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8/6/2012 - 8/12/2012

Soil Fertility Management Issues Following Drought-damaged Crops – Nitrogen

By John Sawyer and Antonio P. Mallarino, Department of Agronomy

The dry growing season in 2012 has raised several soil fertility questions. In some cases, there has been relatively normal crop production and no need for management changes. In other situations with severely damaged crops, there is potential for adjustments for the 2013 corn crop.


Soybean yield and next year corn N rate

In Iowa we no longer use the soybean yield to adjust nitrogen (N) rate recommendations for the next-year corn crop. The reason is that there is no relationship between soybean yield and the rotation effect on corn N fertilization rate for the next year. Would the same hold for drought-damaged soybean in 2012? Yes, even with very low yields. What is important is that soybean was the previous crop. Soybean does not leave “extra” N behind at the end of the season. The reason for the difference in N rate requirement between corn following soybean and corn following corn is complicated, but important reasons deal with the difference in amount and quality of crop residue, and how that affects soil microbial processing, soil mineralization and N for crop residue decomposition. Also, there would not be carryover nitrate-N following soybean. So, just use the normal rate recommendation system (Corn N Rate Calculator, MRTN rate or profitable N rate range) for corn following soybean.


Corn yield and next year corn N rate

This is more complicated than the soybean crop question. In general, as long as plant vegetative growth and/or grain yield was not drastically affected by the dry weather, then use the normal rate recommendation (Corn N Rate Calculator, MRTN rate or profitable N rate range) to determine the needed fertilization rate for continuous corn in 2013. In that situation, corn production of vegetation/grain would use much of the N applied. If there is uncertainty about unused N, and if less than normal rainfall persists into next year, one could use the low end of the Corn N Rate Calculator profitable range for the 2013 rate recommendation.


Residual soil nitrate

If the corn plant vegetation and/or grain yield was drastically affected by drought conditions, then N uptake would have been reduced and unused nitrate-N could be accounted for in determining the N fertilization rate for the 2013 corn crop. There are two methods to estimate carryover N. The direct method is to sample the soil profile (a minimum of 2 feet) after harvest and measure the nitrate-N concentration. Sampling would be by 1-foot increments. If dry conditions persist, most applied N should remain in the top 2 feet. Sampling to 3 feet would be preferable, especially where rainfall was enough to move nitrate deeper in the profile. To add up nitrate-N in the sampled profile, multiply the concentration in each foot by four to get the nitrate-N amount per foot and then add the amounts together. One would not want to account for all of the nitrate-N as a subtraction from the next crop N recommendation as there is always some nitrate in the profile at the end of the season. A suggestion from research conducted in Wisconsin (which should be appropriate for Iowa) is to only account for nitrate-N greater than 40 lb nitrate-N (2 foot depth) or 50 lb nitrate-N/acre (3 foot depth), with the remaining amount then subtracted from the normal rate recommendation. A second method to estimate carryover nitrate-N is to use the 2012 corn grain yield. Take the total N applied for the 2012 corn crop and subtract the 2012 grain yield in bu/acre. Then assume 50 percent of that amount will remain available to the 2013 crop if precipitation is normal or below normal for the fall/winter/early spring. The remaining nitrate-N amount will vary depending on the actual rainfall and potential losses from fall through spring. For example, if the total N application for the 2012 crop was 190 lb N/acre and the 2012 corn yield was 50 bu/acre, then the unused N would be 190 minus 50 = 140 lb N/acre. The 140 lb N/acre times 50 percent leaves 70 lb N/acre to subtract from the 2013 rate recommendation.

As a conservative approach, and due to uncertainty with either estimation method, a minimum rate recommendation of 50 lb N/acre should be considered. If fall/spring precipitation is well above normal, then the carryover nitrate would not be likely, especially in soils with high leaching potential. Sandy soils are not likely to retain carryover nitrate.

Spring soil profile sampling for nitrate-N is an option, especially with concerns about residual nitrate remaining after the fall/winter. In addition, such sampling could allow for a spring preplant or sidedress N application based on spring profile nitrate-N results, and instead of a fall application. Use of the late spring soil nitrate test (LSNT) to determine carryover nitrate may miss considerable nitrate deeper than in just the top foot. Therefore, it would be better to sample the deeper profile before planting.

There could be considerable variation in nitrate levels across fields, due to yield level, banded N application, and soil/topography. Therefore, many cores (at least 12) should be collected per sample, and multiple samples per field from representative areas. Since the cores are by one-foot depths, mixing in the field will be needed to obtain a representative sample for each depth. Keep the soil from each depth as a separate sample to send to the lab.


