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Regional research Nitrogen timing affects corn yield

With corn planting off to a good start, determining final acreage for the state is still very difficult. Last year, acreage was between 320,000 and 340,000 acres. This year the picture is much different with the range most probably being between 500,000 to 600,000 acres.

What is going to determine the final number? Seed availability and weather conditions at planting will be the primary factors.

I was asked by the Louisiana Agricultural Statistics Service what the acreage is going to be. Corn has been the hardest crop to predict.

One thing that I do know is that if the number of acres that will not be planted due to lack of seed were planted, the acreage for the state would be more than 700,000, which would have made it the largest crop since 1998.

Thus far, it has been a relatively smooth planting season. After planting, the next production practice is nitrogen fertilization. Proper nitrogen rate and optimum time of application must be considered. For example, is there a certain growth stage that should be targeted for nitrogen applications? With costs of fertilizer this year, it is imperative that nitrogen applications be made correctly and in a timely fashion.

In the Mid-South, there has been more work published on nitrogen rates than on timing of fertilizer applications. The common practice that Louisiana producers have adopted is to fertilize as early as possible due to the number of acres that are being fertilized and to allow them more time to get ready to plant other crops such as cotton, grain sorghum or soybeans.

Also, farmers often wait until emergence before applying fertilizer so that they can evaluate the stand. Because of this limited window of opportunity, corn is often fertilized at planting or soon after emergence.

Below is a summary of some of the research that has been conducted over the years on nitrogen rates and timings.

Rick Mascagni published some work in 2000 on Commerce silt loam and Sharkey clay at St. Joseph, La., evaluating nitrogen rates and application timings. On Commerce silt loam, maximum yield occurred at nitrogen rates between 150 to 200 pounds per acre, regardless of the time of fertilizer application, at-planting versus sidedress (six- to eight-leaf growth stage), in two of the three years.

Yield responses to starter fertilizer (10 pounds nitrogen per acre applied in-furrow) tended to occur primarily at the lower nitrogen rates (100 pounds of nitrogen per acre and less) and had little effect on optimal nitrogen rates.

Results of this study suggest there is little difference between applying at planting versus sidedress.

When applying the total nitrogen requirement of the plant at sidedress, there was no benefit using a nitrogen starter fertilizer. This suggests that there was adequate native soil nitrogen or carryover nitrogen for plant development between emergence and sidedress.

If nitrogen is applied sidedress, an application earlier than the six-leaf growth stage (about 30 days after emergence) will better ensure adequate fertilizer activation (availability to plants) by the time the nitrogen requirement of the plant exceeds the native soil supply.

The starter fertilizer used in these tests contained only nitrogen. Previous research in Louisiana has indicated a significant early growth response and, in some years, a yield increase from in-furrow application of starter fertilizers that contain both nitrogen and phosphorus — 10-34-0 or 11-37-0 — particularly on the more coarse-textured, sandy soils.

The Sharkey clay soil study had similar results in that neither starter nitrogen fertilizer at planting or timing of fertilizer application had an effect on optimal nitrogen rate.

Although there was a relatively wide range of yield potential across years, approximately 200 pounds of nitrogen per acre was sufficient to produce maximum yield each year.

In addition to these studies, we are in our third year of a study where nitrogen is applied according to the height of corn at the Dean Lee Research and Extension Center in Alexandria, La. The objective of the research is to determine if there is an optimal growth stage or plant height in which corn should be fertilized. The soil type is a Norwood silt loam.

Nitrogen is applied at 180 pounds per acre when corn reaches a height of 3, 6, 9, 12, 15, 18 or 21 inches. Statistically, there have been no differences in yield when nitrogen was applied to corn at a height of 15 inches or less.

Numerically, differences were small but yield decreased as plant height increased. Yield decreased when nitrogen was applied later to 18- and 21-inch corn. This yield decrease can most probably be attributed to root pruning.

Starter fertilizers, especially those with phosphorus, can increase plant growth and, in some years, yield.

Do not apply more than one-third of the total nitrogen application near planting or at crop emergence. The remaining fertilizer needs to be applied within about 30 days after emergence.

From the Alexandria study, nitrogen applied at the 3- to 9-inch corn stage may have a slight advantage over the other applications due to the fact that it allows the nitrogen time to be activated and made available before rapid accumulation of nitrogen begins (about the eight-leaf growth stage).

There have been lots of questions recently regarding additional nitrogen needed by a corn crop in a corn following corn rotation.

Jay Stevens, LSU AgCenter soils Extension specialist, gives a generic recommendation for a corn following corn rotation of 15 to 20 additional units of nitrogen to maximize yield.

In corn following beans and corn following cotton, he is not recommending additional nitrogen.

He does caution that this recommendation is generic and can vary greatly with soil type and the past year's environmental conditions.

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