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Ohio State University Extension


C.O.R.N. Newsletter 2006-04

Dates Covered: 
February 21, 2006 - March 8, 2006
Harold Watters

Nitrogen Consideration for Wheat

Authors: Edwin Lentz, Robert Mullen

Nitrogen rate.
We would still recommend the Tri-State Fertility guide for N rates in wheat. This system relies on yield potential of a field. As a producer, you can greatly increase or reduce your N rate by changing the value for yield potential. Thus, a realistic yield potential is needed to determine the optimum nitrogen rate. Once you have selected a value for yield potential, the recommendation may be based on the following equation for mineral soils, which have both 1 to 5% organic matter and adequate drainage:

N rate = 40 + [1.75 x (yield potential – 50)]

Generally, we would recommend that you subtract from the total (spring N) any Fall applied N up to 20 lb/A. Before N prices were high, producers often ignored this recommendation. We have had an above normal temperature and rainfall for January, so some of this fall N may have been lost, but if temperatures were adequate for nitrification (ammonium N converted to nitrate N) we probably also had N mineralized (released by the soil). Thus we may have gained as much as we lost. Do not take any N credit for any previous soybean crop. In summary, whether you deduct fall N depends how much risk you are willing to take and your anticipated return of investment from additional N.

Application Time
Many of us would like to apply N to wheat now while the ground is still frozen (at least the surface) rather than waiting for proper soil conditions in March. Though we may save time and labor with a February application, we run the risk of losing most of the N (a costly error with existing high N prices). Ohio State University research completed in 2003 revealed that loss of N from a single N application prior to spring green-up may have yield reductions of 19% compared to applications between initial greenup and early stem elongation. This loss occurred for all N sources used in the study (ammonium sulfate, urea or 28%).

Under Ohio conditions, wheat plants use very little N until after stem elongation. Stem elongation generally begins around the latter part of April. Thus the longer the time from application to stem elongation the greater the risk for N loss. Some individuals believe that N needs to be applied very early to increase the number of tillers. This philosophy is one of our rural myths. The tillers that provide the largest yield in our wheat crop were established last fall. Granted we may get more small tillers from early Spring N but their heads will only make a small contribution to overall yield. Keep in mind that N applied around March 15 (initial greenup) is still early spring. In a three-year study at Ohio State University, yield did not drop off from a single N application until growth was past early stem elongation (late stem elongation to boot stage). As a result of this research, we would recommend that spring N not be applied until after spring greenup (generally mid March) but before early stem elongation.

Nitrogen Source
The longer the time between application and early stem elongation, the more time for N loss. In general, 28% has the largest potential for loss, then urea, followed by ammonium sulfate. Heavy rains shortly after application or extended saturated conditions may increase the potential for N loss from 28%, but band applications (e.g. dribble bars) would minimize loss. Volatilization losses from urea and 28% are generally low during March and April (cool and moist conditions) so we would expect little loss potential and little need for urease inhibitors. Some may ask do we need a nitrification inhibitor, but the wheat plant generally has a high demand for N by the time ammonium-N converts to nitrate-N, so we would expect little benefit from a nitrification inhibitor. Also, the nitrification inhibitor cannot protect the original nitrate portion in the 28% and that part has the greatest potential for loss prior to uptake. In summary, we would recommend any of the N sources (and no inhibitor products) with proper management (proper application time and method). Thus price and availability should be used in selecting a N source for wheat in 2006.

Split Application
Split application of spring nitrogen may improve efficiency but adds another trip across the field. In years of N loss, split applications may have larger yields than single applications applied in February and early March. The benefit of a split application would diminish as a single application is applied closer to early stem elongation. Thus applying a single application later would have the same benefit as a split. (As stated earlier, we have adequate N from our organic matter and fall N program for adequate tiller number and spring growth.) In a split program, we would apply only a small amount early (since little is needed and it may be lost) and apply the larger amount later (much like a starter N program for a sidedress system in corn). In summary, a split application of spring N on wheat is a viable option but in most years would not yield more than a single application applied at the proper time, and a split would require another trip across the field.

Nitrogen Placement - How Much N Can I Apply through My Planter?

Authors: Robert Mullen, Edwin Lentz

With high fuel costs this year, some producers may be considering applying the bulk (if not all) of the N budget at planting. The question is what are the risks associated with this application, and if this is considered how far away from the seed should the N be applied?

