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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2009-19

Dates Covered: 
June 23, 2009 - June 30, 2009
Jonah T. Johnson

Applying Urea as a “Sidedress” for Corn

Authors: Robert Mullen, Keith Diedrick, Edwin Lentz

Some producers have been switching to urea as a nitrogen (N) source over the last few years due to price and availability concerns. Those producers may be applying their urea as a “sidedress” (we say sidedress in quotes, because the N is not actually placed to the side of the corn row, so maybe “topdress” is more appropriate) and concerns are being raised about how to manage that sort of application to minimize N losses.

First, remember that when urea hydrolyzes (goes into solution with water in the presence of the urease enzyme) the first product is ammonia, so surface applications are at risk for ammonia loss via volatilization, or vapors rising into the air. There is more to the story than just ammonia loss during the initial hydrolysis. As urea hydrolysis continues for soils with a pH above 6.3, one of the first products of the reaction is ammonium carbonate. Ammonium carbonate will react with any soil calcium to produce calcium carbonate and ammonium hydroxide. Hydrogen will be consumed from soil solution during the process, increasing soil pH. As the soil pH increases, ammonia forms more quickly, and more N could be lost. Simply put, when urea breaks down on the soil surface it is at risk for ammonia vapor loss when there is not adequate enough moisture to incorporate the material. Soils that already have a relatively high surface pH will experience more ammonia losses than a soil with a lower surface pH. Coarse textured soils have a higher potential for ammonia loss than fine textured soils because fine textured soils can more easily replace the hydrogen consumed during urea hydrolysis, so pH does not necessarily increase that much in clayey soils. Additionally, soils that have a lot of surface residue are at more risk for N loss because the residue itself contains urease and the residue produces a higher humidity near the surface.

Now that we are armed with this knowledge of urea reactions, how do we manage the input to mitigate risk of loss? The first and most obvious solution is to slow or stop the hydrolysis reaction by coating urea pellets with a urease inhibitor product. Urease inhibitors essentially buy you time to allow for a rainfall event so that the urea is adequately incorporated to eliminate the risk of loss; they do not work indefinitely (generally last about seven to ten days). Additionally, if a rainfall event occurs within a day or two after the urea application, in all likelihood a urease inhibitor was not necessary.

So what soils and production systems are more likely to benefit from urease inhibitors? Coarse textured soils (low cation exchange capacity, or CEC), fields that have a high surface pH, crop production systems that leave a lot of residue on the surface, and where rain is not expected for an extended period of time.

Scouting for late-season postemergence treatments in soybeans

Authors: Mark Loux

Soybeans should generally be scouted about 10 days after postemergence herbicides have been applied, to start to determine whether a second postemergence treatment will be needed. The activity from contact herbicides has typically been fully expressed by this time. It’s usually possible to assess whether any weed species in the field is failing to respond to systemic herbicides, although a second assessment in another week may be necessary to assess the final level of control. Waiting 3 or more weeks to make an initial assessment allows time for considerable more weed growth, reducing the chance to effectively control the weeds that were completely missed by the first application.

All too often the decision to make a second postemergence treatment occurs too late in the season, when weeds become evident above the soybeans (from the road at 50 mph). Although not a hard and fast rule, our research has fairly consistently shown that many of the weeds that escape a first postemergence treatment can be most successfully controlled when the second postemergence application is made about three weeks after the first. This seems to allow enough time for some recovery and limited regrowth of weeds, but the plants are usually still relatively small. We have heard growers state that three weeks is too early, because weeds are down within the crop canopy and spray coverage will be an issue, or that this is not late enough to control late-emerging weeds. We have not experienced this to be a problem in our research. The spray particles seem to make their way to the weeds, and their small size makes them easier to control, compared with application later in summer to large weeds that are more tolerant of herbicides. Weeds that become evident above the soybeans in late July have usually emerged by early July, and minimal weed emergence occurs from then on.

