The effects of various rates and durations of manure applications on corn silage yields and soil properties

Manure applications to soil can result in increased plant growth but also in accumulations of excess nutrients, of which N and P are of special concern. This experiment was initiated to evaluate the residual effects of dairy manure applications on com silage yields and accumulations of N and P. Rates of 112, 224, and 336 kg manure-N ha^-1, were applied for one, two, or three years to no-till com silage. Inorganic N fertilizer was applied as 84, 168, or 252 kg NH₄NO₃ each year as well as 90 kg P₂O₅ and 168 kg K₂O in accord with soil test recommendations. Manure application rates were derived assuming 75% N availability. Less than 75% of the N in the manure was available when uptake efficiency is defined as (the amount of N recovered - the amount recovered from the control plots) / the amount of N applied.

Soil parameters evaluated were total C, total N, and Mehlich I extractable P and K. Total N and pH were largely unaffected by manure applications during the first two years of the study. After two years of application, in 1995, there were differences in total C only in the top 7.5 cm of soil with the highest levels of carbon from the 3M2 (336 kg manure-N applied for two years) treatment. Although the soils have been in no-till for several years, the additional C from the manure has impacted these soils at the surface.

Yields with the highest rate of inorganic fertilizer were significantly higher than all the manure treatments for all three years. The 336 kg manure-N treatment applied for three years (3M3) did produce yields comparable to those with the recommended rate of inorganic fertilizer for silage, 168 kg NH₄NO₃ (2AN). Silage yields did increase within a given manure rate as years of application increased. In almost every instance, yields in the no-till systems were similar to if not the same as, those in conventional till systems.

The pre-sidedress nitrate test although developed for manured situations did not provide a good correlation between silage yield and nitrate test level in this research. The manure and inorganic fertilizer treated soils had nearly equivalent yields after three years but the PSNT values were quite low with the manure and very high in the inorganic treatments. In the treatments that did not receive manure in a given year, the low PSNT values did indicate a nitrogen deficiency which was seen in the low yields. As the years of application increased, the manured yields increased to levels comparable to those from the inorganic treatments, but the PSNT values were still lower than the critical ranges.