The primary risk of this application is the proximity of fertilizer N to the seed (especially for urea-based N fertilizers). Urea hydrolyzes (with the aid of the enzyme urease) to form ammonia (same as anhydrous ammonia) and requires soil water to form ammonium (which is stable). High rates of urea fertilizer will create what is known as an ammonia saturation zone (similar to applied anhydrous). Ammonia can be extremely toxic to germinating seeds, and an application of urea-based fertilizer too close to the seed can result in serious stand losses. Diammonium phosphate and monoammonium phosphate can also release ammonia (DAP represents the greater risk).

As long as the N is kept well away from the seed, planter applied N can be an attractive option. While we do not have well established rules for N placement and application rate (it is a function of how much N is applied and soil moisture), the higher the N rate the further the N needs to be away from the seed. Nitrogen rates below 40 lbs N per acre should be applied at least 2 inches beside and below (2 x 2) the seed furrow. Nitrogen rates higher than 40 lbs N per acre should be further away. The higher the N rate the further away it should be.

Remember the safe distance needed to ensure that germination is not inhibited is also a function of soil moisture. The drier the soil conditions the further away the N should be applied from the seed.

Broadcast Application of Granular Urea for Sidedress?

Authors: Robert Mullen, Edwin Lentz

Ammonia is the precursor to all other forms of nitrogen (N), and with the increased cost of natural gas (over 70% of the cost of ammonia production) in the U.S. compared to other countries around the world, it has become more difficult for U.S. ammonia manufacturing facilities to compete on the global market. Over a quarter of the ammonia manufacturing facilities in the U.S. have shut down because of natural gas costs. It is cheaper to import N (primarily urea) from another country where natural gas is much less expensive than to produce it here in the U.S. As we become more reliant on imported sources of N, dry granular products will likely become a more frequent N source than liquid N or anhydrous ammonia because the latter two products are difficult to transport by barge. So if urea becomes our cheapest source (it may already be in certain markets if we consider application costs), what are the specific concerns of using it as a sidedress material?

Surface application of granular urea is typically not the most efficient way to supply N to a growing crop. It is usually recommended that urea be incorporated to ensure that N is not lost by volatilization. As urea is broken down by the enzyme urease it becomes ammonia and seeks out water to form the stable N form ammonium. When urea is surface applied, there is very little soil surface area and resultant moisture to react with ammonia and keep it from floating away as a gas (volatilization). Volatilization loss potential is increased when urea is surface applied to a no-till soil that has a lot of surface residue, and loss potential is increased by high soil pH. This is why surface application of urea is specifically discouraged on no-till production fields and recently limed fields (specifically no-till fields where lime is not incorporated). Incorporation is the key to ensure volatilization losses are minimized, and this does not necessarily mean mechanical incorporation. Rainfall approaching half an inch can adequately incorporate urea and minimize N loss.

Research conducted in Illinois revealed that in no-till corn production surface applied urea (applied shortly after planting) could result in significantly lower yields than N injected as UAN or anhydrous ammonia when applied at identical rates (Varsa et al., 1999). The average yield of the broadcast urea treatment was 107 bushels per acre while the average yield of the UAN and anhydrous treatments were 146 and 144 bushels per acre, respectively. Addition of an urease inhibitor did result in increased yield (126 bushels per acre), but yields were still lower than the UAN and anhydrous treatments.

Polymer coated urea products are another potential dry source of N that can be applied preplant or sidedress. The semi-permeable polymer coating allows for slow release of N that increases as soil temperature increases. The limited amount of University research available shows that for preplant applications this application method can be an attractive alternative in certain high N loss situations, but the fertilizer material does cost more than urea. To see the economic value of these materials usually requires N rates to be lower than rates of standard urea.

Resistance: the First Line of Defense Against Northern Corn Leaf Blight

Authors: Pierce Paul, Dennis Mills

The importance of selecting corn hybrids with resistance to some of the most important diseases has been addressed in detail in previous C.O.R.N articles. Hence, this article only serves to remind growers that resistance should be the first line of defense against diseases such as northern corn leaf blight (NCLB). As was the case in 2002, 2003 and 2004, NCLB was again severe in many fields planted to susceptible hybrids in 2005, especially in the north western and central parts of the state. Yes, the disease did show up late (after grain fill) in several fields and, as a result, had very little or no direct yield impact. However, history has taught us that several of the major plant disease epidemics occur in years preceded by years with low but consistent levels of disease. This is largely because it may take several years for inoculum (spores) to build up to a level that is high enough to cause major disease damage or for favorable weather conditions to coincide with inoculum buildup at the right plant growth stage.