Stalk nitrate testing

The end-of-season lower corn stalk nitrate test can be useful for determination of excess plant available N from the soil (i.e. concentrations above 2,000 ppm nitrate-N). However, that interpretation is for normal weather and production conditions. In 2012, test results could be abnormally high due to the dry conditions and severe impact on plant growth and grain production. Therefore, it is suggested to not use the stalk nitrate test this year, or to use it as a measure of potential nitrate carryover.


Timing of fall N application

With the potential for early fall harvest this year, carefully consider the risks of early N fertilizer or manure application. With typical warm soils in the late summer and early fall, conversion of fertilizer and manure ammonium to nitrate will be rapid. This places the applied N at risk for loss if wet conditions develop. For many years now the ag industry in Iowa has followed the “wait until 50 ○F and cooling” before anhydrous ammonia application. That would also be a good practice for manure with high ammonium-N content.


Dry periods

Corn yield response to N rate and needed fertilization rate decreases in years with below normal rainfall. This effect can persist across periods (years) of dry conditions, and even for year(s) after rainfall returns to normal (but not excessively wet). If below normal rainfall conditions continue, then consider using the low end of the Corn N Rate Calculator profitable range for corn N rate recommendations.


John Sawyer and Antonio P. Mallarino are professors in the Department of Agronomy with research and extension responsibilities in soil fertility and nutrient management. They can be contacted at and

August 10 Corn Yield Forecast

By Roger Elmore, Department of Agronomy

The USDA/NASS released the first 2012 corn yield forecast on August 10. The current Iowa corn yield forecast is for 141 bu/acre, down from both last year’s 172, and the 30-year trend line value of 180(Figure 1). The August forecast for Iowa yield is 22 percent below trend line; that of the U.S., 123, is 23 percent below trend line.

Figure 1. 30-year Corn Yield Trends, with August 2012 Yield Forecasts. Iowa and United States from USDA-NASS. Full-size image (.pdf)


These forecasts arise from meticulous late-July counts and measurements by the team at USDA/NASS. In late July, enumerators record stand counts and ear counts to obtain an estimate of ears per acre. Statisticians generate yield estimates based on how these counts compare to those of the last five years. 

In late July, final kernel counts per ear and final kernel weights are still uncertain. The assumption in the forecast is that weather during the remainder of the grain filling season is normal when compared to the previous five years. Thus, kernel numbers and kernel weights are ‘normal,’ too. Of course no one knows if that is true or not until after the fact.


Stay tuned for monthly yield forecasts

Each successive monthly forecast through November is a “new” forecast--not simply an update. The USDA/NASS group uses new and expanded observations from a large number of sample locations to formulate expected yields based on field observations for every subsequent yield forecsts. By the time the September yield forecast is published (based on late-August samples), kernel numbers are likely set; thus, yield forecasts more comparable to the final yields for 2012. 

Of course it is possible the weather could be better than average for crop yield and actually increase the yield in subsequent forecasts. This is usually associated with temperatures that extend the filling period beyond normal.

Remember, though, corn that is not harvested because of poor yield potential will not be included in yield forecast reports. Statewide “yield” will actually increase simply because “yield” is defined as “yield per harvested acre.” Thus acres dropped because of drought will not be considered in the harvested area estimates for 2012.
Many groups other than USDA/NASS attempt to get an early assessment of yield, too. Their procedures undoubtedly differ from those of USDA/NASS. But, in any case, the biggest issue with yield forecasts before harvest is obtaining an accurate estimate of kernel numbers per acre (plants per acre x ears per plant x kernels per ear), and final kernel weights. It becomes a numbers game: the more samples taken, the more accurate the estimate; the closer to harvest the estimates are made, the closer the estimate to final yield.


Forecast yield track records

It should not surprise you that the August USDA/NASS yield forecast doesn’t always reflect final annual yield. Figure 2 shows the track record for the August forecasts. With a few exceptions, August yield forecasts are within 10 percent of the final yields for each year.

Figure 2. Historical Track Record for August Forecast Corn Yields and Final Yields. Iowa, 1963-2011; from USDA-NASS. Full-size image (.pdf)


Most of you will remember plausible reasons for the exceptions in the 'Track Record’ chart: e.g. 1983 drought, 1993 floods, 1992 and 2004 long seed-fill periods, etc. As we go into the final stages of kernel fill, the monthly yield forecasts will edge closer to the final annual yield estimates for the state.

Meanwhile, with high-night temperatures and dry conditions during the early seed set and seed-fill period, we can expect fewer kernels per ear and lighter kernel weights than we’ve seen for several years. Cooler night temperatures and periodic gentle rains improve yield potential through increasing seed set and lengthening the seed-fill period.


Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by email at or (515) 294-6655.

Dry Field Conditions Increase Harvest Fire Risks

By Mark Hanna, Department of Agricultural and Biosystems Engineering

Silage harvest is beginning in some areas. It’s always difficult to forecast weather, but if dry field conditions persist, potential for combine and field fires will increase. A high-temperature source in the engine area or an overheated bearing can ignite dry plant material. 

Warm, dry winds rapidly dry plant materials. With potential for an earlier maturing crop and resulting early harvest, air temperatures will likely be warmer than during typical October conditions. Farmers in northwest Iowa experienced a greater than average number of harvest fires during 2011. A recent study by South Dakota State University ag engineers found an average of nearly seven combine fires per county in nine northwest Iowa counties. Although air temperatures were warm and relative humidity was low during much of early harvest, most fires occurred on days with wind speeds averaging 15 mi/h and occasional gusts of 25 to 30 mi/h. This suggests that not only should combine operators be careful when field conditions are dry and air conditions are warm, they should be extra vigilant during windy periods. 

A combine or field fire stops harvest quickly. During harvest periods with increased potential, fires cause millions of dollars in property damage in Iowa, including loss of machinery, crops and time. 

Prevention is a key to avoiding problems, but preparation in case a fire breaks out is also necessary.  Here are some tips for prevention:

  • Keep the machine clean, particularly around the engine and engine compartment. Use a high pressure washer or compressed air to remove caked-on oil, grease and crop residue.
  • Check coolant and oil levels daily.
  • Check the pressurized oil supply line to the turbocharger for wear areas that rub and may start an oil leak.
  • Frequently blow leaves, chaff and plant material from the engine area with compressed air or a portable leaf blower. Remove plant materials wrapped on or near bearings, belts or other moving parts. 
  • Examine exhaust or hot bearing surfaces. Repair leaking fuel or oil hoses, fittings or metal lines immediately.
  • Inspect and clean ledges or recessed areas near fuel tanks and lines. 

In case of fire, carry a cell phone to call the fire department. Two ABC-type fire extinguishers are recommended: a smaller 10-pound unit in the cab and a larger 20-pound extinguisher at ground level on the combine. Invert and shake the extinguishers once or twice a season to ensure machine vibrations don’t compact the powder inside. A shovel to throw dirt can also help. 

Fires may start from plant materials that have smoldered unnoticed for 15 to 30 minutes or more. The ignition source for field fires may have been the earlier passing of a truck, tractor or combine. Flames aren’t apparent until additional oxygen is supplied, perhaps by a gust of wind. Harvest crews may want to discuss a plan for emergency tillage of a fire break should that option become advisable. Keep in mind that personal safety is more important than property loss. 

With current prospects for an early, dry harvest, fire prevention measures will be more important than usual. 


Mark Hanna is an extension agricultural engineer in agricultural and biosystems engineering with responsibilities in field machinery. Hanna can be reached at or (515) 294-0468.

Combine Settings for Drought

By Mark Hanna, Department of Agricultural and Biosystems Engineering

Hot mid-summer field conditions are affecting upcoming harvest. Although soybeans are still developing, corn ear diameter has been established in most fields. Smaller ear diameter and, in some fields, weaker stalks at the base of the corn plant and at the ear shank will affect harvest operations. 


General harvesting

Crop conditions vary across regions, but also within fields in the same farming operation. Harvest won’t be business as usual. Pre-scouting fields and approaching harvest with the right attitude is an important first step. We know that faster combine travel speed helps load the combine and improve grain quality; however, with weak and/or lodged stalks and stems it may be necessary to travel slower to ensure the crop feeds into the combine as easily as possible. Finding the correct travel speed to balance machine field losses (reference PM 574, Profitable Corn Harvesting, to check field losses) with crop quality requires checking both losses behind the combine and grain quality in the tank. Be sure to check before harvest with your crop insurer if losses will be claimed. Warmer temperatures associated with an early harvest and dry field conditions increase fire potential. Review your combine and field fire prevention plans.



Ear sizes vary in fields. In many cases, ear diameter (cob and grain) is smaller than normal. On the cornhead, the gap between snapping plates above the stalk rolls should be adjusted so that the ear butt is held on the plates above the rolls but with is enough room for stalks to be pulled through without wedging. A gap of 1.25 inches used in normal years will likely need to be narrowed closer to just over an inch to avoid butt shelling of smaller diameter ears. 

