- Agronomic: corn, hay
- Animal Products: dairy
- Animal Production: feed/forage
- Crop Production: conservation tillage, crop rotation, no-till, nutrient cycling
- Education and Training: on-farm/ranch research
- Soil Management: nutrient mineralization, soil analysis
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.