Alfalfa Yield Response to Nitrogen Applied After Each Cutting

Although alfalfa ( Medicago sativa L.) usually obtains a high percentage of its required N via symbiotic N fixation, additional fertilizer N applied once in the spring can increase forage yields. However, little is known about alfalfa yield response to low N rates (<50 kg N ha −1 ) immediately following each cutting. Low N rates (immediately following each cutting) were evaluated for total alfalfa dry matter production on a Grant silt loam (fine‐silty, mixed, thermic, Udic Argiustoll). This nonirrigated experiment was initiated on a 2‐yr‐old alfalfa stand where sufficient P and K had been applied. Nitrogen rates of 11, 22, and 44 kg N ha −1 were applied immediately following each cutting for 5 yr (4–5 cuttings yr −1 ). After 5 yr of continuous N application, no differences in soil NH 4 –N or NO 3 –N were found at depths >15 cm (0‐ to 240‐cm sampling depth). In 1994, total alfalfa dry matter yield (sum of five harvests) increased 1.29 Mg ha −1 from a total annual N application of 110 kg N ha −1 (22 kg N ha −1 following each cutting). Total forage N decreased from the second to the fifth harvest in most years. By‐harvest dry matter yield increases due to applied N were only found in late‐season harvests, consistent with late‐season decreased N 2 ‐fixing capacity in alfalfa documented by others.