Effect of rotation and cultivation systems on establishment and persistence of annual medics

Abstract

The persistence and annual and seasonal regeneration of 3 annual Medicago species, M. rugosa cv. Paragosa, M. scutellata cv. Robinson and M. truncatula cvv. Cyprus and Jemalong, were compared between 1978 and 1985 over 3 rotation systems, permanent pasture (R1); pasture, barley, pasture, barley repeated (R2) and pasture, fallow, wheat, barley, repeated (R3). Ploughing practices adopted for the various rotations affected persistence and regeneration. Not all ofthe seed that was produced in 1978 produced seedlings. A much lower proportion of the seed reserve of cv. Paragosa produced seedlings compared with cv. Robinson and cvv. Cyprus and Jemalong. The poorer re-establishment of cv. Paragosa could have been due to its more permeable seed compared with the other cultivars. Seed reserves persisted for a maximum of 7 years. Annual re-establishment was greater for all cultivars under the system of permanent pasture than under either the pasture, barley or the pasture, fallow, wheat, barley rotations which disturbed the soil. Unlike the other cultivars, germination and establishment rates of cv. Robinson were low following the first autumn rains in all years. Germination of this cultivar usually occurred from June to November following a delayed breakdown in hardseededness. It also responded to cultivation so that with the exception of R2 in 1980, the highest annual plant populations in R2 and R3 were in the cropping and/or fallowing phases rather than the pasture phases. Although cv. Paragosa produced the highest seed yield in the year of sowing, its reserves were exhausted by the third year in all rotations, whereas the harder seeded cultivars maintained adequate seed reserves to enable good plant re-establishment in the sixth year under all rotations, except in the continuous pasture rotation, where seed reserves of cv. Robinson were exhausted by the fifth year. Cultivation and consequent pod burial in the 2 cropping rotations preserved seeds longer than did the continuous pasture system. Jemalong maintained higher (P< 0.05) seed reserves than cvv. Cyprus and Robinson in the sixth year.