Plant attributes leading to persistence in grazed annual medics (Medicago spp.) growing in rotation with wheat

Attributes of 84 accessions of 12 medics (Medicago spp.) were measured in nursery rows in two successive years. Multiple regression, principal component, and cluster analyses were used to relate 14 attributes of the medics to persistence in the seed bank of grazed pasture growing in rotation with wheat. Principal component analysis distinguished between the attributes of the species. For example, M. rigidula had short petioles, high frost tolerance, many seeds per pod, and large leaves and seeds; while M. polymorpha had long petioles, low frost tolerance, few seeds per pod, and small seeds and leaves. M. noeana produced many flowers per raceme, small pods and seeds, long peduncles, and was hard-seeded and late flowering; while M. aculeata produced few flowers per raceme, large pods and seeds, short peduncles, and was soft-seeded and early flowering. M. trulncatula and M. rotata were intermediate. Long peduncles and high levels of hardseededness were the attributes most closely associated with persistence of the medics in grazed pasture. It was concluded that (1) long peduncles place the flowers above the canopy in spring where they are in full sun light, and (2) hardseededness levels of up to 90% protect seeds against germination in the cereal year. The results also suggest that small leaves and short internodes and petioles protect young plants against over-grazing in winter, and small pods and seeds are less likely than large pods and seeds to be selected and digested by grazing sheep in summer.