This assumes of course that the first application occurred when weeds were relatively small to begin with, and that it had substantial activity on weeds in the field. The exception to the three-week respray period is when the first postemergence application had little or no effect on certain weeds due to high-level herbicide resistance. In this case, the best course of action is probably to respray immediately, since the growth of these weeds will continue unabated and they are still competing with soybeans. Scouting fields 7 to 10 days after application is important so that this can be determined as soon as possible. We have conducted research with glyphosate-resistant populations of common and giant ragweed that have exhibited little response to an initial application of glyphosate, possibly just minor yellowing and very brief cessation of growth within a week or so after application. Immediate retreatment with an alternative to glyphosate is warranted in this situation, before weed size reduces the possibility of successful control. This can be contrasted with resistant giant ragweed populations we have studied that still exhibit about 75% control in response to the first glyphosate application. These plants are greatly stunted and chlorotic, and their growth completely ceases for a while. In this case, the more appropriate course of action is to allow about 3 weeks before application, which allows time for weeds to produce some new growth.

Don’t assume that a second treatment with the same herbicide will necessarily control the weeds that survived the first application. In the first example mentioned above, where there is apparently a high level of glyphosate resistance, a second application of glyphosate is not likely to control ragweed plants that survived the first application without many symptoms. Control will require application of an alternative herbicide with substantial activity on ragweed, such as FirstRate, Classic, Flexstar, or Cobra/Phoenix (although FirstRate or Classic can be used only if the population is known to still be sensitive to ALS inhibitors). It’s also necessary to optimize the adjuvants for these herbicides, even if they are mixed with glyphosate. For example, activity of Flexstar and Cobra/Phoenix is optimized by applying with a crop oil concentrate or methylated seed oil. Failure to include an oil-based adjuvant is likely to result in ineffective control of glyphosate-resistant weeds, because the surfactant in glyphosate products does not optimize the activity of these herbicides.

In the second situation mentioned above, where the glyphosate still had substantial activity on the ragweed plants, it’s possible that a second application of glyphosate can prevent the weeds from interfering with soybean growth and minimize ragweed seed production. However, in our research with this type of resistance, this occurred mainly where the field was weed-free at planting, residual herbicides were used to slow ragweed growth and reduce the population, and a high rate of glyphosate was used in the first postemergence application. Suggesting that a second application of glyphosate will be effective without having detailed information about the weed population of concern makes us nervous, but the key is that the first application should have provided at least fair control. Be sure to apply the highest glyphosate rate possible, based on the rate used in the first application, and when in doubt, combine glyphosate with another herbicide that controls ragweed.
These same principles apply to nonGMO soybeans, with regard to the timing of a second postemergence application. It’s possible that the first postemergence application will adequately control giant ragweed in nonGMO soybeans. However, it’s our experience that this fails to occur fairly frequently in fields where the giant ragweed population is moderate to high, and many nonGMO fields could benefit from a second postemergence application. Where Flexstar has been used in the first postemergence application, it will be necessary to use Cobra or Phoenix in the second application. Our research shows that the 3-week interval between postemergence applications in nonGMO soybeans works well, even in dense ragweed infestations.

Finally, where it’s necessary to mix glyphosate with another herbicide to improve control, whether in a first or second postemergence application, choose a herbicide that has substantial activity on the weeds of concern (substantial activity = 8 or 9 in the OH/IN Weed Control Guide). We have already been made aware of scenarios where glyphosate was applied in combination with other herbicides to control certain weeds, and the other herbicide was not labeled for control or suppression of that weed, and was rated as a “poor” in the Weed Control Guide. We know that there is often an impetus to limit costs. However, adding a “cheap” herbicide to glyphosate applications in an attempt to improve control of certain weeds does little good if that herbicide has little activity on those weeds.

Mixing other herbicides with glyphosate is likely to cause minor leaf burn, chlorosis, and stunting due to the activity of the other herbicide. While we have all become accustomed to the lack of injury from glyphosate applications to Roundup Ready crops, weed populations have adapted to the point that other postemergence herbicides can be required and this results in the possibility for soybean injury. The good news is that it has generally been extremely difficult to prove that injury to soybeans from postemergence herbicides causes yield loss. In our research, the soybeans exhibiting the greatest injury have occasionally out yielded soybeans without appreciable injury symptoms.
Similar to weed control programs before Roundup Ready soybeans, it’s reasonable to expect that postemergence weed control will be accompanied by some injury. There are two situations of concern: 1) we have observed yield loss where late-planted soybeans are injured greatly by postemergence herbicides, followed by an extended period of dry weather that prevents soybeans from adequately growing out of injury (because of the late-planting, the soybeans seem to run out of time to recover and produce maximum yield: and 2) it’s generally not a good idea to really “torch” soybeans that are past the R1 stage. Our experience is that large soybeans can be pretty tolerant of contact herbicides late in the season, however, and it’s possible to just bronze the upper growth without otherwise affecting growth and development.

