Second-year corn following alfalfa
Corn often is planted as the second crop following alfalfa in the Upper Midwest. It was the second-year crop on 50 to 75% of the acres during 2009-2012 in Iowa, Minnesota, Nebraska, South Dakota, and Wisconsin, and on 30% of the acres in North Dakota during this time (Figure 8) (Yost et al., 2014d). Corn can be an excellent second-year crop following alfalfa termination because the grain yield can be equivalent to corn following soybean or about 10% higher than continuous corn (Figure 1).
Planting second-year corn following alfalfa requires attention to residue management. If tillage is used and first-year corn is harvested for grain rather than silage, stalks from first-year corn generally should be shredded prior to tillage. The choice of tillage system typically affects the yield of second-year corn following alfalfa more than that for first-year corn. Full-width tillage systems often produce greater yield than strip-till or no-till systems for second-year corn on fine- and medium-textured soils, especially when early-season growing conditions are cool and wet.
Alfalfa typically provides N to second-year corn, thereby reducing its fertilizer or manure N requirement compared to continuous corn.
Alfalfa N credit guidelines
Guidelines in Minnesota and several other Midwest states indicate that alfalfa N credits of 75, 50, and 0 lb N/acre should be used for second-year corn following good, fair, and poor alfalfa stands, respectively. When these credits are subtracted from the guideline N rates for continuous corn in Minnesota, guideline N rates for second-year corn range from 0 to 155 lb N/acre (Table 4).
Table 4. University of Minnesota Extension N rate guidelines for second-year corn following alfalfa.
|N cost ÷ corn grain priceb|
|plants/ft2||------------------- lb N/acre -------------------|
|High||4 or more||80||65||55||45|
|2 or 3||105||90||80||70|
|1 or fewer||155||140||130||120|
|Medium||4 or more||55||45||35||25|
|2 to 3||80||70||60||50|
|1 or fewer||130||120||110||100|
|Low||4 or more||25||15||5||0|
|2 or 3||50||40||30||20|
|1 or fewer||100||90||80||70|
|aHigh- and medium-productivity soils should have at least 3% organic matter. Irrigated sandy soils are in the high-productivity category.
bRatio is calculated as N fertilizer cost ($/lb N) ÷ corn grain price ($/bu). For example, for urea that costs $460 per ton, the N cost is $0.50 per pound. If grain is worth $5.00 per bushel, then the fertilizer N cost ÷ corn grain price ratio is $0.50 ÷ $5.00 = 0.10.
Figure 8. Second-year corn following alfalfa during 2009-2012 according to combinations of Cropland Data Layers. Percentages indicate percent of total second-year crop that was corn in each state.
Figure 9. Percent adoption (number of respondents) of current N guidelines for second-year corn following alfalfa in Minnesota by region when manure was not (top value in each pair) or was (bottom value) applied. Data are not show for regions with less than five responses.
Adoption of second-year corn N rate guidelines
According to survey responses from 273 growers in 2012, only 29% followed Extension N rate guidelines for second-year corn (Yost et al., 2014a). However, adoption ranged from 17-43% among regions and with the presence or absence of manure (Figure 9). Adoption was slightly higher when manure was not applied (33%) than when it was (25%), but most respondents applied manure (79%). Of the respondents that applied manure, 78% applied it to both corn crops following alfalfa. In these cases, the total N rate for second-year corn includes four major N sources in addition to the N supplied from other soil organic matter: a) second-year manure N credit for manure applied to first-year corn, b) second-year alfalfa N credit, c) first-year manure N credit from newly applied manure, and d) fertilizer N. Because alfalfa N credits for second-year corn are estimated to be about one-half of first-year credits, respondents who applied N only as commercial fertilizer did not exceed the guideline rate by more than 100 lb N/acre when alfalfa N credits to second-year corn were not accurately accounted for. However, when manure was applied to one or both corn crops following alfalfa, almost one-third of the respondents exceeded guideline rates by more than 100 lb N/acre and 18% exceeded guideline rates by at least 200 lb N/acre (Figure 10). Therefore, opportunities exist for growers to improve profits from corn by further crediting N from alfalfa and manure.
Validation of N rate guidelines
Figure 10. Reported rates of N application by survey respondents who use fertilizer only or fertilizer plus manure for second-year corn following alfalfa.
Figure 11. Economically optimum N rates at the N fertilizer cost ($/lb) ÷ corn grain price ($/bu) ratio of 0.10 for 28 on-farm trials conducted in Iowa during 1989-1991 and in Minnesota during 2011-2012.
On-farm research trials were conducted in Iowa between 1989-1991 and in Minnesota between 2011-2012 to determine optimal N fertilizer rates for second-year corn following alfalfa and to confirm alfalfa N credits (Yost et al., 2014b). Results from these 28 on-farm trials showed that: a) no N fertilizer was needed to maximize grain yield on 14 fields, b) the optimum N rate was less than 80 lb N/acre on 5 fields, c) the optimum N rate was less than 120 lb N/acre on 6 fields, and d) the remaining 3 fields needed 175 lb N/acre (Figure 11). What was most striking about these results was that N fertilizer did not increase yield on one-half of the fields. As was the case with first-year corn, alfalfa stand density did not relate well to the size of the alfalfa N credit to second-year corn. The PSNT also had low accuracy in second-year corn, as only 57% of 53 trials in Minnesota and the literature were correctly predicted as being responsive or nonresponsive to fertilizer N (Walker et al., 2014).
Potential field-specific N rate guidelines
In order to identify when second-year corn following alfalfa requires N fertilizer and how much N is needed on responsive fields, the same approach as for first-year corn is being used with 200 trials of second-year corn and is expected to be completed in 2015. These ongoing efforts should be able to identify when corn following alfalfa will need N fertilizer and what N rates to apply.
Phosphorus and potassium
Management of P and K for second-year corn following alfalfa can be according to guidelines for corn following corn.
If manure was applied to first-year corn, be sure to subtract second-year manure N credits to further reduce the amount of N applied for second-year corn as fertilizer or manure. If the second-year alfalfa N credit (up to 75 lb N/acre) plus the second-year manure N credit from first-year corn do not meet the economically optimum N rate for second-year corn, use properly credited manure N or fertilizer N to bring the total N rate up to the Extension guideline rate.
Insects, weeds, and diseases
Insect, weed, or disease management in second-year corn following alfalfa can be according to guidelines for corn following corn.
- University of Minnesota Extension fertilizer recommendations for agronomic crops in Minnesota
- University of Minnesota Extension calculator for alfalfa nutrient needs
- Regional corn N rate calculator
- University of Minnesota Extension manure N credit guidelines
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We appreciate the assistance of several University of Minnesota Extension educators, state agency personnel, private consultants, technical help, the USDA-National Agricultural Statistics Service, and 59 growers in Minnesota, Iowa, and Wisconsin. We also thank Drs. Roger Becker, M. Scott Wells, Daniel Kaiser, and Fabián Fernández for their reviews.
This publication was funded by the Minnesota Agricultural Fertilizer Research and Education Council. The research summarized in this publication was supported by the Minnesota Agricultural Fertilizer Research and Education Council, the Minnesota Corn Research and Promotion Council, the North Central Region-Sustainable Agriculture Research and Education Program, the Minnesota Agricultural Water Resources Center, the Hueg-Harrison fellowship, the University of Minnesota, and the USDA-Agricultural Research Service.