Impact of N Application Rate on Nitrogen Use Efficiency and pH in a Fertilizer-Management Continuous Corn Silage System.

Nitrogen (N) is essential for producing optimum corn (Zea Mays L.) silage yield with high quality. Under-application can impact yield and quality while over-application can result in low nitrogen use efficiency (NUE) and hence loss of N to the environment, a decrease in soil pH, and possibly in soil organic matter. The objectives of this study were to (i) determine the optimum N rate for continuous corn silage production and (ii) assess the influence of under- and over- application of N on yield, forage quality, soil nitrate, organic matter and pH. A 5-yr field study was conducted with six N rates (0, 56, 112, 168, and 224, and 280 kg N ha^-1; sidedressed) in five replications for corn that was established with a 22 kg N ha^-1 starter. Corn silage yield (dry matter basis) varied from year-to-year. In 2002 and 2005 (July drought), corn yield did not increase with N application, averaging 6.2 and 12 Mg ha^-1, respectively. In 2003 and 2004, the most economical N rates (MERN) were 95 and 107 kg N ha^-1, respectively, with yields of 13.3 and 14.1 Mg ha^-1, respectively. Over five years, soil pH was decreased significantly with N application rates higher than 56 kg N ha^-1. These findings suggested that over-application of N can increase lime needs over time. Nitrogen application rates below than MERN (0 and 56 kg N ha^-1) decreased organic matter levels over time, possibly reflecting a reduction in carbon addition through root biomass for lower yielding plots. The results show an optimum N sidedress rate of 100 kg N ha^-1 in good growing seasons versus 0 kg N ha^-1 under challenging growing condition. The results also show that N application cannot overcome weather-related challenges for this soil.