Three conditions are necessary for an epidemic of NCLB to occur: 1) The fungus must be available, abundant, and capable of causing disease on the hybrid that is planted, 2) the hybrid planted needs to be susceptible to the fungus, and 3) favorable weather conditions need to occur at the right time during the growing season. The fungus which causes NCLB (Exserohilum turcicum) survives in corn residue left on the soil surface. Given the fact that we have had fairly high levels of NCLB in some fields over the past few seasons and reduced tillage (which leaves a high amount of residue on the soil surface) is used in most fields, it is reasonable to assume that inoculum (spores of the fungus) will again be available and abundant in 2006.

Ability of the Fungus to Cause Disease (taken from “Northern Corn Leaf Blight Considerations For Ohio Corn Growers” by Patrick Lipps - C.O.R.N Newsletter 2004-36:
The NCLB fungus exists as several different races, each capable of causing disease on corn plants with specific resistance genes. The races are named by numbers (race 0, race 1, race 2) which designate what resistance genes (Ht genes) that particular isolate is capable of attacking. For example race 1 can cause susceptible lesions (large necrotic lesions) on hybrids with Ht1 resistance gene, but Race 0 causes the plant to produce a resistant response (small chlorotic lesions) on the same hybrid. However, race 0 can cause a susceptible lesion (large necrotic lesion) on plants with no Ht resistance gene. It appears that in Ohio we have mostly a mixture of race 0 and race 1.

Hybrid Resistance to NCLB (
There are two different types of genetic resistance to NCLB. These include the race-specific resistance and partial resistance. Race-specific resistance is where certain races of the fungus are prevented from causing severe disease by a single gene in the hybrid. Partial resistance is also know as multiple gene resistance because the resistance response in the plant is conditioned by several different genes in the hybrid and the more of these genes a hybrid has the greater the level of resistance. This type of resistance is effective against all races and works by reducing the size of the lesions, the number of lesions and the amount of spores produced in each lesion. The accumulative effect of partial resistance is to slow down disease spread so that disease levels never get too high during the grain filling period. Partial resistance and race specific resistance can both be very effective in preventing yield losses, but are most effective when used together in a single hybrid.

Both race 1 and race 0 of Exserohilum turcicum are present in Ohio, and some commercial hybrids lack effective resistance genes. Hence, if a susceptible hybrid (one without partial resistance and race-specific resistance) is planted and cool temperatures (66 to 78¬oF), high relative humidity or heavy dew, and rainfall persist before and during the grain filling period, a major epidemic of NCLB and substantial yield loss may occur. To prevent such an epidemic from occurring, growers (especially those who observed high levels of NCLB in their corn fields in 2005) are advised to plant resistant hybrids in 2006. Ask your seed dealer for the highest level of partial resistance available combined with either Ht1 or Ht2 resistance genes. Given that both race 0 and race 1 are present in the state, hybrids with Ht1 or Ht2 resistance gene in combination with a high level of partial resistance should be effective at limiting disease development regardless of the weather conditions.

For more on hybrid selection for resistance to NCLB and other diseases of corn read “Selecting Disease Resistant Hybrids for Planting” by Patrick Lipps and Peter Thomison C.O.R.N newsletter 2005-03: For information regarding NCLB resistance rating scales read “More on Northern Corn Leaf Blight: Selecting Hybrids for Resistance to NCLB in 2005” by Peter Thomison, Patrick Lipps and Allen Geyer (C.O.R.N newsletter 2004-41:

Defining Herbicide Resistance

Authors: Mark Loux

We have spent considerable time in extension education over the past 8 years discussing herbicide resistance, and we like to think that many Ohio growers are well informed on the subject. Recent research we have conducted leads us to believe that, in addition to the readily apparent resistance to glyphosate in many marestail populations, lambsquarters and giant ragweed populations have become less sensitive to glyphosate. However, this appears to be a relatively low level of resistance, which does not necessarily fit our current resistance definitions. Weed scientists continue to debate how to characterize and define resistance, so that everyone eventually gets on the same page, but the definitions and discussion that follow reflect our current thinking.

We typically indicate that we have herbicide resistance biotypes of a given weed species. A “biotype” is a plant or plants within a population that are genetically different from the rest of the population. The genetic change can impart differences in any number of characteristics, including stem and flower color, leaf shape, or more importantly the response to herbicides (sensitive vs resistant). This genetic variability can enable certain individuals within a population of a species to survive an herbicide application, while the rest of the population is sensitive.

The terms “tolerance” and “resistance” have often been used interchangeably to describe the failure of an herbicide to adequately control a weed population. However, tolerance really should be defined as the successful survival of most of the individual plants within a population of a species, and this occurs anytime the herbicide has been used in the field (i.e. it happens the first time the herbicide is applied to this population). Survival as used here implies that the weeds were able to successfully mature and produce seed. Examples of tolerance include: giant ragweed is tolerant of Select, crabgrass species are tolerant of Accent, and fall panicum is tolerant of atrazine. For each of these examples, the weed was never adequately controlled by the herbicide, even following its initial use. One indication of tolerance is that the weed is not listed as being controlled on the herbicide label.

