Thesis Abstract of AGS Students

Intercropping Sesbania rostrata (Brem. and Obrem.) with rice

Nguyen Ngoc De (1992)

Intensive land use in rice-based multiple cropping has led to increases in short term productivity with the trade-off in sustainability. Many attempts have been made to sustain soil fertility, and the use of green manure has been suggested. Sesbania rostrata is one of the potential green manure crops but this potential has yet to be met with the current practices.

The present study was carried out to explore possibilities to incorporate Sesbania rostrata into an intensive rice-based cropping system. A brief field survey with semi-structured interviews were undertaken in four villages of Tien Giang and Dong Thap provinces in the Mekong Delta of Vietnam. Results show that a species of Sesbania can be grown in association with the main rice crop during June to November but its effect on rice was not established. Stems of the Sesbania were also found removed for use as firewood, therefore the contribution of Sesbania as green manure is probably lessen.

A field experiment was conducted to examine the relationships between Sesbania rostrata and rice and their effects on yields and yield components in both species. The contribution of nitro- gen fixation from S. rostrata was also investigated. Both species were grown in mixtures arranged in the replacement series with four different proportions ranging from sole cropping of the two species and intercropping with 25 : 75 and 50 : 50 percents of S. rostrata to rice, respectively. S. rostrata was sown at three dates relative to the rice, i.e., sowing and transplanting at the same time as rice; sowing at 0 and 30 days after transplanting rice. The advantages of intercropping were estimated as Relative Yield Totals (RYT).

Results indicated that S. rostrata severely out competed the rice when established at the same time. The competitive ability of S. rostrata decreased greatly when establishment was delayed. A small proportion of S. rostrata (25%) did not depress rice yields in the delayed establishments of 30 and 60 days relative to rice, but seed production of S. rostrata was highest in the early Sesbania establishments and high proportions in the intercrops. Nevertheless, reasonable amounts of Sesbania seeds, 1.3-2.0 t/ha, could be obtained by delaying Sesbania establishment of 30 days.

The RYT-values for nitrogen yield were greater than 1.0 in the early establishments, i.e., 0 and 30-day differences in seedling ages with rice, suggesting beneficial effects of intercropping in terms of nitrogen nutrition. When S. rostrata was established at the same age as rice (early establishment), no differences in nitrogen fixation were detected in sole and intercrop S. rostrata. The average amount of nitrogen fixed was about 327 kgN/ha. Delaying establishments by 30 and 60 days reduced nitrogen fixation of S. rostrata in intercrops by 47 and 76 percent, respectively.

With exception of the latest Sesbania establishment, returning the residual of total biomass of S. rostrata and rice straw to the field gave positive nitrogen balances of 36-132 kgN/ha. Removal of Sesbania stems as being practiced by farmers, resulted in a negative balance of nitrogen.

This study reveals that S. rostrata can be successfully established as green manure in the rice crop by intercropping. The critical time of nitroduction of S. rostrata into the stand of rice is when the establishment of S. rostrata is delayed by 30 days. Reasonably small proportions (eg. 25%) of S. rostrata in intercropping should be practiced to prevent yield losses in rice and to obtain some nitrogen input and adequate seed production for S. rostrata.

In practice, S. rostrata should be arranged in such a way that it will not interfere during the rice harvest. Strip intercropping of S. rostrata and rice could be evaluated. Clipping may be introduced to reduce the competition from early established S. rostrata on rice. Residuals from the clipping can be readily incorporated as green manure during the growing season of rice. However, effects of clipping on nitrogen fixation in S. rostrata need to be explored.

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