Smaller diameter stalks may be more easily pulled between narrowed snapping plates, but a weakened stalk base makes corn susceptible to lodging by late-season winds. Similarly, weakened ear shanks in some fields may cause ear droop, making it advantageous to scout individual fields for early harvest. Significant amounts of lodged corn may require slower travel speed and/or the use of a reel, cones or divider modifications on the corn head. 

Concave clearance should be adjusted for ear size and material flow. Smaller diameter ears and less plant material suggest narrower concave clearance than normal. Don’t use faster rotor or cylinder speed than necessary for adequate threshing. Check the amount of seed coat cracking to fine-tune concave clearance and rotor speed. Smaller kernel sizes may require smaller than normal sieve openings in the cleaning shoe. Be careful not to adjust so small that significant amounts of grain are recirculated in the tailings return. Air drag is slightly greater on smaller kernels but, unless test weight is low, fan speed should be similar to normal. Kernel size may be larger on ears with significantly fewer kernels requiring larger sieve openings. 



Crop and field conditions still have the potential to change before harvest. Plant heights, however, appear shorter in a number of fields. Some grain platforms allow the cross-auger position to be moved forward. This may be considered to help pull shorter plants away from the cutterbar and into the feederhouse. The reel can be adjusted downward but be careful that reel fingers are not clipped by the cutterbar when it flexes upward to its highest position. 

If biomass amounts are smaller, concave clearance may need to be decreased to allow adequate traction to pull material though the threshing area. Check the grain tank for splits and seed coat cracks. Use only enough rotor speed and only narrow enough concave clearance as required for grain quality and throughput. Low yield areas in some fields may keep plant stems green when beans and pods are mature and ready for combine harvest. These conditions require more attention to adjust aggressiveness of threshing (speed, clearance) for adequate threshing and throughput without causing green discoloration to soybeans. 

Smaller soybeans require slightly narrower sieve openings in the cleaning shoe. Fan speed may be reduced if soybeans are blown out the back. Extremely small soybeans may be brittle during threshing and challenging to clean. 



Don’t assume crop conditions. Inspect fields for variability. Schedule field harvest based on factors of yield and preharvest loss potential, as well as optimizing adjustments required on the combine. Make it a safe harvest. Unscheduled downtime due to accident or fire is more costly than a few extra bushels of preharvest loss.


Mark Hanna is an extension agricultural engineer in agricultural and biosystems engineering with responsibilities in field machinery. Hanna can be reached at or (515) 294-0468.

Carryover Concerns for 2013

By Bob Hartzler and Mike Owen, Department of Agronomy

One of the consequences of the current drought will be reduced herbicide degradation. In some situations, this prolonged persistence will lead to damage to the rotational crop next spring. The risk of carryover injury will vary widely from field to field depending on several factors (Table 1); thus, it is important to evaluate each field individually.


Table 1.  Factors determining risk of carryover injury risk

  1. chemical half-life
  2. rate of herbicide applied
  3. application date
  4. soil characteristics (texture, organic matter, pH)
  5. rainfall (total amount and distribution throughout year)
  6. sensitivity of rotational crop
  7. growing conditions following planting next spring


The relative persistence (half-life) and the rate of herbicides used in the field have the greatest impact on the likelihood of toxic residues. Only a few of the herbicide active ingredients used in corn and soybean have characteristics that may lead to carryover problems in 2013 (Table 2).


Table 2.  Herbicides with carryover potential

High Risk

  • atrazine (numerous products)
  • chlorimuron (Authority XL, Canopy, Envive, Valor XLT, others)
  • imazaquin (Scepter)
  • simazine (Princep, others)

Moderate to Slight Risk

  • fomesafen (Reflex, Flexstar, Prefix)
  • clopyralid (Hornet)
  • cloransulam (FirstRate, Hornet, Gauntlet, etc.)
  • imazethapyr (Pursuit)


  • pendimethalin (Prowl, others)
  • trifluralin (Treflan, others)

HPPD Inhibitors

  • isoxaflutole (Balance Flexx)
  • mesotrione (Callisto, Lumax, Lexar)
  • tembotrione (Laudis, Capreno)
  • topramezone (Impact)


The products listed as ‘High Risk’ have the potential to damage rotational crops under ‘normal’ conditions. Most farmers have learned what rates can be used on their soils safely, but this year’s drought will result in a high risk even with reduced rates. Both chlorimuron and atrazine are more persistent in high pH soils. Preemergence applications of chlorimuron will have a much higher risk of problems because these rates are much higher than when chlorimuron is applied postemergence (Classic). In certain situations, the best option may be to alter rotation plans to avoid planting a susceptible crop. 

