Changes in the size and composition of the seed bank of medic pastures grown in rotation with wheat in north Syria

Abstract

A mixture of 84 accessions of 12 medic (Medicago) species were sown in two successive years into a two-course pasture/wheat rotation in north Syria. Changes in the size and botanical composition of the seed bank were monitored for two cycles of the rotation. At the end of the first year the seed bank consisted of 550 kg ha-1 (sowing 1) and 330 kg ha-1 (sowing 2) of medic seed. Subsequent grazing reduced these seed banks to 225 kg ha-1 and 255 kg ha-1 respectively, and germination in the following wheat crop further reduced it to 205 kg ha-1 and 150 kg ha-1. The seed banks reached 700 kg ha-1 and 790 kg ha-1 respectively, during the second pasture phase of the rotation. The seedling density of medics in the regenerating pasture was 750 m-2 (sowing 1) and 1120 m-2 (sowing 2). Medics comprised about 95% of the pasture by mid-spring in both phases, the remaining 5% being wheat and broad-leaved weeds. The productivity of regenerating pasture was 5 t ha-1 (sowing 1) and 8 t ha-1 (sowing 2). Only three medic species, M. rotata, M. noeana, and M. rigidula, increased as a proportion of the seedbank. The increase was greatest in the first year, though M. noeana and M. rigidula increased further in the next two years. M. littoralis and M. turbinata, initially unsuccessful, became more successful later, while M. scutellata and M. constricta continued to decline throughout the experiment. There were also appreciable differences in the seed bank of individual accessions of several species. The results are discussed in terms of (a) the grazing strategies required in pasture/wheat rotations, and (b) the use of mixtures to select medics suitable for such rotations.