In contrast, we use “resistance” to describe the lack of response of a biotype in a weed population to an herbicide that develops over time. More specifically, resistance occurs when, after repeated use of the same herbicide or herbicides with the same site of action, plants survive the labeled use rate of the herbicide that typically provides adequate control. As in the discussion of tolerance, survival here implies that the weed matured and produced seed. Characterizing a population of a weed species as herbicide-resistant implies that when that herbicide was first introduced and the label developed, the majority of the individuals in all populations of the species were sensitive to the herbicide. If a species is listed on an herbicide label as being controlled, then at least the manufacturer believes the herbicide can effectively control the species. Resistance should therefore be defined as the successful survival of an individual plant (biotype) within a population of a species after the repeated usage of an herbicide at the labeled rate, and future generations of that biotype also survive the labeled rate. For example, research has shown that glyphosate can effectively control marestail (horseweed), but we currently have biotypes or populations of marestail in Ohio that can survive glyphosate. These populations evolved over time due to the repeated use of glyphosate, and the resulting selection for resistant individuals within populations that produced seed and increased the prevalence of resistance in the population.

Prior to our experiences with glyphosate resistance, we dealt mainly with triazine and ALS resistance, which tend to confer a very high level of resistance or immunity to these types of herbicides. For example, application of chlorimuron or cloransulam to ALS-resistant ragweeds fails to induce any response at rates up to 100 times the use rate or higher. However, resistance to glyphosate appears to occur at a much lower level, and does not seem to confer the same lack of response that characterizes ALS and triazine resistance. So, we have started to characterize herbicide resistance as being either high-level or low-level resistance.

- We define high-level resistance as the survival of a biotype (individual plant) at rates higher than roughly ten times the labeled rate of the herbicide. This is characterized by little to no injury to the plant following herbicide application, and the plant matures and produces seed. As a result of this type of resistance, the response of the biotype to the herbicide is not affected by rate, number of applications, coverage, weather conditions, other plant stresses, or plant size and age. Resistance to triazines, ALS inhibitors, and ACCase inhibitors are examples of high-level resistance.

- Low-level resistance is the survival of a biotype (individual plant) at rates less than roughly ten times the labeled rate of the herbicide. Unlike high-level resistance, there is always some level of herbicide activity from a single application, and some injury to the plant. As a result, herbicide rate, coverage, weather conditions, other plant stresses, and plant size and age can influence the activity of the herbicide and the degree of injury to the plant. In other words, plants that have developed a low level of resistance may still be adequately controlled if the herbicide is applied at a certain rate, or when the plant is small, or when environmental conditions are favorable. Conversely, plants with a low level of resistance may be more likely to survive when one or more application parameters are less than optimum (e.g. rate too low, plant too big, etc). Anything that can reduce the activity of an herbicide can allow for the survival of individual plants within a population, and if seed from these plants results in future generations that consistently survive at the labeled rate, then resistance has occurred. Examples of low-level resistance can include glyphosate, PPO inhibitors (Flexstar, Cobra, etc), paraquat, and synthetic auxin herbicides (2,4-D, etc).

Glyphosate-resistant marestail can be characterized as having a relatively low level of resistance. While resistant marestail can survive glyphosate rates in excess of 4 times the often-used glyphosate rate of 0.75 lbs ae/A, it can suffer considerable injury even at 0.75 lbs. The degree of control is dependent upon plant size, glyphosate rate, and environment. Application of glyphosate at 7.5 lbs/A (2.5 gallons of generic glyphosate products!) should control most glyphosate-resistant marestail plants, unless they are very large. In contrast, ALS-resistant marestail survives much higher than labeled rates of ALS-inhibitors such as chlorimuron and cloransulam with minimal injury.

We have also begun to characterize a number of lambsquarters and one giant ragweed population as having a low level of resistance to glyphosate. We have at times characterized these as “glyphosate-insensitive or lower in sensitivity”, but we feel that the use of yet another term to characterize herbicide response in plants only further confuses the issue (it confused us anyway). While control of lambsquarters and giant ragweed with glyphosate can be influenced by a number of factors, we have been able to discern differences among biotypes in their response to glyphosate under controlled conditions in the greenhouse. This low level of resistance has not been expressed to a similar degree across all of the greenhouse and field research we have conducted, but we believe it may be partly responsible for the variability in control of these weeds experienced by some growers, especially those using a single postemergence glyphosate application. We’ll discuss how management affects control of these weeds and the evolution of low-level resistance in the next C.O.R.N. Newsletter.

