Nitrogen management practices for corn have become a popular discussion topic lately among growers, dealers, and crop advisors. Record June-July rainfall (16.25" 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 during June and July. Scenarios where N losses and N-deficient corn were most apparent include: 1) corn following corn, 2) fall-applied N, and 3) poorly to very poorly drained soils. Based on previous research, applying an additional 50 to 60 lb N/A, 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.
Evaluating dry nitrogen fertilizer sources
Narrowing of the cost margin between anhydrous ammonia and urea has also become a conversation topic this fall. With only about a 5-cent per pound of N difference 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?
Time of application
Lets look at research data from studies conducted at Waseca for the dry sources of N (urea and ammonium sulfate). Ammonium sulfate applied at a 180 lb N/A rate in early November and plowed down averaged 160 bu/A compared to a preplant application average of 168 bu/A during a 5-year trial (1997-1981). Urea applied in the late fall for corn after soybeans produced a 2-year average yield of 157 bu/A compared to spring preplant application yielding 164 bu/A (1981-82). When N-Serve was added to the fall and spring-applied urea, yields were 155 and 167 bu/A, respectively. A 3-year study (1997-1999) compared fall-applied urea with and without N-Serve to spring preplant application for corn after soybeans. Yields averaged 152 bu/A for fall urea without N-Serve and 158 bu/A with N-Serve; whereas yields from preplant urea averaged 185 bu/A. 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.
Effect of precipitation
Not only did this study reveal about a 30 bu/A 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 bu/A 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 bu/A 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 bu/A more than fall urea without N-Serve and 45 bu/A more than fall urea with N-Serve.
Studies conducted in 2003-2005 produced 6 to 15 bu/A greater yields in the two years with above-normal April-June rainfall and a 11 bu/A advantage for fall urea in the drier year. In 2006-2009 research was conducted to determine if placement (broadcast vs. 4" 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 4-year yield averages confirmed this with preplant and fall-applied urea yielding 208 bu/A and 206 bu/A, respectively. There was no yield difference between fall urea placed 4" deep in a band vs. broadcast and incorporated by tillage.
In summary, results from 17 years of research, evaluating fall vs. spring preplant applications of urea and ammonium sulfate fertilizers, show very little upside potential for fall application. In fact, the downside potential can be huge, especially when spring rainfall is excessive. Because spring rainfall amounts can not be predicted and since they exert considerable influence on the performance of fall-applied N, fall application of urea becomes a risky management practice. When April-June rainfall was normal or less, there was not a large disadvantage associated with fall application; although yields from fall application seldom exceeded preplant application. When excessive April-June rainfall occurred or when soils were saturated for a period in late May or June, yield losses were large - up to more than 50 bu/A. With corn valued at $4 per bushel, the economic loss could exceed more than $200/A in a wet year, which is a huge economic penalty. Thus, fall application of urea or ammonium sulfate is not recommended for corn in south-central and southeastern Minnesota because of a high potential for economic loss, regardless of the price differential between urea and anhydrous ammonia. 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 UM data exist regarding its performance when fall-applied with urea, it is not recommended at this time.