Products listed under the ‘Moderate to Slight Risk’ have been known to occasionally cause problems or have half-lives that suggest they might cause problems under abnormal conditions. Risks with these products will vary widely from field to field depending on the specific conditions encountered.

To minimize problems next spring, evaluate this year’s herbicide program for compounds that pose a carryover risk. Consider rates, application date, soil characteristics and label restrictions.  Keep in mind that if rainfall returns to normal, this rain will have much less effect on herbicide degradation than had it occurred near the time of herbicide application. Also, while tillage should dilute herbicide residues within the soil profile, past experience has shown that this practice does not consistently reduce crop injury from herbicide residues. The conditions a crop experiences during establishment greatly influence its ability to tolerate residual concentrations of herbicides. Using practices that minimize additional stresses to the crop seedling (planting date, seedbed conditions, etc.) can reduce problems associated with low concentrations of herbicides.


Bob Hartzler and Mike Owen are professors of agronomy and weed science extension specialists with responsibilities in weed management and herbicide use. Hartzler can be reached at or 515-294-1923. Owen can be reached at or phone 515-294-5936.

2012 Corn Crop Races to Maturity: Impacts on Grain-fill Period and Yield

By Roger Elmore, Department of Agronomy

Iowa’s 2012 corn crop races through development stages because of rapid Growing Degree Day (GDD) accumulation (see Table 1 and August 3 ICM News). High minimum daily temperatures largely contributed to the abnormal GDD accumulations. Many wonder and ask how the GDD accumulation rates affect not only maturity but also yield.


Growing Degree Day accumulations outpace norm

On average, GDD accumulated  15 to 16 percent ahead of normal across the state since May 1 and during the month of July (Table 1). Because temperature drives corn development, higher heat unit accumulation means faster cycling through development stages and earlier maturing crops.

Table 1.


During the week of July 27 to August 2, corn accumulated 171 GDD, 12 percent ahead of normal for the week. State average GDD accumulation sits at 1,957 GDD. Looking ahead, if temperatures continue as they were that week, accumulating 24.4 GDD per day, a 2,600 GDD hybrid could mature in 26 days, i.e. August 26 ((2,600-1,957)/24.4). If, however, temperatures return to normal for the week July 27 – August 2 for the remainder of the month, 152 per week and 21.7 per day, the hybrid would mature four days later, August 30. Longer grain-fill period – silk to maturity - increases yield potential.


Historical crop development progress

In the August 3 ICM, I presented data showing the rapid pace of corn development in 2012 relative to last year and the five-year normal and the precipitous drop in crop conditions during July. In records going back to 1986, corn conditions in 2012 rank with 1988 (drought) and 1993 (floods).

Let’s compare specific contrasting years – 1988 drought with yields 29 percent below trend line; 1992, with yields 14 percent above trend line; and 2004 yields, 13 percent above trend line. In terms of crop development, 1992 and 2004 contrast dramatically to those of 1988 (Table 2). Sixteen extra days for seed fill in both 1992 and 2004 dramatically increased yields. Compare these days from silking numbers with the averages for hybrids provided in the Iowa State University Extension and Outreach publication,  Corn Growth and Development (Table 2).  

Table 2.


Figures 1 to 4 show the USDA-NASS Iowa data graphically. The contrast between 1988, 1992 and 2004 is stark. The corn crop this year is developing faster than in the other years. Maturity will likely come sooner than any of these other years.

Figure 1.


Figure 2.


Figure 3.


Figure 4.


Computer model simulations

Hybrid Maize model simulations at Crawfordsville in southeast Iowa show that with a 2,600 GDD hybrid and a May 1 emergence date, grain-fill occurred over a 43-day period in 1988 and 68 days in 2004. According to the model, as of August 2, 2012 grain-fill period at Crawfordsville with the best possible weather occurring for the remainder of the year (like that of 2004), grain fill could take up to 53 days. That is far short of what would favor maximum yields (Table 2). With the worst possible weather we’ve seen recently (like that of 1988), grain fill might be as short as 41 days. That’s less than we experienced statewide in 1988 (Table 2).

Trend lines of Hybrid Maize simulations for grain-fill days and yield at Crawfordsville and in central Iowa (Ames/Gilbert) have slopes that range between 1 and 3 percent per day. That suggests for every day grain-fill is extended, grain yield increases by 1 to 3 percent.  Of course, it works in the opposite direction, too. 

Cooler night temperatures could still help by increasing the grain-fill period in many of Iowa’s corn fields. Likewise, plentiful rain could help retain some of the yield potential that still remains.


Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by email at or (515) 294-6655.

This article was published originally on 8/13/2012 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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