Effect of Tillage System on the Spontaneous Regeneration of Two Annual Medics (Medicago Spp.) After Wheat in North Syria

Abstract

Four tillage treatments were applied to soil on which wheat was to be sown in 1988/89 after pastures of two annual medics, Medicago rigidula and M. polymorpha, had been established in 1987/88. Germination of the medics was monitored in the following year (1989/90) when halfof each plot was sprayed to prevent further seed set. M. polymorpha produced more seed in 1987/88 and three times more regenerated seedlings in 1989/90 than M. rigidula. Regeneration and winter herbage yields of both medic species were least after deep ploughing. Seedling numbers were less in 1990/91 than in the previous year but seedling numbers of M. polymorpha were sufficient to sustain the cereal-pasture rotation except after deep ploughing. Both medics germinated promptly soon after the first effective rains in December 1989, but the first germination in 1990/91 was on 24 November, with three subsequent germinations, the last in early February. About half (40 to 50%) of pods remained in the top 5 cm of soil, except after deep ploughing when only 28% remained near the top. The seeds in the top layer lost their hard-seededness more rapidly than those buried more deeply. M. polymorpha was softer seeded than M. rigidula. The results suggest that deep ploughing, as commonly used in west Asia and north Africa, is unsatisfactory for cereal-pasture rotations.