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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2005-10

Dates Covered: 
April 25, 2005 - May 3, 2005
Steve Prochaska

What to do When You’re Between Preemergence and Postemergence

Authors: Mark Loux

Regarding last week’s article about dry weather and possible herbicide performance problems – never mind. Preemergence herbicides applied prior to the recent rain/snow events should have now received ample moisture to promote effective activity. Questions now, especially in corn, seem to focus on the fields that have been planted but have not been treated with herbicide yet. We assume that most fields planted to corn last week will not be dry enough to sustain traffic until at least the end of this week, and maybe longer if we receive additional rain.

As we mentioned in a C.O.R.N. article two weeks ago (“Weed Management Issues for Early-planted Corn”, April 11), the residual and foliar activity of atrazine and several other corn herbicides (Callisto, Hornet) allows for flexibility in the application window, and they can be applied preemergence or early postemergence. Most effective preemergence control occurs when herbicides are applied at the time of corn planting, since this allows maximum time for rain before weeds start to emerge. Herbicide needs to have moved into the upper inch or two of soil by the time most of the weeds are germinating, or control may be reduced.

There is a situation where we have observed reduced control in our research plots, and it applies to the current scenario in fields that have been planted but not yet treated with herbicide. In these fields, herbicide could be applied within a week or so, when weeds are lurking just below the soil surface, with weed emergence imminent. If weed emergence occurs after herbicide application but before additional rain, control could be reduced because herbicide will not have moved into the soil where it can be taken up into emerging shoots. In other words, there is a window of time where it may be inadvisable to apply preemergence herbicides, and instead shift to an early postemergence application. This may be especially critical for annual grass control, because the acetamide herbicides in most atrazine premix products are immobile in plants and active only through shoot uptake. Some broadleaf herbicides, including atrazine, Hornet, Callisto, and Balance, can move into plants via shoots and roots, so that there is still some possibility of herbicide uptake into small, newly emerged weeds (sometimes know as “reachback” activity).

As a result of this window of time when preemergence herbicides can be ineffective, we try to apply herbicides in our research plots either: 1) as soon as possible after planting to obtain the best chance of effective preemergence activity, or 2) early postemergence, when the first flush of weeds have emerged but are still small. The early postemergence application tends to be more consistently effective than preemergence herbicides applied a week or more after planting that fail to receive rain immediately after application. Most atrazine premix products applied in corn will control small weeds, although additional atrazine or a postemergence grass herbicide should be added if grasses are more than an inch tall. One caution about early postemergence applications – most corn herbicides cannot be applied using 28% as the spray carrier after the corn has emerged. Degree and Degree Xtra are the exceptions to this, and these products can be applied in 28% to corn up to 6 inches tall as long as air temperatures do not exceed 85 degrees. Products and mixtures containing Balance should no be applied after corn has emerged.

Causes of Underground Leafout and Emergence Problems in Corn

Authors: Mark Loux

Considerable progress has been made in the last decade toward safening preemergence acetamide herbicides for use in corn. All acetochlor and metolachlor products are formulated with a safener, which helps protect corn from injury that can occur under unfavorable environmental conditions. We have generally not considered injury from acetamide herbicides to be a major concern in Ohio under “normal”growing conditions. However, prolonged cold and wet conditions following corn planting and acetamide herbicide application can result in problems with emergence and early-season crop development. Symptoms of this injury include underground leafout, failure of plants to emerge, and the failure of the leaves of emerged plants to unfurl. Injury can be more severe when 2,4-D is applied with acetamide herbicides, and is expressed as more severe distortion of plant growth (twisting of roots, etc).

A number of other factors can cause emergence problems in corn and underground leafout, including cloddy seedbeds, cold weather, and crusting. A combination of any of these with acetamide injury would most likely result in greater problems with stand establishment. We do not expect a short period of cold and wet weather following planting to significantly increase the risk of acetamide injury, but continued weather of this nature might be more problematic. Cornfields should be scouted early enough to determine whether emergence problems are occurring (this would correspond roughly to the time of scouting for cutworm problems), so information is available on which to base replant decisions.

Effect of Saturated Cold Soil on Crop Emergence

Authors: Jim Beuerlein, Anne Dorrance

As of April 25, we have a million acres of soybeans planted and plants have emerged in some fields. Since April 22, we have received over two inches of rain in most areas, several inches of snow in other areas, and air temperatures fell to below freezing in northern Ohio. The weather forecast for this week has warmer temperatures and more rain predicted. How will all this affect our bean crop?

Several factors, (soil temperature, soil drainage, fungicide seed treatments, amount of emergence) must all be considered. Soil temperatures at the 2-inch depth are about 40 degrees, so there won’t be any measurable emergence or growth until the soil warms to above 50 degrees. Because the soil is saturated it will warm very slowly because much more energy is needed to warm wet soil than dry soil. The better the drainage in a field the sooner it will warm and allow plant growth to continue. Disease organisms are not active at low soil temperatures but they will regain activity as the soil warms, and the warmer temperatures and wet soil conditions will be ideal for their attack on our crop. There is the possibility that seed treatment fungicides and insecticides may have been diluted to the point that they can’t protect the crop as well as needed. Plants that emerged a few days ago are the most likely to have been killed by low temperatures. The closer the growing point was to the soil surface the less likely low temperatures harmed it. Seeds just under the soil surface should emerge as soon as the soil surface reaches 60 degrees and will likely survive winter’s weekend party. The most recently planted seed is predicted to face an extended period of cool, saturated soil, which is the worst combination for desired rapid germination and a high percent emergence.

