Yellow Corn All Too Common

By Jane Fyksen, Crops Editor
Wednesday, June 25, 2008 12:47 PM CDT

Wet conditions have resulted in tremendous variation in the color and growth of corn across fields. While much of the corn is recovering and beginning rapid growth, it’s very uneven and yellow and stunted in a good number of fields.

“At this time, this variation is mostly due to water, rooting and compaction issues, and much less to nitrogen (N) supply. When corn is small, the total plant N requirement is low and easily met by soil and applied N. As soils dry and rooting depth increases, plants will improve growth and access to N,” says Iowa State University agronomist John Sawyer.

“Crop rotation is also having an impact on early growth of corn this year. At our ‘crop rotation by N rate’ study site at the ISU research farm west of Ames (on a Clarion loam), the corn following corn (CC) is not growing nearly as well as corn after soybean (SC). The CC is not as ‘tall’ and is not as ‘green’ as the SC, even at the highest N rate - 240 pounds of N per acre. This means the growth response difference is not due to simply N supply,” he notes.

“The N in this study is spring preplant incorporated urea. Looking at the corn plants in the SC rotation, there is good response to the applied N and the plants are getting their ‘deep green’ coloration,” he says.

Response to anhydrous timing

“It is very confusing to look at corn plants and decide if the yellow coloration is due to water stress, roots not reaching applied N, or N loss,” Sawyer continues.

To help answer these questions, he visually evaluated corn plants in an anhydrous ammonia rate study being conducted at the ISU research farm between Ames and Boone, Iowa. In this study, anhydrous was applied late fall (Oct. 31, 2007), spring preplant (April 30 this year), and sidedressed (June 18) at different depths of injection and application rates. Corn was planted May 15. The study is “low” on the landscape.

“Like many fields this year, part of the study area is wetter than the rest, with a small part where the corn is dead due to standing water,” notes Sawyer.

As of June 19, the corn is around the V4 growth stage, so it’s still small. Plants on the higher ground are growing better and responding better to applied N, plants on the lower ground are smaller and not showing much growth response to applied N, and plants near the “dead” area are quite small but alive and showing no response to applied N, he details

Looking at color and growth, Sawyer generally sees a better response to the spring-applied ammonia compared to the fall-applied ammonia, especially at lower N rates. “It is too early to tell what amount of N may have been lost this spring, but it is clear either N loss is greater from the fall application, or nitrate from that timing is deeper in the soil and roots have not gotten to the N, or wet soil is restricting deeper root growth, or a combination of all,” he points out.

“It is clear that the corn is not growing as well on the more poorly drained areas compared to the “higher” and better drained areas, and the corn response to fall and spring-applied N is better on the areas where corn is growing better,” he adds.

“What does this all mean? I suspect N losses have occurred with the fall application, less with the spring application, and time will tell how much,” says Sawyer.

“It’s just too early now to make a determination of how much N loss from the visual corn growth and coloration. Also, wet conditions have restricted corn root growth and access to the applied N,” he states.

Rescue N applications

As for rescue N applications, Sawyer says the best approach is to get sidedressed N into the soil. Injection several inches into the soil places N in the root zone, and helps avoid placement on or near the surface where dry soils can limit root uptake. Injection is not always an option and sometimes it is just necessary to surface apply N materials. “In those instances, rain will be needed to move the N into the soil. This (rain) is not exactly what we want right now, but the N needs to get into the soil for plant uptake,” he mentions.

Past research has also shown that sidedress N will not perform as well in dry conditions. “At this time we want corn to grow root systems. Trying to ‘green up’ corn with foliar application of fertilizer materials or application of low nutrient rates will not help promote root growth or help plants recover from poor aeration in the root zone. Corn will soon reach a rapid growth phase, and adequate N must be present in the active root zone to meet plant demands. Corn requires a large amount of N for high yields, and the way this large N demand is met is through root uptake,” he says.

At Purdue University in Indiana, where flooding has also been pronounced, Jim Camberato, Brad Joern and Bob Nielsen in the Agronomy Department say determining the amount of lost N this year isn’t an easy task. “Determining what to do from here on out is even more difficult,” they adds.

Corn in ponded areas may be damaged and incapable of responding to additional N. Corn replanted this late in the season may have a different N requirement than timely planted corn. Otherwise for most remaining cornfields, the decision to add N should be based on an estimate of how much N was lost, the initial application rate, the projected optimum N rate for the field under minimal N loss conditions, N cost, and grain value.

