First-year corn following alfalfa
Figure 3. First-year corn following alfalfa during 2008-2012 according to combinations of Cropland Data Layers. Percentages indicate percent of total first-year crop that was corn in each state.
Corn often is planted as the first crop following alfalfa in the Upper Midwest. It was the first-year crop on about 50% of the acres in the Dakotas and on 75 to 92% of the acres in Iowa, Minnesota, Nebraska, and Wisconsin during 2008-2012 (Figure 3) (Yost et al., 2014d). This section provides suggestions for optimal management of first-year corn.
Due to high water use by alfalfa relative to other crops, soil moisture following alfalfa can be limiting in areas of low precipitation or low soil water holding capacity. This should be a consideration for deciding whether and when to plant corn. On the other hand, if precipitation is adequate for growing corn, water use by alfalfa may allow for earlier corn planting. For fields where anticipated in-season soil moisture is limited, consider: a) terminating alfalfa earlier, b) planting corn early, c) planting drought-tolerant corn hybrids, d) planting shorter-season crops such as wheat, and e) utilizing irrigation.
Through a symbiotic relationship with soil bacteria, alfalfa can gather N from the atmosphere for its own growth and production. During its lifetime, alfalfa sheds and regenerates fine roots, which add N to the soil. The N content of alfalfa herbage and roots combined can be as high as 200 lb N/acre. When alfalfa is terminated, the N in alfalfa residue along with increased soil N and other soil quality improvements that occur during alfalfa production typically supply large quantities of N to one to more years of subsequent crops. This increased N supply often causes first-year corn to require little or no N as fertilizer or manure. The size of the reduction in N rate for first-year corn compared to continuous corn (corn following two or more years of corn) is commonly known as the 'alfalfa N credit.'
Alfalfa N credit guidelines
Guidelines in Minnesota and several other Midwest states indicate that N credits of 150, 75, and 50 lb N/acre should be used for first-year corn following good, fair, and poor alfalfa stands, respectively (Figure 4). When these credits are subtracted from the guideline N rates for continuous corn in Minnesota, first-year corn guideline rates range from 0 to 115 lb N/acre (Table 2).
Table 2. University of Minnesota Extension N rate guidelines for first-year corn following alfalfa.
N cost ÷ corn grain priceb
|Alfalfa stand density||
|plants/ft2||----------------------------- lb N/acre ------------------------------|
|High||4 or more||5||0||0||0|
|2 to 3||55||40||30||20|
|1 or fewer||115||100||90||80|
|Medium||4 or more||0||0||0||0|
|2 to 3||30||20||10||0|
|1 or fewer||90||80||70||60|
|Low||4 or more||0||0||0||0|
|2 to 3||0||0||0||0|
|1 or fewer||60||50||40||30|
|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 4. Current alfalfa N credit guidelines are based on alfalfa stand density measurements at alfalfa termination.
Figure 5. Percent adoption (number of respondents) of N guidelines for first-year corn following alfalfa in Minnesota by region when manure was not (top value in each pair) or was (bottom value) applied.
Adoption of first-year corn N rate guidelines
Across Minnesota, only 35% of respondents followed Extension guidelines for first-year corn, but adoption ranged from 22 to 67% among regions (Figure 5). Adoption rates were slightly higher when manure was not applied to first-year corn (40%) than when it was applied (30%), but the majority of respondents (67%) applied manure. By not fully accounting for alfalfa N credits for first-year corn, growers without manure who exceeded Extension guidelines often (62% of cases) applied 100 to 150 lb N/acre above guidelines (Figure 6). When the combined N credits for manure and alfalfa were not fully accounted for, excessive N rates were even higher; one-third of respondents exceeded guidelines by more than 150 lb N/acre.
Figure 6. Reported rates of N application by survey respondents who use fertilizer only or fertilizer plus manure for first-year corn following alfalfa.
To gain confidence in alfalfa N credits, consider using an 'N-rich' strip (a strip with a high N rate applied) in fields where alfalfa N credits are adopted. If significant differences in plant color or tissue tests occur between the N-rich strip and adjacent corn, a sidedressed N application may be warranted. If sidedressed N is applied, consider leaving a zero-N strip and then compare yields with a yield monitor or weigh wagon to determine whether sidedressed N increased yield.
Validation of N rate guidelines
On-farm research trials were conducted between 2009-2012 to determine economic optimum N fertilizer rates for first-year corn and to confirm alfalfa N credits for modern, high-yielding corn hybrids. The results of 31 on-farm trials showed that alfalfa N credits are reliable and often are LARGER than current guidelines suggest. For example, only 3 of 31 fields required N fertilizer to increase corn grain yield (Figure 7). These three responsive fields had good alfalfa stands at termination, while some non-responsive fields had average stands. These results led to the preliminary conclusions that: i) first-year corn rarely responds to N fertilizer, ii) the response to N is poorly related to final alfalfa stand density, and iii) research needs to identify when first-year corn requires N fertilizer.
