Managing the rotation from alfalfa to corn

Alfalfa termination

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Deciding to terminate

Successful alfalfa termination is essential to fully realize the benefits of alfalfa to subsequent corn crops, as volunteer alfalfa competes with corn for water and N.

Economic reasons to rotate

When possible, the decision about when to terminate alfalfa should be based on economics. Accurate record-keeping of alfalfa establishment costs, production costs, yields, stand quality and value (market or feed) is essential.

Growers can usually recover initial alfalfa establishment costs during the following one or two production years, but can sometimes recover costs in the establishment year if alfalfa is harvested and its price is high.

After the first few production years, alfalfa stand condition and quality can decline at highly variable rates depending on alfalfa genetics, alfalfa management, soil properties, weather conditions and other factors. This variability makes it difficult to define optimal alfalfa stand lengths across many growing conditions.

However, research in Wisconsin, Manitoba and western Canada suggests that net return to alfalfa production often is maximized by rotating alfalfa after an establishment year plus two to four production years.

Other reasons to rotate

Planned alfalfa termination based on economics is not always possible. Sometimes alfalfa is rotated due to winterkill, weedy or diseased stands, changes in government programs, competition with commodity crops, land needed for applying manure or other reasons.

A survey of 421 Minnesota growers in 2012 found that growers rarely rotate alfalfa according to a planned schedule and that the most common reason for terminating alfalfa was the need for a place to apply manure; half of respondents (52 percent) selected this as the top reason for terminating alfalfa.

Therefore, increased manure management options, such as increased storage capacity and increased dispersion to other fields, may promote longer alfalfa stands. The second most common reason for alfalfa termination was thinning alfalfa stands; a third of respondents (36 percent) selected this as the top reason.

Responses were evenly distributed among the remaining reasons for terminating alfalfa, such as weedy alfalfa stands and winterkill. The majority of growers across the Upper Midwest rotate alfalfa after an establishment year plus two to three production years according to survey responses from 421 Minnesota growers in 2012 and an analysis of satellite imagery from 2006 to 2012 (Cropland Data Layers by USDA-National Agricultural Statistics Service).

Stand age upon rotation

However, in some areas of the Upper Midwest, the majority of stands were rotated after six or more years (for example, the western halves of the Dakotas and Nebraska), indicating opportunities for more proactive and frequent rotation of alfalfa to improve net return from alfalfa production (Table 1).

Table 1: Percent acreage distribution of alfalfa stand age by state

State	2-year stand age	3-year stand age	4-year stand age	5-year stand age	+6-year stand age
Iowa	43%	31%	7%	3%	16%
Minnesota	25%	24%	13%	3%	35%
North Dakota	6%	4%	2%	2%	86%
Nebraska	15%	16%	14%	6%	49%
South Dakota	15%	13%	10%	4%	58%
Wisconsin	19%	27%	21%	5%	28%

How and when to terminate

Traditionally, alfalfa stand termination relied on tillage implements such as a moldboard plow or a chisel plow with overlapping sweeps to completely cut off alfalfa roots.

Although effective at terminating alfalfa, these tillage implements may not be suitable for all fields due to concerns about soil erosion. Tillage implements that do not cut the roots from all plants are ineffective for complete alfalfa termination. Therefore, herbicides are needed to provide enhanced termination (Figure 2).

Figure 2 (from top to bottom): (a) Ineffective alfalfa termination by a chisel plow without overlapping sweeps, (b) Effective alfalfa termination by fall-applied herbicide followed by chisel plowing, (c) Fall moldboard plow tillage and (d) Fall-applied herbicide with no tillage in no-till corn.

Fall termination

Advantages to fall alfalfa termination include:

An earlier start to alfalfa residue decomposition.
Potential for earlier drying and warming of soil and an earlier corn planting date.
Potential for earlier N release during first-year corn.

