Abstract
A set of experiments were carried out at the Central
research station of Bangladesh Agricultural Research Institute,
Joydebpur, Gazipur during April through December for four
consecutive years to identify suitable crop(s) or crop
combinations for early kharif season and determine effects of
legumes on the yield performance of succeeding transplanted
aman rice. Monitoring of changes in fertility status of the
experimental soil was also carried out. Aus rice (BR 21), maize
(var. Barnali) and blackgram (var. Barimash) as sole crops and
blackgram as intercrop with maize and strip crop with rice and
Sesbania inter - strip cropped with rice were tested during early
kharif season (March-May). Sesbania and blackgram plants were
harvested and incorporated into the soil at 45 and 75 days after
seeding. After the harvest of early kharif crops, aman rice (var.
BR 11) was transplanted in the middle of August under four
variable N treatments (0, 40, 80 and 120 kg N/ha). Nitrogen
levels were superimposed over the previous cropping treatments
accommodated in a split-plot design.
Aus rice yields varied from 1.16 to 2.40 t/ha depending on
the treatment. The low yield of rice in strip cropping was due to
reduced the land areas occupied by Sesbania and blackgram plants.
Blackgram yields ranged between 0.25 and 0.79 t/ha and maize
yields from 1.72 to 3.90 t/ha. The highest rice equivalent yield (2.99 t/ha) was obtained from maize paired row + blackgram
combination. The same combination also gave highest gross return
(Tk 17801/ha), net return (Tk 9640/ha) and benefit cost ratio
(2.18). Previous cropping exhibited significant influence on the
succeeding transplanted aman (T.aman) rice. Highest grain yield
(3.71 t/ha) was obtained from T.aman when preceded by blackgram
in sole situation. Interaction effects of previous cropping and N
levels were found significant. The highest yield (4.32 t/ha) of
T.aman rice was recorded with 120 kg N/ha when preceded by
blackgram. The highest total rice equivalent yield (7.26 t/ha),
gross return (Tk 41786/ha) and net return (Tk 26180/ha) were
obtained from the cropping sequence of maize paired row +
blackgram - T.aman rice with 120 kg N/ha. However, benefit cost
ratio (2.72) was higher from the same combination with 80 kg
N/ha. Aus rice + blackgram - T.aman rice also showed higher
benefit (2.54) cost ratio than Aus rice - T.aman rice with 80 kg
N/ha cropping sequence. Maize paired row + blackgram T.aman
rice with 40 kg N/ha gave higher benefit (2.53) cost ratio than
Aus rice- T.aman rice with 80 and 120 kg N/ha). Variation in
rice yield in response to preceding crops and N levels accrued
mainly from yield contributing characters i.e. panicles/sq.m and
grains/panicle.
Biomass incorporated into the soil ranged between 2.85 and
7.56 t/ha from Sesbania and 1.30 to 5.74 t/ha from blackgram.
Soil chemical analysis showed that organic matter content of the soil increased (1.32 to 1.42%) in legume- T.aman rice cropping
sequence but decreased (1.20 to 1.10%) in Aus rice - T.aman rice
sequence. A continuous build up of the elements like phosphorus,
potassium and sulphur in the soil was observed in all the
cropping sequence. The maximum (29 P mg/kg, 109 К mg/kg and 45 S
mg/kg) deposition was recorded in legume - T.aman rice cropping
sequence. A positive balance of the nutrients was recorded at the
end of four cycles.