PREPLANT MANURE ON ALFALFA - RESIDUAL EFFECTS ON CORN YIELD AND SOIL NITRATE

Overapplication of N from various sources can be partially attributed to the lack of proper N crediting when organic N sources, such as alfa lfa (Medicago sativa L.) and manure, are part of a farm's cropping sys tem. The objectives of this study were to evaluate: (i) the effect of manure applied prior to alfalfa establishment on subsequent corn (Zea mays L.) grain yield and soil nitrate-N concentrations; and (ii) grain yield response to alfalfa cutting management preceeding plowdown. Pri or to alfalfa establishment, three preplant manure rates (3 000, 6 000 , and 12 000 gal/acre) were applied at Rosemount MN, on a Waukegan sil t loam (fine-silty over sandy or sandy-skeletal, mixed, mesic Typic Ha pludolls) and at Waseca MN, on a Nicollet clay loam (fine-loamy, mixed , mesic Aquic Hapludolls). Fall alfalfa cutting management treatments consisted of either leaving ok removing the fourth-cutting herbage bef ore plowdown. First-year corn grain yields following alfalfa plowdown were similar for all manure treatments applied prior to alfalfa establ ishment. By the second and third year after alfalfa, cam yields declin ed and yield responses to the original manure treatments were less tha n those observed with 30 lb/acre of fertilizer N. Not harvesting the f ourth-cutting herbage before moldboard plowing significantly increased yields by an average of 4.2 bu/acre. In the first year after alfalfa plowdown, the N contribution of the alfalfa, whose N credit can be est imated by the in-season nitrate-N test, overshadowed the contribution of the manure, which ran be partially estimated by the preplant residu al N test or by the in-season nitrate-N test, in the first year after plowdown. By the third year of corn following alfalfa, the manure-N an d alfalfa-N contributions to the mineralizable N pool are exhausted ba sed on soil nitrate-N concentrations and corn grain yield response. Go od stands of alfalfa can supply optimal N for corn in the year after p lowing, regardless of whether preplant manure was applied prior to alf alfa seeding.