Potential field-specific N rate guidelines
In order to identify when corn following alfalfa requires N fertilizer and how much N is needed on responsive fields, results from the 31 on-farm trials were combined with data from all other trials available in the literature and from other researchers (Yost et al., 2014c). With the resulting 259 first-year corn trials, 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 were found to affect the frequency and level of N response in corn (Table 3). These factors were used in predictive equations to estimate when corn will respond to N and what the optimum N rate will be. We found that first-year corn rarely responds to N except on:
- sandy soils
- fine-textured soils when there are prolonged wet early-season conditions
- medium-textured soils when following 1-year-old alfalfa that was direct seeded
- medium-textured soils when following 2-year-old alfalfa (including the establishment year) seeded with a small grain companion crop
- medium-textured soil when following spring-terminated alfalfa
These predictive equations are being validated with on-farm trials across Minnesota beginning in 2014. Current guidelines based on alfalfa stand density (Table 2) should be used until more site-specific guidelines can be developed. Soil tests, such as the pre-sidedress soil nitrate test (PSNT), have low accuracy in first-year corn, as only 60% of 114 trials in Minnesota and the literature were correctly predicted as being responsive or nonresponse to fertilizer N (Walker et al., 2014).
Table 3. Summary of grain yield response to N fertilizer in 259 trials of first-year corn following alfalfa.
|Alfalfa seeding methodb||
|Alfalfa termination time||
Sites responsive to N fertilizer
Range in EONRd
|years||------ % ------||lb N/acre|
|Coarse||D or C||1-3||Fall or spring||96||11||90-210|
|Medium||D or C||3+||Fall||5||86||80-155|
|Medium||D or C||3+||Spring||17||48||40-160|
|Fine||D or C||1-7||Fall or spring||53||19||20-150|
|aCoarse = loamy sand; medium = loam, sandy loam, silt loam, fine sandy loam; fine = clay loam, silty clay loam.|
b D = direct seeded without a companion crop, ND = seeded with a small grain companion crop.
cAlfalfa age at termination includes seeding year.
dThe range in economically optimum N rate (EONR) for the N cost ÷ corn grain price ratio of 0.10.
Figure 7. Economically optimum N rates at the N fertilizer cost ($/lb N) ÷ corn grain price ($/bu) ratio of 0.10 for 31 on-farm trials conducted in Minnesota during 2009-2011.
Phosphorus and potassium
It is important to monitor soil-test potassium (K) towards the end of an alfalfa stand because harvested alfalfa can remove about 160 to 300 lb K2O/acre each year. If K is needed for first-year corn following alfalfa, applying K ahead of corn rather than ahead of last-year alfalfa will reduce luxury consumption of K by alfalfa and maximize K use efficiency for first-year corn (Yost et al., 2011). University of Minnesota Extension guidelines suggest that 0 to 255 lb K2O/acre should be applied to corn according to soil-test K concentration in the topsoil and expected corn yield. First-year corn should be fertilized with 0 to 160 lb P205/acre according to soil-test phosphorus (P) concentration in the topsoil and expected corn yield. Be sure to credit N that may be applied with P fertilizers when determining N rates for corn.
Manure often is applied to first-year corn following alfalfa for several reasons, including the need to replenish nutrients removed during alfalfa production, insufficient manure storage capacity, inadequate land area for spreading manure in other crop rotations, and/or inability to distribute or sell manure. However, if possible, avoid manure application for first-year corn following alfalfa because additional N often does not increase corn yield and can cause N loss to the environment. Many fields with a manure history may have adequate or more than adequate soil-test P and K at the end of alfalfa stands, but be sure to soil test. If manure is needed to replenish soil P or K at the end of an alfalfa stand, apply only the minimum rate (based on a manure nutrient analysis) needed to meet P or K requirements. Consider applying solid manure if the P need is greater than K, but liquid manure if the K need is greater than P, because solid manure usually has higher P concentration and liquid manure usually has higher K concentration. Also, consider using P or K fertilizer instead of manure to meet needs of first-year corn so that manure nutrients can be utilized for corn in other rotations or for other crops that need N.
Insects, weeds, and diseases
The potential for soil and residue-borne insects and pathogens that impact corn is usually lower for first-year corn following alfalfa than corn in other rotations. Bt corn hybrids or soil-applied insecticides for protection against corn rootworm are not necessary when following alfalfa because the lifecycle of corn rootworm is disrupted by alfalfa. Healthy alfalfa stands typically suppress many annual weeds that plague corn grown in crop rotations with only annual crops. This can lead to reduced weed pressure in first-year corn, and less need for herbicide.