On-farm research experiments were established at 14 farms in Minnesota and two in Wisconsin to research the effects of no-till and manure application on the alfalfa N credit to first-year corn. When corn was no-till planted into terminated alfalfa, no fertilizer N was required to maximize corn grain or silage yield. Therefore, alfalfa N credits to first-year corn are likely the same across tillage systems. Manure application during fall alfalfa termination did not reduce the amount of fertilizer N needed to optimize yield at the three responsive farms and even had a negative impact on corn grain yield. Growers may want to consider applying manure in the spring or in other rotations where the manure N could be beneficial to crop performance.The presidedress soil nitrate test and the basal stalk nitrate test worked relatively well at identifying the fields that were responsive to fertilizer N.
When alfalfa is integrated into crop rotations in the Midwest, it supports crop diversity, increases sustainability by reducing inputs, improves soil structure, removes excess nitrate, improves soil organic carbon (Baker et al., 2006), increases 5-year economic returns by 100-158% (Singer et al., 2003), and disrupts pest/disease cycles. Many Minnesota farmers are aware of the current University of Minnesota guidelines for an N credit of up to 150 lb N/ac to first-year corn when the previous alfalfa density is greater than 4 plants ft2 (Rehm et al., 2006), yet many do not give the full N credit, and apply manure and/or N fertilizer to corn planted the following year. Shepard (2000) reported that out of 1,928 farmers surveyed in Wisconsin, two out of three applied excess N for corn production. El-Hout and Blackmer (1990) also reported that 17 of 29 corn fields following alfalfa in Iowa had soil nitrate concentrations at least twice the critical amount, and 6 fields had three times the critical amount.
Conservation tillage and manure application complicate the decisions growers make when determining how much of an N credit to apply for corn after alfalfa. Reduced tillage is growing as an option for corn following alfalfa, but the implications on the N credit are not well understood. Reduced tillage may slow N mineralization and reduce the alfalfa N credit. Research is needed to determine the optimum N fertilizer rate for minimum/no-till corn following alfalfa. Some researchers have found no effect of the tillage system used to terminate alfalfa on corn response to N (Triplett et al., 1979; Levin et al., 1987), yet there are concerns that the N credit is smaller with reduced tillage. To build confidence in the alfalfa N credit, farmers may need an independent measurement. The presidedress soil nitrate test (PSNT) is used to determine in-season corn N requirements in some states. Researchers studied PSNT N rate recommendations at 101 sites across Wisconsin and found that recommendations were correct at 58% of the sites, with 36% resulting in over-application of N and 6% in under-application (Andraski and Bundy, 2002). More research is needed to determine the validity of using PSNT in alfalfa-corn rotations under different tillage systems, because the reliability of PSNT seems to be lower in soils that are colder than average, such as occurs with reduced tillage.
Most alfalfa growers in Minnesota and other Midwest states are dairymen and often apply manure when terminating alfalfa to meet soil P and K requirements after several years of intensive alfalfa cropping. Even though most effects of manure are positive, manure can reduce N mineralization due to high carbon (C) content, which can increase the need for fertilizer N to stimulate mineralization. Additionally, excessive manure N additions during the alfalfa rotation may increase the nitrate leaching potential. Research is needed to determine the effects that manure application during the transition to corn has on the N credit from alfalfa.
The objectives of this research were to determine whether adjustments should be made to the alfalfa N credit to account for no-till or manure application, to verify the accuracy of the PSNT in identifying fields that would be responsive to N fertilization, and the basal stalk nitrate test for evaluating whether fertilizer rates were adequate. This research was also aimed at helping farmers become aware of and gain trust in the N credit from alfalfa to first-year corn. This awareness will motivate and prompt farmers to reduce the amount of manure and N fertilizer applied to terminated alfalfa.
We intended to conduct on-farm experiments on at least 20 farms in Minnesota and Wisconsin and had identified 21 farm fields in the fall of 2010. However, we were not able to collect data from three farms with manure treatments because the spring weather either delayed or hindered corn planting and the cooperating growers decided to keep alfalfa and not rotate to corn. We also did not collect data from a no-till farm near Albany, MN, because over 50% of the corn had green-snapped during the early summer of 2011. Results from two other locations (Morristown and Otsego, MN) were excluded due to high variation in corn yield and extremely wet conditions. Therefore, we present data from 6 no-till farms in Experiment (Exp.) One, and from 9 farms with manure treatments in Exp. Two (Figure 1).
