Taking advantage of the cold days on which I prefer to stay inside, I am taking a few minutes to share four things I have learned over more than a decade of research on nitrogen management for corn in Minnesota.
1. Spring applications are better than fall applications.
While there are logistical and practical reasons why nitrogen fertilizers are sometimes applied in the fall, it is important to recognize that those applications are seldom better than doing the same application (same rate, placement and N source) in the spring.
In a 25-site-year study across Minnesota, encompassing corn-after-corn and corn-after-soybean cropping systems and N rates with fall and spring applications of urea, I calculated that a spring application of urea reduced the amount of N needed to achieve the economic optimum N rate by 28 pounds of N per acre and it produced 9 bushels per acre more grain than a fall application.
Clearly, fall applications of urea are a lose-lose situation.
Last year, I looked at various N sources and placements at a sub-optimum N rate. The reason to use a sub-optimum N rate is to more easily see differences resulting from the treatment.
In south-central Minnesota, regardless of the N source or placement I used, a spring application produced anywhere from 9 to 34 bu./acre more than the same application done in fall.
And our fall applications always followed best management practices, applying after soil temperatures at 4 inches were below 50 degrees.
In Lamberton, in southwest Minnesota, there were a few treatments that showed little difference between fall and spring, but two of the treatments showed a substantial advantage (29 to 30 bu./acre) with spring applications.
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