Corn growth “exploding” in many Ohio fields

Authors: Peter Thomison

Last week corn “exploded” in growth across much of Ohio. Most corn has entered the phase of rapid growth and development which usually occurs about the six leaf collar stage (V6 stage). Favorable conditions (abundant rainfall and warm temperatures) are fueling this surge in corn growth. However, there is considerable variability in growth with early planted corn chest high or taller and late May/early June corn not yet knee high. In west central Ohio, most corn planted in late April is at or beyond V11 and corn planted in early June is at about V4. From V10 to tasseling, leaf collar emergence occurs rapidly with approximately one leaf exposed every 50 GDDs; so given the warm temperatures that often occur this time of year, a new leaf can appear every 2 days. By V6 or shortly thereafter, nodal roots are usually established, and by V10, these roots may be over a foot deep and approaching row middles.

There are also some other important processes occurring at this time. As early as the V4/V5 stage, ear shoot initiation is completed and the tassel is initiated on the top of the growing point. During the rapid phase of corn vegetative growth, ear yield components are being determined. Kernel row numbers per ear are generally established by V12. Kernel row numbers are usually less affected by environmental conditions than by genetic background. Corn hybrids characterized by "girthy" ears typically exhibit more kernel rows (about 18 or 20 rows) than hybrids with long tapering ears (about 14 or 16 rows). Determination of kernels per row (ear length) is usually complete about one week before silking (R1) or about the V17 stage. Unlike kernel rows per ear, kernels per row can be strongly influenced by environmental conditions, so the absence of drought stress conditions this year bodes well with regard to the potential number of kernels on developing ears.

Concerns about corn injury from residual herbicides

Authors: Mark Loux

We appear to be having an above-average year for incidents of injury to corn from preemergence herbicides. Purdue weed scientists Glenn Nice and Bill Johnson co-authored an article on this subject in a recent Purdue Pest and Crop Newsletter ( that covers this subject well.

Macronutrients: Foliar Fertilizers vs. Dry Fertilizers

Authors: Keith Diedrick, Robert Mullen, David Henry

Some producers have called us about the in-season use of foliar macronutrients (the nutrients necessary in relatively large quantities: N, P, K, sulfur, calcium, or magnesium), and how well they stack up to the old standby bulk fertilizers in granular form. It is somewhat difficult to apply granular fertilizers in-season, of course, and though foliar fertilizers have their place at this time in the season, one needs to remember that though plant foliage can absorb these nutrients, it cannot absorb a large quantity or concentration of a macronutrient (that may be necessary to correct a deficiency) without burning the plant. With that in mind, it is difficult (and expensive) to provide a crop with all of its fertility needs strictly via foliar fertilization. We recommend the use of foliar fertilizers only if a deficiency is detected and confirmed by tissue test, following the same philosophies of integrate pest management (IPM) for weeds, diseases, and insect pests. In other words, if nutrients are not deficient, yield gains due to an additional fertilizer application are unlikely to occur.

Additionally, we cannot think of too many cases where a liquid fertilizer product is less expensive per pound of nutrient than a bulk granular, so do some easy math to determine price per pound. See our article from last October for examples: . Even though $900-950 for a ton of 0-0-60 seems rather painful and the market seems uncertain, some foliar potash products may be two, three, or even more times the price for the same pound of K2O. Granular fertilizer applications in the fall or spring are more cost-effective than applying a foliar product to deficient plants in the middle of the season. Following the recommendations from the Ohio Agronomy Guide ( and Tri-State Fertilizer Recommendations ( for P and K will reduce the risk of potential deficiencies and subsequent yield loss before they occur. Again, we do not recommend foliar fertilizers as a main source of nutrients, using them only in the case of a deficiency identified by symptoms and a tissue test.

Nutrient Value and Removal of Wheat Straw

Authors: Keith Diedrick, Edwin Lentz, Robert Mullen

As wheat harvest approaches some producers are curious about the economics of baling wheat straw versus leaving it in the field as residue. This leads to the question – what is the nutrient value of the straw being removed and should removal lead to increased fertilizer applications in subsequent years?

