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Influence of defoliation intensity and time of final defoliation on seed yield of paraggio barrel medic

F.N. Muyekho, E.D. Carter and G.K. McDonald

Waite Agricultural Research Institute, University of Adelaide, Glen Osmond SA 5064

High medic seed production followed by sensible summer/autumn grazing is important for the success of a ley farming system (1). Frequent defoliation up to the commencement of flowering increases seed yield in subterranean clover because of increased inflorescence and burr burial (2). Annual medic seed producers and graziers have adopted the same strategy to try and increase seed yield, however, there is little information on whether this strategy increases seed yield in annual medics. An experiment was carried out to examine the effect of intensity and time of final defoliation on the components of seed yield of barrel medic (Medicago truncatula cv. Paraggio).

Methods

The experiment was carried out in a commercial seed crop at Korunye, Adelaide in 1990. The field was sown in late May and heavily grazed during July. The plant density was 580 plants/ m2. The swards were uncut, defoliated at 3 cm (severe) or 6 cm (lenient) at three stages of reproductive growth - start of flowering (6 September), 10 days and 20 days after the start of flowering. The start of flowering was defined as the stage when about 10% of the plants had begun to flower.

Results and discussion

Highest yields of dry matter (DM) and seed were achieved with the uncut control (Table 1). Lenient defoliation at the start of flowering did not significantly reduce seed yields, but delays in time of defoliation beyond the start of flowering and/or severe defoliation significantly reduced seed yield. Seed yield was significantly correlated with leaf DM (r=+0.97), LAI (r=+0.98), total DM (r=+0.99) 40 days after flowering and pods/ha (r=+0.99) at maturity. Seed size was only significantly reduced by severe defoliation whereas number of seeds per pod was stable except with defoliation 20 days after start of flowering.

These results suggest that severe grazing during early vegetative growth followed by lenient defoliation at start of flowering has little effect on seed production of M. truncatula provided the total DM production is restored to that of the uncut control. This finding is of potential importance for the management of regenerating pastures, however further studies are required to establish whether the relationship holds for a wider range of medic cultivars and growing conditions.

Table 1. Effect of height and time of cutting on total DM, leaf weight, LAI 40 days after the start of flowering, seed yield and seed yield components.

References

Carter, E.D. 1982. Proc. 2nd Aust. Agron.Conf., Wagga Wagga. p. 180.

Collins, W.J. 1978. Aust. J. Agric. Res. 29, 789-807.

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