The no-till experiments consisted of four replications of six fertilizer N rates (0, 20, 40, 80, 160 lb N/ac) applied as broadcast ammonium nitrate 1 to 2 weeks after the corn was planted. A sidedress rate of 40 lb N/ac was also applied as banded ammonium nitrate when the corn had reached the V6 leaf stage. We applied both fertilizer applications by hand-spreading. Six to eight soil cores were collected in the subplot that would receive sidedressed N rate immediately before sidedress N application. Soils were sampled to 1 ft depth and analyzed for nitrate-N according to recommendations of the PSNT test. Fertilizer P, K, and S were applied at each location based on background soil tests (Table 1) and University of Minnesota recommendations for corn production.
In the fall of 2011, corn grain was hand-harvested from 10 ft of row within the center of each subplot. Corn ears were dried, shelled, and weighed for moisture to calculate grain yield at 15.5% moisture. We intended to harvest corn stover (stalk + leaves) samples from all locations in order to estimate silage yield, but due to grower’s harvest schedules and time constraints, we were able to harvest stover only from 4 no-till fields. Stover was harvested from the same plants harvested for grain, chipped and subsampled in the field, and dried to determine dry matter yield. Corn silage yield was the calculated at 65% moisture. One to three weeks after corn had reached physiological maturity or black layer, we collected a basal stalk sample (10 stalks) in each subplot according to Iowa State University recommendations (Blackmer and Mallarino, 2000). Basal stalk samples were dried, ground, and analyzed for nitrate-N concentration.
Cow or swine manure was applied to alfalfa by cooperating growers after the last alfalfa harvest had been removed in the fall of 2010. Each grower’s manure application rate was determined by either weighing three tarps on which solid manure was collected during broadcast spreading or by measuring the area covered by one full tank of liquid manure. A manure subsample was collected from the three tarps or directly from the manure applicator tank, frozen, and then sent to a certified commercial lab for determination of C and N concentrations. The N and C concentrations were used to calculate nutrient inputs per acre and C:N ratio of manure (Table 2). After manure application, growers terminated alfalfa with moldboard or chisel plow tillage in the fall. Each farm had four replications of with and without manure treatments. Subplots within these treatments were four fertilizer N rates applied at corn planting (0, 40, 80, and 160 lb N/ac) and a sidedress rate of 40 lb N/ac. Fertilizer application, soil sampling techniques, corn yield sampling, and basal stalk nitrate methods and analysis for the manure fields were identical to the methods used for at the no-till fields. Stover and silage yield were only determined at 5 of 9 farms.
Experiment One: N credit for no-till corn after alfalfa
Corn grain yield ranged from 195 to 239 bu/ac across the six locations with no-till corn planted directly into terminated alfalfa. Corn silage yield ranged from 24 to 33 ton/ac across the four fields where silage yield was measured. Even at these relatively high yield levels, no fertilizer N was required to maximize grain or silage yield (Fig. 1). The results from these six no-till fields suggest that University N credit recommendations should not be altered for no-till corn after alfalfa. It also indicates that when even when precipitation levels in the spring are greatly above average, applying fertilizer N to corn after alfalfa will not always increase corn yield.
The PSNT is one of the tools available for determining when corn might require fertilizer N to maximize yield. Soil nitrate concentrations when the corn at each farm reached the V4-V6 leaf growth stage ranged from 8 to 18 ppm NO3-N. According to the PSNT test, fertilizer N would have been recommended at five of the six locations (according to Iowa State University guidelines). However, yield did not increase with N fertilization suggesting that the critical values of the PSNT may need to be adjusted for no-till corn or that the test is not very robust.
Average basal stalk nitrate (BSN) concentrations across the six no-till locations ranged from 246 to 1273 ppm NO3-N when no fertilizer N was applied to the corn. Four out of the six locations had BSN concentrations that were in the optimum range (adequate N), whereas the other two locations (Norwalk and Lakefield) had marginal concentrations yet fertilizer N did not increase grain or silage yield at these locations.
