Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/1997
Publication Date: N/A
Citation: Interpretive Summary: Farmers and scientists are learning the benefits of diversifying cropping systems. Lower pest infestations, more stable and higher crop yields, and reduced commercial fertilizer requirements are a few of these benefits. Including legumes in crop rotations can significantly reduce the need for purchased nitrogen fertilizer, but many farmers are reluctant to sow long-lived legumes, like alfalfa, because perennial crops limit flexibilit in cropping decisions. Annual medics are closely related to alfalfa but grow only for one season. These are used widely in Australia but represent new crops in the USA. We measured the growth and nitrogen contribution of several annual medics under Upper Midwest conditions and found that they can contribute as much nitrogen in 2 to 3 months as farmers usually apply to corn. As seed supplies of improved varieties of these fast growing medics become available, they may provide high quality forage for farm animals, a rapid means of protecting soils from erosion, and reduce the bill for nitrogen fertilizer on Midwest farms.
Technical Abstract: Assessment of the N2 fixation potential of annual medics (Medicago spp.) in the upper Midwest is important for predicting their net N contribution to cropping systems. Our objectives were to determine the dry matter (DM) accumulation pattern of herbage, roots, and nodules of several annual medic species used as summer annuals and to measure the amount of N derived from symbiotic N2 fixation. Experiments were conducted on a loamy sand at Becker, MN, and a silt loam at Rosemount, MN. The experimental design at both locations was a randomized complete block with 6 replicates. M. trunculata, M. polymorpha, M. scutellata, and M. rugosa were inoculated with commercial Rhizobium and seeded in late May. Herbage, root, and nodule DM of N2 fixing medics increased until about 72 d after planting. Highest herbage, root, and nodule DM yields were 10669, 648, and 169 kg/ha, respectively, from M. polymorpha in 1993. Annual medic herbage at maximum DM contained 86% N derived from the atmosphere, using ryegrass (Lolium multiflorum) as a reference crop, and 79% using noninoculated M. rugosa as the reference. The amount of N2 fixed ranged from 100 to 200 kg N/ha, with M. polymorpha fixing the most and M. rugosa fixing the least. We conclude that annual medics have the potential to contribute a significant amount of N to cropping systems when seeded in the spring and harvested or incorporated into the soil 2 to 3 months later.