On the semi-arid U.S. Great Plains, summer fallow is a dominant practice in the winter wheat agroecosystem. Fallow has stabilized wheat yields but jeopardizes long-term economic and ecological sustainability in many ways, including: low water use efficiency as only 20-40% of precipitation during the 14 month fallow period is stored for the subsequent wheat crop; soil erosion by both water and wind resulting in soil loss and pollution of water and air; loss of soil organic C and N; leaching of fertilizers and pesticides into ground water; generation of saline seeps; and only one harvest every two years. More intensive farming that includes annual legume pastures in the rotation may prove more profitable for producers at the same time that it solves problems of ecological sustainability.

In semi-arid southern Australia, legume pastures have replaced fallow and integrate dryland crop and livestock production. Literally, "ley" means "meadow," and "ley farming" is to grow crops in rotation with self-regenerating annual legume pastures. In the ley farming system, medic pasture (annual species of the genus Medicago) alternates with wheat in a two year cycle. Medics regenerate yearly from a soil seed bank, and in the pasture phase of the cycle provide forage for livestock. In the cereal phase, regenerating medics may briefly furnish forage before seeding the cereal crop.

In America, a ley system might provide many benefits including (1) more profitable cereal production, (2) quality livestock forage, (3) self-regenerating pastures, (4) integrated pest management, (5) reduced fertilizer inputs, (6) improved water use efficiency, (7) improved water quality, (8) improved air quality, (9) soil conservation, (10) improved soil quality, (11) no need for strip farming, and (12) global benefit of increased carbon fixation.

The University of Wyoming, in collaboration with the South Australian Research and Development Institute, is conducting research on ways in which annual Medicago spp. might partially replace fallow either as "summer medics," regenerating and completing a life cycle within a single summer growing season before wheat planting or after wheat harvest, or as "winter medics," regenerating under or after wheat, overwintering, and completing the life cycle the following spring and summer before planting of the subsequent wheat crop.

In this paper I summarize plant breeding research towards achieving a WINTER MEDIC IDEOTYPE for the Great Plains comprised of six traits. (1) Winterhardiness. Medicago rigidula and M. rigiduloides have proven to be the most winterhardy annual medics in Wyoming. Winter desiccation rather than cold per se appears more lethal to medics. (2) Nitrogen Fixation. These species differ markedly in Rhizobium specificity, although distinct within-species differences are also evident. M. rigidula is effectively nodulated with commercial M. sativa inoculum. (3) Regeneration. High levels of hardseededness are a prerequisite to annual legume persistence through crop rotations and both M. rigidula and M. rigiduloides produce hard seed in Wyoming. Breakdown of hardseededness and effectiveness of regeneration from soil seed banks will require many years of evaluation. (4) Competitiveness. Medics must compete with aggressive weeds on the Great Plains. Although well-established medics can suppress weeds, successful establishment and regeneration of medics may prove to be more a function of wheat, residue, weed and grazing management than of genetics. (5) Forage Production. Insofar as medic pasture replaces fallow which provides little grazing except for wheat residue, this is an "easy" trait. (6) Early and Prolific Seed-Set. Better lines of M. rigidula and M. rigiduloides survive winter to produce sufficient seed to sustain a soil seed bank. Seed is mature by early July to provide a brief period of true fallow before wheat planting.