Experiment Two: Manure effect on alfalfa N credits to corn
When manure with N content ranging from 10 to 219 lb total Kjeldahl N (TKN)/ac was applied during alfalfa termination (Table 2), corn grain yield across all nine locations was not improved. In fact, at three farms that responded to fertilizer N, manure application during alfalfa termination decreased corn grain yield by 10 bu/ac and increased the amount of N needed to maximize grain yield by 80 lb N/ac. Across the five locations where silage yield was measured, corn silage yield increased by 1.4 ton/ac (4%) with manure application. Two of these locations were responsive to fertilizer N while the others were not.
Fertilizer N was required to maximize grain yield at 3 of 10 locations with manure (Fig. 2). Grain yield across these three farms (Howard Lake, St. Rosa, and Waseca) increased by 33 bu/ac (21%) when 160 lb N/ac was applied at planting. Sidedress N application of 40 lb N/ac at these three locations produced the same yield as 80 lb N/ac at planting. We currently have no explanation as to why the fertilizer N increased grain yield at these three locations but not the two organic sources (manure and alfalfa). This is especially surprising because these locations had good alfalfa stands, and high amounts of manure N were injected at Waseca and broadcast at Howard Lake. It is possible that the manure C tied up N that otherwise would have been available to the corn; however, the manure C:N ratios at Howard Lake and Waseca were much lower than the other locations (Table 2). High precipitation in the spring is another possible explanation for the yield response to fertilizer at these locations. The Howard Lake farm had 2, 3.5, and 3.8 inches of excess precipitation in April, May, and July, respectively, in relation to the long-term normal precipitation, whereas the Waseca location only had 2 inches of excess in July. Therefore, excess precipitation may explain part of the reason why organic sources were not sufficient. However, this work highlights the need for more accurate predictors of fertilizer N response in first-year corn after alfalfa are needed.
The economically optimum fertilizer N rate for corn silage yield (assuming $0.45 lb/N and $28/ton) across five locations where yield was measured was 160 lb N/ac and yield was 4.4 ton/ac (11%) higher than the nonfertilized corn. The sidedress N rate of 40 lb N/ac was equivalent to 90 lb N/ac of fertilizer applied at planting for corn silage yield. Therefore, the sidedress rate was much more efficient than the application at planting. When growers suspect that corn after alfalfa will require fertilizer N to maximize yield (excess precipitation, poor alfalfa stands, etc.), sidedress N applications are likely to be more efficient than N applications at planting when there is excessive precipitation during the time between planting and sidedress N application. In addition, this data along with results from similar experiments highly suggest that alfalfa N credit guidelines for first-year corn may need to be altered for silage corn.
The PSNT worked well at identifying the three farms that were responsive to fertilizer N as soil nitrate concentrations in nonfertilized corn plots were only 6 ppm NO3-N. One other farm (Faribault) had low concentrations (8 ppm NO3-N), but did not respond to N, however, the other five locations had concentrations ?16 ppm NO3-N. Therefore, the test worked well at 8 of 9 farms with manure application.
Manure application during alfalfa termination increased corn BSN concentration by 2505 ppm NO3-N (44%) across three locations (Medford, Plainview-2, and Randolph). These three locations had an average of 60 lb/ac of total manure N with 20 lb/ac of NH4-N applied as broadcast manure at Plainview-2 and Randolph, and injected at Medford (Table 2). Manure had no effect on BSN concentration at the St. Rosa farm where solid manure was broadcast applied, even though the manure N content was similar. It was surprising that injected swine manure (131 and 108 lb/ac of TKN and NH4-N, respectively) applied in the fall of 2010 (30 October) at Waseca did not increase BSN concentration or grain yield.
Fertilizer N application, at the time of corn planting, increased BSN concentrations across five locations with manure application. At the Medford and Waseca farms, only small amounts of fertilizer N 6 and 20 lb N/ac, respectively, was needed to raise the BSN above 700 ppm NO3-N, whereas the St. Rosa location needed about 100 lb N/ac (Fig. 3). Corn with concentrations above this value is considered to have had adequate N for optimal growth (Blackmer and Mallarino, 2000). The BSN concentration across two other locations (Randolph, Plainview-2) was in the excessive category (5470 ppm NO3-N), even when no fertilizer N was applied. When the highest rate of N was applied (160 lb N/ac), BSN concentration rose by 3300 ppm NO3-N across these two locations (Fig. 3). Basal stalk samples were not collected at the remaining four manure locations because the cooperating growers harvested the corn for silage. Overall, the BSN test was successful in identifying the responsiveness and non-responsiveness of corn grain yield to fertilizer N at these five farms.
