Under‐ or Over‐Application of Nitrogen Impact Corn Yield, Quality, Soil, and Environment

CORE IDEAS: Nitrogen application mid‐season cannot overcome drought stress later in the season. Except for crude protein, under‐application of N did not impact forage quality. Soil organic matter decreases in a chisel‐disked corn silage system regardless of N fertilizer rate. Use of compost, cover crops, and conservation tillage can offset soil organic matter losses. Under‐applying N by 30 kg N ha⁻¹ was economically more detrimental than over‐applying. Under‐ or over‐application of N fertilizer to corn (Zea mays L.) has adverse economic and environmental consequences. A 5‐yr study was conducted to determine the impact of N fertilizer on corn silage yield, quality, soil properties, farm economics, and nitrogen‐use efficiency (NUE). Corn silage yields were 12.9, 14.2, and 14.7 Mg ha⁻¹ with most economic rate of nitrogen (MERN) of 90, 95, and 114 kg N ha⁻¹ in 2001, 2003, and 2004 (the three responsive years), respectively. In 2002 and 2005 (non‐responsive years), yields averaged 9.1 Mg ha⁻¹. Yield increased by 3.3 Mg ha⁻¹ with each 10 cm of precipitation in July and August. At the MERN, NUE ranged from 16 (2001) to 25.8 kg DM kg N⁻¹ (2004), reflected in greater soil NO₃–N (0–20‐cm depth) at harvest in 2001 as well (23 vs. 8.9 mg kg⁻¹ in 2004). Soil NO₃–N at silage harvest in responsive years ranged from 8.9 (2004) to 23 mg kg⁻¹ (2001) in 2001 and in non‐responsive years averaged 22 mg kg⁻¹. Soil NO₃–N at harvest was not a good indicator of crop N responsiveness or NUE. Nitrogen addition beyond the MERN decreased NUE and soil pH, and increased crude protein (CP). Under‐application decreased CP and yield in N‐responsive years and increased NUE. Soil organic matter (SOM) was decreased regardless of N rate. Overall, application of 30 kg N ha⁻¹ below the MERN was economically more detrimental than fertilizing the same amount above the MERN.