Improving Predictability and Adoption of Alfalfa N Credits for Corn


Start Date: 2012

Principal Investigator: Jeff Coulter

Organization: University of Minnesota, Department of Agronomy and Plant Genetics

Status: Complete

Background Information:

Some of the most frequent and extreme cases of nitrogen (N) over-fertilization occur when N fertilizer is applied to corn following alfalfa. The majority of research studies support the conclusion that on medium-textured soils, alfalfa typically provides most or all of the N needed to maximize grain yield of the following corn crop. This reduction in fertilizer N requirement for corn following alfalfa is known as the N fertilizer replacement value, or N credit. Although it is more common for first-year corn to require additional N fertilizer on sandy and fine-textured soils, no reliable methods exist to predict which fields need to be fertilized. This uncertainty, in combination with the concern that alfalfa will not provide sufficient N to support the high corn yields common today, has limited the adoption of alfalfa N credits on Minnesota farms. This project focuses on improving the predictability and adoption of alfalfa N credits to corn by: 1) determining the factors that cause corn following alfalfa to respond to N fertilizer; and 2) providing education for Minnesota growers and their advisors about the benefits and reliability in crediting N from alfalfa.

Key Findings:

A total of 259 field trials from the northern and central U.S., including 31 on-farm trials conducted in Minnesota from 2009 to 2011, were analyzed to determine the underlying factors affecting the response to N fertilizer in first-year corn following alfalfa. Results indicate that response to N fertilizer is strongly influenced by soil texture, alfalfa stand age at termination, and termination time. On coarse-textured soils (loamy sands), N fertilizer was needed to optimize grain yield of first-year corn 96% of the time. On fine-textured soils (clay loams, silty clay loams, and sandy clay loams), N fertilizer was needed 53% of the time. On medium-textured soils (loams, silt loams, sandy loams, and fine sandy loams), the response to N was dependent on the time of alfalfa termination. On medium-textured soils, first-year corn following an alfalfa stand that was at least three years old (establishment year plus one production year) responded to N fertilizer only 5% of the time when alfalfa was terminated in the fall, compared to 17% of the time when alfalfa was terminated in the spring. More frequent response to N fertilizer on medium-textured soils occurred when corn followed younger alfalfa stands. These results were shared with Minnesota growers and their advisors through meetings and field days, articles in the farm press, and an extension bulletin (

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