From a pure fertilizer value, wheat straw contains very little in the way of phosphorus (P2O5) but moderate amounts of nitrogen (N) and potassium (K2O). The actual amounts of N, P2O5, and K2O contained in a ton of wheat straw are 11, 3, and 20 pounds, respectively. A sixty bushel wheat crop might produce upwards of 2.2 tons of straw per acre (assuming a harvest index of 0.45 – meaning that 45% of the total biomass grown in the field was grain), removing 30 pounds of N, 9 pounds of P2O5, and 44 pounds of K2O. Thus, straw does have some fertilizer value especially with regard to potassium and may require some additional fertilizer input in subsequent years, but, soil testing should be conducted to validate the need for additional nutrients.

Wheat straw residue also contains organic matter that when returned to the soil does have value, but it is difficult to put a dollar value on it. Continued removal of the above-ground biomass may have negative repercussions in the long-run in the form of decreased organic matter, especially if some organic residue is not returned to the soil.

Do you work with a crop advisor?

Authors: Harold Watters

As crop production has progressively gotten more complicated, more and more producers are hiring a crop advisor. Often a farmer is good at one of the necessary tasks on the farm, but also knows who to call for advice on those topics that he or she knows a little less about.

In Ohio, we have a number of groups who work to advise farmers on crop production. The newest of these organizations is the internationally known Certified Crop Adviser (CCA) program - international because the program operates in the US and Canada. Currently there are over 500 participants in Ohio and almost 14,000 internationally.

Certified Crop Advisers are required to keep up on their education – with 40 continuing education units required every two years. They also demonstrate their education and experience before being certified by a board of their peers. In Ohio, CCAs work for co-ops, retailers, seed companies, suppliers, government agencies (including OSU Extension) as well as private independent consultants.

Two reasons why this is a topic of concern now:
• The Ohio Farm Bureau contributes its efforts to an award program called “Excellence in Crop Advising;” any CCA in Ohio is eligible. If you know or work with a CCA who is deserving of this award go to this site for the form:, and a description of the nomination process. In order to qualify for the award, at least one of the nominators must be an Ohio Farm Bureau member. Send nominations to: Jennifer Baldwin, Ohio Farm Bureau, P.O. Box 182383, Columbus, OH 43218-2383. The deadline for nominations is June 26th.
• If you or your crop advisor is not already a Board Certified Crop Adviser, then for information on becoming certified you can go to the Ohio AgriBusiness Association website: There you can find information on registering for the international and the local exam and joining over 500 others in Ohio who are already certified. The deadline for registration for the two exams (given August 7th in Columbus) and to begin the certification process is June 26th.
o If you want some tidbits on preparing for the Ohio and International CCA Exams, the OSU Extension Agronomic Crops Team provides an Exam Preparation Course. The course this year will be held July 23 & 24, 2009 in Sidney Ohio. Contact Wesley Haun with OSU Extension in Logan County, for more information: Wesley Haun, OSU Extension – Logan County, 120 E. Sandusky Ave. Suite #1, Bellefontaine OH 43311, phone 937-599-4227 or by email

Weather Update

Authors: Jim Noel

The next 2 weeks will see a greater than 80% chance of above normal temperatures. Most of this will be concentrated in overnight low versus daytime highs. Overall, expect highs 86-91 and lows 65-70 for many of the days over the next 2 weeks.

There is 20-40% chance of a “two-inch” rain event and about a 60% chance of a “one-inch” rain event over the state. Normal is about two inches. Hence, below normal rainfall is expected in most areas. The best chance of heavier rain will be in the north versus the south which has a better chance for little rainfall.

The percentages above come from the North American Ensemble Forecasting System.

El Nino looks on track to develop; I still expect a switch to a warmer and drier pattern by fall. We will have to watch if this switch occurs earlier in August too.

Archive Issue Contributors: 

Pierce Paul, Anne Dorrance, and Dennis Mills (Plant Pathology), Ron Hammond, Andy Michel and Bruce Eisley (Entomology), Peter Thomison (Corn Production), Robert Mullen & Keith Diedrick (Soil Fertility), Mark Loux (Weed Science), and Jim Noel (NOAA). Extension Agents and Associates: Roger Bender (Shelby), Howard Siegrist (Licking), Glen Arnold (Putnam), Greg LaBarge (Fulton), Harold Watters (Champaign), Mike Gastier (Huron), Mark Koenig (Sandusky), Wes Haun (Logan), Jonah Johnson (Clark), Les Ober (Geauga), Alan Sundermeier (Wood), Suzanne Mills-Wasniak (Montgomery) and Tim Fine (Miami).

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.