These benefits likely have been realized by growers in Minnesota because two-thirds of 421 growers in 2012 indicated they terminated alfalfa in the fall. The major disadvantage with fall termination is that it does not allow for the opportunity to assess whether an alfalfa stand will survive the winter and be productive for another year.

Fall herbicide applications to terminate alfalfa should occur before the first killing freeze (28 degrees Fahrenheit or lower for a few hours), but when new alfalfa regrowth since the last cutting is at least 4 to 6 inches. Herbicide applied to plants with less than 4 inches of regrowth or to freeze-damaged plants may result in an ineffective kill due to poor herbicide translocation to roots.

To determine if a killing freeze occurred, use the fingerprint test the morning after a freeze: If your fingerprint remains after squeezing alfalfa leaves between your fingers, the epidermis of the leaves has ruptured due to a killing freeze.

In some fields, fall tillage is desired after an herbicide application for alfalfa termination in order to relieve soil compaction and produce a soil surface that dries more rapidly in the spring. Tillage can generally begin within three to four days following herbicide application, since the majority of the herbicide is translocated within the plant by this time.

Be aware that some labels require a longer interval before tillage. If tillage that completely cuts all alfalfa roots is used to terminate alfalfa, such as a moldboard plow or a chisel plow with overlapping sweeps, herbicides typically are not needed prior to tillage.

A highly effective herbicide option for fall termination of alfalfa is 2,4-D amine or ester used alone or tank-mixed with dicamba products. If grasses are present with alfalfa, a good option is to tank-mix glyphosate with 2,4-D.

However, using high rates of glyphosate alone for alfalfa termination often results in only partial kill (60 to 90 percent kill with fall applications, or 40 to 80 percent kill with spring applications). Glyphosate will not control alfalfa that is tolerant to glyphosate.

Spring termination

Advantages to spring termination of alfalfa include the ability to assess alfalfa winter survival and the opportunity for an additional harvest of alfalfa in late May followed by delayed planting of short-season silage or grain corn. Another advantage is soil cover to prevent erosion during winter months.

Disadvantages to spring termination of alfalfa include delayed decomposition of alfalfa residue and N release during first-year corn, the potential for delayed corn planting, and increased difficulty in terminating alfalfa with herbicides.

Spring termination of alfalfa with herbicide can be challenging because the optimal time to plant corn typically occurs before alfalfa regrowth is greater than 4 to 6 inches. Tillage alone or in combination with herbicide can be used to terminate alfalfa in the spring.

When herbicides are used preemergence, glyphosate tank mixed with dicamba and 2,4-D provides consistent control. However, typical dicamba or 2,4-D rates require two weeks or more after application before planting corn to reduce the chance of crop injury during emergence.

To control alfalfa postemergence in corn, products containing 2,4-D or dicamba can effectively control volunteer alfalfa. Products that contain clopyralid also can be used for suppression before or after corn planting.

Be aware that the time available for application is short and corn injury can occur if herbicide labels are not followed. Also, injury to non-target crops such as soybean can occur from volatilization of many postemergence growth-regulator herbicides.

Relying solely on a postemergence herbicide program to terminate alfalfa is discouraged due to increased risk of crop injury and incomplete alfalfa kill.

Managing first-year corn

Map of Dakota, Nebraska, Minnesota, Iowa, and Wisconsin. Areas all across each state highlighted. — Figure 3: First-year corn following alfalfa from 2008-12, from Cropland Data Layers. Percentages indicate percent of total first-year crop that was corn

Corn often is planted as the first crop following alfalfa in the Upper Midwest. It was the first-year crop on about 50 percent of the acres in the Dakotas and on 75 to 92 percent of the acres in Iowa, Minnesota, Nebraska and Wisconsin during 2008 to 2012 (Figure 3).

Planting

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:

Terminating alfalfa earlier.
Planting corn early.
Planting drought-tolerant corn hybrids.
Planting shorter-season crops such as wheat.
Utilizing irrigation.