Generally, well-drained fields will have better plant stands than poorly drained fields. Fields receiving less rain will likely fare better that fields receiving more rain. Snow would have protected emerged soybeans and likely will not have any negative impact on un-emerged seedlings because it is expected to melt rapidly.

Impact of Freezing Temperatures and Snow on Corn Survival

Authors: Peter Thomison, Patrick Lipps

The freezing rains and snows that much of Ohio experienced over the weekend have led to questions about the impact of these weather conditions on the corn crop. As of last Friday, many of the major corn producing counties were reporting that 70% or more of their corn was planted. Although it's too early to determine how individual corn fields will respond to the recent weather, the effects of the low temperatures on corn survival will probably be negligible for the most part.

In past years, we have observed that early planted corn that was in the process of germinating or as far along as the V1 stage (one leaf collar visible) survived freezing soil temperatures in late April with little impact on crop performance or plant stand. Agronomists generally downplay the impact of low temperature injury in corn because the growing point is at or below the soil surface until V6 (six leaf collars visible), and thereby relatively safe from freezing air temperatures. Moreover, the cell contents of corn plants can sometimes act as an "antifreeze" to allow temperatures to drop below 32 degrees F before tissue freezes, but injury to corn is often fatal when temperatures drop to 28 degrees F or lower for even a few minutes.

To assess the impact of these freezing temperatures on emerged corn, check plants about 5 days after the freezing injury occurred (and preferably when growing conditions conducive for re-growth have occurred). New leaf tissue should be emerging from the whorl. You can also observe the condition of the growing point (usually located « in to 3/4 in below the soil surface) by splitting seedlings lengthwise. If the growing point appears white to light yellow and firm several days after the frost, prognosis for recovery is good.

Of greater concern with regard to the viability of germinating and emerging corn is how long soils will remain saturated. Cool temperatures and wet weather provide the right conditions for the development of seedling blight diseases. Cold temperature injury can play a significant role in predisposing plants to root infection and blight. Under normal conditions plants can continue to grow and produce new roots, but when other injuries occur, new roots cannot develop rapidly and Pythium and other soil fungi can kill stressed plants. Seed treatment fungicides generally remain effective from 10 to 14 days but under saturated conditions the duration of protection may be shorter.

Subsequent rainy weather can cause problems to freeze damaged corn. Bacterial soft rots can destroy the corn growing point and this often occurs when rains splash bacteria into frost damaged leaf whorls. If growing conditions are favorable, i.e. warm and dry after the freezing event, the plants typically outgrow bacterial damage, but if weather remains cold, wet and cloudy following the freezing event, the potential for this bacterial damage increases.

Injury from freezing can also prevent the leaf from unfurling normally resulting in tied leaf whorls - this frost damage sometimes resembles the "buggy whipping" and tight leaf rolling associated with certain herbicide injury. Generally plants exhibiting such symptoms resume normal growth when growing conditions improve. Mowing fields to cut off the tied leaf whorls, and thereby allow normal expansion of undamaged leaf tissue is usually of limited benefit.

Nitrogen Rates and Nitrogen Prices - Deciding What to Do

Authors: Robert Mullen, Edwin Lentz

As producers face higher nitrogen prices, the question often arises “What is the most economical rate based on today’s prices?” While this question may seem simple and straightforward, the solution is complex and confusing. If potential yield for a given growing season could be identified in time to adjust nitrogen rates and the amount of nitrogen released “naturally” from soil organic matter could be estimated, nitrogen recommendations could be given with an exceptional level of precision. As we all know, we currently do not have a good method of estimating potential yield (not early enough in the season to matter) nor do we have a good method of estimating nitrogen mineralization rates (not yet), so nitrogen rate decisions are based on historical crop performance. Weather is the variable that we can not get a handle on because it can and does change dramatically from one year to the next (which any producer can attest). Potential yield and nitrogen mineralization are directly controlled by weather - primarily soil temperature and moisture. Tri-State nitrogen recommendations utilize yield goal to generate N recommendations. Yield goal (synonymous with potential yield in my language) is the realistic yield that could be generated in a specific field and is typically the average yield for that specific field over the last five growing seasons. Current recommendations given in the Tri-State Fertilizer guide are designed to ensure that enough nitrogen is present to maximize yield. The discussion presented below will provide a basis for decision-making with regard to fertilizer cost and corn price based on 8 site-years of information collected from the Hoytville research station (maintained by Greg LaBarge).