Educational & Outreach Activities
To highlight and extend the results of this and other related research, extension summer field days were conducted at the Medford, Red Wing, and Albany locations. Field days consisted of three to four research presentations, a farmer panel presentation made up of cooperating growers, and a plot tour. These field days were attended by nearly 60 individuals managing more than 48,000 acres. Evaluations indicate that there was a knowledge gain by these field day participants, and that they anticipate reducing their nitrogen fertilizer rates for first- and second-year corn after alfalfa.
The results of this study were also presented at the Minnesota Department of Agriculture-Nutrient Management research conference where over 300 individuals (mostly farmers and farm advisers) were in attendance and at an University of Minnesota Extension workshop in Cologne, MN. The results of this study will be published as a technical manuscript in the spring of 2012.
The results of these two experiments indicate that alfalfa N credits should not be altered for first-year corn after alfalfa when grain or silage corn is planted in a no-till compared to a conventional tillage system. Even though we did not have a side-by-side tillage treatment to compare no-till and conventional tillage, grain and silage yields in the no-till fields were high and comparable with conventional tillage yields. This suggests that there is great potential for no-till corn after alfalfa in Minnesota. When manure is applied during fall termination of alfalfa with a stand of at least 4 plants ft2, it had minimal and potentially negative effects on the following year corn N requirements for grain yield, however silage yield was increased. If first-year corn after alfalfa will be harvested for grain as compared to silage, growers should consider applying less manure to increase the efficiency of manure N and reduce the potential for greater N loss. The PSNT and BSN tests were relatively successful in identifying which corn fields would respond to fertilizer N following alfalfa. Therefore, growers should utilize these tests where appropriate to guide their N applications to corn following alfalfa. Sidedress N applications were usually twice as efficient at fertilizer applied at planting from an agronomic standpoint.
Andraski, T.W., and L.G. Bundy. 2002. Using the presidedress soil nitrate test and organic nitrogen crediting to improve corn nitrogen recommendations. Agron. J. 94:1411-1418.
Baker, J.M., T.E. Ochsner, R.T. Venterea, and T.J. Griffis. Tillage and soil carbon sequestration – What do we really know? Agric. Ecosyst. Environ. 118:1-5.
Blackmer, A.M., and A.P. Mallarino. 2000. Cornstalk testing to evaluate nitrogen management. Available at http://www.extension.iastate.edu/Publications/PM1584.pdf). Iowa State Univ. Ext., Ames.
El-Hout, N.M., and A.M. Blackmer. 1990. Nitrogen status of corn after alfalfa in 29 Iowa fields. J. Soil Water Conserv. 45:115-117.
Levin, A., D.B. Beegle, and R.H. Fox. 1987. Effect of tillage on residual nitrogen availability from alfalfa to succeeding corn crops. Agron. J. 79:34-38.
Rehm, G., G. Randall, J. Lamb, and R. Eliason. 2006. Fertilizing corn in Minnesota. FO-3790-C. Available at http://www.extension.umn.edu/distribution/cropsystems/DC3790.html. Univ. of Minnesota, St. Paul.
Shepard, R. 2000. Nitrogen and phosphorus management on Wisconsin farms: Lessons learned for agricultural water quality programs. J. Soil Water Conserv. 55:63-68.
Singer, J.W., C.A. Chase, and D.L. Karlen. 2003. Profitability of various corn, soybean, wheat, and alfalfa cropping systems. Crop Manage. doi: 10.1094/CM-2003-0130-01-RS.
Triplett, G.B., F. Haghiri, Jr., and D.M. Van Doren, Jr. 1979. Plowing effect on corn yield response to N following alfalfa. Ohio Agric. Res. and Dev. Ctr. I-79.
Each growers that participated in this research was surveyed about their management of corn following alfalfa. There were 6 of 16 growers that did not apply more than a starter fertilizer application of N to corn following alfalfa. The other 10 growers applied between 45 to 100 lb N/ac. The results of this research will be shared and discussed with cooperating growers and should help them make more informed management decisions about the rotation from alfalfa to corn.
Areas needing additional study
Additional research is needed to identify when corn following alfalfa will require fertilizer N to economically optimize corn grain and silage yield. In most cases, fertilizer N is not needed, however when fertilizer is needed but not applied, farmers lose profit and trust in the alfalfa N credit to corn.