Nitrogen

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 pounds of N per 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.” For specific guidelines, visit Nitrogen management for first- and second-year corn following alfalfa.

Adoption and validation of N guidelines

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Adoption of first-year corn: N rate guidelines

Across Minnesota, only 35 percent of respondents followed Extension guidelines for first-year corn, but adoption ranged from 22 to 67 percent among regions (Figure 4). Adoption rates were slightly higher when manure was not applied to first-year corn (40 percent) than when it was applied (30 percent), but the majority of respondents (67 percent) applied manure.

By not fully accounting for alfalfa N credits for first-year corn, growers without manure who exceeded Extension guidelines often (62 percent of cases) applied 100 to 150 pounds of N per acre above guidelines (Figure 5). When the combined N credits for manure and alfalfa were not fully accounted for, excessive N rates were even higher; a third of respondents exceeded guidelines by more than 150 pounds of N per acre.

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.

Map of minnesota with 5 regions — Figure 4: Percent adoption of N guidelines for first-year corn following alfalfa in Minnesota by region when manure was not (top) or was (bottom) applied.

two pie charts, one without manure, one with manure — Figure 5: Reported rates of N application by survey respondents who use fertilizer only or fertilizer plus manure for first-year corn following alfalfa.

Validation of N rate guidelines

On-farm research trials conducted between 2009 and 2012 aimed to determine economic optimum N fertilizer rates for first-year corn and to confirm alfalfa N credits for modern, high-yielding corn hybrids.

Bar chart with 3 bars around 80 lbs/ac N — Figure 6: Economically optimum N rates at the N fertilizer cost ($/lb N) / corn grain price ($/bu) ratio of 0.10 for 31 on-farm trials in MN 2009-11.

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 three of 31 fields required N fertilizer to increase corn grain yield (Figure 6). These three responsive fields had good alfalfa stands at termination, while some non-responsive fields had average stands.

These results led to these preliminary conclusions:

First-year corn rarely responds to N fertilizer.
The response to N is poorly related to final alfalfa stand density.
Research needs to identify when first-year corn requires N fertilizer.

Potential field-specific N rate guidelines

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.

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 affected the frequency and level of N response in corn (Table 2).

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.

Current guidelines based on soil texture, stand age and termination time 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 percent of 114 trials in Minnesota and the literature were correctly predicted as being responsive or nonresponsive to fertilizer N.

Table 2: Summary of grain yield response to N fertilizer in 259 trials of first-year corn following alfalfa.

Soil texture^a	Alfalfa seeding method^b	Alfalfa age^c	Alfalfa termination time	Sites responsive to N fertilizer	Total sites	Range in EONR^d (lbs. N/acre)
Coarse	D or C	1-3 years	Fall or spring	96%	11	90-210
Medium	D	1 years	Fall	56%	16	50-200
Medium	C	2 years	Fall	35%	54	50-240
Medium	D	2 years	Fall	8%	25	50-150
Medium	D or C	3+ years	Fall	5%	86	80-155
Medium	D or C	3+ years	Spring	17%	48	40-160
Fine	D or C	1-7 years	Fall or spring	53%	19	20-150

Phosphorus and potassium

It is important to monitor soil-test potassium (K) toward the end of an alfalfa stand because harvested alfalfa can remove about 160 to 300 pounds of K₂O per 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.

We suggest that 0 to 255 pounds of K₂O per 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 pounds of P₂0₅ per 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

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 used 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.

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 percent of the acres from 2009 to 2012 in Iowa, Minnesota, Nebraska, South Dakota and Wisconsin, and on 30 percent of the acres in North Dakota during this time (Figure 7).

Map of Upper Midwest — Figure 7: Second-year corn following alfalfa from 2009-12, from Cropland Data Layers. Percentages indicate percent of total second-year crop that was corn.

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 percent higher than continuous corn (Figure 1).