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Applying urea in fall can bring risks

Nitrogen management practices for corn have become a popular discussion topic lately.

Record June-July rainfall (16.25 inches at Waseca) placed intense pressure on N availability for corn, resulting in considerable acreage of lighter-green to yellowish-green corn in southern Minnesota. This appearance indicates a shortage of N, likely due to denitrification losses of N from the saturated soils.

Key Points

• Research shows nitrogen loss with fall-applied urea.

• An unpredictable wet spring carries away fall-applied nutrients.

• The economic loss with corn valued at $4 per bushel is very high.


Scenarios where N losses and N-deficient corn were most apparent include: corn following corn, fall-applied N and poorly to very poorly drained soils. Based on research, applying an additional 50 to 60 pounds of N per acre, especially in the fall, under these high-N-loss conditions would not have been sufficient to meet the N demand of this year’s corn.

Needed N when?

Narrowing of the cost margin between anhydrous ammonia and urea has also become a conversation topic this fall. With only about a 5-cent difference per pound of N between the two products, some growers and fertilizer dealers may be tempted to apply urea this fall because of the ease and reduced time and effort of application. Is fall application of urea a wise decision in south-central Minnesota?

Let’s look at research data from studies conducted at Waseca for the dry sources of N (urea and ammonium sulfate). Ammonium sulfate applied at a rate of 180 pounds of nitrogen per acre in early November and plowed down averaged 160 bushels per acre compared to a preplant application average of 168 bushels per acre during a five-year trial (1997-1981). Urea applied in the late fall for corn after soybeans produced a two-year average yield of 157 bushels per acre compared to a spring preplant application yielding 164 bushels per acre (1981-82).

When N-Serve was added to the fall- and spring-applied urea, yields were 155 and 167 bushels per acre, respectively. A three-year study (1997-1999) compared fall-applied urea with and without N-Serve to spring preplant application for corn after soybeans. Yields averaged 152 bushels per acre for fall urea without N-Serve and 158 bushels per acre with N-Serve; whereas yields from preplant urea averaged 185 bushels per acre. Fertilizer N recovered by the corn ranged from an average of 76% for spring preplant urea to 47% and 43% for fall urea with and without N-Serve, respectively, indicating tremendous loss of fall urea N.

More yield with spring-applied N

Not only did this study reveal about a 30-bushel-per-acre average yield advantage for spring application, it also showed a huge effect of spring (April-June) precipitation. Excess rainfall during this period before rapid uptake of N by corn can cause substantial loss of N via denitrification and leaching. In 1997, when April-June rainfall was 26% below normal, corn yields were 14 bushels per acre lower for fall application with no difference between with and without N-Serve. In 1998, when April-June rainfall was 4% above normal with a very wet period late in May, yields were 25 bushels per acre lower for fall application with no effect of N-Serve. In 1999, when April-June rainfall was excessive (39% above normal), preplant urea yielded 56 bushels per acre more than fall urea without N-Serve and 45 bushels per acre more than fall urea with N-Serve.

Studies conducted in 2003-05 produced greater yields of 6 to 15 bushels per acre in the two years with above-normal April-June rainfall and a 11-bushel-per-acre advantage for fall urea in the drier year. In 2006-2009, research was conducted to determine if placement (broadcast vs. 4-inch-deep band) would affect the performance of fall-applied urea. Precipitation for April-June was not excessive in any of the years, averaging 90% of normal with a high of 9% above normal one year. Under these conditions, N loss and yield depression with fall-applied urea were not expected. The four-year yield averages confirmed this with preplant and fall-applied urea yielding 208 bushels per acre and 206 bushels per acre, respectively. There was no yield difference between fall urea placed 4 inches deep in a band vs. broadcast and incorporated by tillage.

In summary, results from 17 years of research on fall vs. spring preplant applications of urea and ammonium sulfate fertilizers show very little upside for fall application. Because spring rainfall amounts can’t be predicted, fall application of urea becomes a risky management practice.

With corn valued at $4 per bushel, the economic loss could exceed more than $200 per acre in a wet year.

In addition, the yield response data do not support fall application of urea impregnated with N-Serve. A new inhibitor, Instinct, has recently been approved for use with urea. Because Instinct is chemically similar to N-Serve and because no U-M data exist regarding its performance when fall-applied with urea, it is not recommended at this time.

Randall is with the Southern Research and Outreach Center, University of Minnesota.

This article published in the November, 2010 edition of THE FARMER.

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