When does corn following alfalfa need nitrogen?
Over the past 5 years, researchers from the University of Minnesota and USDA-Agricultural Research Service partnered with over 40 Minnesota growers to conduct on-farm research trials to determine optimal nitrogen (N) fertilizer rates for the first- and second-crop of corn following alfalfa and to confirm alfalfa N credits for modern, high-yielding corn hybrids. Current guidelines indicate that about 10, 60, and 110 lb N/acre should be applied to first-year corn following good (4 or more plants/ft2), average (2-3 plants/ft2), or poor alfalfa stands (1 or fewer plants/ft2), respectively. Similarly, the guidelines for second-year corn are 85, 110, and 160 lb N/acre when following good, average, and poor alfalfa stands.
Nitrogen needs in first-year corn
In 2012, after 31 on-farm trials in first-year corn had been completed, results showed that only 3 of 31 fields required N fertilizer to increase corn grain or silage yield. For the three fields with a response to N, the economically optimum N rate (EONR) was less than 80 lb N/acre. Surprisingly, these three responsive fields did not have poor alfalfa stands at termination. In fact, they had good stands while some other non-responsive fields had average stands. These results led to the initial conclusions that: i) first-year corn rarely responds to N fertilizer, ii) the response to N is not related to alfalfa stand density, and iii) more research is needed to determine when first-year corn requires N fertilizer.
Nitrogen needs in second-year corn
Similar conclusions were reached for second-year corn following alfalfa in 2013. Results from 11 on-farm trials of second-year corn showed that: i) 3 of 11 corn fields required no N fertilizer for maximum yield; ii) the optimum N rate was 65 lb N/acre on five fields; and iii) the remaining three fields needed 175 lb N/acre. What was most striking about these results was that 3 of 11 fields did not require N fertilizer for maximum yield. Alfalfa stand density again did not relate well to the size of the alfalfa N credit.
Identifying when corn needs N following alfalfa
In order to identify when corn following alfalfa requires N fertilizer and how much N is needed on responsive fields, we combined the data from our on-farm trials with that from all other trials available in the literature and from other researchers. With 259 first-year corn trials, we found that combinations of soil textural class (fine, medium, or coarse), age of alfalfa at termination, alfalfa termination timing (fall vs. spring), and weather conditions between alfalfa termination and corn planting affect the frequency and level of N response in corn (Table 1). These factors were used in predictive equations to estimate when corn will respond to N and what the optimum N rate will be.
Table 1. Summary of grain yield response to nitrogen fertilizer in 259 trials of first-year corn following alfalfa.
|Soil texture||Alfalfa termination time||Alfalfa age1 (years)||Alfalfa seeding method||Sites responsive to N fertilizer||Total sites|
|1Alfalfa age includes seeding year.|
These predictive equations are currently being validated with 15 on-farm trials across Minnesota this year. The same approach is being used with 200 trials of second-year corn following alfalfa and is expected to be completed later this year. These ongoing efforts should be able to identify when corn following alfalfa will need N fertilizer and what N rates to apply.
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