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 alfalfa (Medicago sativa L.) and manure, are part of a farm's cropping system. 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. Prior to alfalfa establishment, three preplant manure rates (3 000, 6 000, and 12 000 gal/acre) were applied at Rosemount MN, on a Waukegan silt loam (fine-silty over sandy or sandy-skeletal, mixed, mesic Typic Hapludolls) and at Waseca MN, on a Nicollet clay loam (fine-loamy, mixed, mesic Aquic Hapludolls). Fall alfalfa cutting management treatments consisted of either leaving or removing the fourth-cutting herbage before plowdown. First-year corn grain yields following alfalfa plowdown were similar for all manure treatments applied prior to alfalfa establishment. By the second and third year after alfalfa, corn yields declined and yield responses to the original manure treatments were less than those observed with 30 lb/acre of fertilizer N. Not harvesting the fourth-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 estimated by the in-season nitrate-N test, overshadowed the contribution of the manure, which can be partially estimated by the preplant residual 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 and alfalfa-N contributions to the mineralizable N pool are exhausted based on soil nitrate-N concentrations and corn grain yield response. Good stands of alfalfa can supply optimal N for corn in the year after plowing, regardless of whether preplant manure was applied prior to alfalfa seeding. Research QuestionThe transition from alfalfa to corn in a dairy producer's cropping system brings about important crop management decisions, such as fall regrowth and N fertilizer management. There is a great deal of uncertainty about the interaction between manure and alfalfa management on the subsequent corn crop. The objectives of this study were to: (i) evaluate the effects of manure rates applied prior to establishment of a 2-yr alfalfa crop on subsequent corn grain yields and soil N concentrations after alfalfa plowdown; and (ii) determine the influence of fall management (leaving or removing herbage from last cutting) on corn grain yields. Literature SummaryFertilizer recommendations for commercial N suggest little or no N when corn is grown following alfalfa. When manure is applied onto alfalfa before plowdown, there is an additive effect of the two organic (manure and alfalfa) N sources; however, when manure is applied prior to alfalfa establishment, N crediting is less certain because it may affect the amount of plant N fixation. Study DescriptionThree sites in southeastern Minnesota were used in this study. Three rates of manure (3 000, 6 000, and 12 000 gal/acre), along with a control, were applied prior to establishment of a 2-yr stand of alfalfa. Plots were then split in half before alfalfa plowdown, with the fall herbage removed on half and left unharvested on the other half. Corn was then grown for 3 yr with measurements made on: soil nitrate-N before planting and at the V4 stage of growth, and corn grain yields at maturity. Applied QuestionsWhat effect did residual N from the manure have on corn grain yields? Corn grain yields in the first year after alfalfa plowdown were not statistically affected by manure rates up to 12 000 gal/acre or 924 lb N/acre applied before establishing alfalfa (Fig. 1). This indicates that good stands of alfalfa can supply optimal N for corn even though the symbiotic N fixation by the alfalfa may have been affected by the manure rates. Residual manure treatments produced significant grain yield differences at one location in the second year of corn following alfalfa but the effects did not continue in the third year of corn. When corn grain yield response to second and third year alfalfa credits and residual manure credits occurred, these effects were all less than that observed with a 30 lb N/acre fertilizer rate. Did fall cutting management have an effect on corn grain yield? First-year corn yields were consistently increased when the last (fourth) cutting of alfalfa herbage was not removed from the plots compared with when the alfalfa was cut and removed (Fig. 1). Although the herbage left in these higher yielding plots contributed an additional quantity of N for the subsequent corn crop to use, the lack of a fertilizer N response in adjacent N “equivalency” plots indicates that some factor other than N is important in this yield response. What effect did manure treatments have on preplant and in-season soil nitrate-N tests? Preplant soil samples collected from a depth of 2 ft and analyzed for nitrate-N were not statistically influenced by manure treatments except at one location in the second year after alfalfa plowdown (Fig. 2). Using an in-season soil nitrate-N test, relatively high nitrate-N concentrations were measured the first year following alfalfa plowdown, yet the nitrate-N concentrations in subsequent years were relatively low and represent background concentrations (Fig. 2). Fig. 1Mean corn grain yields from three locations as influenced by previous manure rate treatments for alfalfa and fall alfalfa cutting management. Fig. 2The residual effects of alfalfa and manure, which had been applied prior to alfalfa establishment, on soil nitrate-N measurements during subsequent corn crops after alfalfa plowdown measured preplant (2 ft) and in-season (1 ft).