Annual medics, principally cultivars of the barrel (Medicago truncatula), strand (M.littoralis), burr (M.polymorpha) and disc (M.tornata) medic species, have been widely used in Australian ley farming systems since the 1950’s. However, the arrival of both spotted alfalfa aphid (SAA) and blue-green aphid (BGA) in Australia in the late 1970’s resulted in massive damage to medic pastures throughout the continent, with all cultivars available at that time being highly susceptible to one or both pests. In consequence, considerable effort has been devoted to the breeding and selection of multi-aphid resistant cultivars.

Phase 1; cultivars developed directly from introductions.

Mass screening for SAA and BGA resistance of about 12 000 accessions in the medic species mentioned above resulted in the identification of a small number of barrel medic lines only with resistance to BGA and some resistance to SAA. Out of the 12 000, less than 0.1% exhibited sufficient resistance to warrant field test, but from these, the cultivars Paraggio (1984), Sephi (1986), and Parabinga (1987) were released.

Phase 2; cultivars developed by hybridisation and selection.

Unlike alfalfa, medics are very highly self-fertile, generally diploid and technically difficult to hybridise, so that breeding and selection systems for these annuals are quite contrasting to the perennials. On the minus side, this necessitates medics needing to undergo generally at least 6 generations of segregation post hybridisation before lines with acceptably high levels of homozygosity can be isolated. On the plus side, resistance can be fixed in all plants of a cultivar, and cultivars are much more readily definable and distinguishable from each other.

Paraggio, Sephi and Parabinga all have significant agronomic deficiencies, and hence efforts were subsequently focussed on breeding and selection of resistant cultivars without those weaknesses. This process has been highly successful; SAA and BGA resistance genes have not only been moved around within the barrel medic species, but have also been transferred into strand and disc medics. The success of this process can be gauged against the fact that the newer, bred cultivars produce up to 20% more herbage and as much as 40% more seed than the cultivars they replace in the first year alone; even in the absence of aphids. The cultivars in this category are Harbinger AR strand (1990), Mogul barrel (1992), Caliph barrel (1994) and Herald strand (1996) medics. Pending (unnamed) aphid resistant releases include another barrel, as well as a disc medic.

With respect to the North American scene, it appears likely from the Australian experience that pest resistance, particularly to the already present alfalfa aphids, will be an advantage for at least some regions.