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On-farm validation of Alfalfa N Credits to Corn

Fri, May 7th, 2010
Posted in Agriculture

There is, what I consider, a very interesting research plot in Fillmore County this year. Jeff Coulter, Extension corn agronomist, and Michael Russelle, USDA-ARS soil scientist are following up on 2009 research in five other locations with research of "On-Farm Validation of Alfalfa N Credits to Corn." They are addressing nitrogen credit for corn following alfalfa, which is one of the most common questions I receive every year. A related potash study by Craig Sheaffer, professor of forage management, and Dan Kaiser, Extension nutrient management specialist, is also of interest. I also find this interesting, because a potash rate study was the first plot I was involved with in Fillmore County.

Rotating alfalfa with corn is useful to: 1) reduce soil erosion, 2) enhance soil tilth and carbon storage, 3) reduce weed seedbanks, 4) disrupt the life cycles of disease and insect pests of corn, and 5) supply nitrogen (N) to the subsequent corn crop. University of Minnesota N-credit guidelines for corn following alfalfa are currently based on alfalfa stand density and are 150 lb N/acre for ≥ 4 alfalfa plants/sq ft, 100 lb N/acre for 2 to 3 plants/sq ft, and 40 lb N/acre for ≤ 1 plant/sq ft. Respective second year alfalfa N credits are 75, 50, and 0 lb N/acre.

To adjust N fertilizer rates for corn following alfalfa, subtract the appropriate alfalfa N credit from the N fertilizer rate you would apply to corn following corn. The cost savings are huge for farmers that accept these alfalfa N credits and reduce fertilizer input accordingly.

Minnesota-specific N-fertilizer guidelines for corn following corn and other crops are available at: www.extension.umn.edu/Corn/nitrogen.html.

Table 1. First-year CORN-silage and grain yields following alfalfa as affected by final-alfalfa-year spring potash rates. Data are averages over five Minnesota locations in 2009, and five N-fertilizer rates applied to corn. (NS = yields were not statistically different among K fertilizer rates.)

With the high cost of N fertilizers, improved corn genetics, higher yields, and tight economic returns, validation of the University's alfalfa N-credit guidelines is important to ensure grower success.

In spring 2008, with funding from the Minnesota Agriculture Fertilizer Research and Education Committee, o n - f arm experiments were established in third or fourth-year alfalfa fields at five locations from southeast to central Minnesota.

All sites had medium soil test potassium (K) levels. Soils included loams, silt loams, and an irrigated sandy loam. Alfalfa stand density ranged from 4 to 9 plants/sq ft.

The objectives were to determine: 1) whether alfalfa's yield and quality respond to top-dressed K applied in the spring of its final production year, and 2) the N-fertilizer requirement of corn following alfalfa that received different rates of K fertilizer during its last year of production.

In these experiments, soil fertility was maintained according to UMN recommendations for all nutrients except N and K. At each farm, replicated rates of 0, 20, 50, 100, and 200 lb potash/acre were applied to final-year alfalfa in early spring or after the first harvest.

Alfalfa was harvested three or four times after potash application. Total alfalfa hay yield and average relative forage quality (RFQ) did not differ with the rate of potash application.

These alfalfa fields were terminated with tillage in the fall or spring depending on grower preference, and planted to corn in 2009.

After corn planting, five N fertilizer rates (0, 20, 40, 80, and 160 lb N/acre) were broadcast as ammonium nitrate within each of the larger plots that received the different rates of K fertilizer the previous year of alfalfa.

At all locations, corn silage and grain yields were high and unaffected by the rate of potash applied to alfalfa (Table 1) or the rate of N fertilizer applied to corn (Table 2). The lack of corn response to N fertilizer was consistent across all five potash rates applied to the previous alfalfa crop at all locations.

These results demonstrate the ability of reasonable alfalfa stands to supply adequate N to first-year corn over a range of soil and climatic conditions. In 2010, research will be conducted at an additional five locations to improve our confidence in these results.

Table 2. First-year CORN-silage and grain yields following alfalfa as affected by N fertilizer rate applied to corn. Data are averages over five Minnesota locations in 2009, and five potash rates applied to final-year alfalfa. (NS = yields were not statistically different among N fertilizer rates.)

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