problems.
- Always follow the following order of mixing when adding products to the sprayer: 1) water; 2) ammonium sulfate; 3) manganese; and 4) glyphosate.

G) Manganese Problems in Soybeans - Lentz CORN Questions

Occasionally northwest Ohio will have pockets in soybean fields where soils are deficient in manganese. These pockets generally occur in glacial lakebed soils, soils developed in glacial outwash areas, and in peat and muck soils. Soil pH is the most important factor affecting manganese availability because it is less soluble at higher pH levels, but other factors such as organic matter, soil type, and weather may magnify the problem. On medium to fine-textured soils (silts and clays), it seldom occurs when soil pH is below 6.5. It may occur on sandy soils that are high in organic matter with a pH as low as 6.2. It sometimes occurs on muck and peat soils down to pH 5.8. Pale yellow to nearly white leaves with distinct green veins are the most visual symptoms of manganese deficiency in soybeans. In more severe cases, the plants may also be stunted. Producers who surf the web or read national trade magazines may confuse manganese deficiency with iron chlorosis since deficiency symptoms are similar between the two nutrients, and both problems occur on high pH soils. However, Ohio soils tend to be naturally high in iron as most soils east of the Mississippi River. Thus, manganese deficiency generally occurs east of the Mississippi River and iron chlorosis occurs west.

Generally, supplemental manganese is not recommended unless deficiency symptoms are observed or a field has a history of manganese problems. Keep in mind that short term deficiencies may be observed in dry years but soon recover after adequate rainfall without yield reductions. Since only small areas are generally affected, spot treatments would be adequate to correct the problem rather than treating the whole field. Foliar applied products have the most consistent results in correcting deficient situations. Foliar rates should be adjusted so that 1 to 2 pounds of actual manganese is applied per acre. Multiple applications may be needed when both the surface and sub-soil have high pH values. If the manganese product will be mixed with an herbicide, producers should examine the herbicide label to insure that the product selected will not interfere with the activity of the herbicide.

H) Plant Analysis for Soybeans - Lentz CORN Questions

Many soybeans fields in the state have begun to flower or will in the next week. This is the time to consider plant analysis. Plant analysis is one way to determine whether the soil can routinely provide certain nutrients without additional fertilizer, such as sulfur and micronutrients. For the test, collect the leaflets from the uppermost, fully expanded leaf of thirty plants. Place leaves in paper bags and ship or deliver to a reputable lab. Leaf stems (petioles) should not be included in the samples. Most labs that receive soil samples also run plant analysis. Adequate levels of nutrients would be as follows: 4.25 - 5.50 % for nitrogen, 0.30 - 0.50 % for phosphorus, 2.01 - 2.50 % for potassium, 0.36 - 2.00 % for calcium, 0.26 - 1.00 % for magnesium, 0.21- 0.40 % for sulfur, 21 - 100 ppm for manganese, 51 - 350 ppm for iron, 21 - 55 ppm for boron, 10 - 30 ppm for copper, 21 - 50 ppm for zinc, and 1.0 - 5.0 ppm for molybdenum. Several years of plant analysis would be recommended before adopting a non-traditional fertility program or a program that encourages the use of other nutrients besides nitrogen, phosphorus and potassium.

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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: Harold Watters        Web Editor: Nathan Watermeier

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