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Nitrogen for corn following alfalfa: when and how much?

Matt Yost, Jeff Coulter and Michael Russelle

Growers usually enjoy a yield boost when corn follows alfalfa. The major reasons: reduced pest and disease pressure, better soil structure that enhances root growth and water infiltration, and an altered soil microbial community. In addition, when corn follows alfalfa the economically optimum nitrogen (N) fertilizer rate is much less than when corn follows other crops. When compared to continuous corn, current University of Minnesota guidelines suggest that growers reduce N fertilizer applications for first-year corn after alfalfa by 150 pounds N per acre when there is a good stand of alfalfa (at least 4 alfalfa plants per square foot) at the time of termination. The guidelines also suggest that the N fertilizer rate for second-year corn after alfalfa be reduced by 75 pounds N per acre when a good alfalfa stand is terminated. These reductions in N fertilizer for corn following alfalfa are known as N credits, and the N passed onto corn is largely due to additions of N-rich inputs from alfalfa to the soil. These inputs include things such as alfalfa leaves and stems lost during harvest, alfalfa stand losses over time, turnover of thin alfalfa roots, and substances that exude out of alfalfa roots.

Recent research on 31 farms in Minnesota and western Wisconsin from 2009 to 2011 has confirmed N credits for first-year corn following alfalfa and investigated how these credits are affected by potassium fertility during the last alfalfa year, amount of alfalfa regrowth at termination, termination time (fall vs. spring), no-tillage, and fall manure application. Surprisingly, none of these factors affected the economically optimum N rate for first-year corn after alfalfa and N fertilizer increased grain yield on only 3 of the 31 farms. This research supports university recommendations and indicates that 90% of the time, N from fertilizer or manure is not needed for first-year corn after alfalfa. On the 3 of 31 farms where N was needed, grain yield was optimized with 80 pounds N per acre when applied near planting or with just 40 pounds N per acre when sidedressed. The three responsive farms had fine-textured soils and excessive early-season precipitation.

Prior to the 31 recent on-farm trials in Minnesota and western Wisconsin, results from 200 other N rate trials in first-year corn after alfalfa were gleaned from the literature. Together, these 231 trials were analyzed to determine the underlying factors affecting N response in first-year corn. Results indicate that the response to N is strongly influenced by soil texture. On coarse-textured soils (loamy sands), N fertilizer was needed to optimize grain yield of first-year corn 96% of the time. The high frequency of response to N on coarse-textured soils is likely due to high potential for leaching of mineralized N. On fine-textured soils (clay loams, silty clay loams, and sandy clay loams), N fertilizer was needed 50% of the time. Response to N on fine-textured soils typically occurred in years when there was excessive early-season soil moisture, which likely slowed mineralization of N by soil microorganisms due poor soil aeration. On medium-textured soils (loams, silt loams, sandy loams, and fine sandy loams), the response to N was dependent on the time of tillage for alfalfa termination. On medium-textured soils, first-year corn responded to N fertilizer only 4% of the time when alfalfa was terminated in the fall, compared to 20% of the time when alfalfa was terminated in the spring. More frequent response to N when alfalfa was terminated in the spring on medium-textured soils was likely due to less time for mineralization of N before rapid N uptake by corn.

To better understand and predict N credits from a good stand of alfalfa to second-year corn, 11 on-farm trials were conducted across southern and central Minnesota in 2011 and 2012. We were surprised to find that N fertilizer did not increase the grain yield of second-year corn on 4 of 11 farms. The economically optimum N rate varied among the seven responsive farms, but was often at least half as large as that for continuous corn. As in first-year corn, sidedress applications of N allowed growers to reduce fertilizer rates without sacrificing yield. These results indicate that N credits for second-year corn after alfalfa are reliable in most cases and are sometimes much higher than expected.

When possible, growers should consider planting two years of corn after alfalfa to take full advantage of alfalfa N credits. Current research is focused on developing predictive models to determine the conditions that cause first- and second-year corn after alfalfa to respond to N fertilizer, and to better predict the economic optimum N rates in fields where a response to N is expected.

The authors greatly appreciate the financial support for this research, which was provided by the Minnesota Corn Growers Association, the Minnesota Agricultural Fertilizer Research and Education Council, the Minnesota Agricultural Water Resource Center, the USDA-Sustainable Agriculture Research and Education Program, the Hueg-Harrison Fellowship, and the USDA-Agricultural Research Service.

Published in Dairy Star February 23, 2013

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