On average, to maximize yield, it required 196 lb N/acre to produce 178 bu of corn per acre. Recognize that the 196 lb N/acre and 178 bu/acre are averages and actual optimum N recommendations and yield change from one year to the next. The average N rate is within 11 lb of the Tri-State recommendation of 185 lb N/acre for this yield goal. Using a corn price of $2.50/bu and a nitrogen cost of $0.25/lb, the economic optimum fertilizer rate would be 150 lb N/acre or 19% less than the Tri-State Fertilizer guide recommendation. Remember, however, that optimum N rates can and do change from one year to the next. If the average economic optimum N rate of 150 lb N/acre (based on $2.50/bu corn and $0.25/lb N) had been applied in each of the 8 site-years, in 3 of those years additional revenue could have been generated with additional N (greater than 150 lb/acre). The three years where additional revenue could have been generated with additional N were years with exceptional yields (greater than 180 bu/acre). With the exception of one year (2003), additional revenue was not generated by application of N greater than the 196 lb N/acre.

Take home message: Should nitrogen rates decline as the price of nitrogen increases and corn price remains the same? Absolutely, this makes economical sense. How much should they decrease? This is not as easy to identify as one might think. Be reasonable when it comes to decreasing the amount of N applied this year (if you are decreasing). Applying 10-20% less than normal (assuming you are applying more than 160 lb N/acre) because of high N prices should not hurt field productivity too much, but the risk is there for some yield loss. Just for reference, the data presented above (second paragraph) was for sidedress N only.

Marestail (Horseweed) Has Bolted – What are the Options?

Authors: Mark Loux

Marestail initially grows as a rosette, a plant with a very short stem and numerous leaves close to the soil, similar to a dandelion. When enough nutrients have been stored in the root, then an upright stem begins to develop. This process of going from a rosette to an upright stem is called bolting. Shortly after the stem bolts and reaches 1 to 2 inches in height, marestail becomes progressively more difficult to control with herbicides, and appropriate selection of herbicides becomes more important.

At this time, the minimum amount of glyphosate that should be applied to any marestail biotype is 1.1 pounds acid equivalent/acre (lb ae/A), which corresponds to 30 to 48 ounces/A, depending upon the glyphosate formulation. For biotypes known to be resistant to glyphosate, the addition of 2,4-D ester (0.5 pound lb ai/acre - 0.67 to 1 pt/A, depending upon the formulation) to the appropriate rate of glyphosate usually provides effective burndown control. This program will also be effective on ALS resistant marestail biotypes, which can be found throughout most of Ohio.

Another effective herbicide burndown program at this time is a mixture of Gramoxone Max (1.5 pt/A) plus 2,4-D ester (0.5 lb ai/A) plus Sencor (8 oz/A). When the stems of marestail reach 4 inches in height, increase the Gramoxone Max rate to 1.7 pt/A and when the stems are greater than 7 inches tall, increase the Gramoxone Max rate to 2.1 pt/A.

Two additional burndown strategies, especially for ALS and glyphosate resistant marestail or for marestail with greater than 7 inch stems: glyphosate (at least 1.1 lb ai/A) plus 2,4-D ester (0.5 lb ai/A) plus either cloransulam (FirstRate, Gangster) or chlorimuron (Canopy XL, Synchrony XP); or glyphosate (at least 2.25 lb ae/A) plus 2,4-D ester (0.5 lb ai/A).

The application of burndown herbicides before May 10th to control marestail should include a residual herbicide in order to control later-emerging marestail. The most effective products, regardless of the type herbicide resistance, include Sencor (8 oz/A), Valor (2.0 oz/A), and Gangster (1.8 oz/A). If the marestail is not ALS resistant, the addition of cloransulam, chlorimuron, or Python to the burndown should provide residual marestail control.

Tillage can also be used to control emerged marestail plants prior to planting. However, be sure that tillage is thorough enough to completely disrupt plants. Plants that survive and recover following tillage are not likely to be controlled by any subsequent herbicide treatments.

Cold and Snow Has Not Hurt Wheat

Authors: Patrick Lipps

There recent colder weather and snow has prompted several phone calls about possible injury to the wheat crop. The wheat in Ohio is currently in the stem elongation growth stage. The first node is visible in most fields (Feekes growth stage 6), but in southern Ohio some fields may be in flag leaf emergence (Feekes growth stage 8). Wheat at these growth stages can tolerate cold temperatures down to about 24 to 26 degrees F. The temperatures in Ohio did not much below 30 degrees over the past three days in any part of the state. Based on this the wheat crop should see no damage due to cold injury.

Archive Issue Contributors: 

Mark Loux and Jeff Stachler (Weed Science), Pat Lipps, Anne Dorrance and Dennis Mills (Plant Pathology), Jim Beuerlein (Soybean Production), Peter Thomison (Corn Production), Robert Mullen ( Soil Fertility), Extension Agents and Associates: Roger Bender (Shelby), Alan Sundermeier (Wood), Steve Bartels (Butler), Ed Lentz (Seneca), Steve Foster (Darke), Gary Wilson (Hancock), Dusty Sonnenberg (Henry), Mark Koenig (Sandusky), Howard Siegrist (Licking), Harold Watters (Champaign), Tammy Dobbels (Logan), Glenn Arnold (Putnam) and Steve Prochaska (Crawford)

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.