Current HRWW production utilizes primarily native organic N which is depleted as N is mineralized and soil organic matter is lost (Schuman et al. 1994). N applied as commercial fertilizer is also used which adds to production costs and public concern about environmental pollution.
In some regions of the world annual legumes have replaced fallow in cereal agroecosystems. Self-regenerating pastures of annual medics (Medicago spp.) grown in rotation with wheat contribute to sustainable agriculture in the cereal/pasture zone of southern Australia by providing organic N, conserving and building soil, and providing a break from cereal pests and diseases.
This wheat/sheep system is a short-term annual system of pasturing in a 330 to 450 mm annual precipitation zone which occupies as much as 30 million hectares (Crawford et al. 1989). The legumes naturally regenerate annually from seed. They are prostrate in growth habit, making them relatively easy to manage in the wheat crop. Pasture phases are short term (1 to 3 years) followed by wheat. Soil nitrogen and water-stable soil aggregates increased with each pasture phase and decreased with each wheat phase (Reeves 1987); however, in the integrated system both nitrogen and water-stable aggregates are maintained above the previous continuous cropping levels.
Pest control benefits that result from crop rotation can also have a significant economic and environmental impact. For example, in Australia, cereal cyst nematode (Heterodera avenae Woll.) can be a serious problem. Rotation to annual grass-free legume pasture can effectively control this pest as well as the disease, take-all (Gaeumannomyces graminis var. tritici Walker) (Tow and Schultz 1991). On the Great Plains 2- or 3-years rotations with legumes will result in control of Cephalosporium stripe [Cephalosporium gramineum Nis. & Ika. (syn. Hymenula cerealis Ell. & Ev.)] (Wiese 1977). Additionally, winter annual grass weeds such as downy brome (Bromus tectorum L.) can be effectively managed in legume rotations.
In northern Idaho, fall-planted Austrian winter pea [AWP, Pisum sativum spp. arvense (L.) Poir] has also been shown to have the potential to improve soil quality, producing in excess of 10 t ha-1 of organic matter (Auld et al. 1982). On the Western High Plains, winter annual legumes could play a similar role in rotation with wheat by partially replacing the typical 14 month fallow period if materials can be identified with sufficient cold-tolerance to survive the harsh winters of this region. The objective was to determine winter survival potential of cultivars and/or experimental lines of annual medic and Austrian winter pea on the Western High Plains.
Our results indicate that M. rigidula offers the greatest potential for winter survival of all the annual medics evaluated in this region. Our study confirms results of Abd El Moneim and Cocks (1986) and Cocks and Ehrman (1987) at the International Center for Agricultural Research in the Dry Areas, Allepo, Syria. These researchers found that in two severe winters (1983 and 1984), Medicago rigidula exhibited more cold tolerance than other annual Medicago spp.
Recently M. rigidula has been differentiated into the two species M. rigidula (L.) All. and a new species, M. rigiduloides E. Small. (Small et al. 1990; Small 1990). Subsequently, research at the South Australian Research and Development Institute (R.W. Groose, R. Ballard, N. Charman, A.W.H. Lake, unpublished results) has demonstrated that the two species have very different Rhizobium requirements. Commercial strains of Rhizobium recommended for various annual medic species in South Australia effectively nodulated M. rigidula lines but failed utterly with lines of M. rigiduloides. Conversely, some experimental Rhizobium strains effectively nodulated M. rigiduloides but were only weakly effective with M. rigidula.
Lines of M. rigidula and M. rigiduloides from high elevations and the northern limits of the ranges of these species in Europe and Asia have been selected and used as parents in a breeding program to produce winter-hardy medics adapted to the Western High Plains that will combine enhanced cold-tolerance, optimum Rhizobium specificity, reduced pod spininess and high productivity.
|M. littoralis cv. Harbinger AR (strand medic)||15||0|
|M. truncatula cv. Caliph (barrel medic)||44||0|
|M. truncatula cv. Paraggio (barrel medic)||9||0|
|M. sativa cv. Ladakz (alfalfa)||28||11|
|M. scutellata cv. Sava (snail medic)||16||0|
|M. polymorpha cv. Santiago (burr medic)||260||0|
|M. polymorpha SC03077 (burr medic)||260||7|
|M. rigidula SCO3075||260||143|
|M. sativa cv. Ladakz (alfalfa)||260||208|
|M. truncatula cv. Paraggio (barrel medic)||260||7|
|M. truncatula SCO7078 (barrel medic)||260||14|