Manure's nutrient contribution holds in 'bin buster' year
by Gina Hanson, manure management specialist, NEIDP

For the past five years the Northeast Iowa Demonstration Project staff has conducted field demonstrations to answer producer questions about crediting for manure applications. Four years of demonstration results on 17 farms showed that a typical manure application without added nitrogen (N) is sufficient to obtain optimum yields, but crop years 1994-1997 never produced a bin buster (table 1).

How would manure hold up in a year when many farmers recorded yields near 200 bushels an acre? This was the year NEIDP staff and producer-cooperators had been waiting for.

Seven area producers, Richard Thompson, Larry Friedlein, David Reierson, Kirk Snitker, Duane Hageman, Duane Kuehner and Ed Ruff, cooperated this year with the NEIDP staff to establish manure management demonstrations on their own farms and refine their manure management for crop production. As in previous years, the replicated manure and nitrogen plots were placed in fields that had no manure applied the prior year. These fields were selected to reduce the carryover effects of previous manure applications.

The only variables in these demon strations were manure and N rates. Project staff determined manure application rates through spreader calibration and had manure samples analyzed to determine nutrient content.

From this information the first year N contribution was calculated, which averaged 161 pounds per acre at the seven locations. Corn yields and cornstalk residual nitrate N levels from the seven sites are shown in table 2.

The cornstalk nitrate nitrogen test measures the amount of nitrate N in the corn plant after it matures. Corn plants remove N from the lower cornstalks and leaves during gra!nhll. Corn plants that have access to more N than needed will have more nitrate in their lower stalks. (See previous article for an explanation of the cornstalk nitrate nitrogen numbers.)

This year's demonstration results indicate the application of 50 pounds per acre of N in addition to manure provided sufficient N for the corn crop even in a high- yielding environment. The 50 pound N application also compen sates for the variability in nutrient content and spreading uniformity. As in previous years, the 100 pound N application in addition to manure did not produce a sufficient yield increase to justify its use. This remains true when all 24 demonstrations for the past five years are averaged (table 3).

Overall, these demonstrations indicate that manure provides a significant N resource for crop production. The type of manure applied, amount, method and uniformity of application are all factors that need to be considered when determining whether or not to supplement a manure applica tion with additional N. The demonstrations show that applying more than 50 pounds per acre of N isn't justified when using typical manure application rates. Farmers who don't feel comfortable relying on manure for all of their N needs can be assured that manure can make up a significant part of their fertility program.

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