The projected economic optimum N rate (EONR) differs from field to field. For corn grown after soybeans, the EONR was 150 pounds of N per acre (calculated at $6.50 a bushel corn and N at 75 cents a pound) in 31 studies conducted in the past two seasons. Nitrogen was sidedressed in many of these trials and N loss was considered minimal. Although the average EONR was 150 pounds of N per acre, some fields required as little as 100 pounds of N per acre to achieve the economic optimum yield while others needed as much as 200 pounds of N per acre.

The average yield at 150 pounds of N per acre was 184 bushels per acre. Yield at 105 pounds of N per acre was 173 bushels per acre, representing an 11 bushel per acre yield decrease with 50 pounds of N per acre less. As the N rate approaches the EONR, it takes 7 to 8 pounds of N to produce the last additional bushel of yield with 75 cents per pound N and $6.50 per bushel grain. Yield may continue to increase above the EONR, but the cost of N exceeds the value of grain produced.

Corn grown after corn required about 30 pounds of N per acre more than corn after soybeans to achieve economic optimum yield, but the yield decrease with 50 pounds less N per acre was the same 11 bushels per acre.

“To fertilizer or not to fertilize, that is the question,” this trio quips. Here are their opinions:

- Consider 30 to 60 pounds of N per acre for fields where urea or 28 percent UAN was applied one to two weeks prior, or anhydrous ammonia was applied three to four weeks prior, to excessive rain, especially if: silt loam field areas were saturated for more than three days and the crop survived; poorly drained field areas were ponded for more than three days and the crop survived; sandy fields received 4 to 8 inches of rainfall that infiltrated the soil.

- Additional N likely is not required in some situations, such as fields where N loss is estimated to be less than 30 pounds of N per acre and the projected optimum N rate or higher was used initially. For example, these include: Fields where N was applied two to seven days (urea or 28 percent) or three weeks (anhydrous ammonia) before excessive rain; silt loam field areas that were saturated for less than three days; poorly drained field areas that were ponded for less than three days; and sandy fields receiving less than four inches of rainfall.

If you sidedressed with urea or 28 percent within 24 hours of the heavy rains on sandy soils, urea may have leached below the root zone. The amount of additional N required is dependent on how much rain fell on the field. To sandy fields that received 4 to 8 inches of rainfall that infiltrated the soil, apply 60 to 120 pounds of N per acre. To sandy fields that received 4 to 8 inches of rainfall that infiltrated the soil, they suggest applying 30 to 60 pounds of N per acre. On sandy fields that received less than 4 inches of rainfall, it’s likely no additional N is necessary.

Penn State University’s Doug Beegle says farmers have the Pre-Sidedress Soil Nitrate Test (PSNT) to fall back on for an indication of the N availability status of corn fields just before the peak N demand by the corn crop. For optimum N nutrition for corn, the soil nitrate-N level in the PSNT should be above 21 parts per million (ppm) when the corn is 12 inches tall or around leaf stage 6. If it is above this level, there is adequate N in the soil to meet the needs of the crop. However, if it is below this critical level then sidedressing additional N is recommended, even if adequate N was applied earlier.

Beegle offers couple of important points to keep in mind when using the PSNT:

- Wait until the corn is 12 inches tall; sampling too early can give misleading results

- Do not sample immediately following heavy rains; wait two to three days.

- Take a 12-inch deep soil sample

- Dry the sample the same day as it is collected

- Use a quick test kit or send the dry sample to a soil testing lab for NO3-N analysis

“Sampling several fields that are representative of different N management strategies on your farm this spring can be very helpful in making a decision about whether additional sidedress N is needed. N is too expensive to just put some extra on just in case. Sidedress N recommendations based on the PSNT are, on average, twice as accurate as recommendations made without the PSNT,” Beegle points out.

If less than 15 pounds of fertilizer N was applied around planting time, the chlorophyll meter can be used to test the N status of the crop. This is especially helpful if you are counting on manure to meet most or all of the crop N needs. A new fact sheet on using the chlorophyll meter is available at http://cmeg.psu.edu/. There is also a video demonstrating how to use the chlorophyll meter at that same website. (This same website also has more information about the PSNT and a related video on that.)

The chlorophyll meter is a tool that can be used in some situations just before sidedressing to significantly improve N sidedress recommendations. The recommended procedures for using the chlorophyll meter must be strictly followed if the results and recommendations are to be valid. Some key instructions include:

- Take samples only after the corn has reached at least the six-leaf stage

- The chlorophyll meter cannot be used if more than 15 pounds of N per acre was applied at planting

- Take readings on the fifth leaf about three-quarters of the way out the leaf between the edge of the leaf and the midrib

- Take readings on at least 30 plants in a field.

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