Bulletin 545 - Control of Insect Pests of Field Crops Revised

Authors: Bruce Eisley, Ron Hammond

Bulletin 545, Control of Insect Pests of Field Crops, has been revised and contains information about labeled insecticides for insect pests on Alfalfa, Corn, Small Grains and Soybean. The insecticides are listed by crop and insect and in tabular form. The information for each insect contains the name of the insecticide, rate applied per acre, placement of the insecticide if necessary, pre-harvest interval and a short statement about treatment thresholds for each insect. The bulletin is currently on the web in pdf format at and should be available in hard copy from OSU Extension offices or the Media Distribution Office at OSU Extension in the near future.

Top Ten Issues and Developments Impacting Crop Management During Past 50 Years

Authors: Peter Thomison

The following article was adapted from CSSA SOCIETY NEWS February 2006 V51 No. 2 CSA News 29, A “Top Ten List” of Issues and Developments Impacting Crop Management and Ecology over the Past 50 Years

Dr. R. Kent Crookston from the Department of Plant and Animal Sciences at Brigham Young University recently surveyed the Crop Science Society of American (CSSA) members asking them what they felt were the most important developments and issues impacting crop management and ecology over the past 50 years. He received nearly 100 responses, and from those responses, developed a “top ten list. ” He presented this list at the November 2005 American Society of Agronomy Meeting. The following list starts with number 10 and proceeds to the number 1 development.

10. Alternative crops and alternative uses for crops (18 votes). Examples provided by respondents included soybeans, sunfowers, and canola being grown as alternative oilseed crops, and crop lands being used to produce nutra-ceuticals and biofuels.

9. Improved water management (20 votes). One respondent said, “Think about it. Fifty years ago, irrigation was all flood/furrow; today we have an array of efficient center-pivot, trickle, and buried-drip technologies. Fifty years ago, water was applied by perception; today we understand critical growth stages and have sensors to monitor the water status of both crop and soil.”

8. Improved mechanization (31 votes). Examples included the development of small-plot equipment to facilitate plant breeding, low-drift nozzles and air-assist sprayers to minimize off-target drift, and NIRS evaluation of forage quality and all the digital devices for making plant measurements.

7. Precision technology (GIS and GPS) (32 votes). Some felt that precision technology yielded great advantages and others didn’t. “Lots of money invested, little return,” one respondent said.

6. Shift from land grants to industry (enter “big ag”) (35 votes). One respondent said companies like Pioneer have developed programs that allow them to almost be the sole source of information for their growers. Another said, “When universities started hiring biotechnology people instead of field production people, it initiated a shift of the Land Grant purpose and domain to industry. Rapid adoption of Roundup Ready crops was not driven by university but by industry—with the approval of growers.”

5. Conservation tillage (40 votes). “Conservation tillage (CT) has obliterated the moldboard plow,” said one member. “According to the Natural Resources Conservation Service, cropland erosion was reduced by one-half from the mid-1980s to the mid-1990s because of CT.”

4. The environmental movement and sustainability (43 votes). According to one respondent, Rachel Carson’s Silent Spring triggered the environmental movement and an interest in ecology. Another said sustainable agriculture has done little to increase production but has increased costs.

3. Government programs and policies (45 votes). One member suggested that the USDA grain program has been used to support multinational grain companies more than improve land management, crop diversity, or the economy of the average producer.
2. Substitution management (replacement of crop rotations and animals with chemical fertilizers, herbicides, and pesticides) (96 votes). “Commercial fertilizers knocked crop rotations for a loop,” said one respondent. “Monoculture increased (more herbicides and pesticides), and soil quality went into decline.” Another said, “The disassociation of animal and crop production has led to the end of the family farm.”

1. Improved crop genetics (112 votes). Said one member, “Plant breeding and genetic improvements account for about 50% of the yield increases we’ve seen over the past 50 years.” Another said worldwide storages of genetic resources greatly extended the genetic pool in breeding programs.

Archive Issue Contributors: 

Anne Dorrance, Pierce Paul and Dennis Mills (Plant Pathology), Peter Thomison (Corn Production), Mark Loux and Jeff Stachler (Weed Science), Robert Mullen (Soil Fertility), Ed Lentz (Agronomy) and Ron Hammond and Bruce Eisley (Entomology). Extension Agents and Program Assistant: Gary Wilson (Hancock), Howard Siegrist (Licking), Harold Watters (Champaign), Bruce Clevenger (Defiance), Glen Arnold (Putnam) and Woody Joslin (Shelby).

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.