Siratro — a tropical pasture legume bred from Phaseolus atropurpureus

Abstract

A new perennial tropical pasture legume, that has been named Siratro, has been bred from a cross between two Mexican strains of the wild species Phaseolus atropurpureus DC. It is a composite of three high yielding selections which are transgressive segregates from the cross with a marked intensification of the stoloniferous character. The breeding method wed was to select superior recombinants in the F2 population and then subject plots of the F3 and F4 families from them to a system of heavy intermittent grazing with cattle. This mass-selected those plants able to spread and produce seed under high grazing pressure. Siratro has inherited the high resistance of the parents to root-knot nematode and the virus disease legume little leaf. These resistances are necessary in pasture legumes grown in northern Australia. Under grazing, Siratro not only outyields the parents but also all other introduced strains of P. atropurpureus available. The marked stoloniferous habit of the selections in Siratro gives greater persistence and density and high yielding ability. This in turn results in the selections maintaining the Rhodes grass sown with them. The composite nature of Siratro should give it sufficient variability for its adaptation to the spear grass zone Queensland and other coastal and sub-coastal areas of northern Australia.