The small-plot experiment station results indicate that N from fall applied swine manure, which has a high proportion of N in the inorganic, ammonium form, is rapidly converted to nitrate. The earlier the application, the higher the proportion of ammonium that is converted and available for leaching. The total N remaining in the top three feet of the soil profile by June of the cropping year was less for the earlier manure applications than for the later applications in 2008 but not as clear in 2009.
In the on-farm strip trials over two years the contrast of early compared with late manure applications showed that the yields from the September (early) manure application were significantly lower than yields from the later (October-November average) application and lower than from the late urea application. The combined analysis clearly shows a significant loss in yield when manure was applied in September relative to later months.
All of these nitrogen processes are controlled by soil type and rainfall patterns. But in general early application of manure increases the risk for leaching and denitrification losses, since there is no crop present for nitrogen uptake. Liquid swine manure, with a high inorganic N content, should be managed much like inorganic fertilizer N sources with either late fall or spring application to delay conversion of ammonium to nitrate and subsequent loss by leaching and denitrification.